Find out number of stereoisomers obtained when 3-methylhex-2-ene reacts with HBr in presence of peroxide.

The correct answer is 4.

Let's break down the reaction step-by-step to understand why 4 stereoisomers are obtained.

1. Structure of the reactant:
The reactant is 3-methylhex-2-ene. Its structural formula is:
CH₃-CH=C(CH₃)-CH₂-CH₂-CH₃

2. Reaction conditions:
The reaction is the addition of hydrobromic acid (HBr) in the presence of peroxide. This reagent combination undergoes a free-radical addition mechanism, commonly known as the Anti-Markovnikov addition.

3. Regiochemistry of addition:
In an anti-Markovnikov addition of HBr to an unsymmetrical alkene, the bromine atom (Br) attaches to the less substituted carbon of the double bond (C2), and the hydrogen atom (H) attaches to the more substituted carbon of the double bond (C3). Let's locate the double-bonded carbons:
- Carbon-2 (C2) is bonded to one H and one methyl group: -CH(CH₃)- (less substituted).
- Carbon-3 (C3) is bonded to a methyl group and a propyl group: -C(CH₃)(C₃H₇)- (more substituted).
Therefore, the addition yields:
CH₃-CH(Br)-CH(CH₃)-CH₂-CH₂-CH₃ (2-bromo-3-methylhexane)

4. Analyzing stereocenters (chiral centers):
Let us examine the structure of the product, 2-bromo-3-methylhexane, to identify its chiral carbons (carbons with four different groups attached):
- C2: It is bonded to -H, -Br, -CH₃, and the -CH(CH₃)CH₂CH₂CH₃ group. Since all four groups are different, C2 is a chiral center.
- C3: It is bonded to -H, -CH₃, -CH(Br)CH₃, and -CH₂CH₂CH₃. Since all four groups are different, C3 is also a chiral center.

5. Calculating the number of stereoisomers:
The product contains 2 chiral centers ($n=2$).
Because the molecule is unsymmetrical, there is no plane of symmetry or center of inversion. The number of stereoisomers is calculated using the formula:
$\text{Number of stereoisomers}=2^n$
Substituting $n=2$:
$2^2=4$ stereoisomers.

These 4 stereoisomers consist of two pairs of enantiomers: (2R, 3R), (2S, 3S), (2R, 3S), and (2S, 3R).