1.2 Typology and structure of hydrocarbons

1.2.1 Aliphatic hydrocarbons IUPAC (International Union of Pure and Applied Chemistry) nomenclature entails the use of a common The first important class of hydrocarbons is aliphatic suffix to refer to any given class of compounds; for the hydrocarbons, from the Greek ¨leifar, ‘oil, fat’, which alkanes this suffix is -ane. The first four alkanes are comprises the alkanes, alkenes and alkynes. named methane, ethane, propane and butane; when dealing with compounds containing 1, 2, 3 or 4 carbon Alkanes atoms, the prefixes meth-, eth-, prop- and but- are always used. Starting from alkanes with 5 carbon Structure atoms, prefixes are used which simply indicate the Alkanes are hydrocarbons with sp3 hybridized carbon number of carbon atoms present in the molecule:

atoms and the general formula CnH2nϩ2. They are described pentane, hexane, heptane and so forth. The as ‘saturated’ because in their molecules, the four possible nomenclature for alkanes with a large number of bonds of carbon – arranged in space according to a regular carbon atoms is reported in Table 1. tetrahedral structure – are simple and saturated with hydrogen Molecules with an identical molecular formula but atoms or other carbon atoms. The bond angles are identical to different structure are known as structural isomers which one another and measure 109.5°. The alkane series is have different chemical-physical properties and chemical described as ‘homologous’ because the molecules differ from reactivity. one another by a constant amount: to move from one alkane For non-straight chain alkanes, IUPAC nomenclature sets

to the subsequent one a CH2 unit is always added. a series of rules for their identification: Generally speaking, hydrocarbon molecules are described • Identify the longest straight chain containing only by various equivalent descriptive systems: all the atoms carbon atoms in the molecule and all the alkyl residues belonging to the molecule may be reported explicitly, or only bound to it. the carbon atoms, implying that all the free valences of these • Number each carbon atom in this chain progressively so atoms are saturated with hydrogen atoms. Alternatively, only that the substituents are given the lowest numbers. If a )the skeleton of the intramolecular bonds may be shown: substituent recurs several times in the structure, the prefixes di-, tri-, tetra-, penta- and so forth are used. CH 3 C C • Prefix the alkyl residues with the number of the carbon C4H10 CH C of the longest chain to which they are attached. • If two chains of identical length can be identified, the H3C CH3 C one with the most substituents is used.

Table 1. IUPAC nomenclature for alkanes as a function of the number of carbon atoms

Carbon atoms Name Carbon atoms Name Carbon atoms Name 10 decane 22 docosane 60 hexacontane 11 undecane 23 tricosane 70 heptacontane 12 dodecane 30 triacontane 80 octacontane 20 icosane 40 tetracontane 90 nonacontane 21 henicosane 50 pentacontane 100 hectane

VOLUME V / INSTRUMENTS 9 NATURE AND CHARACTERISTICS OF HYDROCARBONS

polarized light to the right; the symbol (S) (from the 2 2 Latin sinister) if it rotates light to the left. The 24 1 313 135 existence of these isomers is extremely important for biological compounds like aminoacids, but it is purely 2-methylpropane 2,2-dimethylpropane 2,2,4-trimethylpentane academic for alkanes (except in some specific conditions). Often, however, IUPAC nomenclature is not used, and many compounds are named in accordance with Methane the rules in force before its introduction. In the case of The simplest alkane is methane, discovered in 1776 by the molecules shown above, for example, the most Alessandro Volta who, during a boat trip on Lago Maggiore common names are isobutane for 2-methylpropane, near Angera, noticed gas bubbles rising from the muddy neopentane for 2,2-dimethylpropane and isooctane for bottom of the lake. Volta later collected this gas, noting its 2,2,4-trimethylpentane; the latter is the chemical inflammable nature and naming it inflammable native swamp compound used to determine the octane number of gas (in this case, the methane was produced by anaerobic fuels (specifically, isooctane is conventionally given an organisms, known as methanogens, on the lake floor). With the Ϫ Ϫ octane number of 100). general formula CH4, it has angles between the H C H The residues formed by alkyl groups with a free bonds which are all identical and measure 109.5°; the CϪH valence deriving from alkanes lacking a hydrogen atom distances measure 1.091 Å, whilst the energy of each bond is are described with the suffix -yl. We therefore have 104 kcal/mol. This is an apolar molecule as it is perfectly methyl, ethyl, propyl and butyl residues, etc. The first symmetrical, and therefore has a dipole moment of zero. four residues are also indicated by the symbols Me, Et, Pr and Bu. The carbon atoms in alkane molecules can Ethane be classified according to the number of hydrogen The higher homologue of methane is ethane, with a

atoms to which they are attached: those attached to general formula of C2H6; the ethane molecule has a covalent three hydrogen atoms are known as primary, those CϪC bond of s type formed by the overlap of sp3 orbitals attached to two hydrogen atoms as secondary; finally, measuring 1.536 Å, and HϪCϪH bond angles of 109.3°. tertiary carbon atoms are attached to one hydrogen The s bond allows for the relative rotation of the methyl atom. Alkyl residues, therefore, can be classified groups without affecting the combination of sp3 orbitals according to the type of carbon on which the free leading to their formation. This allows the molecule to take Ϫ valence is found. In the case of C4H9 residues, a on different arrangements, known as conformations, which distinction can be made between: butyl, sec-butyl and may change into one another without cleaving any bond or tert-butyl, depending on whether the residue is exceeding a significant potential energetic barrier. The study primary, secondary or tertiary: of energy changes in molecules as a result of a change in CH conformation is known as conformational analysis. Since 3 little energy is required to change the conformation, the CH3 CH2 CH2 CH2 CH3 CH2 CH relative rotation of the methyl groups is considered to be free. Fig. 1 shows the transition to different conformations of butyl sec-butyl an ethane molecule. It can be seen that ethane always returns to the same condition after a rotation of 120° around the CH3 CϪC bond. The conformation represented by the three H C C hydrogen atoms superimposed on one another is known as 3 eclipsed, whilst a staggered conformation is obtained by CH3 tert-butyl A further distinction between alkane molecules can be made if one or more chiral carbon atoms are present inside the alkane; a carbon atom is chiral if the four substituents are all different. A chiral compound has the special property of having a non-superimposable mirror image. It therefore has two mirror structures – known as enantiomers – which, like right and left hands cannot be superimposed and 3 kcal/mol

must therefore be considered different. Enantiomeric potential energy compounds have identical physical properties, except for their opposite specific rotatory power, in other words the ability to rotate plane-polarized light impacting upon them to the right or to the left. Thanks to this particular property, a distinction can be made 060120 between two enantiomers which otherwise, according rotation (°) to IUPAC standards, would be identified by the same name. The symbol (R) (from the Latin rectus) is used Fig. 1. Potential energy of the ethane molecule before the name of the alkane if the enantiomer rotates in its different conformations.

10 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

rotating the methyl groups 60° around the bond; between conformation. Fig. 2 shows the variation in potential energy these two structures there is an infinite number of other between the different conformations of the butane molecule. conformations, generically described as skew. Fig. 1 shows In order of molecular weight, butane is the first alkane to that the energetic barrier to rotation (known as torsional possess two different structural isomers: butane (also known

strain) for the ethane molecule is about 3 kcal/mol; this is as normal butane, n-C4H10) and 2-methyl propane (also due to the repulsion of the electron clouds around the known as isobutane, i-C4H10). hydrogen atoms which, in the eclipsed conformation, are As a consequence of the different spatial arrangement of affected to a greater extent by their mutual interaction. The the atoms, the two molecules differ in the type of carbon staggered and eclipsed structures are known as conformers. atoms of which they are composed. In fact, butane has two primary carbon atoms and two secondary carbon atoms, Propane and butane. Conformational analysis whereas 2-methylpropane has three primary carbon atoms The members of the alkane series above ethane are propane and a tertiary carbon atom.

(C3H8) and butane (C4H10). In propane, a free rotation around As the molecular weight of alkanes increases, the the two CϪC bonds can also be observed with a torsional number of isomers rises exponentially from 2 for butane to

strain slightly above the 3 kcal/mol of ethane due to the 75 for C10H22, over 300,000 for C20H42 and above four presence of a methyl group instead of a hydrogen leading to a billion for C30H62. greater repulsion between the two mutually rotating groups. Of particular interest is the butane molecule Higher alkanes Alkanes with a molecular weight from 70 to 240 u are 2 4 liquid under standard conditions (298 K and 1 atm); 1 3 however, if their molecular weight reaches or exceeds 240 u, they are solid and described as waxes. It is also worth noting which, as well as being the first member of the alkane series to the existence of some alkanes with a high molecular weight have four conformers, also presents two different structural (over 1 million daltons) produced by polymerization isomers. reactions. Polyethylene, for example, is an alkane consisting Ϫ Ϫ The conformational analysis of butane shows that of an extremely long chain of CH2 groups which, with Ϫ rotation around the C2 C3 bond leads to the formation of the exception of the two ends of the chain, is composed of the conformers anti I, eclipsed II, gauche III, eclipsed IV, secondary carbon atoms. Polypropylene, by contrast, is an gauche V, eclipsed VI, returning to the conformer anti I when alkane which may have the following structures: the bond has rotated through 360°. The eclipsed II and eclipsed VI structures have identical energy, as do the two gauche conformers. In butane, the maximum potential ] ] energy difference between the anti I structure, the most isotactic polypropylene stable, and the eclipsed IV structure which is the most unstable, is over 5 kcal/mol. Compared to the energetic barrier of the various conformations of ethane, higher values are found for butane. This is due to the fact that in butane the ] ] electronic repulsion is greater, since it occurs between a syndiotactic polypropylene hydrogen atom and a methyl group in the eclipsed II and VI structures and between two methyl groups in the eclipsed IV ] ] eclipsed II eclipsed IV eclipsed VI atactic polypropylene Each carbon atom in the main chain is linked to four different substituents and is therefore chiral; a distinction can thus be made between the different forms of polypropylene by the arrangement of the substituents as well as by molecular weight. Specifically, if the methyl groups are all oriented towards the same side of the chain, the 4.5 polymer is described as isotactic; however, if they are 3.4 kcal/mol kcal/mol oriented in alternating directions or randomly, the polymer is 0.8 described as syndiotactic or atactic respectively. In this case, kcal/mol potential energy the alkane’s stereospecificity (the way in which its spatial molecular structure conditions its properties) is extremely important because the different polymers have different chemical-physical and mechanical properties. gauche III gauche V anti I anti I Cycloalkanes rotation (°) As well as alkanes with straight-chain and branched structures, there are also alkanes which have a cyclic or Fig. 2. Potential energy (not to scale) of the conformers polycyclic structure. The nomenclature for cycloalkanes of butane depending on the angle of rotation. follows that for alkanes, with the addition of the prefix

VOLUME V / INSTRUMENTS 11 NATURE AND CHARACTERISTICS OF HYDROCARBONS

cyclo-. Below, the structures of cycloalkanes containing up to nine carbon atoms are reported: energy l a i 11 kcal/mol cyclopropane cyclobutane cyclopentane cyclohexane potent 5.5 kcal/mol 7 kcal/mol

chairtwist boat half-chair

conformations cycloheptane cyclooctane cyclononane

As for alkanes, the carbon atoms in cycloalkanes are Fig. 3. Energy levels of the conformations of cyclohexane. numbered so that the lowest numbers are given to the substituents. By following this rule, the molecule shown below has the name 1,3-dimethylcyclopentane and not energy difference between these four structures is 1,4-dimethylcyclopentane: about 11 kcal/mol. If, on the one hand, this means that at ambient temperature cyclohexane molecules pass at very high frequency from one structure to another 5 1 2 without impediments of energetic type, on the other it can be said that the probability of finding the molecule 4 3 in the chair conformation is over 90%. Fig. 3 shows the different conformations of cyclohexane and the The simplest cycloalkane is cyclopropane. This is corresponding energy levels. an extremely reactive compound since it has bond In the chair conformation of cyclohexane, two different angles which are highly deviated from the equilibrium types of hydrogen atoms can be identified: value for alkanes of 109.5°. In fact, in cyclopropane the • Axial (shown in the structure below), where the covalent three carbon atoms are planar and form bond angles of bond with carbon is oriented perpendicular to the mean 60°. This variation in the bond angles entails a plane formed by the carbon atoms. reduction in the overlap between the sp3 orbitals • Equatorial, where the hydrogen atoms are oriented leading to the formation of the CϪC bond, and roughly parallel to this plane. therefore a decrease in bond energy. The energy expended to deviate the geometry of the bond angles in cyclopropane, and more generally in all cycloalkanes, is known as angle strain. There is also a ring strain, known as torsional strain, resulting from the fact that, as has already been seen for ethane, the most stable Cycloalkanes in which one or more hydrogen molecular conformation is that in which the hydrogen atoms are substituted by alkyl groups are known as atoms are found in a staggered position with respect to alkyl-cycloalkanes. The presence of the ring prevents one another, whilst the structure of cyclopropane forces the rotation of the s bonds, making it possible to the hydrogen atoms into an eclipsed conformation. In distinguish between two compounds which differ in cyclopropane, the contribution of angle strain is the position of the substituents with respect to the nevertheless far greater than that of torsional strain, ring, and which are known as cis diastereoisomers or although generally speaking the energy associated with trans diastereoisomers (in Latin cis means «on the torsional strain is significant. The carbon atoms in the same side» and trans «on the other side»). Unlike cyclobutane molecule are not all on the same plane and enantiomers, cis and trans diastereoisomers have have CϪCϪC bond angles of 90°; to minimize the different chemical-physical properties. For example,

torsional strain, one carbon atom is located slightly out decalin – an alkane with the general formula C10H18, of the plane whilst the bond angles have a value of reported in Fig. 4 and consisting of two condensed about 88°. cyclohexane rings which share two consecutive The stability of cycloalkanes continues to increase carbon atoms – exists both as a cis isomer and a trans from cyclopentane to cyclohexane, which has zero isomer. However, cis-decalin, also known as angle strain (in this molecule, all the carbon atoms lie cis-decahydronaphthalene, has a melting point of 242 at the corners of a tetrahedron with bond angles of K and a boiling point of 460 K, whereas trans-decalin 109.5°). However, the torsional strain in cyclohexane is has a melting point of 230 K and boiling point of a function of the conformation adopted by the 466 K. molecule: in the chair conformation the torsional strain Whereas alkanes such as decalin or decahydroazulene is zero since all the hydrogen atoms are staggered, are described as bicyclic, the fusion of several rings leads whereas the value of the strain increases in the twist, to the formation of compounds which are generically boat and half-chair conformations. The maximum described as polycyclic; these include

12 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

Fig. 4. Structure of the cis and 10 2 trans isomers of decalin. 9 1 3

8 6 4 7 5 trans

cis

perhydrophenanthrene which consists of three condensed rule, the adamantane molecule is also known as rings containing six carbon atoms: tricyclo[3.3.1.1(3,7)]decane: H 7 8 10 H 9 3 5 4 H decaline decahydroazulene perhydrophenanthrene 1 6 2 The nomenclature for polycyclic alkanes obeys the H following rules: Steroidal hydrocarbons are a particular category of • The carbon atoms are numbered and the compound is cycloalkanes. These saturated hydrocarbons contain the given the name of the straight-chain alkane with the ring of perhydrocyclopentanophenanthrene and are same number of carbon atoms. known as steroids; most of these, in addition to the • A prefix is added which indicates the number of rings hydrocarbon chain, contain other oxygenated which must be broken in order to obtain a hydrocarbon substituents (e.g. cholesterol and testosterone) but there without cyclic chains (bicyclic, tricyclic, tetracyclic, etc.). are some which consist exclusively of carbon and • The number of carbon atoms in between the shared hydrogen such as androstane and cholestane. atoms is specified in square brackets. Using these rules, decalin can be given the systematic Alkenes

name bicyclo[4.4.0]decane, since the shared atoms are C1 and C6 and between these are two groups of four atoms Structure and one of zero atoms, since C1 and C6 are consecutive. Alkenes are compounds which have at least one double Below, the structures of some bicyclic compounds CϭC bond in their molecules. These compounds are consisting of rings sharing two non-consecutive carbon generically known as olefins or unsaturated hydrocarbons atoms are shown: (the latter name derives from the fact that the double bond 6 7 can be interpreted as the outcome of a dehydrogenation reaction of the corresponding alkane). The carbon atoms 3 4 participating in the double bond are sp2 hybridized and, in 4 3 5 addition to the carbon atom with which they share the double 2 1 1 bond, are bonded to two other carbon or hydrogen atoms. 5 2 6 The three bonds of the unsaturated carbon atom are planar and form bond angles of 120°. bicyclo[1.1.2]hexane bicyclo[2.2.1]heptane Alkenes have the general formula CnH2n and are 8 identified by the suffix -ene. The first three members of the 8 7 series – in accordance with IUPAC rules – are ethene, 4 5 4 propene and butene, but they are more commonly known as 5 1 3 ethylene, propylene and butylene. There are no exceptions 7 3 6 1 for the other alkenes, which bear the double bond on the first 2 2 6 two carbon atoms of the chain, starting from pentene. The nomenclature used for alkenes in which the double bond is bicyclo[3.2.1]octane bicyclo[2.2.2]octane found inside the chain follows the rule stating that the name If there are three rings, the nomenclature introduces of the molecule is preceded by the position (as low as two numbers which indicate which of the shared atoms possible) of the carbon bearing the double bond. In is the last shared carbon atom. In accordance with this accordance with this rule, the alkenes shown below are

VOLUME V / INSTRUMENTS 13 NATURE AND CHARACTERISTICS OF HYDROCARBONS

described by the IUPAC name of 1-pentene (or simply 120° and the length of the CϪC bond is 1.34 Å, less than pentene) and 2-pentene. If substituents are present, the same the 1.54 Å observed for the ethane molecule. rules used for alkanes apply. Finally, if two or more double Unlike alkanes, the residue obtained from an ethylene bonds are present, the position of the carbons bearing the molecule which is deprived of a hydrogen atom does not double bond is indicated followed by the name of the alkene follow the rules of IUPAC nomenclature and is known as with the addition of di-, tri- etc before the suffix -ene. By vinyl: following this rule, the alkene shown in the diagram below is known by the IUPAC name of 1,3-butadiene: H2C CH

Propylene Propylene is the higher homologue of ethylene, has a 1-pentene 2-pentene 1,3-butadiene general formula of C3H6 and, like ethylene, is a gas under Under normal conditions, the presence of a double bond standard conditions which, however, melts at 88 K and boils prevents the molecule from rotating around this bond. This at about 225 K. peculiarity makes it possible to differentiate between two The residue obtained from propylene deprived of a unsaturated molecules which, even though they contain the hydrogen atom, known as allyl, is of particular interest and is same groups of atoms, differ in their arrangement around the very important in hydrocarbon chemistry: double bond. However, for this distinction to be possible, neither of the two sp2 hybridized carbons must have two H2C CH CH2 identical substituents; in this case, a distinction can be made The structure of this molecule will be discussed below between two different molecules bearing the two groups on (see Section 1.2.3). the same side (cis) or on opposite sides (trans); this is the case for hexene: Dienes Hydrocarbons with two double bonds are generically known as dienes; they can be subdivided into allenes, conjugated dienes and isolated dienes. cis-3-hexene trans-3-hexene Allenes bear the double bonds on the same carbon atom The melting points of these two molecules are 136.6 K and are also known as cumulated dienes. The simplest of and 159.6 K respectively, whilst the boiling points are 339.8 these compounds is 1,2-propadiene, a gaseous molecule with K and 340.2 K. Given these properties, the two isomers can a melting point of about 240 K and a boiling point of about be separated by fractional crystallization and not by 137 K (Fig. 5). In this molecule, the distance between the distillation, since the two boiling points are almost identical. carbon atoms is shorter than that of the simple double bonds However, this result cannot be generalized, since the two of alkenes and measures 1.31 Å. When two cumulated diastereoisomers of 4,4-dimethyl-2-pentene, for example, double bonds are present, the p orbitals of the central carbon which have boiling points of 353 K and 349 K, can be which form the two p bonds are perpendicular to one separated by fractional distillation. another. This peculiarity means that the molecule is not When the four substituents of the double bond are planar and that the four substituents of the two carbon atoms different, the cis and trans conformations cannot be lie on two perpendicular planes. As a direct consequence of distinguished. To render the nomenclature used for the this, the mirror molecules of an allene which have different diastereoisomers of alkenes universal, the substituents on the allenic carbon atoms are not Cahn-Ingold-Prelog rule is therefore used; this assigns superimposable and are therefore optically active. There are increasing priority to the substituents linked to the double thus two different enantiomers even though there is no chiral bond (for saturated residues, priority is assigned on the centre. basis of the molecular weight of the residue, with the Conjugated dienes are thus called because they have highest priority being given to the residue with the highest alternating double and single bonds. The simplest of these is molecular weight). If the two sp2 carbon atoms bear the 1,3-butadiene, a gaseous compound under standard highest priority substituents on the same side, the alkene is conditions with a melting point of 164.3 K and a boiling identified with the symbol (Z) (from the German point of 268.8 K. The distances between the atoms in zusammen, «together»); otherwise it is given the symbol (E) positions 1-2 and 3-4 are 1.34 Å, in line with the 1.337 Å of (from the German entgegen, «opposing»). the simple double bond of ethene; the distance 2-3 is far shorter than the generic length of a single bond and Ethylene The first alkene is ethene or ethylene; under standard conditions it is a colourless and odourless gas with the (ϩ)

general formula C2H4 which melts at about 104 K and boils (ϩ) at about 169 K. Since it has two sp2 hybridized carbon (Ϫ) atoms, its molecule has a planar structure whose bond angles are slightly deviated compared to the 120° associated with (Ϫ) this type of hybridization. This is due to the greater bulk of the electron clouds involved in the formation of the double Fig. 5. Structure of 1,2-propadiene and the bond, which squeeze the two hydrogen atoms linked perpendicular molecular orbitals which generate to the carbon atom; as a result, the HϪCϪH bond angles the two p bonds following the interaction measure about 118°, the two CϪCϪH bond angles about of the p orbitals of the carbon atoms.

14 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

AB C

Fig. 6. 1,3-butadiene molecule: A, highest occupied molecular orbital (HOMO); B, lowest unoccupied molecular orbital (LUMO); C, total electron density.

measures 1.47 Å; this is because the two carbon atoms energy of 1,3-pentadiene, which releases 226 kJ/mol, with forming the bond are both sp2 hybridized. that of 1,4-pentadiene, which releases 252 kJ/mol. The Conjugated dienes have a sequence of carbon atoms with difference in the energy required to hydrogenate molecules p orbitals perpendicular to the plane of the molecule, each with the same number of atoms and double bonds can only occupied by an electron. The presence of these unpaired be due to the conjugation of the double bonds in electrons in partially overlapping orbitals gives the resulting 1,3-pentadiene, which therefore stabilizes this molecule by structure a high degree of stability. It is as if each electron in 26 kJ/mol. However, by analysing the heat of hydrogenation a p orbital contributed to the formation of a bond with the of 1,3-hexadiene and 1,3,5-hexatriene compared to that of 1- two adjacent atoms. This peculiar delocalization, known as hexene, it can be observed that the presence of two and three hyperconjugation, can be easily identified by observing the double conjugated bonds leads to a gain in energy of 24 and total electron density of a conjugated diene which, as 38 kJ/mol. This makes it possible to extend the concept of reported in Fig. 6 for 1,3-butadiene, is uniform over atoms the stability of the delocalization of p electrons not only to which present conjugation. Due to the delocalization of the p dienes but to all compounds which possess n double bonds

electrons occupying the pz orbitals along the skeleton of the alternating with single bonds. molecule, the actual electronic structure of the 1,3-butadiene A conjugated diene of considerable importance in the molecule forms a hybrid of two limit structures. Every time chemistry of unsaturated hydrocarbons is isoprene (or 2- the molecules can be represented in different ways simply by methyl-1,3-butadiene according to IUPAC). Under standard changing the electron occupation of molecular orbitals with conditions, it is a liquid which solidifies at 131 K and boils comparable energy, we are dealing with resonance at 307 K. The isoprene molecule, whose structure forms the structures, which represent limit arrangements of the basis for a long series of unsaturated hydrocarbons, presents electrons in the molecule. The actual arrangement of the hyperconjugation of the p electrons in a similar way to electrons is a hybrid of all the resonance structures which butadiene. The individual isoprene unit, condensed into contribute to the real description of the molecule. The straight-chain, branched and cyclic structures, is found in a concept of resonance, introduced to describe conjugated large number of natural organic compounds known as systems, will be dealt with in greater detail below (see terpenes, classified according to the number of isoprene Section 1.2.2). units of which they are composed: mono-terpenes The stability which hyperconjugation confers on (2 isoprene units), sesqui-terpenes (3 units), di-terpenes conjugated dienes can be measured indirectly through the (4 units), tri-terpenes (6 units), tetra-terpenes (8 units), energetic analysis of hydrogenation reactions. Table 2 shows poly-terpenes (9 or more units). the heats of hydrogenation for various unsaturated Finally, isolated dienes have two double bonds which in hydrocarbons. It is interesting to compare the hydrogenation their molecule occupy positions not adjacent to a carbon

Table 2. Hydrogenation reactions of unsaturated hydrocarbons

Compound Reaction ⌬ Ref. H°r (kJ/mol)

1,3-butadiene ϩ Ϫ᭤ Ϫ236.7Ϯ0.4 Kistiakowsky et al., 1936 2H2

1,3-pentadiene ϩ Ϫ᭤ Ϫ226.4Ϯ0.6 Dolliver et al., 1937 2H2

1,4-pentadiene ϩ Ϫ᭤ Ϫ252.0Ϯ0.6 Kistiakowsky et al., 1936 2H2

1-hexene ϩ Ϫ᭤ Ϫ125.0Ϯ3.0 Linstrom and Mallard, 2003 H2

(Z)-1,3-hexadiene ϩ Ϫ᭤ Ϫ226.0Ϯ1.0 Fang and Rogers, 1992 2H2

(Z)-1,3,5-hexatriene ϩ Ϫ᭤ Ϫ336.0Ϯ1.4 Turner et al., 1973 3H2

VOLUME V / INSTRUMENTS 15 NATURE AND CHARACTERISTICS OF HYDROCARBONS

atom, or alternating with a single bond of s type. Below, the structure of 2,6-octadiene is shown:

Higher alkenes cyclohexene cis-cyclooctene trans-cyclooctene For compounds with more than four carbon atoms, the structural rules outlined above for compounds with a lower The smallest of these hydrocarbons only exist in the molecular weight hold true, bearing in mind that in the case cis form, since a trans structure would create excessive of alkenes, as for alkanes, the number of possible isomers ring strain. The first cycloalkene which can be isolated and diastereoisomers grows exponentially as the molecular in the trans form is cyclooctene. It is interesting to note weight increases. Generally speaking, alkenes with more that the trans-cyclooctene molecule is optically active, than 4 carbon atoms are liquid under standard conditions, since its peculiar structure makes the mirror molecules whilst those with more than 15 carbon atoms are solid. non-superimposable. There are thus two different Below, the alkenes formed by the addition of a high number enantiomers of trans-cyclooctene, although there is no of dienylic molecules will be analysed briefly. The most chiral carbon in the molecule. This peculiarity has important natural compound is the addition polymer of already been seen in compounds with double cumulated isoprene consisting entirely of cis stereoisomers, in other bonds. words natural rubber. It has a high molecular weight As for linear dienes, there are also cyclic systems which (sometimes over 106 u) characterized by the sequence of a have double conjugated bonds. Examples of this class of double bond followed by two single bonds. The homologue compounds are cyclobutadiene, cyclopentadiene and of natural rubber, differing from it due to the presence of cyclooctatriene: double bonds in the trans configuration, is guttapercha. Interestingly, the difference between the cis and trans configurations alone in the structure of the polymer produces two compounds with very different properties. The unsaturated polymer compounds most similar to natural rubber are those derived from the polyaddition of cyclobutadiene cyclopentadiene cyclooctatriene 1,3-butadiene. In this case, too, a material structurally Cyclic alkenes with only double conjugated bonds are similar to polyisoprene is obtained, which takes the generic also known as annulenes (see below). name of elastomer. Generally speaking, cycloalkenes with a high molecular weight and conjugated double bonds are not stable, transforming over time into compounds with condensed ] ] rings, consisting of 4, 5 or 6 carbon atoms per ring. natural rubber Alkynes cis-1,4-polyisoprene Structure Alkynes are unsaturated hydrocarbons containing a triple CϵC bond, in which the carbon atoms are sp hybridized, ] ] linked to only one other atom, of carbon or hydrogen, and form bond angles of 180°.

Alkynes have the general formula CnH2nϪ2 and are gutta-percha indicated by the suffix -yne. The first member of the alkyne trans-1,4-polyisoprene series, although known under the IUPAC nomenclature as ethyne, is normally referred to as acetylene; this is by far the most important alkyne. At ambient temperature it is a gas which liquefies at 189 K; additionally, being an unstable ] ] compound, it explodes easily, producing carbon and elastomer hydrogen. It is generally used to assign a name to the higher alkynes, considered derivatives of acetylene; following these 1,3-polybutadiene rules, for example, propyne is also known as methyl-acetylene. Cycloalkenes For substances in which the triple bond is found inside Cycloalkenes are cyclic molecules which contain one or the molecule, for branched or cyclic molecules, the rules of more double bonds. The nomenclature follows that for nomenclature described for alkenes hold true. Like straight-chain alkenes, with the addition of the prefix cyclo-. acetylene, propyne and 1-butyne are also gases under The structures of some cycloalkenes are shown below: standard conditions (their boiling points are 250 K and 283 K respectively), whilst (again under standard conditions) 2-butyne is liquid, since its boiling point is 300 K. Starting from the alkynes containing four carbon atoms, positional and chain isomers exist whereas, due to the triple bond, there cyclopropene cyclobutene cyclopentene are no stereoisomers.

16 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

1.2.2 Aromatic hydrocarbons even the actual structure of benzene is a hybrid of the possible resonance limit-structures, with greater Aromatic hydrocarbons are the other large class of carbon stability than any one of these, in other words a and hydrogen compounds. The parent of this class is resonance hybrid. benzene, discovered in 1825 by Michael Faraday Each of the six carbon atoms which make up benzene is immediately after he became director of the chemical sp2 hybridized; as a result, they lie on a plane, forming bond laboratory at London’s Royal Institution. Faraday managed angles of 120°. Of the three sp2 orbitals, two are involved in to isolate benzene from the distillation products of an oil the formation of s bonds with the vicinal carbon atoms and obtained as a by-product of the manufacture of illuminant one in the formation of a bond with the hydrogen atom. Each gas. The composition of the benzene molecule was found to carbon atom has a p orbital perpendicular to the plane of the be six carbon atoms and six hydrogen atoms, but the ring, occupied by an electron, formed by two lobes placed arrangement of these atoms in the molecule was unclear. The above and below the plane created by the benzene ring. The problem remained unsolved until 1865, when Friedrich fact that the p orbital of each carbon atom partially overlaps August Kekulé von Stradonitz realized that its structure had with the two p orbitals of the vicinal carbons and that the to be cyclic with three double bonds. The German chemist carbon atoms are fused in a ring generates a continuum thus described how he managed to identify the structure of between the p orbitals, leading to the formation of the parent of aromatic compounds: “I was sitting intent on doughnut-shaped molecular orbitals (Fig. 7) located both writing my treatise, but the work was not progressing; my above and below the ring, inside which the six p electrons thoughts were elsewhere. I turned my chair towards the fire are delocalized. and fell asleep. Again, the atoms started to leap about before The benzene molecule, therefore, is represented as a my eyes, but this time the smaller groups remained modestly hexagon with six hydrogens linked to it, inside which a in the background. My mind’s eye, rendered more acute by circle is drawn to represent the electron delocalization repeated visions of this type, was now able to distinguish of this structure: larger structures, of different sorts, arranged in long rows which in some places were fairly close to one another, all twining and twisting like a pile of moving snakes. Then suddenly one of the snakes, grasping its own tale, whirled ironically before my eyes. As if by a flash of lightning I As a consequence, the carbon atoms are bonded to one awoke... I spent the rest of the night working out the another by the formation of a s bond and half a p bond. consequences of the hypothesis. Gentlemen, let us learn to The delocalization of the six electrons leads to the dream, and perhaps then we will see the truth”. For benzene, formation of an unusually stable structure. Compared to an Kekulé proposed the existence of two equivalent structures unsaturated compound with three double bonds, the in equilibrium with one another: benzene molecule has a stability of 150 kJ/mol, a value calculated from the hydrogenation energy of an unsaturated hydrocarbon (Table 3) as already seen for conjugated dienes. The heat of hydrogenation of a double bond generally has a value of about Ϫ120 kJ/mol; in line with Although it did not fully clarify the molecule’s this value, the hydrogenation of cyclohexene releases properties, this description of benzene’s structure 118 kJ/mol, that of 1,3-cyclohexadiene 224 kJ/mol and that remained in force until the mid-20th century, even after of 1,4-cyclohexadiene 233 kJ/mol (the difference between Linus Pauling had introduced the concept of 1,3-cyclohexadiene and 1,4-cyclohexadiene is due to the resonance in 1931-32. The structures proposed by stability of the conjugation of the two double bonds in Kekulé, thus become two resonance limit structures 1,3-cyclohexadiene). These values would lead one to among all those possible. As for conjugated systems, predict a value of about Ϫ358 kJ/mol for the

ABC

Fig. 7. Benzene molecule: A, highest occupied molecular orbital (HOMO); B, lowest unoccupied molecular orbital (LUMO); C, total electron density.

VOLUME V / INSTRUMENTS 17 NATURE AND CHARACTERISTICS OF HYDROCARBONS

Table 3. Hydrogenation reactions of unsaturated hydrocarbons

Compound Reaction ⌬ Ref. H°r (kJ/mol)

ϩ Ϫ᭤ Ϫ Ϯ 1-cyclohexene H2 118.0 6.0 Linstrom and Mallard, 2003

ϩ Ϫ᭤ Ϫ Ϯ 1,3-cyclohexadiene 2H2 224.4 1.2 Turner et al., 1973

ϩ Ϫ᭤ 1,4-cyclohexadiene 2H2 Ϫ233.0 Roth et al., 1991

ϩ Ϫ᭤ Ϫ Ϯ benzene 3H2 205.3 0.6 Kistiakowsky et al., 1936

hydrogenation of benzene; however, experimentally, it Benzene has been found that the heat released is only 205 kJ/mol. Under standard conditions, benzene is a liquid which The difference between the two heats of reaction is due melts at 278.6 K and boils at 353.3 K. All its atoms are to the stability conferred by the phenomenon of coplanar with a CϪC bond length of 1.39 Å, in between the aromaticity. Additionally, it is interesting to note that 1.47 Å of a single bond between two sp2 hybridized carbon aromaticity also leads to an accentuation of stability in atoms and the 1.33 Å of an isolated double bond in an comparison to straight-chain conjugated systems. alkene. The length of the CϪH bonds is 1.10 Å and the Referring to Table 2, in fact, it can be seen that the bond angles between three consecutive atoms in the presence of three conjugated double bonds in molecules measure 120°. 1,3,5-hexatriene gives the molecule a stability of 40 kJ/mol, a modest value when compared to the Arenes 150 kJ/mol found for benzene. This evidence allows us Arenes are compounds whose molecules contain both to conclude that the phenomenon of delocalization is a aromatic and aliphatic groups. They can be divided into necessary but not sufficient condition to explain the three subclasses: aromaticity of benzene. • Alkylbenzenes, consisting of an aromatic group linked to In addition to benzene, other molecules consisting an aliphatic group. of aliphatic groups linked to benzene rings, of • Alkenylbenzenes, consisting of an aromatic part and a condensed benzene rings and especially of compounds group containing at least one double bond. with no similarities to the hexagonal ring of benzene, • Alkynylbenzenes, consisting of an aromatic group linked have the peculiarity of being aromatic. The rule to a residue with a triple bond. allowing for the identification of aromatic compounds Obviously, only some of the countless molecules was proposed by the German chemist and physicist which can be formed by the combination of aromatic, Erich Hückel. Born in 1896 at Charlottenburg, in the aliphatic and unsaturated groups belong to these Berlin suburbs, Hückel worked immediately after his subclasses. The nomenclature for the simplest doctorate alongside Peter Debye. Together, in 1923, compounds entails assigning names to the residues they formulated the theory of electrolytic solutions linked to benzene, followed by the suffix -benzene. known as the Debye-Hückel theory. During the 1930s, Some specific compounds, however, are generally Hückel’s interests shifted towards quantum mechanics known by their traditional names, as in the case of and in 1931, at the Stuttgart Polytechnic, he formulated methylbenzene, known as toluene; isopropylbenzene his famous rule for the identification of aromatic known as cumene; vinylbenzene known as styrene: compounds. According to this rule, in order for a compound to be aromatic, it must possess a specific number of p electrons, equal to 4nϩ2, in the two clouds of delocalized electrons above and below the plane of its molecule. To understand the reason for this rule, reference should be made to the simplest form of the LCAO (Linear Combination of Atomic Orbitals) toluene cumene styrene theory, according to which molecular orbitals are formed by the linear combination of atomic orbitals. If there is more than one substituent on a single The combination of atomic orbitals gives rise to benzene ring, the six aromatic carbon atoms are bonding and antibonding molecular orbitals. The numbered so as to give the most important substituent number of electrons allowing a molecule to be the lowest number, following the rules of aromatic is that needed to completely occupy all the nomenclature already described for aliphatic molecular bonding orbitals. This makes the cohesion hydrocarbons. Usually, once the most important between the atoms in the molecule as high as possible. substituent has been identified, the positions of the Using Hückel’s rule, it can be predicted that compounds vicinal carbons to which it is linked are described with which have 2, 6, 10, ... p electrons delocalized above and the term ortho- (indicated by the letter o-), the below the plane of the molecule will be aromatic (see also subsequent carbons with meta- (indicated by the letter Section 1.2.3). m-) and those immediately opposite with the term

18 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

para- (indicated by the letter p-). As an example, the structures of p-ethylethylbenzene and m-ethylvinylbenzene are shown below; the dimethylbenzenes, by contrast, are usually known by the name xylenes (o-xylene, m-xylene and p-xylene):

cyclohexylbenzene tetralin

Condensed aromatics The parent of the condensed aromatic compounds consists of two benzene rings which share a bond and takes the name naphthalene; under standard conditions it is a solid p-ethyl- m-ethyl- o-xylene ethylbenzene vinylbenzene which melts at 353 K and boils at 490 K. It consists of 10 planar sp2 hybridized carbon atoms: 8 1 7 2

6 3 5 4 naphthalene

m-xylene p-xylene Its stability can again be studied with recourse to the heats of hydrogenation. The addition of two hydrogen Another way to assign names to arenes consists of molecules leads to the formation of tetralin and the numbering the hydrocarbon chains, as has already production of 125 kJ/mol. The further reduction of tetralin to been shown for alkanes, alkenes and alkynes, and then decalin with the addition of three hydrogen molecules considering the benzene ring as a substituent of this releases 318 kJ/mol. It can thus easily be seen that the hydrocarbon. The residue represented by the benzene addition of each hydrogen molecule to the double bond molecule with one fewer hydrogen atom and a free releases about 63 kJ/mol, a typical value for the valence takes the name phenyl; the residue formed by hydrogenation of benzene (and thus aromatics in general). toluene with a free valence instead of a hydrogen By contrast, the hydrogenation of 1,4-dihydronaphthalene atom in the methyl position is known as benzyl. leads to the release of as much as 113.5 kJ/mol, a value more As an example, the structures of diphenyl, typical of an alkene. The aromaticity of naphthalene results 1,2-diphenylpropane and hexaphenylethane are shown from the two electron clouds located above and below the below: plane of the molecule which, in accordance with Hückel’s rule, are occupied by 10 delocalized p electrons which can be considered as belonging to two distinct electron clouds occupied by six p electrons and thus characteristic of aromatic systems which share a pair of p electrons. The nomenclature for the derivatives of naphthalene is assigned by numbering all the carbon atoms in the compound (also subdivided into a or b atoms) and then indicating the position of the relevant substituents. As an example, the structures of 1,2-dimethylnaphthalene and a-phenylnaphthalene are shown below: diphenyl 1,2-diphenylpropane

1,2-dimethylnaphthalene a-phenylnaphthalene

hexaphenylethane The members above naphthalene are anthracene and phenanthrene, whose molecules are characterized by the

Among the arenes are compounds consisting of both general formula C14H10 and the presence of 14 delocalized p aromatic and aliphatic rings such as, for example, electrons which give them aromaticity. In the nomenclature for cyclohexylbenzene and tetralin: these compounds, the carbon atoms are numbered as follows:

VOLUME V / INSTRUMENTS 19 NATURE AND CHARACTERISTICS OF HYDROCARBONS

6 molecules, known as annulenes; to be considered aromatic, 5 7 these must have delocalized electrons above and below the 8 9 1 4 plane on which the atoms lie and satisfy Hückel’s rule. 8 According to Hückel’s rule, two is the lowest number of 7 2 3 delocalized electrons allowed for an aromatic compound, but 6 3 2 9 there are no neutral hydrocarbons with this number (as will 5 10 4 1 10 be seen in Section 1.2.3, the cyclopropenyl cation is aromatic). The subsequent compounds require six electrons, anthracene phenanthrene and are represented by benzene. On the other hand, there are no aromatic compounds with 10 delocalized p electrons. In As the number of condensed rings increases, the number of fact, [10]annulene, although it satisfies Hückel’s rule, is not different isomers also increases. There are five other isomers of aromatic since the molecule is not planar; the interaction of naphthacene, formed by the condensation of four benzene rings, the hydrogens at the centre of the cycle forces two double shown below: 1,2-benzanthracene, 3,4-benzophenanthrene, bonds in the ring into the trans conformation, thus chrysene, 9,10-benzophenanthrene and pyrene. preventing the molecule from adopting a planar geometry. However, in accordance with Hückel’s rule, we find that [14], [18] and [22]annulene are particularly stable compounds thanks precisely to their aromaticity. The diagram below shows the structures of some aromatic annulenes:

naphthacene 1,2-benzanthracene

[14]annulene [18]annulene

3,4-benzophenanthrene chrysene

[22]annulene

9,10-benzophenanthrene pyrene 1.2.3 Cationic, anionic and radical hydrocarbons All these compounds have the general formula C18H12 and an H/C ratio of 0.67. The H/C ratio decreases inversely Carbocations to the size of polycondensate aromatic compounds. It passes Carbocations are positively charged molecules which from a value of 1 for benzene to 0.8 for naphthalene, 0.71 have an electron gap on a carbon which has its three for compounds with three aromatic rings, 0.67 for those with valence electrons in sp2 hybridized orbitals and one empty four aromatic rings and 0.63 for those with five condensed p orbital oriented perpendicularly to the plane described rings. The H/C ratio continues to fall with subsequent by the three occupied orbitals. These are unstable condensations, tending towards zero in graphite. As far as compounds which, except in some very specific hydrocarbons in general are concerned, it can be observed circumstances under which they can be studied directly, that the maximum H/C ratio is that of the methane molecule are usually chemical intermediates which cannot be (equal to 4) and that in general this value ranges from 4 to 2 isolated. The resulting geometry of the cation is, as for in saturated aliphatic molecules, has a value of 2 in alkenes, trigonal planar. molecules with a double bond and cycloalkanes, falling In saturated systems the stability of carbocations below 2 for unsaturated and polyunsaturated compounds, to decreases passing from cations localized on tertiary, 1 for benzene and below 1 for condensed aromatics. secondary and primary carbon atoms until it reaches the ϩ methyl group CH3 , which is the most unstable. This scale of Annulenes stability is due to the effect of the R electron-donating In addition to benzene and the condensed aromatic groups which tend to fill the cation’s electron gap, thus compounds, there is a different class of cyclic aromatic stabilizing it.

20 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

Fig. 8. A, total electron density of the cyclopropenyl ion; B, total electron density of the tropilium cation; C, lowest unoccupied molecular orbital (LUMO) of the cyclopentadienyl anion; D, highest occupied molecular orbital (HOMO) of the triphenylmethyl radical.

AB

CD

In unsaturated or polyunsaturated systems, there are with this rule, the most stable carbocations are those which carbocation structures which are particularly stable, as is the have a large number of conjugated double bonds which make case for the allylic and benzylic carbocations. The allylic it possible to delocalize the positive charge as much as carbocation possible. Finally, it is worth describing some specific instances in CH CH ϩ ϩ which the presence of a positive charge makes the molecules H2CHCH2 2C CH2 extremely stable, as in the case of aromatic carbocations. As although characterized by the presence of a charge on a has already been stated, there are no neutral aromatic primary carbon, has a stability between that of secondary molecules with two delocalized p electrons. The and tertiary carbocations. This is due to the presence of two cyclopropenyl ion, on the other hand, is trigonal planar with resonance hybrids. The positive charge is thus delocalized three sp2 hybridized carbon atoms and has two electron onto the two carbon atoms, which are therefore equivalent. clouds occupied by two p electrons above and below the The benzylic carbocation is a resonance hybrid of the plane of the molecule. This cation is a resonance hybrid of five structures shown below: three equivalent structures. As such, in addition to being an ϩ ϩ aromatic molecule, the ion is stabilized by the conjugative CH2 CH2 CH2 CH2 CH2 effect (Fig. 8 A). ϩ A particularly important cation is cycloheptatrienyl, also CH HCϩ known as the tropylium ion (Fig. 8 B); it has seven planar sp2 CH hybridized carbon atoms and two electron clouds occupied ϩ by a number of p electrons which satisfies Hückel’s rule. The delocalization of the positive charge onto four The positive charge is delocalized onto as many as seven carbon atoms makes the benzylic carbocation more stable carbon atoms which, as also shown by Nuclear Magnetic than a tertiary radical, although in formal terms the charge is Resonance (NMR) analysis, are equivalent. found on a primary carbon. Generally speaking, it can be stated that the stability of Carbanions carbocations is due to two factors: the inductive effect Hydrocarbons whose molecules have a negative charge and the conjugative effect. The former stabilizes a on a carbon atom are known as carbanions; like carbocation if the groups linked to the positive carbon are carbocations, they are unstable and reactive. In saturated electron-donating groups which stabilize the electron gap. systems, the stability scale of carbanions is exactly the Ϫ The latter, on the other hand, stabilizes the carbocations opposite to that of carbocations: the CH3 group is the most increasingly the more the resonance effect delocalizes the stable, followed by carbanions with a charge localized on charge. For hydrocarbons specifically, the conjugative effect primary, secondary and tertiary carbon atoms. This is due to always predominates over the inductive effect. In accordance the electron-donating inductive effect of the alkyl

VOLUME V / INSTRUMENTS 21 NATURE AND CHARACTERISTICS OF HYDROCARBONS

substituents of carbon which, by increasing the electron high stability of this compound thus makes it possible to density of the already negatively charged atom, compromise isolate it, keeping it stable for a relatively long time. its stability. Moreover, the stability of anions increases Hexaphenylethane, even when kept at ambient temperature, passing from sp3 hybridized carbon atoms to sp2 hybridized has such a weak single CϪC bond that it breaks atoms, up to anions with negative charges localized on sp spontaneously, giving rise to an equilibrium with two hybridized carbon atoms. Given the greater s characteristic triphenylmethyl radicals. of these orbitals – and thus their ability to attract electrons – they are the most stable. Experimental confirmation of the Carbenes stability scale presented here has come from the analysis of Hydrocarbons characterized by the presence of at least the energy of the formation reaction of the anion and the H+ one neutral divalent carbon atom, which thus forms only two ion starting from the corresponding neutral hydrocarbon. bonds, are known as carbenes; the parent carbene is The energy needed for the three reactions reported in Table 4, methylene, whose electronic structure is anything but in fact, decreases constantly from an sp3 hybridized simple. This compound has three electronic structures, two molecule (ethane) to an sp hybridized one (acetylene). of which are singlet states whilst one is a triplet state. The For carbanions, too, the most important factor two singlet states can be differentiated into a low energy determining stability is the conjugative effect, which allows singlet state (where the non-bonding electrons with paired the negative charge to be delocalized onto several atoms. spin occupy an sp2 hybrid orbital leaving a p orbital empty) Aromatic compounds are among the most stable carbanions. and an excited singlet state (where the electrons with paired The addition of an electron, and thus of a negative charge, spin occupy separate p orbitals). The triplet state, which is allows some hydrocarbons to meet the requirements for the most stable of the three, is characterized by the presence being aromatic. The first aromatic anion is the of the non-bonding electrons with parallel spin in two Ϫ cyclopentadienyl anion (Fig. 8 C), with the formula C5H5 and different p orbitals; in this state, therefore, the methylene five equivalent hydrogen atoms (as shown by NMR molecule is a biradical. In the low energy singlet state, the analysis); it also satisfies Hückel’s rule since it is planar and carbon atom is sp2 hybridized; as a consequence, the has six electrons in p orbitals, as compared to five in the geometry of the molecule is planar with HϪCϪH bond neutral molecule. Aromatic anions with 10 delocalized p angles of about 120°. By contrast, in the excited singlet or electrons include the cyclooctatetraenyl anion and the triplet states, the molecule is linear with an HϪCϪH bond cyclononatetraenyl anion. In the molecule of the former, two angle of 180°, since the carbon atom is sp hybridized. negative charges are delocalized onto eight atoms, whereas in the latter a charge is delocalized onto nine carbon atoms. 1.2.4 Physical properties Free radicals of hydrocarbons Free radicals are uncharged chemical species which have at least one orbital containing a single unpaired electron. In The main physical properties of hydrocarbons, such as the the case of hydrocarbons, the unpaired electron is localized melting point, boiling point, critical parameters or density, on a carbon atom. The stability of free radicals follows the depend on their molecular and electronic structure, which same scale as carbocations: radicals on sp3 hybridized has been discussed above in general terms for all carbon atoms are more stable than those on sp2 and sp hydrocarbons. atoms. For free radicals, too, the conjugative effect plays a The boiling point is strictly correlated with the crucial role in determining stability. Thus the allyl radical is molecular weight of the hydrocarbon. For homologous more stable than the tertiary radical because it delocalizes compounds – which differ in their molecular weight – the the unpaired electron onto two carbon atoms; the benzyl boiling point increases in line with the latter. For saturated radical is yet more stable because it delocalizes the unpaired compounds with the same number of carbon atoms, the electron onto four carbon atoms. Although free radicals are boiling point increases from straight-chain to branched unstable and extremely reactive, some of them, by virtue of molecules, with cyclic compounds having the highest inductive and conjugative stabilization effects, may exist in boiling points. In the presence of unsaturations (in other appreciable concentrations even at ambient temperature. words double and/or triple bonds), both the melting point This is true of the triphenylmethyl radical, where the and the boiling point are generally higher. This is due to the unpaired electron is localized on a tertiary carbon stabilized presence of sp2 and sp hybridized carbon atoms which, by the inductive effect and which is a resonance hybrid of as being more electronegative than sp3 hybridized carbons, many as 13 limit structures, in which the radical can be generate a charge imbalance which increases delocalized onto as many as 10 carbon atoms (Fig. 8 D). The intermolecular forces.

Table 4. Deprotonation reactions of hydrocarbons

Compound Reaction ⌬ Ref. H°r (kJ/mol)

Ϫ ϩ ethane Ϫ Ϫ᭤ Ϫ ϩ Ϯ DePuy et al., 1989 CH3 CH3 CH3 CH 2 H 1758.0 8 Ϫ ϩ ϭ Ϫ᭤ ϭ ϩ Ϯ ethylene CH2 CH2 CH2 CH H 1703.0 13.0 Graul and Squires, 1990 Ϫ ϩ acetylene CHϵCHϪ᭤ CHϵC ϩH 1580.0Ϯ20.0 Linstrom and Mallard, 2003

22 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

The melting point is not correlated in an equally direct way with molecular structure. Generally speaking, the rule Table 6. Flammability limits (vol. %) stating that the melting point rises as molecular weight of some hydrocarbons in air, determined increases holds true but there are numerous exceptions; in under standard conditions fact, comparing the melting point of hexene and (Weast, 1987; Hunter and Lias, 2003; Lias, 2003) 4-methylpentene with the 2-butenes or 2-methylpropene, it can be observed that alkenes with four carbon atoms actually Molecule Lower limit Upper limit have a higher melting point than alkenes with six carbon methane CH 5.00 15.00 atoms. Normally, the melting point for molecules with an 4

identical number of carbon atoms rises proportionally with hexane C6H14 1.18 7.40 the compactness and symmetry of the molecule. In these ethene C H 2.75 28.60 instances, packing in the solid state is facilitated, with a 2 4

consequent increase in phase stability and therefore the 1-butene C4H8 1.65 9.95 melting point. For example, cyclobutane melts at 183 K as ethyne C H 2.50 80.00 compared to the 136 K of butane, and benzene at 278 K as 2 2

compared to the 120 K of 2-methylpentane. benzene C6H6 1.40 7.10 Table 5 shows the electric dipole moment and the toluene C H 1.27 6.75 diamagnetic susceptibility of some hydrocarbons. 7 8 The dipole moment, in particular, identifies the distribution of the electrical charge in the molecule and of hexane, octane and decane at 20°C is 0.326, 0.542 and depends on its structure and symmetry as well as on the 0.920 cP respectively. Cyclic hydrocarbons are more viscous presence of carbon atoms with different hybridizations. For than the corresponding straight-chain hydrocarbons: for example, methane, ethane, cyclopropane, ethene, example, hexane and cyclohexane have a viscosity of 0.326 1,3-butadiene, ethyne, p-xylene are perfectly symmetrical and 1.02 cP. Unsaturated compounds, on the other hand, molecules which, as such, have a dipole moment of zero and usually have lower viscosity than saturated hydrocarbons: are apolar compounds. By contrast, asymmetrical molecules cyclohexane 1.02, cyclohexene 0.66 and benzene 0.652 cP. such as propene or 1-hexyne, which additionally has a Hydrocarbons are flammable organic compounds which charge imbalance due to the presence of two sp hybridized burn in the presence of a comburent. The ratio of fuel to atoms, have dipole moments of 0.35 and 0.89 debye. The comburent, however, must fall within a certain range in order dipole moment influences phase changes since, as it for combustion to take place; this range depends on both increases, intermolecular attractive interactions increase and constituents and is identified by a lower and upper limit. The the melting and boiling points rise. lower and upper flammability limits indicate the minimum The magnetic properties of a substance also provide and maximum volume percentages of the fuel in the information on the behaviour of the molecular electrons. comburent, above and below which, in the presence of a Specifically, the study of diamagnetic properties trigger, the mixture catches fire. Table 6 shows the limits for (summarized in the value of diamagnetic susceptibility) due some hydrocarbons in air. Table 7 reports the melting and to the emergence of an induced molecular magnetic moment boiling points, density and refractive index for various and which therefore do not depend on either magnetic, alkanes, alkenes, alkynes and aromatic compounds. For orbital or spin moments, provide information on electronic homologous series, density increases as molecular weight configurations. increases and, given an identical number of carbon atoms, it is The viscosity of homologous series increases as higher for cyclic than for straight-chain or branched molecular weight rises; in alkanes, for example, the viscosity compounds. Among the different classes of hydrocarbons, however, density increases, given an identical number of carbon atoms, from alkanes to alkenes, alkynes and aromatics. Table 5. Dipole moment and diamagnetic susceptibility (Weast, 1987) Spectroscopic characterization methods The spectroscopic characterization methods used for Diamagnetic Dipole moment hydrocarbons are essentially mass spectrometry (or mass Molecule susceptibility spectroscopy), infrared spectroscopy and nuclear magnetic m (debye) Ϫ и 6 ( cm 10 ) CGS resonance spectroscopy. methane CH 0.0 12.2 Mass spectra can be considered a sort of identity card for 4 molecules. When molecules are bombarded with high energy

ethane C2H6 0.0 27.3 electrons they shatter, generating a characteristic distribution of positively charged fragments from which it is possible to ethene C H 0.0 1.0 2 4 measure mass and relative abundance, since they are deflected by electric and/or magnetic fields. The results of cyclohexene C6H10 0.55 57.5 these measurements are reported in a diagram, known as a ethyne C2H2 0.0 12.5 mass spectrum, whose x-axis shows the ratio of the ϩ propyne C H 0.72 – fragment’s mass to its charge, generally equal to 1, whilst 3 4 the y-axis shows the relative intensity registered by the benzene C6H6 0.0 54.84 instrument (mass spectrometer) proportional to the toluene C H 0.36Ϯ0.03 66.11 abundance of fragments detected. The spectrum presents 7 8 peaks with a very modest relative abundance, which in some

VOLUME V / INSTRUMENTS 23 NATURE AND CHARACTERISTICS OF HYDROCARBONS

Table 7. Physical properties of hydrocarbons (Weast, 1987)

Molecule M.P. (°C) B.P. (°C) Density* Refractive index* Ϫ Ϫ Ϫ164 methane CH4 182.5 161.5 0.415 – Ϫ Ϫ Ϫ108 0 ethane C2H6 182.8 88.63 0.572 1.0769 Ϫ hexane C6H14 95 68 0.6594 1.0749 Ϫ Ϫ Ϫ79 cyclopropane C3H6 126.6 33 0.720 –

cyclohexane C6H12 6.5 81 0.7791 1.4266 Ϫ Ϫ 0 100 ethene C2H4 169.2 104 0.00126 1.363 Ϫ Ϫ Ϫ25 1,3-butadiene C4H6 108.9 4.4 – 1.4292 Ϫ cyclohexene C6H10 103.5 82.9 0.8110 1.4465 Ϫ Ϫ Ϫ82 0 ethyne C2H2 81.8 83.6 0.6181 1.0005 15 20 benzene C6H6 5.5 80.1 0.8787 1.5011 Ϫ toluene C7H8 95 110.6 0.8669 1.4961 Ϫ 20 20 cumene C9H12 96 153 0.864 1.4911 77 77 diphenyl C12H10 70 255.9 1.9896 1.588

* The superscript numbers indicate the temperature in °C at which the measurement was made

cases may be due to fragments produced in very low identified: symmetrical (or in phase) and asymmetrical (out quantities but which more frequently indicate fragments in of phase); in both cases, bond stretching is indicated by the which the heavy isotopes of carbon and/or hydrogen are letter n. There are two different types of bending: one in the present. In fact, 98.89% of the carbon making up plane (indicated with the letter d) and one outside the plane hydrocarbons consists of the 12C isotope with the remaining (indicated with the letter g). Bending in the plane may lead 1.11% being of the 13C isotope; 99.985% of the hydrogen is the bonds to converge in the same direction, in which case we present as the 1H isotope and 0.015% as deuterium D (the 2H speak of rocking; alternatively, the bonds may diverge in isotope). This means that in the mass spectrum of a opposite directions, in which case we have scissoring hydrocarbon, peaks will also be found due to the presence of vibration. Bending outside the plane is classified as either fragments which contain these heavier isotopes. These twisting or wagging, depending on whether the bonds move peaks, with a value of mass number incremented by one or in two different opposing directions or in the same direction. more units depending on the number of 13C and D present in Generally speaking, in the IR spectrum of hydrocarbons, a the molecule, are found immediately after the peaks created series of peaks are observed which present medium by fragments without heavy isotopes; the relative abundance absorption intensities of infrared radiation, others which of these fragments is nonetheless very low given the tiny present high absorption intensities and a series of low percentage of these isotopes present in nature. intensity absorptions which it is difficult to assign to specific Infrared spectroscopy is a powerful tool for classifying bond vibrations in the molecule since they are due to and identifying hydrocarbons. It is based on the ability of particularly complex internal torsional vibrations. InfraRed (IR) radiation impacting on a chemical compound The medium-intensity absorption peaks refer to to excite the vibrational frequencies of some chemical stretching vibrations; specifically, at decreasing wave bonds; as a result, depending on the bonds present in the numbers we find the stretching of CϪH bonds, triple CϵC molecule, a sample being examined absorbs specific bonds, double CϭC bonds and finally single CϪC bonds. frequencies of the infrared spectrum. More generally, the IR The peaks which have high density absorption are correlated spectra obtained give the wave number rather than the with the bending in and out of the plane of CϪH bonds. wavelength absorbed; the relationship between_ these two The NMR spectroscopy of a hydrocarbon provides magnitudes is given by the equations nϭcրl, nϭ1րl, where l detailed information on the bond state of the hydrogen and is the_ wavelength, c is the speed of light, n is the frequency carbon atoms, through which it is possible to clarify or and n is the wave number. identify the structure of the chemical compound analysed. As in the case of the mass spectrum, each IR spectrum Briefly, this technique is based on exciting the nuclear spin can be correlated with a single compound. The range of states by using radio frequency impulses and on measuring wavelengths which defines the infrared spectrum of the energy absorbed as a function of the frequency applied. As electromagnetic radiation lies between 400 and 4,000 cmϪ1. far as hydrocarbons are concerned, the atoms which can be The main vibrational frequencies which absorb in the infrared studied using NMR are hydrogen and the 13C isotope. Since field are bond stretching vibrations and bond bending the frequency of radiation absorption depends on the density vibrations. Two different types of bond stretching can be of the electrons surrounding the nucleus, the resonance

24 ENCYCLOPAEDIA OF HYDROCARBONS TYPOLOGY AND STRUCTURE OF HYDROCARBONS

frequency reflects the state of hybridization and bonding of References the atom under investigation. In the benzene molecule, for example, there is only one type of carbon and hydrogen atom; DePuy C.H. et al. (1989) The gas phase acidities of the alkanes, as a consequence, in the 13C NMR spectrum there is a single «Journal of American Chemical Society», 111, 1968-1973. type of signal due to the six equivalent nuclei of the C atoms. Dolliver M.A. et al. (1937) Heats of organic reactions. V: Heats of Similarly, in the PMR (Proton Magnetic Resonance) hydrogenation of various hydrocarbons, «Journal of American spectrum, there is a single signal due to the six equivalent Chemical Society», 59, 831-841. protons of the hydrogen atoms. By contrast, in the 13C NMR Fang W., Rogers D.W. (1992) Enthalpy of hydrogenation of the spectrum, propane has two different signals due to the hexadienes and cis- and trans-1,3,5-hexatriene, «Journal of Organic Chemistry», 57, 2294-2297. secondary carbon and the two primary carbons (which in this Graul S.T., Squires R.R. ( case are equivalent). From the examples provided, it can be 1990) Gas-phase acidities derived from threshold energies for activated reactions, «Journal of American deduced that the number of peaks shown by the NMR Chemical Society», 112, 2517-2529. spectrum corresponds to the different number of atoms Hunter E.P., Lias E.G. (2003) Proton affinity evaluation, in: Linstrom present, whilst the position of these absorption peaks in the P.J., Mallard W.G. (editors) NIST Chemistry WebBook, National spectrum is a function of the electronic structure characteristic Institute of Standards and Technology, Standard reference database, of these atoms, which differs significantly depending on 69. whether they are aromatic, aliphatic, benzylic, vinylic, allylic, Kistiakowsky G.B. et al. (1936) Heats of organic reactions. IV: primary, secondary, tertiary, etc. This is due to the fact that the Hydrogenation of some dienes and of benzene, «Journal of American magnetic field applied produces a shift of the electrons inside Chemical Society», 58, 146-153. the molecule, so that the nuclei are subjected to an effective Lias S.G. (2003) Ionization energy evaluation, in: Linstrom P.J., Mallard field which is greater or less than that actually applied, W.G. (editors) NIST Chemistry WebBook, National Institute of leading to a shift in the absorption frequency which would Standards and Technology, Standard reference database, 69. have been obtained had the atomic nucleus not been shielded. Linstrom P.J., Mallard W.G. (editors) (2003) NIST Chemistry This shift, known as chemical shift, is measured with respect WebBook, National Institute of Standards and Technology, Standard to a reference signal (in the case of PMR the absorption reference database, 69. frequency of the hydrogen atoms of tetramethylsilane); Roth W.R. et al. (1991) Die Berechnung von Resonanzenergien; das generally reported as a relationship to the frequency of the MM2ERW-Kraftfeld, «Chemische Berichte», 124, 2499-2521. spectrometer, it is in the order of parts per million (ppm). Turner R.B. et al. (1973) Heats of hydrogenation. X: Conjugative interaction in cyclic dienes and trienes, «Journal of American Chemical Society», 95, 8605-8610. Weast R.C. (editor in chief) (1987) CRC Handbook of chemistry and Bibliography physics. A ready-reference book of chemical data, Boca Raton (FL), CRC Press. Atkins P.W. (1994) Physical chemistry, Oxford, Oxford University Press. Carlo Cavallotti Atkins P.W., Friedman R. (1997) Molecular quantum mechanics, Oxford, Oxford University Press. Davide Moscatelli Graham Solomons T.W. (1993) Chimica organica, Bologna, Dipartimento di Chimica, Materiali Zanichelli. e Ingegneria chimica ‘Giulio Natta’ Morrison R.T., Boyd R.N. (1969) Chimica organica, Milano, Casa Politecnico di Milano Editrice Ambrosiana. Milano, Italy

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