Question

Predict the structure of the major product formed by 1,2 -addition of \(\mathrm{HCl}\) to 2 -methyl- 1,3 -butadiene (isoprene).

Short answer

Question: Predict the structure of the major product formed by the 1,2-addition of HCl to 2-methyl-1,3-butadiene (isoprene). Answer: The major product for the 1,2-addition of HCl to isoprene is 3-chloro-2-methyl-but-1-ene: ``` CH3 \ H C-Cl // C=C ```

Step-by-step solution

Write down the structure of the reactants

Draw the structure of isoprene (2-methyl-1,3-butadiene) and the structure of HCl. Isoprene: ``` CH3 \ C=C-C=C ``` HCl: H-Cl

Identify the mechanism for the addition of HCl to isoprene

The addition of HCl to the conjugated diene (isoprene) will follow an electrophilic addition mechanism.

Determine the stability of the carbocation intermediate

The first step in the electrophilic addition involves the attack of the π bond of the conjugated diene (isoprene) onto the electrophile (H^+). Following the Markovnikov rule, the more stable carbocation will form. The two possible carbocations which can be formed are: 1. Primary carbocation (on C1, including the methyl group as substituent): ``` CH3 \ C^+-C=C ``` 2. Secondary carbocation (on C2, between the two carbon double bonds): ``` CH3 \ C-C^+-C=C ``` The secondary carbocation (on C2) is more stable due to inductive and resonance stabilization provided by the adjacent carbons.

Complete the electrophilic addition

Now that we have determined the more stable carbocation as the C2 carbocation, attack the Cl^− ion (nucleophile) on the C2 carbocation, resulting in the 1,2-addition product 3-chloro-2-methyl-but-1-ene: ``` CH3 \ H C-Cl // C=C ```

Write down the major product and double-check

Write down the structure of 3-chloro-2-methyl-but-1-ene, which is the major product for the 1,2-addition of HCl to isoprene: ``` CH3 \ H C-Cl // C=C ``` Double-check the product by ensuring that the addition has taken place at the more stable carbocation to predict the structure of the major product of the 1,2-addition of HCl to isoprene.