Question

In Mass spec, Why are there smaller fragments such as 71, 43, 29, 15 for 1-pentanol, but no such fragments for 3-heptanol such as at 57 or 29?

Answer 1

In 1-pentanol the hydroxyl group is at the end of the five-carbon chain. There are two significant odd-electron fragment ions, one at m/z=71 (loss of water), and the other at m/z=43 (loss of water and ethene). The fragment ion at m/z=29 is probably due to a methyl radical loss from the m/z=70 ion. The m/z=15 ion may be a protonated formaldehyde ion, formed by alpha-fragmentation.

where as 3-heptanol shows two significant fragment ions. Alpha-fragmentation (loss of an butyl radical is more stable than m/z=87 which is due to loss of ethyl radicals because of greater inducative effect) forms the m/z=57 base peak.

When bond breaking is occured 2 stable species ion and neutral fragments are formed i.e at m/z=57 base peak. and m/z=29.