Octane

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 Point [°C]	Boiling Point [°C]	Density [g/cm3]
n-octane	-57	126	0.703
2-methylheptane	-109	118	0.698
3-methylheptane	-121	119	0.702
4-methylheptane	-121	118	0.705
3-ethylhexane		119	0.714
2,2-dimethylhexane	-121	107	0.695
2,3-dimethylhexane	-110	116	0.691
2,4-dimethylhexane		110	0.696
2,5-dimethylhexane	-91	109	0.691
3,3-dimethylhexane	-126	112	0.710
3,4-dimethylhexane		118	0.715
3-ethyl-2-methylpentane	-115	116	0.719
3-ethyl-3-methylpentane	-91	118	0.727
2,2,3-trimethylpentane	-112	110	0.716
2,2,4-trimethylpentane (isooctane)	-107	99	0.688
2,3,3-trimethylpentane	-101	115	0.726
2,3,4-trimethylpentane	-110	113	0.719
2,2,3,3-tetramethylbutane	-101	107	0.824

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).