1. Identify distinct types of hydrogens in methylcyclohexane

Methylcyclohexane has the following non-equivalent positions for replacing one H by Cl:

• on the methyl group attached to the ring
• on C1 of the ring (the carbon bearing the methyl group)
• on C2/C6 of the ring
• on C3/C5 of the ring
• on C4 of the ring

So monochlorination can give 5 constitutional isomer types.

2. Now count stereoisomers for each constitutional isomer

We must include stereoisomers.

(i) Chlorination on the methyl group

This gives chloromethylcyclohexane.

Structure: cyclohexylmethyl chloride. There is no stereogenic center formed, because the ring carbon attached to methyl still has two equivalent ring paths.

So this gives: $1 \text{ isomer}$

(ii) Chlorination at C1

This gives 1-chloro-1-methylcyclohexane.

At C1, the carbon now bears:

• Cl
• CH$_3$
• one ring bond one way
• one ring bond the other way

The two ring paths are equivalent in an unsubstituted cyclohexane ring, so C1 is not a stereogenic center.

So this gives: $1 \text{ isomer}$

(iii) Chlorination at C2/C6

This gives 1-methyl-2-chlorocyclohexane.

Now there are two stereocenters: C1 and C2. Because substituents are different (methyl and chloro), no meso form is possible.

Hence stereoisomers are:

• cis: one enantiomeric pair
• trans: one enantiomeric pair

Total: $4 \text{ stereoisomers}$

(iv) Chlorination at C3/C5

This gives 1-methyl-3-chlorocyclohexane.

Again there are two stereocenters, and substituents are different, so no meso form.

Thus:

• cis: one enantiomeric pair
• trans: one enantiomeric pair

Total: $4 \text{ stereoisomers}$

(v) Chlorination at C4

This gives 1-methyl-4-chlorocyclohexane.

Here also there are stereoisomers:

• cis-1-methyl-4-chlorocyclohexane
• trans-1-methyl-4-chlorocyclohexane

Since substituents are different, these are not meso-related to each other, but each form is achiral due to symmetry of the 1,4-disubstituted ring framework.

So total here is: $2 \text{ stereoisomers}$

3. Add all isomers

Total number of isomers obtained on monochlorination: $1+1+4+4+2=12$

4. Final Answer

$\boxed{12}$

This matches the stored correct answer.