3,3-dimethylheptane
2,3-dimethylheptane
2-methylheptane
3-methylheptane
4-methylheptane
The Correct Answer and Explanation is:
Here is the chirality status of each molecule:
- 3,3-dimethylheptane – Not chiral
- 2,3-dimethylheptane – Chiral
- 2-methylheptane – Not chiral
- 3-methylheptane – Not chiral
- 4-methylheptane – Not chiral
Explanation
Chirality in organic molecules occurs when a carbon atom is bonded to four different groups, creating a stereocenter (also called a chiral center). A molecule with at least one chiral center and no internal plane of symmetry is considered chiral—it cannot be superimposed on its mirror image.
Let’s analyze each compound:
1. 3,3-Dimethylheptane:
The carbon at position 3 has two identical methyl groups (–CH₃), so it cannot be a chiral center because it lacks four different groups. The rest of the molecule also lacks any carbon with four different substituents. Therefore, it is not chiral.
2. 2,3-Dimethylheptane:
This molecule can have a chiral center at carbon 3, which is bonded to:
- A hydrogen,
- A methyl group,
- An ethyl chain (from carbon 2 and back to carbon 1),
- A longer alkyl chain (from carbon 4 onward).
Since carbon 3 has four different groups, it is a stereocenter. Therefore, 2,3-dimethylheptane is chiral.
3. 2-Methylheptane:
Here, carbon 2 is attached to:
- A methyl group,
- A hydrogen,
- An ethyl chain (toward carbon 1),
- A longer alkyl chain (toward carbon 3–7).
It may seem chiral, but the two chains are not different enough—upon closer analysis, the molecule is not chiral due to symmetry or lack of a true stereocenter.
4. 3-Methylheptane:
Carbon 3 appears similar to 2-methylheptane’s situation—when analyzing the attached groups, they are not all distinct. Therefore, this molecule is not chiral.
5. 4-Methylheptane:
Likewise, carbon 4 is not bonded to four different groups, so there’s no chiral center. It is not chiral.
In summary, only 2,3-dimethylheptane is chiral because it has a carbon with four distinct substituents.
