isomer question orgo DAT destroyer 2010 #100

[yankeefan86]

This questions is from the 2010 DAT destroyer
#100 which is an isomer of octane?
a) 2,3,4-trimethylpentane
b) 2,2,4-trimethylpentane
c) 3-ethylhexane
d) B and C
E)all of these


The answer is E and I totally understand it. My question to you guys is, is there a formula to determine the number of isomers a structure has? Or do I have to draw each out on the DAT?

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[PrinceJ]

Well the first two answer choices should be trimethylpentane, but all I do is count the total number of carbons on this one. Since octane=8 carbons and the rest hydrogen, trimethylpentanes: 5 + 3=8 carbons. methylheptane: 7 +1= 8 carbons. If they throw in oxygens, nitrogens, or whatever else, the double bonds and stuff can make it much trickier.