McLafferty rearrangement requirements

Requirements for the McLafferty rearrangement in a broad sense i) the atoms A, B, and D can be carbons or heteroatoms, ii) A and B must be connected by a double bond, hi) at least one y-hydrogen is available that iv) is selectively transferred to B via a six-membered transition state, v) causing alkene loss upon cleavage of the p-bond. 

In addition to the already described benzylic and phenylic cleavages (Chap. 6.4), phenylalkanes may undergo alkene loss by a mechanism that is perfectly analogous to the true McLafferty rearrangement, provided the alkyl substituent fulfills all requirements. The y-hydrogen is transferred to the ortto-position where the aromatic ring serves as the accepting double bond ... 

When investigating proton transfer in a series of dialkyl-N-nitros-amines which requires an intramolecular rearrangement analogous to the first step in the McLafferty rearrangement ... 

Many other examples of McLafferty-like rearrangements will be encountered with various classes of compounds. In general, this rearrangement requires only a double bond and a hydrogen atom in the proper relative positions, as shown by Equation 2.65, where the letters represent various types of atoms. The hydrogen migration may occur before cleavage of the C-D bond, but it cannot, of course, occur afterward. 

Ortho Rearrangement The ortho effect is observed in those bis-substituted aromatic compounds that have the two appropriate substituents in the ortho positions. Similar to the McLafferty rearrangement, this reaction also requires the transfer of a hydrogen atom from one of the substituents to the acceptor group of the other substituents through a six-membered transition state from which a neutral molecule is eliminated via an inductive effect for example HX in the reaction... 

The first of these reactions requires a y-hydrogen, and thus at least one Cs-alkyl substitutent. It may either be regarded as the even-electron analogy of the McLafferty rearrangement McL, Chap. 6.7) as it will be treated here [77,78,126], or alternatively, as a retro-ene reaction [127,128]. 

Thus, the secondary C3HeO radical cations created by the McLafferty rearrangement of the molecular ions of hexan-2-one (Scheme 7) correspond to the ionized enols of acetone and not acetone radical cations, and further fragmentation may require re-ketonization . Similarly, elimination of an alkene fragment from the molecular ions of alkylbenzenes gives rise primarily to an isotoluene radical cation (Scheme 8) which differs in reactivity from the toluene molecular ion. In most cases these differences alter the details of the fragmentation mechanisms, but not the principal routes of a further fragmentation. 

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... 

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