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:
Answers:
- 3,3-Dimethylheptane — Not chiral
- 2,3-Dimethylheptane — Chiral
- 2-Methylheptane — Chiral
- 3-Methylheptane — Chiral
- 4-Methylheptane — Not chiral
Explanation:
Chirality in organic molecules occurs when a carbon atom is attached to four different substituents, creating a chiral center or stereocenter. Such a carbon atom lacks a plane of symmetry, and the molecule cannot be superimposed on its mirror image. This is key to chirality.
Let’s analyze each molecule based on this:
1. 3,3-Dimethylheptane
This molecule has two identical methyl groups attached to carbon 3. Because carbon 3 has two identical groups, it cannot be a chiral center (it does not have four different groups). Also, no other carbon in the molecule has four distinct groups attached, so the molecule is achiral.
2. 2,3-Dimethylheptane
At carbon 2, there is a methyl substituent, and at carbon 3, there is a methyl substituent. Carbon 2 is attached to four different groups: a hydrogen, a methyl group, and two alkyl groups differing due to the position of methyl at carbon 3 and the rest of the chain. Similarly, carbon 3 also has four different groups. This molecule has at least one chiral center, so it is chiral.
3. 2-Methylheptane
The methyl substituent on carbon 2 creates a carbon with four different groups attached (a hydrogen, a methyl, and two different alkyl chains on either side). Hence, carbon 2 is a stereocenter, and the molecule is chiral.
4. 3-Methylheptane
Similarly, the methyl group on carbon 3 generates a carbon with four different substituents: hydrogen, methyl, and two different alkyl chains (parts of the heptane chain on either side). This makes carbon 3 a chiral center. So, the molecule is chiral.
5. 4-Methylheptane
The methyl substituent is exactly in the middle of the chain. Due to symmetry, carbon 4 does not have four different groups attached. The groups on either side of carbon 4 are identical, making this molecule achiral.
Summary:
Chirality arises from having at least one carbon with four different substituents. Molecules 2, 3, and 4 have such centers and are chiral. Molecules 1 and 5 lack such a center due to identical substituents or symmetry, so they are achiral.
