Q.12 The total number of chair conformations possible for 1,2-dimethylcyclohexane is _____.
The correct answer is: the total number of chair conformations possible for 1,2-dimethylcyclohexane (considering both cis and trans, each with its ring-flip chair pair) is 4.
Concept Of 1,2-Dimethylcyclohexane
1,2-Dimethylcyclohexane is a disubstituted cyclohexane where two methyl groups are attached to adjacent carbons (C-1 and C-2) of the cyclohexane ring. For such systems, both configuration (cis or trans) and conformation (axial/equatorial arrangements in chair form) must be considered separately.
Cis And Trans Isomers Of 1,2-Dimethylcyclohexane
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In cis-1,2-dimethylcyclohexane, both methyl groups lie on the same side of the ring (both “up” or both “down”). In any chair conformation, one methyl is axial and the other is equatorial for the cis-1,2 isomer.
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In trans-1,2-dimethylcyclohexane, one methyl is “up” and the other is “down”, meaning they are on opposite sides of the ring. For the trans isomer, one chair conformation has both methyls equatorial and the ring-flipped form has both methyls axial.
Thus, there are two configurational isomers (cis and trans), each capable of existing in more than one chair shape due to ring flipping.
Chair Conformations: Axial And Equatorial
In the cyclohexane chair:
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Each carbon carries one axial bond (roughly vertical) and one equatorial bond (roughly around the ring equator).
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Ring flipping interconverts axial and equatorial positions: an axial substituent becomes equatorial and vice versa, but “up” remains “up” and “down” remains “down”.
This means that for a fixed configuration (cis or trans), two interconverting chair conformations are always possible due to ring flip.
Total Number Of Chair Conformations
Cis-1,2-Dimethylcyclohexane
For the cis isomer:
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In one chair, methyl at C-1 may be axial up and methyl at C-2 equatorial up (or both down analogously); this satisfies the “same side” requirement.
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On ring flip, these positions swap: the axial methyl becomes equatorial and the equatorial methyl becomes axial, still remaining cis (same side).
So cis-1,2-dimethylcyclohexane has 2 chair conformations related by ring flipping, both with one axial and one equatorial methyl.
Trans-1,2-Dimethylcyclohexane
For the trans isomer:
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In one chair, both methyl groups can occupy equatorial positions (one up, one down), giving a more stable diequatorial form.
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On ring flip, both methyl groups become axial, giving a less stable diaxial form, but still representing a distinct chair conformation.
Thus trans-1,2-dimethylcyclohexane also has 2 chair conformations: one diequatorial and one diaxial.
Final Count
Combining both stereoisomers:
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Cis: 2 chair conformations
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Trans: 2 chair conformations
Therefore, the total number of chair conformations possible for 1,2-dimethylcyclohexane is 4.
Stability Of The Different Conformations
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For cis-1,2-dimethylcyclohexane, both chair conformations have one axial and one equatorial methyl, so their steric strain and stability are similar; neither is strongly preferred.
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For trans-1,2-dimethylcyclohexane, the diequatorial chair is significantly more stable than the diaxial form because axial methyl groups suffer from 1,3-diaxial interactions.
From a conformational analysis viewpoint, the most stable overall conformer is the trans-1,2 diequatorial chair, but the exam question about “total number of chair conformations” counts all four distinct chairs (cis: 2, trans: 2).
