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 examine if it has at least one chiral center (a carbon atom bonded to four different groups) and no plane of symmetry.
Analysis of Each Molecule
- 3,3-Dimethylheptane
- Structure: Heptane backbone with two methyl groups on the third carbon.
- The central carbon (C3) has two identical methyl groups attached, so it cannot be chiral.
✅ Not chiral
- 2,3-Dimethylheptane
- Structure: Methyl groups on C2 and C3.
- Look at C2 and C3:
- C2 is bonded to: a methyl group, a hydrogen, C1 chain, and C3 chain — these are all different.
- C3 is similarly bonded to different groups.
- One or both of these centers can be chiral depending on the exact substituents.
- In this case, C2 is chiral because the substituents are all different; C3 may or may not be.
✅ Chiral
- 2-Methylheptane
- Structure: Methyl group on C2.
- C2 is bonded to: a hydrogen, a methyl group, and two alkyl chains (C1 and C3–C7).
- The two alkyl chains differ, so C2 has four different groups.
✅ Chiral
- 3-Methylheptane
- Structure: Methyl group on C3.
- C3 is bonded to: a hydrogen, a methyl group, and two different alkyl chains (C1–C2 vs. C4–C7).
- Again, these are different, so C3 is a chiral center.
✅ Chiral
- 4-Methylheptane
- Structure: Methyl group on C4.
- C4 is attached to: a hydrogen, a methyl group, and two different alkyl chains (C1–C3 vs. C5–C7).
- These groups are all distinct, so C4 is chiral.
✅ Chiral
Conclusion
| Molecule | Chiral? |
|---|---|
| 3,3-Dimethylheptane | ❌ No |
| 2,3-Dimethylheptane | ✅ Yes |
| 2-Methylheptane | ✅ Yes |
| 3-Methylheptane | ✅ Yes |
| 4-Methylheptane | ✅ Yes |
Chirality in alkanes like methylheptanes arises from the presence of asymmetric (chiral) carbon centers. If a carbon atom is bonded to four distinct groups and the molecule lacks symmetry, the molecule will be chiral. Only 3,3-dimethylheptane is achiral due to symmetrical methyl substituents on the same carbon.
