Propose a mechanism for each of the following reactions:
b.

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Step 1: Identify the type of reaction. This is an electrophilic addition reaction where HCl adds across the double bond of the alkene.

Step 2: Protonation of the alkene. The π-electrons of the double bond attack the hydrogen atom of HCl, leading to the formation of a carbocation intermediate. The more stable carbocation will form, which in this case is the tertiary carbocation due to hyperconjugation and inductive effects.

Step 3: Formation of the carbocation. The double bond breaks, and the hydrogen from HCl attaches to the less substituted carbon of the double bond, leaving the more substituted carbon as the carbocation.

Step 4: Nucleophilic attack by the chloride ion. The chloride ion (Cl⁻) attacks the positively charged carbocation, forming a new bond and resulting in the final product.

Step 5: Verify the product. The final product is 2-chloro-2-methylpropane, where the chlorine atom is attached to the tertiary carbon.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electrophilic Addition Reactions

Electrophilic addition reactions involve the addition of an electrophile to a nucleophile, typically across a double bond. In the context of alkenes, the π bond acts as a nucleophile, attacking an electrophile, which leads to the formation of a more stable carbocation intermediate. This mechanism is crucial for understanding how alkenes react with acids and halogens.

Carbocation Stability

Carbocation stability is a key concept in organic chemistry, as the stability of the carbocation intermediate influences the reaction pathway. Carbocations are classified as primary, secondary, or tertiary based on the number of alkyl groups attached to the positively charged carbon. Tertiary carbocations are the most stable due to hyperconjugation and inductive effects, which play a significant role in determining the product of electrophilic addition reactions.

Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX (where X is a halogen) to an alkene, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This rule helps predict the major product of electrophilic addition reactions, as it accounts for the formation of the more stable carbocation intermediate, guiding the regioselectivity of the reaction.

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