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Home , Hydroxy group, Molecular geometry, Norbornane, Structural isomer

ORGANIC [MH5; Chapter 22, Tutorial Notes]

• The common feature of all organic compounds is that they contain the element carbon together with only a few other elements, principally , and nitrogen. • The number of known organic compounds already exceeds ten million, a number vastly larger than that of all other elements taken together (with the exception of hydrogen), and the possible number is virtually limitless. • For this reason one modern definition of is the chemistry of carbon compounds.

Features of Organic Compounds • Organic compounds are molecular, rather than ionic.

• Each carbon always forms a total of 4 covalent bonds.

• Carbon may be bonded to each other, or to other non metal atoms; commonly hydrogen, a halogen, oxygen or nitrogen.

– 379 – SHAPES OF ORGANIC • The arrangement of atoms bonded to a carbon atom follows VSEPR theory......

3 CH4: four single bonds tetrahedral sp hybrids

2 C2H4: two single bonds trig. planar sp hybrids one double bond

CO2: two double bonds linear sp hybrids

C2H2: one single bond, linear sp hybrids one triple bond

– 380 – • Carbon occurs as three allotropes, with three different structures. • The incredible hardness of diamond results from a three- dimensional network of C—C bonds; it is in fact a single giant ; each carbon is bonded to four other carbons.

• In contrast, graphite is built up from two-dimensional sheets of sp2 hybridized C atoms which can easily slip and slide over one another.

• The third allotrope of carbon exists as a network of 60 carbon atoms arranged in a sphere; this form is commonly known as a “buckyball”.

• Organic molecules exhibit isomerism,both structural isomerism and stereoisomerism. • This means two distinct and different compounds can have the same molecular formula.

– 381 – Structural Classification of Organic Compounds • Most organic compounds fall into a small number of groups. • Within each of these groups, all compounds have similar chemical and physical properties and can be synthesized by similar reactions. • So...... we can concentrate on learning the characteristics of these groups, or families without discussing individual compounds in detail.

• These "groups" or can be classified according to similarities in structure in two basic ways. • The first is the Skeletal classification. •The backbone of all organic compounds (except for those having only one C atom) is a skeleton of C atoms linked to each other in chains or rings. • These C chains or rings are quite stable; which means that they survive unchanged throughout most chemical reactions. • The majority of the remaining bonds to Carbon are satisfied by H atoms. • Compounds that contain only C and H, the , are regarded as the parent structures.

Hydrocarbons

aliphatic aromatic

acyclic cyclic

alkanes alkenes alkynes

• The second classification is that of Functional Groups. • All other compounds may be derived from the parent structures by replacement of one or more H atoms by other atoms or "groups of atoms". • It is the nature of these other atoms which determines the characteristic chemical properties or functionality of the compounds. THE SKELETAL CLASSIFICATION

– 382 – • We begin with the hydrocarbons, compounds containing only C and H atoms. • The first subdivision is the aliphatic hydrocarbons, which may be divided into acyclic compounds (having no rings of C atoms) and cyclic compounds (having rings of C atoms).

Acyclic alkanes [MH5; 22.1] • The distinctive feature of the acyclic category is that the C atoms are linked in chains only. • This basic skeletal structure may be characterized further, according to the presence or absence of either branches in the chain or of multiple bonds linking the C atoms. • A continuous sequence of C atoms in an unbranched chain has often been called a straight chain; not really correct in view of the 109.5E bond angles!!

• When all of the C atoms are linked by single bonds only, the compound is said to be saturated, and is called an . • The first eight members of the acyclic alkane family are:

– 383 – • A particularly important characteristic of any such homologous series of compounds is that it can be represented by a general molecular formula.

• The general formula for acyclic aliphatic hydrocarbons is CnH2n+2 (where n = 1,2,3.... the number of Carbon atoms).

• Let’s take a look at butane, C4H10:

• We see that TWO different arrangements of the atoms are possible, one with an unbranched chain, and the other with a branched chain.

• Both have molecular formula C4H10 but different molecular structures; they are , and the general phenomenon is termed isomerism.

• Isomers are categorized as either structural isomers or stereoisomers.

– 384 – ISOMERISM IN ALKANES Structural Isomerism • When the differences between isomers arise from a difference in which atoms are bonded to which other atoms, the isomers are termed structural isomers. • If one looks at compounds of C and H, it becomes apparent that

three of the simplest compounds, CH4, C2H6 and C3H8, can have only one bonding arrangement. • Each C atom forms 4 bonds; each H atom 1 bond.

– 385 – •For C4H10, we saw that two non-equivalent bonding arrangements are possible, depending upon how the C atoms are linked.

A: CH3CH2CH2CH3..... m.p. !138E b.p. 1E density 0.579 g mLG1

B: CH3CH(CH3)2 ..... m.p. !159E b.p. !12E density 0.549 g mLG1

AB

• Each has its own unique physical and chemical properties, which can differ greatly.

• For the two isomers of C4H10, the only way that isomer A could possibly be converted into isomer B would be if two single bonds

were broken, CH3 and H were to exchange positions, and two new bonds were to form. (This does not happen under normal conditions.) • Because the energy required to break many single bonds is about 300!400 kJ molG1 at ordinary temperatures, structural isomers are normally stable and distinct chemical species. • When you are drawing structural isomers you must remember that one or more bonds must be broken to change one bonding arrangement into another. • This is characteristic of all structural isomers.

– 386 – Drawing Structural Isomers

C6H14

C4H9CR

– 387 – • The number of structural isomers possible for large molecules can be enormous, as the following data show:

CH4 none C6H14 5

C2H6 none C7H16 9

C3H8 none C8H18 18

C4H10 2C10H22 75

C5H12 3C20H42 366,319

Cyclic or Cyclo Alkanes, CnH2n • It is also possible for an unbranched chain of Carbon atoms to form a ring through loss of two H atoms, and formation of a C - C sigma (δ) bond.

EXAMPLE:

Hexane is C6H14:

It can form a ring:

• As a result, , C6H12 has two less H atoms than C6H14. • The smallest number of carbon atoms that can form a ring is three,

which results in cyclopropane, C3H6.

– 388 – ORGANIC CHEMISTRY SHORTHAND • The most common method for depicting rings is by simple polygons, in which each corner is a C atom. • Hydrogen atoms are not shown; the number of H’s at each corner C is the difference between 4 and the number of line-bonds shown. • Also, each line bond is assumed to terminate in a C atom.

• This is illustrated in the following example where the number of H atoms at each C (identified by number) is indicated:

8 3

2 7 4 1 5

6

There are at: C1 0 H atoms

C2 1 H atoms

C3,C4,C5 2 H atoms

C6, C7, C8 3 H atoms

– 389 – • Each type of ring system is classified also according to the number of rings (mono- vs poly-cyclic) and, more importantly, the degree of unsaturation (discussed later).

CH2 CH2 or CH3 or CH2 CH2 C4H8 C6H12

Linked Ring Decalin Fused Rings

Norborane Bridged Ring System

• Polycyclic molecules can differ in the structural relationship between rings. • The rings may be separate and independent and simply linked together or they may share one or more atoms. •Thus fused ring systems share two adjacent atoms (e.g. decalin) and bridged ring systems share at least three atoms (e.g. norbornane).

– 390 – Bond Rotation and Conformers • In addition to structural isomers, differing short-lived arrangements of atoms in a molecule, termed conformations, can result by rapid rotation of atoms, or groups of atoms, around single bonds.

• When two atoms are connected by a single bond, the atoms are free to spin about the bond axis. • For simple molecules such as HCR, this rotation of the two atoms with respect to the bond axis has no effect on the , and has no structural consequences. • For a bond to persist, overlap between orbitals must be maintained. Rotation about the H —CR bond ( a σ-bond ) does NOT break the overlap. • For molecules containing more than two atoms, however, the rotation about a single bond axis may change the molecular geometry. • When a rotation around a single bond does result in a change in the molecular geometry, the structures which can be drawn are called conformations, and the molecules are conformers.

– 391 – •The concept of a preferred conformation is simply illustrated using

, C2H6:

A B C

HH H H H H H H C C H C C C C H H H H H H H H H

•Figure A shows ethane as a flat molecule (which we know is not so!!).

•Figure B shows ethane in 3-D (recall the wedges indicate the bond coming forward and the dotted line indicate the bond going away).

•Figure C shows a different conformation of ethane; one “end” of the molecule has rotated, so the positions of the Hydrogen atoms with respect to each other have changed.

• There is another notation system known as Newman Projections; these show more effectively the radial distribution of the atoms attached to two adjacent atom centres.

– 392 – H H H H

H H H H

H H H H

Eclipsed Staggered

• The Newman diagrams for ethane show the spatial orientations observed when looking along the C—C bond axis - where the central point and its three spokes represent carbon atom 1 (in front), and the circle represents carbon atom 2 (behind). • For a simple symmetrical molecule such as ethane there are two extreme conformations obtained by twisting rotation about the C—C single bond.

•In the eclipsed conformation, each H atom bonded to C atom 1 is directly aligned with ("eclipses") one bonded to C atom 2. •In the staggered conformation (obtained by rotation of the rear o CH3 group through 60 ), each H atom on carbon 1 is spaced equidistantly between two H atoms on carbon 2.

– 393 – • All other conformations are intermediate in geometry and energy between these two extremes.

• A staggered conformation is of slightly lower energy than the eclipsed, so at any given instant, the majority of molecules will be found to be roughly staggered. • Because of their rapid interconversion at room temperature, conformations cannot normally be separated or isolated and are therefore not isomers.

• Consider the molecule dichloroethane, CR!CH2CH2!CR, and some of its conformations

Cl Cl Cl H H H

H H H Cl H H

Cl H Cl H H H

• These are different molecular geometries, but because they result from rotation about the C—C single/, they are conformers, not isomers. • They cannot be separated as distinct molecules with unique chemical and physical properties. • They are simply two different conformations of the same molecule.

– 394 – Conformations of Cyclic Compounds • There many compounds, both naturally-occurring and synthetic, which contain rings of atoms. • The cyclic structure limits the rotation about the bonds linking the ring atoms, and has extremely important consequences for the conformations and hence the shapes of a molecule.

• The most frequently occurring ring structures contain five or six atoms bonded together to form a ring. • The reason for this becomes apparent when one considers the normal bond angle anticipated for polyvalent atoms on the basis of VSEPR rules. • Atoms like carbon prefer to place their four bonding shell pairs at the corners of a , thus creating bond angles of 109.5o.

• If we think about the internal angles for a series of regular, planar polygons, it is clear that only in the case of the five - membered ring will the angle be close to the ideal tetrahedral angle. • This is illustrated by the following diagrams, the deviation from ideal bond angles is shown below:

49.5E 19.5E 1.5E 10.5E

– 395 – • In order to avoid these unfavourable bonding angles, many rings larger than 3-membered are not planar, and, more significantly, if the ring contains six C atoms, the tetrahedral bond angle can be exactly achieved by adoption of a non-planar conformation.

Cyclohexane, C6H12 • This molecule is unique in that it can adopt two basic conformations in which all bond angles are perfectly tetrahedral, referred to as the "chair" and "boat" conformations.

• Even more importantly, in one of these two conformations, the chair, all bonds to any two adjacent carbon atoms are perfectly staggered ! • This is shown in the Newman projection of the chair conformer, structure A:

A B

• While at room temperature there is relatively easy and rapid inter- conversion between these two extreme conformations, the chair is slightly lower in energy than the boat, so at any given instant the majority of molecules will be found in the chair conformation.

– 396 – • If we look at the geometry of 6-membered ring systems more closely, two important characteristics of the chair become evident...... • Despite its non-planar nature, the ring approximates a planar system having 'top' and 'bottom' sides with one of the two H atoms on each C atom on top and the other on the bottom. (Structure B). • Of the twelve H atoms, six lie close to the average plane of the ring, and are described as equatorial, while the other six lie above and below the plane, and are called axial hydrogen atoms.

– 397 – ALKENES AND ALKYNES [MH5; 22.2] • When one or more multiple bonds (C=C or C/C) are present, the molecule is said to be unsaturated because such compounds always contain fewer H atoms than the saturated analogs, the alkanes.

• The presence of a double bond (C = C) means the compound is an

; general formula CnH2n. • If the molecule contains a double bond, it will have very different chemical properties than those of the corresponding alkane. • The double bond is classified as a ; in addition to the skeletal classification.

•If a triple bond (C/C) is present, the compound is an ; of

general formula CnH2n-2.

• You may have noticed that for each multiple bond introduced, two H atoms are lost from the fully saturated alkane parent....

• For example, the two C atom compounds: Ethane

Ethene ()

Ethyne ()

are considered to have zero, one and two degrees of unsaturation, in that ethane is fully saturated, ethene is short 2 H atoms, while ethyne is missing 4 H atoms.

– 398 – • 2 H atoms = one degree of unsaturation. • How else can a compound “lose” 2 H atoms?? •When it forms a ring; as we saw with cyclohexane.

• Look at a compound of molecular formula C8H14...... • This molecule is short of two pairs of H atoms from the

corresponding alkane (C8H18).

• It could contain either a triple bond (2-octyne), two double bonds (1,3-octa-diene), a ring and a double bond (cyclooctene), or even two rings.

CH3(CH2)4C/CCH3

2-octyne 1,3-octadiene

cyclooctene bicyclo

• Shown in the figures above are some of the methods used to depict

– 399 – the structural formula of a compound. •In the condensed structural formula, the H atoms (or other atoms/groups of atoms) are written on the right hand side of the

C to which they are attached (e.g. CH3CH2CH2CH3). • You may also see:

(CH3)2CHCH2CH3 or CH3(CH2)4CH3

• How do we count the number of rings ?

•Consider decalin...... there are two obvious six-membered rings, fused together, but is there also a third, formed by ignoring the fused bond? • Is this molecule tricyclic ? • The answer is no - it's bicyclic. • This is indicated by the fact that only two bonds in the structure need to be broken to make it acyclic.

– 400 – Decalin

• The skeletal classification may be extended by the introduction of H O S O O N hetero-atoms, usually N, O and S, as depicted below for a number of heterocyclic species

STEREOISOMERISM [MH5; 22.5] • Structural isomers are those which arise from the differing bonding connections among atoms in a molecule. • Stereo isomers are isomers in which the bonding connections remain unchanged, but the spatial arrangements of specific groups relative to one another differ. • One type of stereoisomerism is know as geometric isomerism.

Geometric Isomerism in Alkenes • The most common structural feature which gives rise to geometric isomers in carbon compounds is the carbon/carbon double bond. • For the molecule 1,2 - dichloroethene, CRCH=CHCR, two bonding arrangements exist, one in which both chlorine atoms are on the same side of the double bond, and one in which they are on opposite sides.

-401- Cis Trans

-402- • There is no rotation at room temperature about a double bond, so conversion of cis to trans would require that bonds be broken and reformed. • Because there is no rotation, the isomers do not interconvert and a mixture of them can be separated because of their differing physical and chemical properties. • Therefore, they are not conformations, but are actually different compounds.

• Geometric isomers exist only when the two carbon atoms of the double bond each bear two different groups.

• For example, the molecule 1,1 - dichloroethene, CH2= CCR2 (a of 1,2-dichloroethene) does not have geometric isomers...

1,1-dichloroethene

• Although many compounds containing double bonds have geometric isomers, it is incorrect to assume that all do.

-403- • As a further example, the formula CH3CH=CHCH3 has geometric

isomers, but (CH3)2C=CHCH3 does not......

CH3CH=CHCH3

geometric isomers

(CH3)2C=CHCH3

no geometric isomers

• For each double bond in a molecule, a maximum of two geometric isomers is possible.

-404- • So for compounds containing two double bonds, a maximum of four geometric isomers can result; for the formula

CH3CH=CH-CH=CHCH2CH3......

• All four of these isomers have different chemical and physical properties, and to convert any one into one of the others requires the breaking and reforming of one or more bonds.

-405- Geometric Isomerism in Cyclic molecules • The presence of a ring of atoms in a molecule can also give rise to geometric isomers. • If one of the H atoms in cyclopentane, A, is replaced by another

atom/group (e.g. CH3 or F), there is no difference between it being on the top, B or the bottom side C. Structures B and C are identical.

H F F

F H F

A B C D

• If the second H atom on that same Carbon atom is replaced by a F atom in D, there is again no stereoisomerism possible.

-406- • But if the second F atom is bonded to a different Carbon atom, then the possibility of geometric isomerism arises; the second F atom can be on the same side of the ring, E, or on the opposite side, structure F, from the first.

F

FF F E F

E: cis!1,2!difluorocyclopentane F: trans!1,2!difluorocyclopentane

• Structures E and F are obviously different; they are two geometric isomers each with its own characteristic properties.

• Note also that E and F are both structural isomers of D, because the connectivity of the atoms is different in D, though E is not a structural isomer of F.

• As is the case with double-bond geometric isomers, the essential requirement for geometric isomerism in cyclic molecules is that there be two different groups on each of two different ring atoms.

-407- Optical Isomers • Optical isomers are another example of stereoisomerism. • Optical isomers occur when a carbon atom in a molecule is bonded to four different atoms or groups. • This type of bonding arrangement always results in two different forms of the molecule, and the forms are mirror images of each other. • The carbon atom in such molecules is called the chiral centre ( or sometimes the stereocentre) ; and these molecules are said to be chiral. • The two different forms of the molecule are called enantiomers. • Chiral molecules have no plane of symmetry; molecules that do have a plane of symmetry therefore cannot be chiral.

• A molecule may have more than one chiral centre, in this case, there will be more than one pair of enantiomers. • Stereo isomers which are not mirror images of each other are called diastereomers.

-408- • Because molecules that exist as pairs of enantiomers have the same tetrahedral structure and bonded groups, they exhibit nearly identical chemical properties. • However, they may behave differently when they react with other chiral molecules.... • Most biochemical reactions involve chiral molecules (some with several chiral centres) and the “fit” between molecules is crucial. • So, if chiral molecules exist as pairs of enantiomers, how do we tell them apart ? • A characteristic of chiral molecules is their ability to rotate a plane of polarized light; one isomer will rotate the light clockwise (to the right) and the other isomer will rotate the light counter clockwise (or to the left). • The direction of rotation must be determined experimentally. • We often label these molecules “R” (right handed) and “S” (left handed); this designation has to do with the orientation of the various groups bonded to the chiral carbon.

EXAMPLES:

• In the course of a which produces molecules with a chiral centre, often a 50:50 of the two isomers is formed. • This is called a racemic mixture; it has no effect on plane polarized light.

-409- Finding chiral carbons (or the chiral centres) in a molecule:

OH H H l l l

H3C - CH2 - C - CH = CH2 H3C - C - CH2 - C - CH2CH3 l l l

CH2CR OH CH2CH3

CH3 CH3

OH OH

CH3 CH3

OH H3C

CH2OH H O H H OH H OH OH H OH

-410- Aromatic Hydrocarbons or Arenes • When rings are highly unsaturated their chemical behaviour is different from the corresponding saturated compounds, so their chemistry is usually treated separately. • Since most of the examples first identified had distinctive smells or aromas, they became known as aromatic compounds.

• The simplest (and probably most discussed!!) arene, , C6H6 was discovered in 1825 by Michael Faraday. • All aromatic compounds are cyclic by definition.

C 6 H 6 benzene C 12 H 10 biphenyl

OH Oil of wintergreen

COOCH3

• Their most distinctive feature is that the structure cannot be adequately represented by a single .

• Thus benzene (C6H6) is depicted by the two contributing structures:

• Benzene is considered the parent aromatic , with four degrees of unsaturation (three double bonds and one ring). • The benzene ring is an exceptionally stable structure, even when it is a part of a larger compound.

FUNCTIONAL GROUP CLASSIFICATION

-411- • Now that we know all (!!??) about the structural skeletons of organic molecules, we will look at what happens when one or more H atoms of a hydrocarbon is replaced by other atoms or groups of atoms. • These other atoms/groups - the functional groups - are what determine both the chemical and the physical properties of the "families" of compounds. • A tabular summary of some common functional groups arranged according to the heteroatoms involved follows on page 415 - 416.

• The first families listed are the hydrocarbons. • The presence of a C = C functional group or a C/C functional group in the unsaturated hydrocarbons gives the molecule characteristic chemical properties. • Also shown is a short-hand device employed by organic chemists in writing structural formulas. • Because the focus of attention is usually the functional group, the hydrocarbon skeleton to which it is attached is frequently abbreviated to the symbol, R (for residue, or the rest of the molecule).

• The families containing O are very important. • Those containing a saturated O group (i.e. singly bonded, R—O—X) may be considered as though it came from water by successive replacement of the H atoms by hydrocarbon groups (R = group,

e.g. CH3, CH3CH2, etc.). • This analogy is particularly apt in the case of which exhibit many chemical and physical properties similar to water. •An , R - OH, can be represented by the general molecular

formula, CnH2n+2O (insertion of the O atom has no effect on the number of H atoms present).

-412- •Water, H - O - H and an alcohol, R - O - H

• have the general formula: R - O - R'

• The remaining families of O-containing compounds are characterized by the presence of a , where two H atoms attached to a C atom of a hydrocarbon have been replaced by an O atom.

CO

• The chemical properties of the carbonyl group are very dependent on the nature of the other groups attached to the carbonyl carbon. • Therefore, the carbonyl group appears in many different types of compounds. • Unless a molecule has only one C atom (the carbonyl carbon), one of the two atoms attached to the C atom of the carbonyl group is always a C atom (of an alkyl or aryl group).

-413- • If the second atom is a C the compound is a .

CO

• If the second atom is a H atom, the compound is an .

CO

• Due to the C=O bond these compounds are said to be unsaturated. • So, the general formula for acyclic aliphatic and

is CnH2nO.

• The other families form a sub-set in that the second atom attached to the carbonyl C atoms is a heteroatom (i.e. N, O, X). • When that second atom is the O atom of a , the ombination is called a carboxyl group.

CO

• Because of their acidity, this family of organic compounds is known as the carboxylic .

-414- • The other members of this sub-set (carboxylate salts and ) are referred to as derivatives because they each can be derived from carboxylic acids.

• The last two families listed in the table contain N atoms. •The are derivatives of where the H atoms have been replaced successively by C atoms of (alkyl or aryl groups), • Amines exhibit chemical properties similar to ammonia; in particular they are weak bases. •The contain both O and N, with a carbonyl group bonded to an amino group. • This linkage is of great importance in polyamide polymers such as nylon and forms the that links two amino acids in naturally occurring .

-415- The Functional Group Classification

Alkanes: or C C C C C C

Alkenes: or CC

C C Alkynes: or

Arenes: Alcohols: R OH

O Ethers: R R'

O O Aldehydes: or C R H R H

-416- O O Ketones: or C R R' R R'

O O Carboxylic Acids: or C R OH R OH

O O Esters: R' or C R' R O R O

Amines: R NH2 ; R NH R' or R N R'

R"

O O Amides: or R' C R' R N R N

H H

-417- • You can see that in several of the functional groups there are oxygen atoms, some of which are double bonded, or a nitrogen atom. • Determination of degrees of unsaturation is trickier when there are heteroatoms in the formula of a molecule......

• Oxygen - inclusion of singly bonded O has no effect on the number of H atoms when compared with the corresponding hydrocarbon. • In calculations of degrees of unsaturation, one simply ignores O atoms, if the molecule correctly corresponds to the fully saturated hydrocarbon.

EXAMPLE: C5H12O

• If the molecule does not correspond to the fully saturated hydrocarbon, we use the same method to determine degrees of unsaturation, then we deal with the O.

EXAMPLE: C4H8O

-418- • Halogen atoms, F, CR, Br, and I : When a halogen atom is present it is considered to have replaced a H atom, and therefore the number of halogen atoms and H atoms must be added to arrive at the 'base hydrocarbon'.

EXAMPLE: C4H5Br3

• Nitrogen - An (R3N, R2NH, RNH2), has one more hydrogen

atom than the corresponding hydrocarbon: CH3NH2 vs CH4.

EXAMPLE: C5H9N

-419- REACTIVITY of ORGANIC COMPOUNDS • Most reactions of organic compounds involve a second reactant, called a reagent. • For a reaction to take place usually two species must collide and do so in such a way that one or more covalent bonds are broken and/or one or more bonds are made. • These reactions occur typically through a number of separate and discrete steps. • Reaction mechanism is the term applied to the description of the detailed course of the overall reaction - any mechanism must explain and account for all observable, experimental facts. • These include such things as reaction conditions (heat, light, catalysts), formation of intermediates or of by-products through side reactions, and ultimately, why these changes occur. • When a breaks it can do so in two ways which differ in the fate of the shared electron pair......

(1) HOMOLYTIC BOND BREAKING A—B ! A• + B• (radicals) e.g. the free chlorination of

(2) HETEROLYTIC BOND BREAKING A—B ! A+ + :B G () e.g. a

• Heterolytic bond breaking is the more common of the two and usually involves a polar covalent bond between two atoms of different electronegativities. • The more electronegative atom always acquires the electron pair.

-420- ELECTRONEGATIVITIES OF SOME ELEMENTS

HC NOF 2.1 2.5 3.0 3.5 4.0

Si P S CR 1.8 2.1 2.5 3.0

• Many organic reactions take place via initial ionization to give a reactive, ionic intermediate. • When the positive or negative charge is located on a C atom, the intermediate is called a carbocation or a , respectively.

l — C — CR º CR + l C

CARBOCATION

l

— C— H + :B º HB + C: l

CARBANION • Many of the reactions involve a conversion of one functional group into another while the C skeleton remains intact and unchanged. • Most of the reactions we will consider in the sections which follow can be classified in one of three categories:

-421- Substitution • An atom or group of atoms is substituted for another atom or group of atoms attached to a C atom, without any change in the number of double or triple bonds if any are present.

CH3CH2OH + HBr ! CH3CH2Br + H2O

• Above, a bromo "group", Br, replaces a hydroxy group, OH, or a H atom (below).

H Br Br2

FeBr3

Addition • Atoms or groups of atoms are added to the compound without any loss of atoms from it. • There is an increase in the number of atoms attached to at least one C atom:

Pd catalyst

CH3CH 4 CH2 + H2 ! CH3CH2CH3

• Here 2 H atoms are added, one to each of the two C atoms of the C = C group.

-422- Elimination • Atoms or groups of atoms are eliminated, or removed from the compound without any atoms being added to it. • There is a decrease in the number of atoms attached to at least one C atom, with a corresponding increase in the number of double or triple bonds.

H2SO4 catalyst

CH3CH2CH2—OH ! CH3CH 4 CH2 + H2O

• Here water is eliminated; a hydrogen atom is removed from one C atom and a hydroxy group from another.

REACTIONS OF THE FUNCTIONAL GROUPS

Acyclic and cyclic alkanes • The outstanding feature of saturated hydrocarbons is their general lack of reactivity. • This is due to the relatively strong, yet non-polar, C—C and C—H bonds. • In the absence of any reactive site (or functional group) alkanes do not react with common acids and bases, or oxidizing and reducing agents. •Because of this inertness, alkanes can often be used as solvents for reactions of other substances.

• However, under certain conditions alkanes will react with O2, CR2, or

Br2.

-423- Oxygen • The most important use of alkanes is as fuels. • When initiated (e.g. with a spark), alkanes burn in an excess of oxygen according to the combustion equations discussed in earlier parts of the course...

C8H18 (R ) + 25/2 O2 (g) ! 8 CO2 (g) + 9 H2O (R )

• Although an initial input of energy is required, once initiated the reaction proceeds spontaneously and exothermically. • These combustion reactions are the basis for the use of hydrocarbons for heat ( natural gas and fuel oil) and for power (gasoline). • Virtually all organic compounds are combustible in air, so this is not a characteristic reaction of only alkanes.

Chlorine • When illuminated with light, or heated to 300!400EC, a mixture of methane and chlorine gases reacts vigorously to form chloromethane and :

• This is a substitution reaction, in which a H atom has been replaced by a CR atom.

-424- •The CH3CR formed can then undergo a second substitution reaction to

form dichloromethane, CH2CR2, which in turn can form CHCR3

(), and yet again to yield , CCR4....

• It is very difficult to limit chlorination of methane to the monosubstituted product.

• Mixing one mole of CR2 with one mole of CH4 might be expected to

give one mole of CH3CR and one mole of hydrogen chloride, but this is not the case. • At the start of the reaction there is only methane for chlorine to

react with, as is desired, but as the reaction progresses and CH3CR is formed in increasing amounts, there is an increasing chance of

chlorine reacting with CH3CR instead of with the diminishing amount of methane. • Even using a large excess of methane gives a mixture of products. • Fortunately methane and chloromethane have greatly differing boiling points (!161 and !24EC, respectively) and can be separated by distillation.

-425- Alkenes and alkynes • In contrast to alkanes, unsaturated aliphatic hydrocarbons react readily with the halogens, acidic reagents and a variety of oxidizing and reducing agents. • Reaction is characterized by addition of reagent X—Y to the double or triple bond......

• This reaction usually occurs by means of a multi step mechanism......

-426- • Once addition to an alkene has occurred the product is saturated, so further addition is not possible. • The most common alkene addition reactions of practical importance are outlined below. • Alkynes react in the same way; often the only essential difference is that further addition can occur since after one addition to an alkyne, the product is an alkene......

H2

CH3C/CH ! Pd

Hydrogen, H2 • The addition of hydrogen converts an alkene to an alkane. •This saturation or hydrogenation of the double bond is an important reaction both in the research laboratory and in commercial applications. • Although this addition reaction is strongly exothermic, it is very slow if no catalyst is used.

Pd catalyst

CH3CH 4 CH2 + H2 !

• The common catalysts are finely divided metals such as palladium, nickel or platinum. • The heterogeneous catalyst lowers the activation energy barrier to reaction by adsorbing both reactants on its surface, which facilitates both the bond-breaking and the bond-making steps.

-427- Halogens, X2

• In contrast to hydrogen, both CR2 and Br2 react rapidly with alkenes in the absence of a catalyst.

CH3CH 4 CHCH3 + CR2 !

CH3CH 4 CH2 + Br2 !

• The reaction with Br2 is a useful qualitative test for the presence of a carbon-carbon double or triple bond.

• Solutions of Br2 in most solvents are coloured red-brown; alkenes and dibromoalkanes are typically colourless.

• The rapid disappearance of the reddish colour of the Br2 solution is a characteristic reaction of an alkene or alkyne, and provides a simple visual method of detection.

Water, H2O • Water adds to alkenes in the presence of an acid (catalyst) to form alcohols:

H2SO4 catalyst

CH3CH 4 CHCH3 + H - OH !

-428- •If the alkene is unsymmetrical (the two C atoms of the double bond do not bear the same groups), addition can to two structurally isomeric products.

H2SO4 catalyst

CH3CH 4 CH2 + HOH !

• Both products are formed but, in most instances, one tends to be strongly favoured over the other.

Hydrogen Halide, HX • Hydrogen halides (HCR, HBr, HI) add to alkenes to give alkyl halides:

CH3CH 4 CHCH3 + H—X !

• Just as in addition of water, addition of hydrogen halide to an unsymmetrical alkene gives a mixture of two products, although once again, the formation of one particular isomer tends to predominate.

CH3CH 4 CH2 + HCR !

-429- Arenes, or Aromatic compounds

• Unlike the addition reaction of alkenes and alkynes, benzene and other aromatic compounds undergo substitution reactions in which a H atom is replaced by a variety of atoms or groups of atoms

Cl, Br NO2 OH alkyl alkenyl

Br NO2 OH CH3 CH CH2

• Substitution is limited to one H atom replaced per carbon atom, but there are three possible sites where a second could go.....

Br Br Br

Br

Br

Br

• The six carbon aromatic ring DOES NOT react !!!

-430- Alcohols • The system which has been developed for classifying alcohols specifies the location of the hydroxy group —OH on the C skeleton.

• An alcohol is designated as primary, secondary, or tertiary, when the C atom bonded to the OH is attached to one, two or three other C atoms

H R’ R’ l l l R - C - OH R - C - OH R - C - OH l l l H H R”

primary secondary tertiary

• A familiar is Ethyl alcohol, or , C2H5OH.

• This is the one that you drink! (, CH3OH is the one you do NOT drink!)

Acid/Base Properties of Alcohols • Like water, alcohols are very weak acids, but, depending on their structure, usually somewhat weaker still. • A solution of an alcohol in water is neutral.....

+ 16 HOH º H + OH G Ka = 1.8 × 10G + 16 19 ROH º H + RO G Ka = 10G to 10G

• As water does, alcohols react with the alkali metals (eg. Li, Na, K)

to liberate H2 gas......

-431- + HOH + Na ! Na OHG + ½ H2 (g) + ROH + Na ! Na ORG + ½ H2 (g)

• The resulting salt, a metal alkoxide, is a very strong base which hydrolyses extensively in water:

RO G + H2O ! ROH + OH G

• Water is not only a very weak acid but also a very weak base. • In the presence of a strong acid it accepts a proton to form the + , H3O :

+ H2O + HCR ! H3O + CR G

• Similarly, alcohols behave as bases and can react with strong acids to form oxonium ions, the equivalent of the hydronium ion:

+ ROH + H2SO4 ! ROH2 + HSO4G

• This behavior can be attributed to their related structures -

methanol, CH3OH , is simply water with a H atom replaced by a . • The —OH functional group, with its lone pairs of , is unchanged.

-432- Oxidation of Alcohols • Oxidation of alcohols is the first organic oxidation reaction we will study. • While we know that oxidation means “loss of electrons”, in organic chemistry it means other things too...... • Oxidation also means: loss of 2 H’s or gain of O. • Primary and secondary alcohols oxidize to form carbonyl compounds: H l [ O ] \

— C —OH ! C = O + H2O l /

•The [O] refers to an oxidizing agent; the most commonly used

oxidants are chromic , CrO3 and dichromate salts Na2Cr2O7 in acidic solution. • In this reaction is 2 H atoms are eliminated. • This requires that the hydroxy-bearing C atom have at least one H atom. • The ease of reaction and the nature of the product formed is dependent on the type of alcohol. • Primary alcohols oxidize extremely readily and the initially formed product is an aldehyde. • However, aldehydes are very easily oxidized.....

O O ll ll

CH3CH2OH ! CH3 C—H ! CH3 C—OH Ethanol Acetic acid

-433- • Unless the oxidation is carried out under carefully controlled conditions, the product isolated from the oxidation of a primary alcohol is the corresponding . • The fact that the alcohol and the aldehyde are being oxidized here is most easily recognized by drawing analogy to the one-C series

CH3OH ! CH2O ! HCO2H methanol

• The oxidation numbers of C go from !2 through 0 to +2 in this series. • How do we determine the Oxidation Number of Carbon in an ?

• For each bond between Carbon and an atom less electronegative than Carbon (usually Hydrogen), assign a -1.

• For each bond between Carbon and an atom of equal electronegativity (another Carbon or Sulfur), assign a 0.

• For each bond between Carbon and an atom of greater electronegativity (Oxygen, Nitrogen or Halogens), assign a +1.

• Add all these numbers together, being sure to keep the signs straight.

• The result is the Oxidation Number of Carbon.

-434- EXAMPLE:

CH3OH

CH2O

HCOOH

-435- • Oxidation of a secondary alcohol gives a ketone:

CH3CH — CH3 ! CH3—C—CH3 l ll OH O

• Ketones do not usually undergo further oxidation.

• Under the same conditions, where primary and secondary alcohols react easily, tertiary alcohols, which lack a H atom at the C atom bearing the OH group, are not oxidized at all:

CH3 l

CH3— C—OH ! NO REACTION l

CH3

• As no reaction occurs, the red-orange colour of the acidic CrO3 solution remains.

• In contrast, oxidation of 1E and 2E alcohols by CrO3 results in a change in colour from red-orange to the green of Cr3+(aq). • This difference is the basis of a qualitative test to distinguish primary and secondary alcohols from tertiary alcohols, and was also used in the “breathalyzer” test for inebriated drivers.

-436- Reduction of Aldehydes and Ketones • It should come as no surprise that the oxidation reactions described above can be reversed with a suitable reducing agent, converting aldehydes and ketones into alcohols.

• Sodium borohydride, NaBH4 and lithium aluminum , LiAlH4

are commonly used in the laboratory, but H2 gas (with a catalyst) is used for industrial scale reactions. • If oxidation reactions lose 2 H atoms, then reduction reactions must gain 2 H atoms!!

Ethers • The general formula of an is R—O—R’ and they are usually named by stating the nature of R and R’

symmetrical unsymmetrical

CH3 — O — CH3 C2H5 — O — CH3 ethyl methyl ether

• Ethers are weakly polar, with a solubility in water comparable to that of alcohols. • Symmetrical ethers are important industrial solvents. • Ethers are synthesized by dehydration of alcohols:

conc. H2SO4 2 C2H5OH ! C2H5— O — C2H5 + H2O heat to 140EC

-437- • Ethers are quite non-reactive; they do not undergo reduction, elimination, oxidation, or reaction with bases, so they are popular in the laboratory as solvents. • Because they do not have the — OH functional group, ethers molecules do not to each other; this results in quite low boiling points. (For , the b.p. is 35o C)

Carboxylic Acids and Their Derivatives • A carboxylic acid is a combination of a carbonyl group and a hydroxy group:

• Because of the unsaturated and dipolar character of this combination, acids undergo a variety of reactions, nearly all of which are reactions of the — OH group made possible by the presence of the carbonyl group.

-438- Acid/Base Properties of Carboxylic Acids [MH5; 13.4] • The most distinctive chemical property of these compounds is their acidity. • All are weak acids undergoing partial ionization in water to give weakly acidic solutions:

+ 5 R — COOH º H + R — COO G Ka . 10G

• For most of these acids, the magnitude of the acid ionization 5 constant, Ka, is in the vicinity of 10G . • Why are the carboxylic acids so much stronger acids than water or alcohols - compounds that also contain an —OH group ?

+ 16 CH3CH2— O — H º H + CH3CH2O G Ka = 10G ethyl alcohol ethoxide ion

• The difference is attributed to stabilization of the conjugate base of the acid by resonance through the carbonyl group. • In the ethoxide ion the lone pairs of electrons are localized on a single O atom, whereas in the acetate ion electrons are delocalized equally over both O atoms......

-439- • As there are two contributing structures; each O atom has a formal charge of !½. • Since this delocalized structure is of lower energy than a localized one, equilibrium for the formation of the carboxylate lies farther to the right than that for the formation of an alkoxide ion.

• of an aqueous solution of the acid with one equivalent of base gives a solution of the salt...

and the pure carboxylate salt can be isolated in crystalline form by evaporation of the water.

• Carboxylic acids are strong enough acids to liberate CO2 gas from

bicarbonate salts (e.g. NaHCO3).

-440- Formation of Carboxylic Acid Derivatives • Carboxylic acids can be converted into a number of structurally related derivatives, compounds in which the —OH group is replaced by other groups. • One important derivative is the .

Esters • When a carboxylic acid and an alcohol are heated in the presence of

an acid catalyst (usually conc. HCR or conc. H2SO4) an equilibrium is established with an ester and water:

O O ll ll

CH3 C—OH + HO - CH3 º CH3 - C—O—CH3 + H2O acetic acid methanol methyl acetate

• The yield of the ester can be maximized by continuous removal of the water as it is formed; this is an example of an elimination reaction. • The most important reaction of esters, and of all other acid derivatives, is hydrolysis to the corresponding carboxylic acid, which is simply the reverse of ester formation:

-441- • Volatile esters generally have pleasant odours, and are of importance in the perfume industry and as artificial flavourings.

• The last two families of compounds to be considered in this brief summary, amines and amides, contain a N atom.

Amines • Amines are derivatives of ammonia and are classified as primary, secondary or tertiary according to the number of C atoms attached to the nitrogen atom......

H - N - H R - N - H R - N - R’ R - N - R’ l l l l H H H R” ammonia primary secondary tertiary

• Note that the terms primary, secondary and tertiary are applied to amines in a different way than they are applied to alcohols. • As is the case for ethers and ketones, the R groups may be the same or different, and the nitrogen atom may be part of a ring of atoms.

-442- Basicity of Amines [MH5; 13.5] • Like ammonia, amines are weak bases and their aqueous solutions are basic:

+ 5 NH3 + H2O º NH4 + OHG Kb = 1.8 × 10G

+ 4 CH3NH2 + H2O º CH3NH3 + OHG Kb = 5.0 × 10G

• As bases, amines react quantitatively with acids to form substituted salts:

+ CH3NH2 + HCR ! CH3NH3 CR G methylamine methylammonium chloride

+ (CH3)3N: + CH3COOH ! (CH3)3NH CH3COOG trimethylamine trimethylammonium acetate

• For most aliphatic amines the magnitude of the base ionization 4 5 constant, Kb, is about 10G , close to that of ammonia, 1.8 x 10G

• In contrast to esters, amines usually smell awful ! • Methylamine has a sharp odour like that of ammonia, and trimethylamine smells like dead fish.

-443- Amides • Amides are compounds formed from the reaction of a carboxylic acid with an amine. • In these compounds, the —OH of the carboxylic acid is replaced by

—NH2 or —NHR or —NRR'. • So, an contains a carbonyl group linked to an amino group....

• Amides are generally prepared by a two step process:

1)

-444- 2)

• Amides are hydrolysed when heated with aqueous acids or aqueous bases, reforming the carboxylic acid and the amine. • Depending upon the pH of the reaction, either the acid or the amine is obtained as a salt.

• The amide linkage, sometimes called the peptide bond, is particularly important in some synthetic polymers and proteins. • The analgesic acetaminophen is an amide.

-445- ORGANIC POLYMERS [MH5;22.6] • Polymers are giant molecules (from the Greek, 'many parts' poly meros ) made by joining many small molecules often referred to as monomers. • Polymer molecules can have molecular weights ranging from thousands to millions.

• Polymers may be naturally occurring, such as proteins, and nucleic acids. • Synthetic, or man made polymers include many plastics, polyesters, polyamides and composites.

• Polymers are often classed according to their method of synthesis......

• Addition polymers such as polyethylene, polystyrene (PS)and polyvinylchloride (PVC) are made by adding together the simple

alkenes: CH2=CH2 , C6H5CH=CH2 and CH2=CHCR respectively.

-446- • Heating ethylene to 100-250EC at 1000-3000 atmospheres in the presence of a catalyst gives polymers with molar masses of several million. • A polymer with molar of one million would contain almost 36,000 ethylene molecules!!!!

• Depending upon the reaction conditions, different molecular structures result, with different properties. • Polymer chains can be linear (HDPE),

branched (LDPE),

-447- or cross-linked (CLPE)

• HDPE is dense, hard and strong, due to the close packing formed by the long, linear chains. • LDPE is soft and flexible; it has a lower density due to the branching of the polymer chains. • CLPE is rigid and inflexible; the cross linking adds to the rigidity of the polymer.

• Teflon is polytetrafluoroethylene:

• Styrofoam (food and beverage containers) is made by cooling foamed, molten polystyrene

C C

-448- • Condensation polymers such as polyesters and polyamides are made by elimination reactions. • The reaction between terephthalic acid and ethylene glycol gives the polymer PETE, polyethylene terephthalate, by elimination of a water molecule.

HO O

C C + HOCH2CH2OH

O OH

OH O

+ H2O C C

O OCH2CH2OH

HOCH2CH2OO

C C

O OCH2CH2O O C C

OH O

• Soft drinks are sold in PETE bottles. • PETE is an example of a polyester...... when recycled, 5 large pop bottles will make a T- shirt!!

-449- •Nylon is a polyamide prepared by elimination of H2O in the reaction of adipic acid and hexamethylenediamine:

O H H OH + N N HO H O H

H O

N H HO N

O H

H H O O N N OH N N H O O H H

• You will note that the linkage in nylon is another example of an amide linkage.

-450- Natural Polymers: Polypeptides and Proteins • Proteins contain an amide linkage formed by elimination of a water molecule in a condensation reaction between two amino acids. • Amino acids (with one exception) are chiral molecules......

•When two amino acids form an amide linkage, it is known as a peptide bond; the molecule formed is known as a dipeptide.

• When the resultant molecule becomes really large (consisting of hundreds or thousands of amino acids), the polymer becomes known as a . • The properties of a protein depend on the sequence of amino acids that make up the backbone of the molecule. • Enzymes (biological catalysts) are polypeptide molecules and may be rendered useless if even one in the polymer chain is missing or out of place.

-451-