Total number of isomers (including stereoisomers) obtained on monochlorination of methylcyclohexane is ______.

We need to find the total number of isomers (including stereoisomers) obtained on monochlorination of methylcyclohexane.

Methylcyclohexane has a cyclohexane ring with a methyl group at C1. The distinct hydrogen positions are:

- $$CH_3$$ group (methyl hydrogens): 3 equivalent H atoms

- C1 (tertiary hydrogen): 1 H atom

- C2 and C6 (equivalent by symmetry): each has 2 H atoms

- C3 and C5 (equivalent by symmetry): each has 2 H atoms

- C4: 2 H atoms

(i) Chlorination at the methyl group:

Replacing one H on $$CH_3$$ gives a $$CH_2Cl$$ group attached to the ring.

No chiral center is created. This gives 1 product.

(ii) Chlorination at C1 (tertiary position):

Cl replaces the tertiary H. C1 now bears $$CH_3$$, Cl, and two ring carbons. The molecule has a plane of symmetry through C1-C4, so no chirality.

This gives 1 product.

(iii) Chlorination at C2 (or C6):

This produces 1-methyl-2-chlorocyclohexane. Both C1 and C2 become stereocenters.

- cis-1-methyl-2-chlorocyclohexane: The two substituents are on the same side of the ring. C1 and C2 are both chiral centers with no internal plane of symmetry, giving a pair of enantiomers = 2 stereoisomers.

- trans-1-methyl-2-chlorocyclohexane: The two substituents are on opposite sides. Again, C1 and C2 are chiral centers giving a pair of enantiomers = 2 stereoisomers.

Subtotal from C2/C6 position: 4 stereoisomers.

(iv) Chlorination at C3 (or C5):

This produces 1-methyl-3-chlorocyclohexane. Both C1 and C3 are stereocenters.

- cis-1-methyl-3-chlorocyclohexane: Enantiomeric pair = 2 stereoisomers.

- trans-1-methyl-3-chlorocyclohexane: Enantiomeric pair = 2 stereoisomers.

Subtotal from C3/C5 position: 4 stereoisomers.

(v) Chlorination at C4:

This produces 1-methyl-4-chlorocyclohexane. The molecule has a plane of symmetry through C1 and C4.

- cis-1-methyl-4-chlorocyclohexane: Has a plane of symmetry, so it is achiral = 1 stereoisomer.

- trans-1-methyl-4-chlorocyclohexane: Also has a plane of symmetry, so it is achiral = 1 stereoisomer.

Subtotal from C4 position: 2 stereoisomers.

Therefore, the total number of isomers (including stereoisomers) is $$\boxed{12}$$.