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Author: Hans Lohninger

Octane, C8H18, is an alkane with 18 structural isomers. The most important isomer is 2,2,4-trimethylpentane (isooctane) which is used - together with n-heptane - as a reference value in the octane rating scale. Isooctane is a component of gasoline and reduces engine knocking.(1)

Following is a list of the octane isomers and selected physical properties:

Isomer Melting
n-octane -571260.703
2-methylheptane -1091180.698
3-methylheptane -1211190.702
4-methylheptane -1211180.705
3-ethylhexane 1190.714
2,2-dimethylhexane -1211070.695
2,3-dimethylhexane -1101160.691
2,4-dimethylhexane 1100.696
2,5-dimethylhexane -911090.691
3,3-dimethylhexane -1261120.710
3,4-dimethylhexane 1180.715
3-ethyl-2-methylpentane -1151160.719
3-ethyl-3-methylpentane -911180.727
2,2,3-trimethylpentane -1121100.716
2,2,4-trimethylpentane (isooctane) -107 990.688
2,3,3-trimethylpentane -1011150.726
2,3,4-trimethylpentane -1101130.719
2,2,3,3-tetramethylbutane -1011070.824

2,2,4-Trimethylpentane is synthesized industrially by dimerization of 2-methylpropene, in the presence of 60% sulfuric acid at 70 C. The resulting iso-octenes are hydrogenated to form 2,2,4-trimethylpentane.

(1) Considering its price the best anti-knock agent would be tetraethyl lead, and in fact, was heavily used until the 1990ies. However, since lead is poissonous and does harm to the environment, the gasoline manufacturers were forced to switch to 2,2,4-trimethylpentane (at least in the US and in Europe).

Last Update: 2011-02-21