Write the structure of 2-methylbutane

2-Methylbutane is: $\mathrm{CH_3-CH(CH_3)-CH_2-CH_3}$

Let us identify different types of hydrogen atoms where monochlorination can occur.

Find distinct positions for substitution

In 2-methylbutane, there are four carbon positions, but some are equivalent:

• Two equivalent methyl groups attached to C-2
• One terminal methyl group at the other end
• One methylene carbon
• One tertiary hydrogen at C-2

So monochlorination can occur at the following distinct positions:

(i) Chlorination at one of the two equivalent methyl groups attached to C-2

Product: $\mathrm{CH_2Cl-CH(CH_3)-CH_2-CH_3}$ Now check chirality at C-2:

C-2 is attached to:

• $\mathrm{CH_2Cl}$
• $\mathrm{CH_3}$
• $\mathrm{CH_2CH_3}$
• $\mathrm{H}$

These are four different groups, so this carbon is chiral. Hence this gives one chiral compound, which exists as a pair of enantiomers.

(ii) Chlorination at the terminal methyl group on the other end

Product: $\mathrm{CH_3-CH(CH_3)-CH_2-CH_2Cl}$

Check C-2: It is attached to:

• $\mathrm{CH_3}$
• $\mathrm{CH_3}$
• $\mathrm{CH_2CH_2Cl}$
• $\mathrm{H}$

Since two groups are identical ($\mathrm{CH_3}$ and $\mathrm{CH_3}$), it is not chiral.

(iii) Chlorination at the methylene carbon

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

Now the carbon bearing Cl is attached to:

• $\mathrm{Cl}$
• $\mathrm{H}$
• $\mathrm{CH_3}$
• $\mathrm{CH(CH_3)CH_3}$

These are four different groups, so this carbon is chiral. Hence this gives one chiral compound, again existing as a pair of enantiomers.

(iv) Chlorination at the tertiary hydrogen (C-2)

Product: $\mathrm{CH_3-C(Cl)(CH_3)-CH_2-CH_3}$

The carbon bearing Cl has two identical $\mathrm{CH_3}$ groups, so it is not chiral.

Count chiral compounds

We found two monochloro products that are chiral:

• $\mathrm{CH_2Cl-CH(CH_3)-CH_2-CH_3}$
• $\mathrm{CH_3-CH(CH_3)-CH(Cl)-CH_3}$

Each chiral compound exists as two enantiomers.

Therefore total number of chiral compounds possible: $2 \times 2 = 4$

Match with options

Correct option is: $\boxed{\text{C: 4}}$

Compare with stored answer

Stored correct answer is B: 2, but that counts only the number of chiral products/structures, not the number of chiral compounds.

Since the question asks "how many chiral compounds are possible," enantiomers should be counted separately. Hence the correct answer should be 4.