Animated Solution for Chemistry - Organic Chemistry: The major product formed in the following reaction is

Welcome to one of the most fascinating concepts in organic chemistry: the battle between kinetic and thermodynamic control! In this problem, we are given a conjugated diene, specifically isoprene (2-methylbut-1,3-diene), and we are reacting it with hydrogen bromide (HBr).

The very first thing you must notice is the word "(excess)" written right below the diene. This is a massive hint! It tells us that the diene is in excess, which makes HBr the limiting reagent. Because HBr is limited, we are only going to add one equivalent of HBr across the conjugated system. We will not form a dibromide. This immediately eliminates any options that show two bromine atoms attached to the carbon chain.

When a conjugated diene reacts with one equivalent of a hydrogen halide, it undergoes an electrophilic addition reaction. The first step is the protonation of one of the double bonds to form the most stable carbocation possible.

If we protonate the terminal carbon of the more substituted double bond, we generate a tertiary allylic carbocation:

This intermediate is incredibly stable because it is both tertiary and allylic. But the magic of conjugated systems doesn't stop there. Because it is an allylic carbocation, the positive charge is delocalized via resonance. The adjacent $\pi$ electrons can shift over, moving the positive charge to the primary carbon at the end of the chain:

Now we have two distinct electrophilic sites where the bromide ion (${\text{Br}}^{-}$) can attack: the tertiary carbon and the primary carbon.

If the bromide ion attacks the tertiary carbon, we get the 1,2-addition product. This attack happens very quickly because the bromide ion is physically closer to the tertiary carbon immediately after the protonation step. Therefore, the 1,2-addition product is the kinetic product.

If the bromide ion attacks the primary carbon, we get the 1,4-addition product. Let's look closely at the double bonds in our two potential products. The 1,4-addition product features a trisubstituted double bond, whereas the 1,2-addition product only has a monosubstituted double bond. According to Zaitsev's rule and hyperconjugation principles, more substituted alkenes are significantly more stable. Therefore, the 1,4-addition product is the thermodynamic product.

Since the reaction conditions in the problem do not specify a low temperature (like $-80^{\circ }\text{C}$), we must assume standard or elevated temperatures. Under these conditions, the reaction is under thermodynamic control, meaning the system has enough energy to equilibrate and favor the most stable product.

Thus, the major product is the 1,4-addition product: 1-bromo-3-methylbut-2-ene. This perfectly matches option (a).