Question

Which of the following compounds have chiral carbon ?
A. 1, 1-Dibromopentane
B. 2-Chloro-2-methylpropane
C. 2-Bromopentane
D. 3-Bromopentane
E. 1-fluoro-2-ethylpentane
Choose the correct answer from the options given below :

• A. B and D only
• B. C and E only
• C. A and C only
• D. B and E only

Hint:

Always draw out the full structure, especially the alkyl chains. A very common trap is placing a substituent exactly in the middle of a symmetric chain (like 3-bromopentane or 4-heptanol), which results in two identical alkyl groups, rendering the molecule achiral.

Answer 1

The Correct Option is B

Step 1: Concept:
The objective is to analyze the chemical names provided, deduce their chemical structures, and identify which of them possess at least one chiral carbon atom (a stereocenter).

Step 2: Key Formula or Approach:
A carbon atom is considered chiral (or an asymmetric stereocenter) if it is $sp^3$ hybridized and bonded to four distinctly different atoms or chemical groups. We must draw the structures and examine the attachments to every carbon.

Step 3: Step-by-step Explanation:

• A. 1,1-Dibromopentane: Structure is $CH(Br)_2-CH_2-CH_2-CH_2-CH_3$. Carbon-1 is attached to a hydrogen, a butyl chain, and two identical bromine atoms. Because it has two identical groups, it is not chiral. None of the other carbons are chiral either.

• B. 2-Chloro-2-methylpropane: Also known as tert-butyl chloride. Structure is $C(Cl)(CH_3)_3$. The central carbon is attached to a chlorine and three identical methyl groups. Not chiral.

• C. 2-Bromopentane: Structure is $CH_3-CH(Br)-CH_2-CH_2-CH_3$. Let's look at Carbon-2. It is attached to: 1) a Hydrogen atom ($-H$), 2) a Bromine atom ($-Br$), 3) a Methyl group ($-CH_3$), and 4) a Propyl group ($-CH_2CH_2CH_3$). All four groups are distinctly different! Therefore, C-2 is a chiral carbon.

• D. 3-Bromopentane: Structure is $CH_3-CH_2-CH(Br)-CH_2-CH_3$. Let's look at Carbon-3. It is attached to: 1) $-H$, 2) $-Br$, 3) an Ethyl group ($-CH_2CH_3$), and 4) another identical Ethyl group ($-CH_2CH_3$). Because two groups are the same, it is achiral.

• E. 1-fluoro-2-ethylpentane: Structure is $CH_2(F)-CH(CH_2CH_3)-CH_2-CH_2-CH_3$. (Note: IUPAC name might be 3-(fluoromethyl)hexane, but the given name clearly denotes the structure). Let's examine Carbon-2. It is attached to: 1) a Hydrogen atom ($-H$), 2) a Fluoromethyl group ($-CH_2F$), 3) an Ethyl group ($-CH_2CH_3$), and 4) a Propyl group ($-CH_2CH_2CH_3$). All four groups are unique. Therefore, C-2 is a chiral carbon.
The molecules with chiral carbons are C and E.

Step 4: Final Answer:
Compounds C and E only contain chiral carbons, which points to option (B).