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 each of the following molecules is chiral or achiral, we must assess whether they contain a chiral center—a carbon atom bonded to four different groups—and lack a plane of symmetry.
Molecule-by-Molecule Chirality Analysis:
1. 3,3-Dimethylheptane – Achiral
- Structure: The carbon at position 3 has two identical methyl groups (-CH₃) attached.
- Analysis: Since this carbon is bonded to two of the same groups, it cannot be a chiral center.
- Conclusion: The molecule has no chiral centers and is achiral.
2. 2,3-Dimethylheptane – Chiral
- Structure: Methyl groups are on carbon 2 and carbon 3.
- Analysis:
- Carbon 2 is bonded to:
- A methyl group (-CH₃)
- A hydrogen
- An ethyl group (C1)
- The rest of the carbon chain (via C3)
- These are four different groups → Carbon 2 is a chiral center.
- Conclusion: Since there’s at least one chiral center and no internal symmetry, the molecule is chiral.
3. 2-Methylheptane – Chiral
- Structure: A methyl group is attached to carbon 2.
- Analysis:
- Carbon 2 is bonded to:
- A methyl group (-CH₃)
- A hydrogen
- A longer alkyl chain (C3–C7)
- A shorter chain (C1)
- All four substituents are different → Carbon 2 is chiral.
- Conclusion: The molecule is chiral.
4. 3-Methylheptane – Chiral
- Structure: A methyl group is on carbon 3.
- Analysis:
- Carbon 3 is bonded to:
- A methyl group
- A hydrogen
- A longer chain (C4–C7)
- A shorter chain (C1–C2)
- All groups are different → Carbon 3 is a chiral center.
- Conclusion: The molecule is chiral.
5. 4-Methylheptane – Achiral
- Structure: Methyl group on carbon 4 (the center of the chain).
- Analysis:
- Carbon 4 is bonded symmetrically to:
- Two similar carbon chains (C1–C3 and C5–C7).
- These chains are mirror images → Carbon 4 is not a chiral center.
- Conclusion: The molecule is achiral.
Final Answers:
- 3,3-Dimethylheptane – Achiral
- 2,3-Dimethylheptane – Chiral
- 2-Methylheptane – Chiral
- 3-Methylheptane – Chiral
- 4-Methylheptane – Achiral
Summary:
Chirality in organic molecules depends on the presence of a carbon atom bonded to four distinct groups and the absence of internal symmetry. Branched alkanes like methylheptanes can have chiral centers when the branching leads to unique substituent arrangements.