Why is the structolure of isooctane not like 2-merhylheptane?
Answers
Because the trivial name for isooctane does not relate to the traditional meaning of iso. If it would, the compound would indeed be 2-methylheptane as suggested by the OP.
Besides the traditional meaning the prefix iso is also used as short for ‘isomer’. (see e.g. isophthalic acid as one of the isomers of phthalic acid.)
Isooctane is by far the most important octane isomer, so historically the name isooctane was coined to
2,2,4-trimethylpentane
Isooctane is one of the petroleum compounds that is used in the ‘anti knocking’ or octane rating scale. 100% heptane is 0, 100% isooctane is 100.
Isooctane is one of the distillation compounds of petroleum, but an additional explanations of the name might relate to the fact that it can be can be made out of isobutene dimerisation and subsequent reduction of the resulting octene mixture.