Nomenclature of Unsaturated Hydrocarbons

Unsaturated Hydrocarbons

If a hydrocarbon contains at least one double or triple bond, the hydrocarbon is called an unsaturated hydrocarbon.

Unsaturated hydrocarbons, like saturated hydrocarbons may be cyclic (closed chained) or acyclic (open chained).

Alkenes and alkynes

Open chain unsaturated hydrocarbons are either alkenes or alkynes depending upon whether the hydrocarbon contains double bonds or triple bonds.

Alkenes

Open chain unsaturated hydrocarbons, containing carbon-carbon double bonds are known as alkenes. The primary suffix of alkenes is ene.

Examples of alkenes : Ethene (CH₂=CH₂), Propene (CH₃CH=CH₂) etc.

Alkynes

Open chain unsaturated hydrocarbons, containing carbon-carbon triple bonds are known as alkynes. The primary suffix of alkynes is yne.

Examples of alkynes : Ethyne (CH≡CH), Propyne (CH₃C≡CH) etc.

Rules for IUPAC nomenclature of Unsaturated Hydrocarbons

The following additional rules are followed while naming unsaturated hydrocarbons.

1) According to 2013 IUPAC recommendation (the latest one), the longest carbon chain is selected as the parent chain. Earlier, the chain containing the multiple bond used to be selected as the parent chain regardless of the length of the chain.

Preference is not given to the multiple bond in IUPAC nomenclature anymore, example is 2-Ethylpent-1-ene is now 3-Methylidenehexane

2) If both double and triple bonds are present, the numbering of the parent chain should always be done from that end which is nearer to the double or triple bond.

Double bond is preferred over triple bond in hept-2-en-4yne

3) However, if there is a choice in numbering, the double bond is always given preference over the triple bond.

Double bond is preferred over triple bond in iupac nomenclature, example is pent-2-en-4yne

4) Locants (position of multiple bonds) are always placed before the suffix.

But-2-ene

5) If both double and triple bonds are present, their positional numbers are written before their respective suffixes; furthermore, the letter 'e' from the suffix 'ene' is dropped while writing the complete name of the organic compound.

Pent-3-en-1-yne

Compare :

Pent-3-ene
Pent-3-en-1-yne (not Pent-3-ene-1-yne)

Note : Organic compounds are named as derivatives of alkyne if both double and triple bonds are present.

6) If the parent chain contains more than one double or triple bonds, numerical prefixes such as di (for two), tri (for three), tetra (for four) etc. are added to the primary suffix.

TYPE OF CARBON CHAIN	PRIMARY SUFFIX	GENERAL NAME
Unsaturated with two double bond	-diene	Alkadiene
Unsaturated with two triple bonds	-diyne	Alkadyne

Buta-1,3-diene

Note : Extra 'a' is added to the word root if primary suffix contains di, tri etc.

Compare :

But-2-ene
Buta-1,3-diene (not But-1,3-diene)

7) If the parent chain can be selected in more than one ways, then the chain with multiple bond is given higher priority than the chain with alkyl groups.