Question

Identify the false statement with respect to McLafferty rearrangement in mass spectroscopy.

• A. It is a fragmentation reaction
• B. A six-membered cyclic transition state is involved
• C. It occurs in any compound that has a hydrogen atom on the \(\gamma\)-carbon
• D. A radical cation and a neutral alkene fragments are formed

Hint:

For a McLafferty Rearrangement, you need two essential structural features: 1. A \(\text{C=O}\) group (or similar unsaturated double bond system). 2. An accessible \(\gamma\)-hydrogen atom available to migrate through a 6-membered ring.

Answer 1

The Correct Option is C

Concept: The McLafferty rearrangement is a well-established intramolecular fragmentation pathway that occurs during mass spectrometry for radical cations containing an unsaturated functional group (such as ketones, aldehydes, esters, or alkenes).

Step 1: Evaluate Statement A and D The McLafferty process is a unimolecular fragmentation reaction driven by electron ionization. It begins with a radical cation precursor and undergoes a cyclic redistribution of bonds, breaking an axial carbon-carbon bond to yield two distinct fragments: an enol radical cation species (which is detected) and a complementary neutral alkene fragment molecule. Thus, Statements A and D describe true aspects of the mechanism.

Step 2: Evaluate Statement B The spatial arrangement required for this rearrangement involves a sterically favorable six-membered cyclic transition state. The radical site on the unsaturated heteroatom reaches across to abstract a hydrogen atom from the carbon at the gamma (\(\gamma\)) position relative to the carbonyl group: \[ \text{[C}_\gamma\text{-H]} \cdots \text{[O=C}_\alpha\text{]}^{+\bullet} \] Because it proceeds through this specific geometry, Statement B is true.

Step 3: Evaluate Statement C Statement C claims the reaction occurs in *any* compound that has a hydrogen atom on the \(\gamma\)-carbon. This statement is overly broad and incorrect. For a McLafferty rearrangement to take place, the molecule must also contain an unsaturated, polarizable double-bond group (such as a carbonyl \(\text{C=O}\), an imine \(\text{C=N}\), or an alkene \(\text{C=C}\)) to accept the migrating proton. A pure saturated alkane containing a \(\gamma\)-carbon lacks this functional group requirement and cannot undergo this rearrangement. Since Statement C is false, it is the correct answer to this question.