Indicate whether or not the following molecules are chiral.
3,3-dimethylheptane
2,3-dimethylheptane
2-methylheptane
3-methylheptane
4-methylheptane
The Correct Answer and Explanation is:
To determine whether a molecule is chiral, we must check for the presence of a chiral center (a carbon atom bonded to four different groups) and no internal plane of symmetry.
Let’s analyze each compound:
1. 3,3-Dimethylheptane
Not chiral
- The carbon at position 3 has two methyl groups attached (hence “dimethyl”), meaning it is bonded to identical groups.
- There are no chiral centers, and the molecule has a plane of symmetry.
- Conclusion: Achiral.
2. 2,3-Dimethylheptane
Chiral
- Focus on carbon C-3, which is attached to:
- A hydrogen
- A methyl group (CH₃)
- An ethyl group (C₂H₅)
- A longer but different alkyl chain from the other side of the backbone
- These four groups are all different, making C-3 a chiral center.
- There’s no plane of symmetry, and no superimposable mirror image.
- Conclusion: Chiral.
3. 2-Methylheptane
Not chiral
- Examine the substitution at C-2, where the methyl group is added.
- All the carbon atoms are connected in a way that does not create a chiral center. Any carbon with four bonds will have at least two hydrogens or identical alkyl groups, failing the chiral center criteria.
- Conclusion: Achiral.
4. 3-Methylheptane
Not chiral
- Carbon C-3 is bonded to:
- Hydrogen
- Methyl group
- Two similar alkyl chains (left and right of the backbone are similar enough to not create a chiral center).
- The carbon is not connected to four different groups, so there is no chiral center.
- Conclusion: Achiral.
5. 4-Methylheptane
Not chiral
- Examine C-4 with the methyl group:
- It’s connected to:
- A hydrogen
- Methyl
- Two similar carbon chains (from positions 1–3 and 5–7)
- These branches are not sufficiently different to create four unique substituents.
- Conclusion: Achiral.
✅ Final Summary:
| Molecule | Chiral? |
|---|---|
| 3,3-Dimethylheptane | ❌ No |
| 2,3-Dimethylheptane | ✅ Yes |
| 2-Methylheptane | ❌ No |
| 3-Methylheptane | ❌ No |
| 4-Methylheptane | ❌ No |
Only 2,3-dimethylheptane is chiral due to the presence of a carbon atom (C-3) with four distinct substituents, meeting the criteria for chirality.
