Structural isomerism and E/Z isomerism

Same formula, different molecule

Ethanol and methoxymethane are both C₂H₆O. Ethanol, CH₃CH₂OH, is the liquid in wine and boils at 78 °C. Methoxymethane, CH₃OCH₃, is a gas at room temperature, boiling at −24 °C, and is used as an aerosol propellant. The difference is where the oxygen sits. In ethanol it is at the end of the chain with a hydrogen on it, so the molecules hydrogen-bond to each other. In methoxymethane it sits between the two carbons and has no hydrogen to offer.

Compounds like these are isomers: the same molecular formula, a different arrangement of atoms. There are two families. In structural isomers the atoms are joined in a different order, so the structural formulae differ. In stereoisomers the atoms are joined in the same order and the structural formula is identical; only the arrangement in space differs.

E/Z isomerism is one kind of stereoisomerism. Optical isomerism, the other, depends on a carbon with four different groups, whose two mirror-image arrangements cannot be superimposed. Both kinds keep the structural formula and change only the arrangement in space, so a definition of stereoisomers has to say both things: same structural formula, different arrangement of atoms in space.

Isomers

Compounds with the same molecular formula but a different arrangement of atoms.

Structural isomers

Compounds with the same molecular formula but a different structural formula.

Stereoisomers

Compounds with the same structural formula but a different arrangement of the atoms in space.

Chain, position and functional group isomers

Chain isomers have the same functional group and a different carbon skeleton. C₄H₁₀ has two: butane, CH₃CH₂CH₂CH₃, an unbranched chain, and 2-methylpropane, CH₃CH(CH₃)CH₃, a chain of three with a methyl branch. Both are alkanes and react in the same way. The branched molecule is more compact, so neighbouring molecules touch over a smaller surface, the van der Waals forces between them are weaker, and it boils at −12 °C against −1 °C for butane.

Position isomers have the same carbon skeleton and the same functional group, with the group on a different carbon. On an unbranched chain, C₄H₉Br gives 1-bromobutane, CH₃CH₂CH₂CH₂Br, and 2-bromobutane, CH₃CH₂CHBrCH₃. There is no 3-bromobutane or 4-bromobutane: numbered from the correct end, they are the same two molecules.

Functional group isomers have the same molecular formula and different functional groups, so they belong to different homologous series and react differently. C₃H₆O includes propanal, CH₃CH₂CHO, an aldehyde, and propanone, CH₃COCH₃, a ketone. Warmed with acidified potassium dichromate(VI), propanal is oxidised and the solution turns from orange to green; propanone leaves it orange. Ethanol and methoxymethane are functional group isomers of the same sort, an alcohol and an ether.

Some general formulae always cover two series. CₙH₂ₙ fits the alkenes and the cycloalkanes, so propene and cyclopropane are both C₃H₆. CₙH₂ₙO fits the aldehydes and the ketones. CₙH₂ₙO₂ fits the carboxylic acids and the esters: butanoic acid, which smells of rancid butter, and ethyl ethanoate, which smells of nail-varnish remover, are both C₄H₈O₂.

Describe the drawing

Five skeletal formulae of C₄H₈O. Butanal sits in the centre of the top row, linked on the left to 2-methylpropanal as chain isomers and on the right to butanone as functional group isomers. Below, but-3-en-1-ol and but-3-en-2-ol are linked to each other as position isomers, the OH moving from carbon one to carbon two, and as a pair to butanal as functional group isomers.

Chain isomers

Structural isomers with the same functional group but a different arrangement of the carbon skeleton.

Position isomers

Structural isomers with the same carbon skeleton and functional group, the functional group being attached to a different carbon atom.

Functional group isomers

Structural isomers with the same molecular formula but different functional groups.

The same method finds the chain isomers of an alkane. Start with the unbranched chain, then shorten it by one carbon and put that carbon as a methyl branch on each inner carbon in turn, then shorten it by two. For C₆H₁₄ this gives hexane; 2-methylpentane and 3-methylpentane; and 2,2-dimethylbutane and 2,3-dimethylbutane: five isomers. A methyl group on carbon 1, or an ethyl group on carbon 2, only lengthens the chain again and gives a name already on the list.

Bending a chain on paper changes nothing. The two ends of a C–C single bond rotate freely at room temperature, so CH₃CH₂CH₂CH₂Br drawn in an L shape is still 1-bromobutane. The name settles it: two drawings with the same correct name are the same compound.

Why a double bond does not rotate

A C=C double bond is different. It is made of two bonds: a σ bond, formed by overlap of orbitals directly between the two carbons, and a π bond, formed by the sideways overlap of a p orbital on each carbon, above and below the plane of the molecule. Twisting one end relative to the other would pull the p orbitals out of line and break the π bond. That costs about 265 kJ mol⁻¹, the difference between the mean bond enthalpies of C=C, 612 kJ mol⁻¹, and C–C, 347 kJ mol⁻¹, and collisions at room temperature come nowhere near supplying it.

The phrase that carries the idea is restricted rotation about the planar C=C double bond. Use it in full: 'the double bond is rigid' or 'the molecule cannot move' does not say what is restricted. The two carbons of the double bond and the four atoms joined to them lie in one plane, at about 120° to each other, and a group on one side of the double bond stays on that side.

Describe the drawing

Two three-dimensional drawings of but-2-ene. On the left the two carbons of the double bond and their four groups lie in a grey plane with both CH₃ groups on the same side, a σ bond joins the carbons, and a vertical p orbital on each carbon overlaps sideways with the other to give a π bond drawn as one lobe above the plane and one below. On the right the right-hand carbon has been twisted by ninety degrees, its p orbital now lies at right angles to the other, there is no overlap, and the π bond is marked broken at a cost of about 265 kilojoules per mole. A crossed rotation arrow sits between the panels.

Restricted rotation is necessary for E/Z isomerism but not enough on its own. Each carbon of the C=C must also carry two different groups. But-2-ene, CH₃CH=CHCH₃, qualifies: each carbon carries a CH₃ and an H, so the two methyl groups can be on the same side of the double bond or on opposite sides, and those are two different compounds that cannot turn into each other.

But-1-ene, CH₂=CHCH₂CH₃, does not qualify. Carbon 1 carries two hydrogen atoms, and swapping two identical groups produces the same molecule. Nor does 2-methylpropene, (CH₃)₂C=CH₂, which has two identical groups on both carbons. One carbon with a matching pair is enough to rule out E/Z isomers.

E/Z isomerism

A form of stereoisomerism caused by restricted rotation about a C=C double bond, where each carbon of the double bond carries two different groups.

Assigning E and Z by priority

When the groups on a C=C are all different, 'same side' needs a rule saying which groups to compare. The Cahn–Ingold–Prelog (CIP) priority rules give it. Look at each carbon of the double bond separately. Of the two atoms attached directly to that carbon, the one with the higher atomic number has the higher priority.

If the two higher-priority groups are on the same side of the double bond, the isomer is Z, from the German zusammen, together. If they are on opposite sides it is E, from entgegen, opposite. The letter goes at the front of the name, joined by a hyphen: Z-but-2-ene. IUPAC prints it in brackets, as (Z)-but-2-ene, and both forms mean the same isomer.

When the atoms attached directly are the same element, look one atom further out. List the atoms attached to each, highest atomic number first, and decide at the first point of difference. CH₃ carries (H, H, H) and CH₂CH₃ carries (C, H, H). Carbon beats hydrogen at the first position, so ethyl outranks methyl. CH₂OH carries (O, H, H) and outranks both.

Describe the drawing

Z-2-chloropent-2-ene drawn with its double bond horizontal and a dashed line dividing the two carbons. On the left carbon chlorine, atomic number 17, is ringed above as higher priority over CH₃, atomic number 6, below. On the right carbon C₂H₅, atomic number 6, is ringed above over H, atomic number 1, below. A bracket over the top joins the two ringed groups and reads same side: Z. Beneath, the E isomer is drawn smaller with chlorine up and C₂H₅ down, labelled opposite sides: E.

Cis and trans are the older names, and they describe the same idea for a simple case. When each carbon carries one hydrogen, cis means the two other groups are on the same side, so for but-2-ene cis is Z and trans is E. The two systems part company once a carbon carries no hydrogen. In CH₃CCl=CHCH₃, the isomer with both methyl groups on the same side is cis by its carbon chain. On carbon 2, though, Cl outranks CH₃, so the higher-priority groups are on opposite sides and the same isomer is E. Where a question gives a cis or trans name, work out the priorities before converting it to E or Z.

Cahn–Ingold–Prelog priority rule

On each carbon of the double bond, the group whose first atom has the higher atomic number has the higher priority.

Cis–trans isomerism

A special case of E/Z isomerism in which the two carbons of the C=C carry one identical group each, such as H; cis has those groups on the same side, trans on opposite sides.

Why the shape matters

E and Z isomers have the same bonds, so they take part in the same reactions, but their shapes differ, and so do the properties that depend on shape. Z-1,2-dichloroethene boils at 60 °C and E-1,2-dichloroethene at 48 °C. In the Z isomer both C–Cl dipoles point in the same direction and the molecule is polar, so permanent dipole–dipole forces add to the van der Waals forces. In the E isomer the dipoles point in opposite directions and cancel.

Butenedioic acid, HOOCCH=CHCOOH, shows a larger effect. The E isomer melts at about 287 °C and the Z isomer at 135 °C. The E molecules are symmetrical and pack closely, held by hydrogen bonds between neighbours. In the Z isomer the two COOH groups are on the same side, close enough to hydrogen-bond to each other inside one molecule. Heated, the Z isomer loses water and closes into a five-membered ring; the E isomer cannot, because its COOH groups point away from each other and the double bond will not rotate to bring them together.

The same effect decides whether a fat is solid or liquid. Most unsaturated fatty acids in plant oils have Z double bonds, and each one puts a kink in the hydrocarbon chain. Kinked chains cannot pack closely, the van der Waals forces between them are weaker, and olive oil is a liquid. Partial hydrogenation, once used to harden oils into margarine, turns some of the double bonds into the E form. E chains are nearly straight and pack like saturated chains. These trans fats raise the level of LDL cholesterol in the blood, and their use in food has been cut back sharply for that reason.

Other boards: the π bond, cis–trans names and optical isomers

OCR, Edexcel and Cambridge expect the reason for restricted rotation to be given in terms of the π bond: sideways overlap of p orbitals above and below the plane, which twisting would break. An answer on those papers that says only 'restricted rotation' has named the effect without its cause. AQA's statement stops at restricted rotation about the planar double bond, and the π bond explanation is extra there, not wrong.

Cambridge calls this kind of stereoisomerism geometrical isomerism and names the isomers cis and trans, with the same two conditions: a C=C that cannot rotate, and two different groups on each of its carbons. For the alkenes Cambridge sets, each carbon usually carries one hydrogen, and cis and trans then agree with Z and E. Where they would not agree, as in CH₃CCl=CHCH₃, the CIP rules are the safer way to decide.

Cambridge also teaches optical isomerism in the first year, alongside geometrical isomerism, as the second kind of stereoisomerism: a carbon bonded to four different groups gives two non-superimposable mirror-image forms. The other three boards keep it for the second year.