Mass Spectrum Example 3

Example 3: pentan-2-one and pentan-3-onePentan-2-one, CH₃COCH₂CH₂CH₃, and pentan-3-one, CH₃CH₂COCH₂CH₃, are position isomers with the formula C₅H₁₀O. Both have M_r = 86 and a molecular ion peak at m/z 86. Ketones usually break at the C-C bonds on either side of the C=O group, so the fragments show where the C=O group is.Pentan-3-oneThe molecule is symmetrical, with an ethyl group on each side of the C=O. Losing an ethyl radical (86 – 29 = 57) gives C₂H₅CO⁺, the base peak at m/z 57.C₂H₅COC₂H₅⁺ → C₂H₅CO⁺ + C₂H₅•There is also a large peak at m/z 29 from C₂H₅⁺. There is no large peak at 43, because there is no CH₃CO group.Pentan-2-oneLosing the propyl radical (86 – 43 = 43) gives CH₃CO⁺, the base peak at m/z 43.CH₃COC₃H₇⁺ → CH₃CO⁺ + C₃H₇•Breaking the bond on the other side of the C=O, with loss of a methyl radical, gives C₃H₇CO⁺ at m/z 71 (86 – 15).Conclusion: a C₅H₁₀O ketone with peaks at 86, 57 and 29 but no large peak at 43 is pentan-3-one. One with a base peak at 43 and a peak at 71 is pentan-2-one.