Question

How many chiral carbon atoms are present in 2-Bromo-3,4,5-trimethylhexane?

• A. 2
• B. 3
• C. 4
• D. 1

Hint:

When scanning a carbon chain for chirality, look out for the terminal ends. A $\text{-CH(CH}_3)_2$ group at the end of a chain immediately makes that specific branch carbon achiral since it holds two identical methyl twins!

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We are given the IUPAC name of a branched haloalkane molecule. We need to construct its structural formula and evaluate the total number of chiral (asymmetric) carbon centers present.

Step 2: Detailed Explanation: Let's draw out the structural framework of 2-Bromo-3,4,5-trimethylhexane: \includegraphics[width=0.7\linewidth]{images/48sol.png}
The structural line formula is: $$ \overset{1}{\text{C}}\text{H}_3 - \overset{2}{\text{C}}\text{H}(\text{Br}) - \overset{3}{\text{C}}\text{H}(\text{CH}_3) - \overset{4}{\text{C}}\text{H}(\text{CH}_3) - \overset{5}{\text{C}}\text{H}(\text{CH}_3) - \overset{6}{\text{C}}\text{H}_3 $$ A carbon atom is classified as chiral if it is bonded to four completely different atoms or groups of atoms. Let's analyze each carbon along the main chain:

• C1 and C6: Form part of $\text{-CH}_3$ groups (three identical hydrogens), so they are achiral.

• C2: Bonded to: -H, -Br, $\text{-CH}_3$ (C1), and the large group $\text{-CH(CH}_3)\text{CH(CH}_3)\text{CH(CH}_3)\text{CH}_3$ (C3 onwards). All 4 groups are unique.

(Chiral Center 1)

• C3: Bonded to: -H, $\text{-CH}_3$, the left group $\text{-CH(Br)CH}_3$ (C2), and the right group $\text{-CH(CH}_3)\text{CH(CH}_3)\text{CH}_3$ (C4 onwards). All 4 groups are unique.

(Chiral Center 2)

• C4: Bonded to: -H, $\text{-CH}_3$, the left group $\text{-CH(CH}_3)\text{CH(Br)CH}_3$, and the right group $\text{-CH(CH}_3)_2$. All 4 groups are unique.

(Chiral Center 3)

• C5: Bonded to: -H, the left group (C4 onwards), and two identical methyl ($\text{-CH}_3$) groups (C6 and the branch attached to C5). Because two attached groups are the same, C5 is achiral.
Summing them up, carbons C2, C3, and C4 are asymmetric centers.

Step 3: Final Answer: The total number of chiral carbon atoms present in the molecule is 3, matching option (B).