Q26E - Organic Chemistry (Mcmurry)

Question

What fragments might you expect in the mass spectra of the following compounds?

(a)

(b)

(c)

Step-by-Step Solution

Answer

(a) This ketone shows mass spectrum fragment that are due to alpha cleavage and to the McLafferty rearrangement. The molecular ions occur at = 148, and the major fragments have m/z = 120, 105 and 71.

(b) The fragment in the mass spectrum of this alcohol result from dehydration and alpha cleavage. Major fragments have m/z values of 128, 110 and 99 and the molecular ions occur at = 128

(c) Amines fragmented by alpha cleavage. In this problem cleavage occur in the ring, producing the same value of fragment m/z as molecular ion that is 99.

Step 1: McLafferty Rearrangement.

The McLafferty rearrangement is a reaction observed in mass spectrometry during the fragmentation and dissociation of organic molecules.For example

Fragmentation of hexan-2-one, producing acetone enol radical cation and a neutral molecule of propene.

Step 2: Alpha cleavage

Alpha cleavage is defined as the cleavage of the bond in molecular ion that is alpha with respect to the charge radical site. This cleavage is characteristic mode of fragmentation of alkyl halides and ethers and also the most common fragmentation of amines is alpha cleavage to give a resonance stabilized cation-an iminium ion.

          Alpha-cleavage                                    iminium ion

 

Step 3: Fragments of the given compound in mass spectra

(a) This ketone shows mass spectrum fragment that are due to alpha cleavage and to the McLafferty rearrangement. The molecular ions occur at = 148, and the major fragments have m/z = 120, 105 and 71.

(b) The fragment in the mass spectrum of this alcohol result from dehydration and alpha cleavage. Major fragments have m/z values of 128, 110 and 99 and the molecular ions occur at = 128

 

(c) Amines fragmented by alpha cleavage. In this problem cleavage occur in the ring, producing the same value of fragment m/z as molecular ion that is 99.