Q.Choose the most suitable reagent to convert the methyl ketone CH3-CH=CH-CH2-CO-CH3 (hex-4-en-2-one) into the carboxylic acid CH3-CH=CH-CH2-COOH (pent-3-enoic acid) - that is, a reagent that removes the terminal CH3CO- (methyl-ketone) carbon and installs a -COOH group while leaving the C=C double bond intact.

Concept

A compound bearing a CH3-CO- group (a methyl ketone) undergoes the haloform reaction: the three alpha-hydrogens of the methyl are replaced by halogen, then base cleaves the C-C bond to give a carboxylate and the trihalomethane (here iodoform, CHI3).

Applying it

Substrate: CH3-CH=CH-CH2-CO-CH3 (a methyl ketone, the CH3-CO- at the right end).

• I2/NaOH triiodinates the terminal methyl to give CH3-CH=CH-CH2-CO-CI3.
• Hydroxide attacks the carbonyl and cleaves the C-CI3 bond.
• Products: CH3-CH=CH-CH2-COO(-) (sodium salt) + CHI3 (yellow iodoform precipitate). Acidification gives CH3-CH=CH-CH2-COOH (pent-3-enoic acid). …

Method: Recognising the Haloform (Iodoform) Reaction on a Methyl Ketone

Core Concept

Any compound bearing a CH3-CO- (methyl ketone) group reacts with a halogen in base (I2/NaOH) to iodinate all three alpha-hydrogens of that methyl group; hydroxide then cleaves the resulting C-CI3 bond to release a trihalomethane (iodoform, CHI3) and leave a carboxylate with one fewer carbon -- the standard one-step route that shortens a methyl ketone by exactly its terminal CH3CO- carbon while leaving other unsaturation untouched.

Steps

1. Identify the CH3-CO- group in the substrate: CH3-CH=CH-CH2-CO-CH3 has its methyl ketone at the right-hand end.
2. Recognise I2/NaOH as the classic haloform reagent combination for methyl ketones (and methyl carbinols, via prior oxidation).
3. Mechanistically: base deprotonates one of the three methyl alpha-hydrogens; the resulting enolate attacks I2, and this repeats three times to give -CO-CI3.
4. Hydroxide then adds to the carbonyl and expels the very stable CI3(-) (soon protonated to CHI3, the characteristic yellow iodoform precipitate), leaving the rest of the molecule as a carboxylate. …