Question

Which of the following are chiral molecules? 

Choose the correct answer from the options given below: 
 

• A. (A) and (D) only
• B. (A), (C) and (D) only
• C. (A) and (B) only
• D. (B) and (D) only

Hint:

To identify a chiral molecule, look for a carbon atom with four different substituents attached. Draw the molecule out and check each carbon. If you find one such carbon (a chiral center) and the molecule has no internal plane of symmetry, it is chiral. Be aware that questions in exams can sometimes be flawed.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
A molecule is chiral if it is non-superimposable on its mirror image. The most common cause of chirality in organic molecules is the presence of a chiral center (or stereocenter), which is a carbon atom bonded to four different groups.
Step 2: Detailed Explanation:
Let's examine each molecule:
(A) CH\(_3\)CH(Br)OH (1-bromoethanol): The central carbon atom is bonded to four different groups: a hydrogen atom (-H), a bromine atom (-Br), a hydroxyl group (-OH), and a methyl group (-CH\(_3\)). Since all four groups are different, this carbon is a chiral center, and the molecule is chiral.
(B) CH\(_3\)CH(OH)CH(OH)CH\(_3\) (2,3-butanediol): This molecule has two potential chiral centers (C2 and C3). Each of these carbons is bonded to -H, -OH, -CH\(_3\), and the other -CH(OH)CH\(_3\) group. The molecule exists as stereoisomers. It has a pair of enantiomers (which are chiral) and a meso form (which is achiral). Since the molecule can exist in chiral forms, it is considered a chiral molecule. The structure drawn represents one of the chiral enantiomers.
(C) CH\(_3\)CH\(_2\)CH(OH)CH\(_2\)CH\(_3\) (pentan-3-ol): The central carbon (C3) is bonded to -H, -OH, and two identical ethyl groups (-CH\(_2\)CH\(_3\)). Since two of the groups attached to the carbon are identical, it is not a chiral center. The molecule has a plane of symmetry and is achiral.
(D) CH\(_3\)CH(Br)CH\(_2\)CH\(_3\) (2-bromobutane): The second carbon (C2) is bonded to four different groups: -H, -Br, a methyl group (-CH\(_3\)), and an ethyl group (-CH\(_2\)CH\(_3\)). Therefore, C2 is a chiral center, and the molecule is chiral.
Step 3: Final Answer:
Based on the analysis, molecules (A), (B), and (D) are chiral, while molecule (C) is achiral. None of the options correctly list (A), (B), and (D). This indicates a probable error in the question or the provided options. However, in such situations, one must choose the best-fitting option. Both options (1), (3), and (4) list subsets of the correct answer. Given the options, and assuming there might be a reason to group certain molecules, (A) and (B) are both chiral alcohols. There is no clear chemical reason to exclude (D). However, if forced to choose from the given options, and acknowledging the question's flaw, we select an available option that contains correct entries. Option (3) lists (A) and (B), which are both indeed chiral.