Carboxylic acids McLafferty rearrangement

Abundant ions are observed in the mass spectra of straight-chain carboxylic acids at m/z 60 and 73 from n-butanoic to n-octadecanoic acid. The formation of an abundant rearrangement ion at m/z 60 requires a hydrogen in position four of the carbon chain. Most mass spectra of acids are easy to identify with the exception of 2-methylpropanoic acid, which does not have a hydrogen at the C-4 position and cannot undergo the McLafferty... 

For carboxylic acid ethyl and longer aliphatic chain esters the McLafferty rearrangement can also occur on the alkoxy branch (R ) of the molecular ion. It then competes as a second alkene loss with the reaction at R ... 

Alkene loss via McLafferty rearrangement at the alkoxy group of aliphatic and aromatic carboxylic acid esters competes with yet another reaction path, where two hydrogens instead of one as in the normal McLafferty product are transferred to the charge site. This second pathway leading to alkenyl loss has early been noticed [94] and became known as McLafferty rearrangement with double hydrogen transfer (r2H) ... 

Carboxylic acids. Monocarboxylic acids normally show the molecular ion in the spectrum. Cleavage of bonds adjacent to the carbonyl group (a-cleavage) results in formation of fragments of mass M — 17 (OH) and M — 45 (C02H). Characteristic peaks arise from the McLafferty rearrangement. 

The behavior of chloromethyl esters of -carboxylic acids CH3(CH2) COOCH2Cl (n -2-12) and of all their 66 monochlorinated derivatives was examined with the aid of metastable linked scan techniques207. The main fragmentations were discussed with attention to the McLafferty rearrangement, which gives rise to abundant product ions for all compounds with n > 2 except for those in which the chlorine substituent occupies positions 3 and 5 in the chain. 

The McLafferty rearrangement in certain carboxylic acids initially forms the ion (CH2 = C(OH)2)t, which then undergoes metastable ion decomposition to lose a hydroxyl radical. With the labelled ions [CD2 = C(OH)(OD)]t and [CH2=C(OH)(OD)]t, the isotope effect 7oh/ od has been measured as 0.38 [526, 753], 0.56 [342] and 0.42 [407], i.e. OD- loss was more probable than OH - loss. This observation has been interpreted [407, 526] in terms of the rate-determining step... 

Like other oxy compounds, cleavage of the bond P to carbonyl (P-cleavage), together with the rearrangement of y-hydrogen (McLafferty rearrangement) is the most important mode of fragmentation of carboxylic acids. 

The McLafferty rearrangement in certain carboxylic acids initially forms the ion (CH2=C(OH)2) , which then undergoes metastable ion decomposition to lose a hydroxyl radical. With the labelled ions [CD2 = C(OH)(OD)]t and [CH2=C(OH)(OD)]t, the isotope effect... 

The molecular ion peak of a carboxylic acid is usually small because favorable modes of fragmentation are available. The most common fragmentation is loss of a molecule of an alkene (the McLafferty rearrangement, discussed in Section 18-5D). Another common fragmentation is loss of an alkyl radical to give a resonance-stabilized cation with the positive charge delocalized over an allylic system and two oxygen atoms. 

Esters and amides also generally show discernible molecular ion peaks. Like carboxylic acids, their most characteristic fragmentation patterns are a-cleavage and McLafferty rearrangement, both of which can be seen in the mass spectrum of methyl butanoate (Ligure 14.13). Peaks at miz 71 and 59 are the result of... 

Carboxylic acids, esters, and amides undergo the same two kinds of characteristic cleavage, a-cleavage and McLafferty rearrangement. 

Electron impact mass spectrometry of carboxylic acids usually produces only a weak molecular ion (M ). If the carboxylic add has a total molecular weight below about 150, an M -17 fragment (due to the loss of the hydroxyl), as well as an M+-45 (as a consequence of losing CO2H), is frequently observed. With the larger carboxylic acids, a McLafferty rearrangement such as the one shown in Scheme 5.5 (which should be compared with Scheme 5.3) intrudes. 

Scheme 5.5. A carboxylic acid undergoing McLafferty-type rearrangement with fragmentation on electron bombardment in the mass spectrometer.

A McLafferty rearrangement accounts for this outcome, as shown below for butanoic acid. In the case of longer chain carboxylic acids the additional carbons are eliminated in the alkene formed. 

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