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 the given molecules are chiral, we need to check if they contain a chiral center. A chiral center (stereogenic center) is a carbon atom bonded to four different groups, leading to non-superimposable mirror images (enantiomers).
Analysis of Each Molecule
- 3,3-Dimethylheptane
- The carbon at position 3 has two identical methyl groups (-CH₃), meaning it does not have four different groups.
- No chiral center → Achiral ✅
- 2,3-Dimethylheptane
- The carbon at position 2 is bonded to a methyl (-CH₃), ethyl (-C₂H₅), and a longer hydrocarbon chain, making it potentially chiral.
- The carbon at position 3 is bonded to two methyl groups, making it not chiral.
- Presence of a chiral center at C-2 → Chiral ✅
- 2-Methylheptane
- The carbon at position 2 is bonded to a methyl (-CH₃), an ethyl (-C₂H₅), and two different hydrocarbon chains.
- This results in four different groups around C-2, making it a chiral center.
- Chiral ✅
- 3-Methylheptane
- The carbon at position 3 is bonded to a methyl (-CH₃), an ethyl (-C₂H₅), but has two identical hydrocarbon chains (one shorter and one longer but not sufficiently different).
- No chiral center → Achiral ✅
- 4-Methylheptane
- The carbon at position 4 has two identical hydrocarbon chains, meaning it does not have four different groups.
- No chiral center → Achiral ✅
Final Answer:
- Chiral: 2,3-Dimethylheptane, 2-Methylheptane
- Achiral: 3,3-Dimethylheptane, 3-Methylheptane, 4-Methylheptane
Conclusion
Chirality arises when a carbon atom has four distinct groups. In 2,3-dimethylheptane and 2-methylheptane, at least one carbon satisfies this condition, making them chiral. The other molecules lack a chiral center, making them achiral.