Question

On irradiation using UV light (\(>300\) nm), compounds X and Y predominantly undergo which photochemical pathway? (Structures as shown in the figure)

• A. \(X\): Norrish type I reaction and \(Y\): Norrish type II reaction
• B. \(X\): Norrish type II reaction and \(Y\): Norrish type I reaction
• C. Both \(X\) and \(Y\): Norrish type I reaction
• D. Both \(X\) and \(Y\): Norrish type II reaction

Hint:

Look for an accessible, properly aligned \(\gamma\)-H (six-membered TS) \(\Rightarrow\) Type II likely.
If \(\gamma\)-H is missing or geometrically blocked, expect \(\alpha\)-cleavage \(\Rightarrow\) Type I.
Aldehydes often favor Type I; bulky \(\gamma\)-substitution disfavors Type II.

Answer 1

The Correct Option is B

Step 1: Identify possibilities. Norrish type I = \(\alpha\)-cleavage next to \(C{=}O\) (acyl–\(\alpha\) bond scission). Norrish type II = intramolecular \(\gamma\)-H abstraction by the excited carbonyl oxygen through a six-membered transition state, giving a 1,4-biradical and alkene migration/fragmentation.
Step 2: Analyze \(X\). In \(X\), the carbonyl has a properly aligned \(\gamma\)-hydrogen on the side chain (tertiary/benzylic-like) and the conformation allows the classic six-membered cyclic abstraction. This strongly favors the Type II pathway over \(\alpha\)-cleavage.
Step 3: Analyze \(Y\). In \(Y\), the carbonyl is embedded such that an accessible \(\gamma\)-H is \emph{absent/misaligned} (substitution places a quaternary/sterically hindered center at \(\gamma\)), suppressing the Type II abstraction. Aldehydes/ketones in such cases undergo facile \(\alpha\)-cleavage (Type I), giving acyl and \(\alpha\)-alkyl radicals.
Step 4: Hence \(X\) predominantly shows Norrish II, while \(Y\) undergoes Norrish I \(\Rightarrow\) option (B).