Question

C. D. Iron(II) carbonate Lead(II) carbonate 4 The diagram shows the mass spectrum of an organic compound. 60 100 Relative abundance 73 27 41 88 0+ 0 20 40 60 80 100 m/z Which compound was analysed?

Answer 1

This diagram shows mass spectrum of n-Butanoic acid :

M : m/z : 88 (molecular formula : C₄H₈O₂ ) : The molecular ion peak from a carboxylic acid is generally observed, although it is often very weak : same is the case.

Best diagnostic peak is base peak at m/z 60 it is the result of the McLafferty rearrangement product, with elimination of neutral molecule alkene (ethene : 28) ; it is easy to diagnose as even mass from even mass molecule is indication of such rearrangement in which neutral molecule is lost : giving radical ion.

Other peaks like at m/z 45 is alpha-cleavage of the carboxyl group to give the ion [COOH]⁺.

m/z 73 is due to loss of methyl radical from parent ion.

m/z 43 due to loss COOH as radical, and it futher lose H` to give m/z 42 and 41; loss of CH<sub>2</sub>` radical from m/z 41 gives rise to m/z -27.