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 look for chirality centers (also called stereocenters or asymmetric carbons). A carbon is chiral if it is bonded to four different groups, and the molecule has no plane of symmetry.
Molecule 1: 3,3-Dimethylheptane
- Structure: Two methyl groups are attached to carbon 3.
- Analysis: Carbon 3 has two identical groups (methyl), so it cannot be a chiral center. There is no other carbon with four different groups.
- ✅ Conclusion: Not chiral
Molecule 2: 2,3-Dimethylheptane
- Structure: A methyl group on carbon 2 and one on carbon 3 of heptane.
- Analysis: Carbon 2 is bonded to:
- A hydrogen
- A methyl group
- A –CH(CH₃)–(CH₂)₃–CH₃ group (toward C3)
- A –CH₂–CH₂–CH₂–CH₃ group (toward C1) These four groups are different ⇒ chiral center at carbon 2.
- Carbon 3 may also be a chiral center, depending on symmetry.
- ✅ Conclusion: Chiral
Molecule 3: 2-Methylheptane
- Structure: A methyl group on carbon 2.
- Analysis: Carbon 2 is bonded to:
- A methyl group
- A hydrogen
- A propyl group (toward C1)
- A butyl group (toward C3) All four are different ⇒ chiral center at carbon 2.
- ✅ Conclusion: Chiral
Molecule 4: 3-Methylheptane
- Structure: A methyl group on carbon 3.
- Analysis: Carbon 3 is bonded to:
- A methyl group
- A hydrogen
- An ethyl group (toward C2, C1)
- A propyl group (toward C4, C5) All four are different ⇒ chiral center at carbon 3.
- ✅ Conclusion: Chiral
Molecule 5: 4-Methylheptane
- Structure: A methyl group on carbon 4.
- Analysis: Carbon 4 is bonded to:
- A methyl group
- A hydrogen
- An ethyl group (toward C3, C2)
- An ethyl group (toward C5, C6) Two side chains are identical ⇒ not chiral.
- ✅ Conclusion: Not chiral
Final Answers:
- 3,3-Dimethylheptane – Not chiral
- 2,3-Dimethylheptane – Chiral
- 2-Methylheptane – Chiral
- 3-Methylheptane – Chiral
- 4-Methylheptane – Not chiral
Summary of Key Concepts:
- Chirality requires a carbon bonded to four different substituents.
- Symmetry or duplicate groups prevent chirality.
- In branched alkanes, carefully compare the branches to assess uniqueness.