[Kalibasa]

This is probably something very simple I'm forgetting, but I'm confused. We are given mass spectra for methylcyclohexane and ethylcyclopentane and told to distinguish them, and I don't have any general problems with reading mass spectra. I just don't understand the fragmentation pattern- according to the spectra (and the answer) the methylcyclohexane loses its methyl group to yield the main product while the ethylcyclopentane loses the ethyl group.

It seems logical, but I can't actually explain it- I don't see why the ring should necessarily be left intact in both cases, as there are no double bonds, there is no resonance, and the hybridization of all the atoms is the same. If anything, I thought that the ethylcyclopentane would lose just one carbon (becoming a methylcyclopentane cation) and then undergo a carbocation rearrangement to get the positive charge on the tertiary carbon, which is stabler.

Why isn't this the case?

[orgopete]

This is a good question. It also reminds me of why I was always reluctant to require students to understand fragmentation reactions as students generally weren't as familiar with other reactions. If you follow complete fragmentation routes, many unexpected and even hard to predict reactions may take place. Consequently, MS, one of the oldest instrumental methods, did not gain in popularity until "softer" ionization methods became common.

Back to the question. Your question is correct, but the assumptions may not be (because the result is different). Remember, you are bombarding the compound with a stream of electrons to knock an electron off (or electron pair?). Which electrons should be most susceptible to removal?  It isn't a magical process by which a methyl group is lost from methyl cyclohexane. Perhaps the electron that is lost is from the tertiary carbon and a rebalance of electrons results in the loss of the methyl radical and formation of the cyclohexyl carbocation (the base peak).

If a similar process occurred in ethylcyclpentane, then loss of an ethyl radical would seem more likely than loss of a methyl radical to form a primary carbocation and rearrangement.

Note, that I am using the MS as data. Rather than presuming how the reaction must occur, I am trying to think what the data means.