Write the structure of the alkene

$3$-Methylhex-$2$-ene has the structure: $\mathrm{CH_3-CH=C(CH_3)-CH_2-CH_2-CH_3}$

Numbering:

• C$_2$ and C$_3$ are the double-bond carbons.
• A methyl group is attached to C$_3$.

Reaction with HBr in presence of peroxide

In presence of peroxide, HBr adds by the free-radical (anti-Markovnikov) mechanism.

Therefore:

• $\mathrm{Br}$ adds to the less substituted carbon of the double bond.
• $\mathrm{H}$ adds to the more substituted carbon.

So the product is: $\mathrm{CH_3-CH(Br)-CH(CH_3)-CH_2-CH_2-CH_3}$

This is 2-bromo-3-methylhexane.

Identify stereocentres

Product: $\mathrm{CH_3-CH(Br)-CH(CH_3)-CH_2-CH_2-CH_3}$

• C$_2$ has four different groups:

  • $\mathrm{H}$
  • $\mathrm{Br}$
  • $\mathrm{CH_3}$
  • rest of chain $\mathrm{-CH(CH_3)CH_2CH_2CH_3}$

  So C$_2$ is a chiral centre.

• C$_3$ also has four different groups:

  • $\mathrm{H}$
  • $\mathrm{CH_3}$
  • left side $\mathrm{-CH(Br)CH_3}$
  • right side $\mathrm{-CH_2CH_2CH_3}$

  So C$_3$ is also a chiral centre.

Count possible stereoisomers

Number of stereocentres $=2$.

Maximum stereoisomers: $2^2 = 4$

Check for meso form: not possible because the molecule has no plane of symmetry.

Hence total possible stereoisomers $=4$.

Compare with stored answer

Derived answer: $4$

Stored correct answer: $4$

They agree.