Draw the three constitutional isomers having molecular formula C7H14 that contain a ve-membered ring and two methyl groups as substituents. For each constitutional isomer that can have cis and trans isomers, draw the two stereoisomers.
The Correct Answer and Explanation is:
To satisfy the molecular formula C₇H₁₄, we must draw constitutional isomers containing:
- A five-membered ring (i.e., a cyclopentane core),
- Two methyl substituents,
- Saturation level appropriate for one ring (CₙH₂ₙ formula),
- Total of 7 carbon atoms and 14 hydrogen atoms.
✅ Three Constitutional Isomers of C₇H₁₄ with a Five-Membered Ring and Two Methyl Groups:
1. 1,2-Dimethylcyclopentane
- Two methyl groups on adjacent carbons (C-1 and C-2).
- Can have cis and trans stereoisomers.
Drawings:
- Cis-1,2-dimethylcyclopentane
- Trans-1,2-dimethylcyclopentane
2. 1,3-Dimethylcyclopentane
- Methyl groups on carbons 1 and 3 (alternating).
- Can have cis and trans stereoisomers.
Drawings:
- Cis-1,3-dimethylcyclopentane
- Trans-1,3-dimethylcyclopentane
3. 1,1-Dimethylcyclopentane
- Both methyl groups attached to the same carbon (C-1).
- No cis/trans isomerism possible (symmetrical at one carbon).
✏️ Explanation
The molecular formula C₇H₁₄ corresponds to a saturated hydrocarbon with one degree of unsaturation, which can be a ring or a double bond. Since the question specifies a five-membered ring, this satisfies the one degree of unsaturation through a cyclopentane ring.
With 5 carbons in the ring, we need 2 more carbon atoms to reach 7. These are added as methyl (–CH₃) groups, giving rise to various constitutional isomers depending on their positions.
- 1,2-Dimethylcyclopentane has methyl groups on adjacent carbons. Since the ring is not planar and has restricted rotation, this molecule exists in cis and trans forms, where:
- Cis means both methyls are on the same face of the ring,
- Trans means they are on opposite faces.
- 1,3-Dimethylcyclopentane places methyls on alternate ring carbons. Like 1,2-, this configuration also results in cis and trans isomers due to ring puckering and limited rotation.
- 1,1-Dimethylcyclopentane has both methyls on the same carbon, removing the possibility of stereoisomerism, as there’s no spatial difference for cis or trans designations.
These structures differ not only in methyl positions but also in connectivity, making them true constitutional isomers. The presence or absence of stereoisomerism depends on whether two different ring atoms are substituted and their relative positions.
These structures differ not only in methyl positions but also in connectivity, making them true constitutional isomers. The presence or absence of stereoisomerism depends on whether two different ring atoms are substituted and their relative positions.
