Page 46 - FPGEE Medicinal and Organic Chemistry Q&A Book

Krisman

500. (c) Gasoline is a complex mixture of hy-
drocarbons. The branched-chain alkanes give a
relatively smooth performance when burnt in
the internal combustion engine of motor cars,
hence they are excellent fuels for cars. In con-
trast, normal alkanes detonate and cause knock-
ing of the engine, and are bad fuels. The knock
property of various gasolines is stated in terms
of octane number or octane rating. Isooctane,
which has no tendency to knock, is assigned an
octane number of 100 and heptane, which
causes great knocking, is assigned the octane
number 0.
The octane number of a particular gaso-
line is the percentage of isooctane in a mixture
of isooctane and heptane, which gives
equivalent knock performances. For example,
the knock performance of an unknown sample
of a gasoline is found to be the same as that of a
mixture of 65% isooctane and 35% of heptane.
The unknown compound should have an octane
number of 65.

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