9.3 Mass Spectroscopy
- Analysis of Aldehydes: Aldehydes usually show weak molecular ion peak for aliphatic aldehydes but the peak is quite strong for aromatic aldehydes. Cleavage of one of the bonds to the carbonyl group, sometimes called α-cleavage occurs very often.
Thus, the peak corresponding to m/z =71 in butyraldehyde is due to loss of H . .
Similarly, they may undergo fragmentation of R group in what is called β-cleavage.This gives the peak at m/z = 29 for butyraldedyde.
The third major fragmentation pathway for aldehydes is McLafferty rearrangement. The fragment has m/z =44.
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Analysis of Ketones: In case of ketones, there is a strong molecular ion peak. For aliphatic ketones, fragment resulting from loss of a methyl, ethyl and so on groups occur in the mass spectrum. The McLafferty rearrangement also occurs to give its corresponding peak. Cyclic ketones may undergo loss of carbonyl group. Thus in cyclohexanone the following fragmentation occurs.
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Aromatic ketones undergo α-cleavage to lose the alkyl group and form ArCO+. This ion loses CO to form Ar+ ion. They also undergo McLafferty rearrangement. The fragmentation pattern of acetophenone is shown below.
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Several elements exist as isotopes in nature with their own relative natural abundance. Chlorine is normally a 3:1 mixture of 35 Cl and 37 Cl while bromine is an almost 1:1 mixture of 79 Br and 81 Br. Mass spectrometry separates these isotopes so that one gets true not average molecular weights. The molecular ion in the E.I. mass spectrum of the N-(4-bromophenyl)acetamide has two peaks at 213 and 215 of roughly equal intensity.