This reaction involves the addition of chloroform (CHCl₃) to propene (Me-CH=CH₂) under aqueous KOH and heat, leading to the formation of a major product. This is an example of the Carbylamine Reaction or Isocyanide Test, but here it is applied in a different context to form an alkyne derivative.

Step 1: Generation of Dichlorocarbene
Under aqueous KOH, chloroform undergoes α-elimination to produce dichlorocarbene (:CCl₂), a highly reactive electrophilic species. $CHC l_{3} + KO H \overset{Δ}{\to} :C l_{2} + KC l + H_{2} O$

Step 2: Addition to Alkene
The electrophilic dichlorocarbene adds to the double bond of propene (an alkene) to form a 1,1-dichlorocyclopropane derivative. This is a cyclopropanation reaction. $Me - CH = {CH}_{2} + :C l_{2} \to (Me - CH - {CH}_{2} C_{l_{2}})$ The product is 1-methyl-2,2-dichlorocyclopropane.

Step 3: Rearrangement under Excess KOH
Under excess aqueous KOH and heat, the dichlorocyclopropane ring undergoes hydrolysis and rearrangement. The geminal dichloro group is unstable and leads to ring opening, followed by elimination, to form the major product: 2-Butyne-1-al (Me-C≡C-CHO). This is because the methyl group (Me-) makes the alkene unsymmetrical, and the rearrangement favors the formation of the terminal alkyne aldehyde.

Final Answer: The major product A is 2-Butyne-1-al, which has the structure Me-C≡C-CHO. Among the given options, the correct structure is the one showing this aldehyde with a triple bond.

Related Topics & Formulae

Carbene Chemistry: Dichlorocarbene (:CCl₂) is an electrophilic carbene generated from chloroform and base. It adds to alkenes to form cyclopropanes.

Rearrangement in Alkynes: Under strong base, certain cyclopropanes with geminal dihalides can rearrange to alkynes, especially when forming terminal alkynes is favorable.

Reaction Condition: Aqueous KOH is used for hydrolysis and elimination steps, leading to the final alkyne product.