Classification, Nomenclature and Isomerism: Quick Revision

A brief note on Classification, Nomenclature and Isomerism. Read it before a test. Try each question at the end before you open its answer.

Shapes of carbon compounds

• Carbon forms four bonds, and joins to itself in chains and rings.
• $sp^3$: tetrahedral, 109.5 degrees. $sp^2$: planar, 120 degrees. $sp$: linear, 180 degrees.
• Every bond has one sigma bond. A double bond adds one pi bond, and a triple bond adds two.
• Bond lengths: C-C 154 pm, C=C 134 pm, C≡C 120 pm.

Writing a structure

• Complete: every atom and bond. Condensed: $\text{CH}_{3}\text{CH(OH)CH}_{2}\text{CH}_{3}$.
• Bond-line: corners and ends are carbons, and the hydrogen atoms on carbon are not drawn.
• Wedge and dash: a wedge comes towards you, a dash goes away.

Classification

• Acyclic (open chain) or cyclic. Cyclic: alicyclic, aromatic or heterocyclic.
• Saturated (single bonds only) or unsaturated.
• A homologous series has one functional group and one general formula, and each member differs by $\text{CH}_{2}$, 14 u.
• Alkanes $\text{C}_n\text{H}_{2n+2}$, alkenes $\text{C}_n\text{H}_{2n}$, alkynes $\text{C}_n\text{H}_{2n-2}$.

Naming

• Word roots: meth, eth, prop, but, pent, hex, hept, oct, non, dec.
• Longest chain, holding the principal group and the multiple bonds.
• Lowest locants at the first point of difference: principal group first, then multiple bonds, then branches.
• Branches in alphabetical order, ignoring di and tri.
• Seniority: acid, ester, amide, nitrile, aldehyde, ketone, alcohol, amine, alkene, alkyne.
• Suffixes: -oic acid, -oate, -amide, -nitrile, -al, -one, -ol, -amine, -ene, -yne.
• Prefixes only: chloro, bromo, nitro, methoxy, alkyl.

Isomerism

• Structural: chain, position, functional group, metamerism, tautomerism.
• Geometrical: each end of a double bond has two different groups, giving cis and trans.
• Optical: a chiral carbon holds four different groups. Enantiomers turn polarised light in opposite directions.
• Degrees of unsaturation: $\dfrac{2\text{C} + 2 + \text{N} - \text{H} - \text{X}}{2}$.
• Up to $2^n$ stereoisomers for n different stereocentres.

A worked example

Name $\text{CH}_{3}\text{CH(CH}_{3}\text{)CH}_{2}\text{CH(OH)CH}_{3}$.

1. Chain. The longest chain has five carbons: pentane.
2. Principal group. The alcohol gives the suffix -ol. Number from the right, so that OH is on carbon 2, not 4.
3. Branch. The methyl group is then on carbon 4.
4. Name. 4-methylpentan-2-ol.

Practice with new numbers

A program to try

The program builds the name for methyl groups on a chain, with the lowest locants.

Mistakes to avoid

• Not taking the longest chain.
• Choosing the lowest sum of locants, not the lowest at the first difference.
• Forgetting that the principal group comes before the multiple bonds and the branches.
• Saying every alkene has cis and trans forms. Each end needs two different groups.
• Calling a carbon chiral when two of its groups are the same.