Isomerism

One formula, many compounds

Ethanol, the alcohol in a spirit, and methoxymethane, a gas used as a solvent, both have the formula $\text{C}_{2}\text{H}_{6}\text{O}$. The atoms are the same, but the way they are joined is different. Isomers are compounds with the same molecular formula and different structures. Structural isomers differ in the order in which the atoms are joined. Stereoisomers are joined in the same order but sit differently in space.

Structural isomerism

Chain isomerism. The carbon skeleton is different. $\text{C}_{5}\text{H}_{12}$ has three isomers.

Draw the carbon skeleton first, then fill in hydrogen atoms. A bent or backwards drawing is the same molecule. Check it again by the longest chain and the branches.

• Position isomerism: the same skeleton, but a group or a double bond sits elsewhere. Examples: 1-chloropropane and 2-chloropropane; but-1-ene and but-2-ene.
• Functional group isomerism: different groups. Examples: ethanol and methoxymethane; propanal and propanone, $\text{CH}_{3}\text{COCH}_{3}$.
• Metamerism: members of one family with different alkyl groups on each side of the group. Ethoxyethane, $\text{CH}_{3}\text{CH}_{2}\text{OCH}_{2}\text{CH}_{3}$, and methoxypropane, $\text{CH}_{3}\text{OCH}_{2}\text{CH}_{2}\text{CH}_{3}$, are ethers with the formula $\text{C}_{4}\text{H}_{10}\text{O}$.
• Tautomerism: two functional group isomers that change into each other quickly by the move of a hydrogen atom. Propanone is in balance with a trace of its enol form, $\text{CH}_{2}\text{=C(OH)CH}_{3}$.

Counting isomers

The number of alkane isomers climbs quickly. Try the slider.

The program counts the skeletons. It builds every possible tree of carbon atoms, throws away any that give a carbon more than four bonds, and merges those that are the same molecule drawn differently.

Degrees of unsaturation. A saturated open-chain hydrocarbon is $\text{C}_n\text{H}_{2n+2}$. Each ring and each pi bond takes away two hydrogen atoms.

For $\text{C}_{4}\text{H}_{8}$ the value is 1, so it is an alkene or a cycloalkane. Oxygen does not change the count.

Geometrical isomers

Atoms cannot turn about a double bond. If each carbon of the double bond holds two different groups, there are two arrangements. In cis, the two similar groups are on the same side. In trans, they are on opposite sides.

But-2-ene has cis and trans forms. But-1-ene does not, because one doubly bonded carbon has two hydrogen atoms. 1,2-Dichloroethene does, but 1,1-dichloroethene does not. The two forms differ in physical properties. In 1,2-dichloroethene the C-Cl dipoles cancel in the trans form and add in the cis form. Rings also give cis and trans forms.

Optical isomers

A carbon with four different groups is a chiral carbon, or stereocentre. Butan-2-ol has one, carbon 2, which holds $\text{CH}_{3}$, $\text{CH}_{2}\text{CH}_{3}$, OH and H. The molecule cannot be placed on top of its mirror image, just as your left hand cannot be placed on your right. The two mirror images are enantiomers.

• They have the same melting point, boiling point and density.
• They turn plane-polarised light by equal angles in opposite directions, (+) and (-).
• An equal mix is a racemic mixture, and it does not turn the light.

A molecule with a plane of symmetry is not chiral. Propan-2-ol has two identical $\text{CH}_{3}$ groups on its middle carbon, so it is not. Lactic acid, $\text{CH}_{3}\text{CH(OH)COOH}$, is chiral. Each different stereocentre doubles the number of stereoisomers, up to $2^n$ for n centres.

Practise with new numbers

Common mistakes

• Counting a bent drawing as a new isomer. Rotate or flip it. If the longest chain and the branches match, it is the same molecule.
• Mixing up chain and position isomerism. Chain isomers have a different skeleton. Position isomers have the same skeleton.
• Saying every alkene has a cis and a trans form. Each doubly bonded carbon needs two different groups.
• Calling a carbon chiral when two of its groups are the same. It needs four different groups.

Remember these

• Isomers share a molecular formula and differ in structure.
• Structural: chain, position, functional group, metamerism and tautomerism.
• Geometrical needs two different groups on each end of a double bond. Optical needs a chiral carbon.
• $\text{C}_4\text{H}_{10}$ has 2 isomers, $\text{C}_5\text{H}_{12}$ has 3, and $\text{C}_6\text{H}_{14}$ has 5.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What kind of isomerism is shown by $\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{2}\text{OH}$ and $\text{CH}_{3}\text{CH}_{2}\text{CH(OH)CH}_{3}$?

2. Does pent-2-ene have cis and trans forms? Does pent-1-ene?

3. Which carbon, if any, is chiral in $\text{CH}_{3}\text{CH}_{2}\text{CH(CH}_{3}\text{)CH}_{2}\text{OH}$?

4. A molecule has the formula $\text{C}_{5}\text{H}_{10}$. How many rings and double bonds does it have together?