Indicate whether or not the following molecules are chiral.
1 3,3-dimethylheptane
2 2,3-dimethylheptane
3 2-methylheptane
4 3-methylheptane
5 4-methylheptane
The correct answer and explanation is :
1. 3,3-Dimethylheptane
Not Chiral.
This molecule has no chiral centers. In 3,3-dimethylheptane, the two methyl groups (–CH₃) are attached to the same carbon at position 3. This creates a symmetry, meaning this carbon is not attached to four different groups. A chiral center requires four distinct substituents. Thus, the molecule is achiral.
2. 2,3-Dimethylheptane
Chiral.
In 2,3-dimethylheptane, there is a chiral center at carbon 3. Carbon 3 is attached to a –CH₂CH₃ group, a –CH₃ group, a –CH₂CH₃ group, and a –H atom, making it attached to four different groups. This lack of symmetry creates chirality, meaning the molecule has a chiral center and can exist as two non-superimposable mirror images (enantiomers).
3. 2-Methylheptane
Not Chiral.
In 2-methylheptane, carbon 2 is attached to a –CH₃ group, a –CH₂CH₃ group, a –CH₂CH₃ group, and a –H atom. Despite the substitution, carbon 2 is not attached to four distinct groups because two of them are ethyl groups (–CH₂CH₃). As a result, this carbon is not a chiral center, and the molecule is achiral.
4. 3-Methylheptane
Not Chiral.
In 3-methylheptane, the chiral center would be carbon 3, but this carbon is attached to a –CH₂CH₃ group, a –CH₃ group, and two hydrogens (because of symmetry in the molecule). Because carbon 3 is not attached to four distinct groups, the molecule is achiral.
5. 4-Methylheptane
Not Chiral.
In 4-methylheptane, the chiral center would be carbon 4, but carbon 4 is attached to a –CH₂CH₃ group, a –CH₃ group, and two hydrogens. This makes it not chiral, as the carbon is not attached to four distinct groups, leading to a lack of chirality.
Summary:
- Chiral molecule: 2,3-dimethylheptane.
- Achiral molecules: 3,3-dimethylheptane, 2-methylheptane, 3-methylheptane, and 4-methylheptane.