Animated Solution for Chemistry - Organic Chemistry: 3-methyl-pent-2-ene on reaction with HBr in presence of peroxide forms an addition product. The number of possible stereoisomers for the product is

Imagine you are in an organic chemistry lab, staring at a flask containing $3$-methylpent-$2$-ene.

We are about to add hydrogen bromide ($HBr$) to this flask, but there is a twist. We are also adding a peroxide.

This small addition of peroxide completely changes the course of the reaction. It is a classic trap in organic chemistry, and we need to be careful.

Normally, the addition of $HBr$ to an alkene follows Markovnikov's rule.

However, the presence of peroxide triggers a free radical mechanism. This is known as the Kharasch effect or the Anti-Markovnikov addition.

According to this rule, the bromine atom will attach to the carbon atom of the double bond that has a greater number of hydrogen atoms.

Let us look at our double bond between carbon-$2$ and carbon-$3$. Carbon-$2$ has one hydrogen atom, while carbon-$3$ has none.

Therefore, the bromine atom will bond to carbon-$2$, and the hydrogen atom will bond to carbon-$3$.

This regioselectivity gives us our final product: $2$-bromo-$3$-methylpentane.

Now that we have our product, we need to determine its spatial geometry. The question asks for the number of possible stereoisomers.

To find this, we must first identify the chiral centers in our molecule. A chiral center is an $s{p}^3$ hybridized carbon atom bonded to four completely different groups.

Let us examine carbon-$2$. It is attached to a hydrogen atom, a bromine atom, a methyl group, and a complex sec-butyl group. Since all four groups are different, carbon-$2$ is a chiral center.

Next, let us look at carbon-$3$. It is attached to a hydrogen atom, a methyl group, an ethyl group, and a complex bromoethyl group. Again, all four groups are distinct, making carbon-$3$ our second chiral center.

So, we have a total of $n=2$ chiral centers.

We are almost at the finish line. We need to apply the formula for calculating stereoisomers.

First, we must check if the molecule is symmetrical. Since one end of the molecule is an ethyl group and the other is a methyl group, the molecule is clearly unsymmetrical.

For an unsymmetrical molecule, the total number of stereoisomers is given by the formula $2^n$, where $n$ is the number of chiral centers.

Substituting our value, we get $2^2=4$.

Therefore, there are exactly $4$ possible stereoisomers for this product.