Predict the product C obtained in the following reaction of 1-butyne. CH3CH2-C CH + HCl B HI C
- CH3-CH2-CH2- Cl | I |C-H
- CH3-CH2- I |CH-CH2Cl
- CH3CH2- Cl | I |C-CH3
- CH3- Cl |CH-CH2CH2I
Answer
(3) CH3CH2- Cl | I |C-CH3
Solution
Step 1: Addition of HCl to the alkyne follows Markovnikov's rule – Cl goes to the carbon having fewer H atoms.
CH3CH2-C CH + HCl CH3CH2- Cl |C=CH2 (B)
Step 2: H+ of HI adds to the terminal CH2; the carbocation formed on the carbon carrying Cl is stabilised by the lone pair of Cl (resonance).
⇒ I- attacks the same carbon, so both halogens are on C-2 (gem-dihalide).
CH3CH2- Cl |C=CH2 HI CH3CH2- Cl | I |C-CH3 (C)
∴ C is 2-chloro-2-iodobutane.
Answer: (3)