The correct answer is D. N-heptane.
Octane number is a measure of a fuel’s ability to resist knocking in internal combustion engines. A higher octane number indicates a fuel that is less likely to knock.
N-heptane is the least branched of the four compounds listed, and it is therefore the most likely to knock. The other three compounds are all more branched, and they are therefore less likely to knock.
2-methyl heptane has one methyl group attached to the second carbon atom. Iso-octane has two methyl groups attached to the same carbon atom. 2,2-dimethyl hexane has two methyl groups attached to adjacent carbon atoms.
The more branched a hydrocarbon is, the more difficult it is for the molecules to line up in a way that allows them to react with each other. This makes it less likely that
the fuel will knock.Therefore, the compound that has the least value for octane number is N-heptane.