An introduction to AS Level organic chemistry

Hydrocarbons, functional groups and the homologous series

A hydrocarbon is a compound made up of carbon and hydrogen atoms only. The alkanes are the simplest hydrocarbons: they are saturated, they contain no functional group at all, and their general formula is CnH2n+2, so the family runs methane CH4, ethane C2H6, propane C3H8 and butane C4H10. Everything else you meet at AS Level is an alkane with a functional group attached, or an unsaturated hydrocarbon such as an alkene.

A functional group is an atom or a group of atoms that gives the molecule its characteristic chemical reactions. It also controls much of the physical behaviour, because it is usually the polar part of the molecule: it decides whether the molecules can hydrogen-bond to each other, whether the compound dissolves in water and how high the boiling point is. A C=C makes a compound react by addition; an OH makes it react by oxidation, by elimination and by esterification; a COOH makes it react with alcohols, with bases and with carbonates.

A homologous series is a family of organic compounds with the same functional group and the same general formula, in which each member differs from the next by one CH2 group. Members of a series have similar chemical properties, because the reactions are the reactions of the functional group, and a gradual change in physical properties, because the chain gets longer and the instantaneous dipole-induced dipole forces between the molecules get stronger. Boiling points therefore rise steadily as you go up the series, and the small members are the ones that dissolve in water.

Formulas and systematic names

You must read and write five kinds of formula for the same molecule. The general formula describes the whole family: CnH2n+1OH is any alcohol. The molecular formula gives the actual number of each kind of atom, so propan-1-ol is C3H8O. The empirical formula gives the simplest whole-number ratio of those atoms, so propan-1-ol is C3H8O (already the simplest ratio) while ethanoic acid is C2H4O2 and has the empirical formula CH2O.

The structural formula, often called the condensed structural formula, writes the atoms group by group on one line, so propan-1-ol is CH3CH2CH2OH. The displayed formula draws every atom and every bond in the molecule, which is the only way to show exactly which atom is joined to which. The skeletal formula draws only the carbon skeleton as a zig-zag line: every vertex and every end of the line is a carbon atom, the hydrogen atoms joined to carbon are left out, and any atom that is not carbon or hydrogen is written in full with its hydrogens.

Worked example. Butan-2-ol, CH3CH(OH)CH2CH3. Count the atoms: C 4, H 10 (three in the first CH3, one on the CH, two in the CH2 and three in the last CH3) and O 1, so the molecular formula is C4H10O. The ratio 4 : 10 : 1 is already the simplest whole-number ratio, so the empirical formula is also C4H10O. Compare ethanoic acid, molecular formula C2H4O2: dividing every number by 2 gives the empirical formula CH2O.

Naming. Find the longest carbon chain that contains the principal functional group; that chain gives the stem - meth-, eth-, prop-, but-, pent- and hex- for one to six carbon atoms. Number the chain from the end that gives the functional group the lowest number, or the first substituent if there is no functional group, and write that number in the name. The functional group becomes a suffix: -ol for an alcohol, -al for an aldehyde, -one for a ketone, -oic acid for a carboxylic acid, -amine for an amine and -nitrile for a nitrile. Halogens and alkyl groups become prefixes such as chloro-, bromo-, iodo-, methyl- and ethyl-. The carbon atom of an aldehyde or carboxyl group is always C1, so propanal and propanoic acid need no number, but an alcohol or a ketone group can sit at more than one position, so propan-1-ol is a different compound from propan-2-ol. Esters are named in two parts: the alkyl group from the alcohol first, then the alkanoate from the acid, so CH3CH2COOCH3 is methyl propanoate, made from methanol and propanoic acid.

• meth-, eth-, prop-, but-, pent-, hex-: 1 to 6 carbon atoms in the longest chain.
• Suffixes: -ol alcohol, -al aldehyde, -one ketone, -oic acid carboxylic acid, -amine amine, -nitrile nitrile.
• Prefixes: chloro-, bromo-, iodo-, methyl-, ethyl-.
• Esters: alkyl alkanoate, for example methyl propanoate, CH3CH2COOCH3.
• Naming is required up to six carbon atoms; an ester may have six carbons on each side, and esters and nitriles are straight-chain only.

The vocabulary of organic reactions

A saturated compound contains only single carbon-carbon bonds, so every carbon is joined to the maximum number of atoms it can take; the alkanes are saturated. An unsaturated compound contains at least one carbon-carbon double or triple bond and can add atoms across it; the alkenes are unsaturated. This is why bromine water is decolourised by an alkene and left alone by an alkane in the dark.

A covalent bond can break in two ways. In homolytic fission the bond breaks evenly, so each atom keeps one electron of the shared pair and two free radicals are formed, each with an unpaired electron. Homolytic fission needs a lot of energy and is favoured by ultraviolet light or a high temperature in the gas phase, which is why a chlorine molecule splits this way in the chlorination of methane. In heterolytic fission the bond breaks unevenly, so one atom keeps both electrons of the shared pair and a positive ion and a negative ion are formed. Heterolytic fission is favoured when the bond is polar and the solvent is polar, and it is how the C-Br bond of a halogenoalkane breaks in aqueous sodium hydroxide. A radical reaction then runs in three named stages: initiation produces the radicals by homolytic fission, propagation is a step in which a radical reacts with a molecule to give a new molecule and a new radical, so one initiation step can start a chain of many thousands of steps, and termination removes radicals from the mixture by joining two of them into a stable molecule.

A nucleophile is a species with a lone pair or a negative charge that it can donate to form a new covalent bond - the hydroxide ion, the cyanide ion, ammonia and water are all nucleophiles - and an attack of this kind is described as nucleophilic. An electrophile is a species that is electron-deficient and accepts a lone pair: the hydrogen atom of H-Br, the bromine molecule Br2 and the nitronium ion NO2+ are electrophiles, and their reactions are described as electrophilic.

In organic chemistry oxidation usually means gaining oxygen or losing hydrogen, and reduction means losing oxygen or gaining hydrogen; the oxidation numbers of the carbons rise and fall in exactly the same way as anywhere else. To keep the equations short, [O] stands for one atom of oxygen taken from an oxidising agent and [H] for one atom of hydrogen taken from a reducing agent. So CH3CH2OH + [O] -> CH3CHO + H2O oxidises ethanol to ethanal with acidified potassium dichromate(VI), CH3CHO + [O] -> CH3COOH oxidises ethanal on to ethanoic acid, and CH3CHO + 2[H] -> CH3CH2OH reduces ethanal back to ethanol with sodium tetrahydridoborate, NaBH4, in water.

• Saturated: single bonds only. Unsaturated: contains a C=C or C#C that can take part in addition.
• Homolytic fission gives two radicals; heterolytic fission gives a positive ion and a negative ion.
• Initiation makes radicals, propagation keeps the chain going, termination removes radicals.
• Nucleophiles donate a lone pair (OH-, CN-, NH3, H2O); electrophiles accept a lone pair (H+ in H-Br, Br2, NO2+).
• Addition, substitution, elimination, hydrolysis and condensation describe what happens to the carbon skeleton.
• [O] is one oxygen atom from an oxidising agent and [H] is one hydrogen atom from a reducing agent.

Reaction mechanisms and curly arrows

A mechanism is a step-by-step account of which bond breaks, which bond forms and in what order. Curly arrows do the bookkeeping: a curly arrow shows the movement of a pair of electrons, and it must start at the source of those electrons - in the middle of a bond, or on a lone pair. Never start a curly arrow on an atom, on a hydrogen atom or on a plus sign, and never stretch one arrow over more than one bond. The head of the arrow shows where the electrons end up, either forming a new bond or landing on an atom to give it a negative charge.

Free-radical substitution begins when ultraviolet light splits a chlorine molecule. In the initiation step the Cl-Cl bond breaks homolytically and two chlorine radicals are formed. In the first propagation step a chlorine radical collides with methane: the unpaired electron on the chlorine forms a bond with a hydrogen atom, which leaves a methyl radical and HCl. In the second propagation step the methyl radical takes a chlorine atom from a chlorine molecule, giving chloromethane and a fresh chlorine radical, so the chain carries on. Termination is any step where two radicals meet: two chlorine radicals give Cl2, a methyl radical and a chlorine radical give chloromethane, and two methyl radicals give ethane, which is why traces of ethane appear in the product.

Electrophilic addition is what a C=C does to an electrophile. The pi bond is a region of high electron density sitting above and below the plane of the molecule, so it is an easy target for an electron-deficient species. The curly arrow starts at the pi bond of the C=C and points to the hydrogen atom of H-Br, and a second curly arrow starts in the middle of the H-Br bond and points to the bromine. The H-Br bond breaks heterolytically, the bromine takes both electrons and leaves as a bromide ion, and the carbon that lost the pi bond is left as a carbocation, a positive ion. The bromide ion has lone pairs, so it is a nucleophile and it attacks the positive carbon to form the second bond; ethene and HBr give bromoethane.

Nucleophilic substitution starts from a polar bond. The carbon of a C-X bond is delta-positive, because the halogen is more electronegative than carbon, so a nucleophile with a lone pair such as OH- or CN- attacks that carbon. The curly arrow starts on the lone pair of the nucleophile and points to the delta-positive carbon, and a second curly arrow starts in the middle of the C-X bond and points to the halogen, so the halogen takes both electrons and leaves as a halide ion. Warm aqueous potassium hydroxide turns 1-bromopropane into propan-1-ol this way, and potassium cyanide in ethanol turns it into butanenitrile. Nucleophilic addition is the same idea aimed at a C=O. The carbon of a carbonyl group is delta-positive and the oxygen is delta-negative, so a nucleophile such as CN- adds across the C=O bond: the arrow from the cyanide lone pair points to the carbonyl carbon, and a second arrow from the pi bond of the C=O points to the oxygen, which takes the negative charge. A hydrogen ion from the acid then joins that oxygen, giving the hydroxynitrile: ethanal and hydrogen cyanide from a KCN and dilute acid mixture give 2-hydroxypropanenitrile, CH3CH(OH)CN, which contains a chiral centre.

Shapes of organic molecules: hybridisation and bond angles

Organic molecules are also described by the shape of the carbon skeleton. A straight-chained molecule has its carbon atoms joined in one continuous line with no branches, like butane, CH3CH2CH2CH3. A branched molecule has a side chain coming off the main chain, like 2-methylpropane, (CH3)3CH. A cyclic molecule has its carbon atoms joined in a ring, like cyclohexane, C6H12, and cyclohexene, C6H10.

The shape around a carbon atom is fixed by how many atoms it is joined to and by the hybridisation of its orbitals. A carbon that forms four single bonds uses four sp3 hybrid orbitals, which point to the corners of a tetrahedron: the bond angle is 109.5 degrees and the arrangement is tetrahedral. Every carbon of ethane, of propan-1-ol and of a cyclohexane ring is sp3. A carbon that forms a double bond uses three sp2 hybrid orbitals and one p orbital; the three hybrid orbitals point to the corners of a triangle at 120 degrees, so the arrangement is trigonal planar, and the carbon atoms of a C=C, the carbon of a C=O and the carbon of a carbocation are all sp2. A carbon that forms a triple bond, or the carbon of a nitrile group C#N, uses two sp hybrid orbitals at 180 degrees, so the arrangement is linear; ethyne, HC#CH, is linear from end to end.

The word planar means that a set of atoms all lie in the same flat plane. Ethene is planar: the two carbon atoms and the four atoms joined to them, two hydrogens on each carbon, all lie in one plane, and the bond angle at each carbon is 120 degrees. This matters because the pi bond is built from p orbitals that stick out perpendicular to that plane, one lobe above and one below, and it is that sideways overlap which holds the molecule flat. A carbonyl group is planar in the same way: the carbon, the oxygen and the two atoms attached to the carbon lie in one plane at about 120 degrees.

Sigma and pi bonds, and why a C=C cannot rotate

A sigma bond is formed by the head-on overlap of two orbitals along the line joining the two nuclei. The electron density is concentrated on the axis between the nuclei, which makes the sigma bond strong, and the two atoms joined by it can rotate about the bond with very little resistance. A pi bond is formed by the sideways overlap of two p orbitals lying parallel to each other, which puts the electron density into two lobes, one above and one below the plane of the sigma framework. Because sideways overlap is less effective than head-on overlap a pi bond is weaker than a sigma bond, and its electrons are further from the nuclei and more exposed, which is exactly why the pi bond of an alkene is attacked by electrophiles.

Count the bonds from the hybridisation. An sp3 atom forms four sigma bonds and no pi bond. An sp2 atom forms three sigma bonds and one pi bond, so the two carbons of a C=C are joined by one sigma bond plus one pi bond, and the carbon and oxygen of a C=O are joined in the same way. An sp atom forms two sigma bonds and two pi bonds, so the C#C of ethyne is one sigma bond plus two pi bonds and the C#N of a nitrile is built the same way.

The pi bond of a C=C has a consequence you will use again and again: rotation about a carbon-carbon double bond is restricted. To twist one end of the C=C relative to the other you would have to swing the two parallel p orbitals out of line and destroy their sideways overlap, and that costs far more energy than a rotation about a single bond. The four groups attached to the two carbons are therefore locked in position, two on one side of the double bond and two on the other, and that is the origin of cis/trans isomerism in alkenes. A C-C single bond has no pi bond to break, so the ends of an alkane chain spin freely.

• A sigma bond is a head-on overlap along the line of the nuclei; it is strong and it allows rotation.
• A pi bond is a sideways overlap of parallel p orbitals, with the electron density above and below the plane; it is weaker and more exposed.
• sp3: four sigma bonds. sp2: three sigma bonds and one pi bond. sp: two sigma bonds and two pi bonds.
• A C=C is one sigma bond plus one pi bond, and the pi bond locks the two ends in place.

Structural isomerism: chain, positional and functional group

Structural isomers have the same molecular formula but different structural formulas. The atoms are joined up in a different order, so different bonds are present, and the isomers are genuinely different compounds with different physical properties and often different chemical behaviour. Structural isomerism divides into three kinds.

Chain isomers have different carbon skeletons, with the same functional group: the chain is straight in one and branched in the other. Butane, CH3CH2CH2CH3, and 2-methylpropane, (CH3)3CH, are both C4H10. Their chemistry is almost identical, because they have the same functional group, but the branched isomer has less contact area between its molecules and therefore weaker instantaneous dipole-induced dipole forces, so it boils at a lower temperature.

Positional isomers have the same functional group in a different position on the same skeleton. Propan-1-ol and propan-2-ol are both C3H8O, and but-1-ene and but-2-ene are both C4H8. Moving the group changes how easily the molecule reacts, because it changes whether the carbon carrying the group is primary, secondary or tertiary: propan-1-ol is oxidised to propanal and then on to propanoic acid, while propan-2-ol can only be oxidised to propanone.

Functional group isomers contain different functional groups. Propanal, CH3CH2CHO, and propanone, CH3COCH3, are both C3H6O; propanoic acid, CH3CH2COOH, and methyl ethanoate, CH3COOCH3, are both C3H6O2. These isomers have completely different chemistry, because chemistry is the chemistry of the functional group. To deduce every isomer of a molecular formula, work in order: write the longest straight chain and move the functional group along it one carbon at a time, then shorten the main chain by a carbon and add a branch, and carry on until the chain is too short to hold the branch. C4H10 gives two isomers, butane and 2-methylpropane, and C4H9Br gives four: 1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane.

• Chain isomers: different carbon skeleton, for example butane and 2-methylpropane, both C4H10.
• Positional isomers: the same group in a different place, for example propan-1-ol and propan-2-ol, both C3H8O.
• Functional group isomers: different functional groups, for example propanal and propanone, both C3H6O.
• Deduce isomers by moving the functional group along the longest chain first, then branching the chain.

Stereoisomerism: geometrical (cis/trans) and optical

Stereoisomers have the same structural formula - the atoms are joined up in the same order - but a different arrangement of those atoms in space. Stereoisomerism divides into geometrical (cis/trans) isomerism and optical isomerism.

Geometrical isomerism in an alkene comes from the restricted rotation about the C=C. It needs one condition: each carbon of the double bond must carry two different atoms or groups. In but-2-ene, CH3CH=CHCH3, the left carbon carries CH3 and H and the right carbon carries CH3 and H, so two arrangements exist: cis, with the two methyl groups on the same side of the double bond, and trans, with them on opposite sides. In propene, CH2=CHCH3, one carbon carries two hydrogen atoms, so the two arrangements are identical and there is no cis/trans isomerism. Use the same test on a ring: a cyclopentane ring cannot rotate, so 1,2-dichlorocyclopentane exists as a cis and a trans isomer, with the two chlorine atoms on the same face of the ring or on opposite faces. E/Z nomenclature is acceptable but is not required.

A chiral centre is a carbon atom joined to four different atoms or groups. A molecule with one chiral centre is not the same as its mirror image: the two forms are optical isomers, or enantiomers, and they cannot be superimposed on each other, in the same way that your left hand cannot be laid exactly on top of your right hand. Butan-2-ol, CH3CH(OH)CH2CH3, has a chiral centre at C2, where the four groups are H, OH, CH3 and CH2CH3; 2-bromobutane and 2-hydroxypropanoic acid (lactic acid) are chiral for the same reason. A carbon in a CH3 or a CH2 group cannot be a chiral centre, because it carries at least two identical hydrogen atoms, and a carbon of a C=C cannot be one either, because it has only three groups attached to it plus its double bond. A molecule can contain more than one chiral centre; you only need to be able to spot them.

Enantiomers have identical physical properties apart from one: they rotate the plane of plane-polarised light in opposite directions, one clockwise and one anticlockwise, and that is how a polarimeter tells them apart. They also behave identically with ordinary achiral reagents but differently in a chiral environment, which is why one enantiomer of a drug can be active while the other is not. An equal mixture of the two enantiomers is optically inactive.

• Geometrical isomers need a C=C with two different groups on each carbon.
• cis: the two named groups are on the same side of the C=C. trans: on opposite sides.
• Cyclic compounds show cis/trans isomerism because the ring prevents rotation.
• A chiral centre is a carbon joined to four different groups, and it gives two enantiomers.
• Enantiomers rotate the plane of plane-polarised light in opposite directions.