Question

Question:(a) What mass spectral fragments are formed by α cleavage of butan-2-ol, ${\mathbf{CH}}_{\mathbf{3}}\mathbf{CH}\mathbf{\left(}\mathbf{OH}\mathbf{\right)}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{3}}$?

(b) What fragments are formed by dehydration of butan-2-ol?

Short answer

Answer

Step-by-step solution

Fragmentation in mass spectroscopy

In mass spectroscopy, fragments formed from the molecular ion of a compound are studied to determine its composition and molecular weight.

The compound is bombarded with a stream of electrons to form the molecular ion. This is highly unstable and fragments into a cation and a free radical.

The cation is detected by a mass spectrometer and therefore will produce a line in the mass spectrum.

Fragmentation in alcohols

The molecular ion peak is of low intensity in primary and secondary alcohols and is absent in tertiary alcohols.

Fragmentation of molecular ions of alcohols generally occurs in two ways

(a) By alpha-cleavage- In this mode of fragmentation, bond is broken and larger alkyl groups are easily cleaved.

(b) By dehydration- in this mode of fragmentation, a water molecule is lost from molecular ion.

Fragmentation of butan-2-ol

Butan-2-ol can also be fragmented in 2 ways:

(a) Alpha cleavage in butan-2-ol can be carried out in 2 ways.

Alpha-cleavage of butan-2-ol

Hence, two mass spectral fragments are formed by alpha cleavage of butan-2-ol with m/z=59 and m/z=45.

(b) Dehydration of butan-2-ol involves the removal of hydroxyl group and hydrogen from an alpha position.

Dehydration of butan-2-ol

Hence, two fragments are formed both with m/z=56