Typology and Structure of Hydrocarbons

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.

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