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ChemistryIsomerismJEE · NEET
  1. 1. Isomer Basics
  2. 2. Functional
  3. 3. Metamerism
  4. 4. Keto-Enol
  5. 5. Other Tautomers
  6. 6. D.B.E. & Counting
  7. 7. Geometrical
  8. 8. Chirality
  9. 9. No Chiral Carbon
  10. 10. Counting Stereoisomers
  11. 11. Racemates & ee
  12. 12. Fischer & D/L
  13. 13. Alkane Conformations
  14. 14. Cyclohexane
Isomerism · Part 14 of 14

Conformations of Cyclohexane

Baeyer assumed flat rings and predicted strain; real cyclohexane puckers into a strain-free chair. This part covers the chair and boat, axial and equatorial positions, and which chair a substituted cyclohexane prefers.

Builds on: Part 13 · Conformations of Ethane, Propane and Butane.

Conformations of CyclohexaneVideo coming soon
Baeyer strain theory

Cyclopropane 24°44′ (most strained, most reactive), cyclobutane 9°44′, cyclopentane 0°44′, cyclohexane −5°16′ if flat. Cyclohexane isn't flat, so it is strain-free.

Table of Baeyer deviations.
Deviation d for flat rings.
3D chair cyclohexane.
Axial H in coral, equatorial in blue.
Chair cyclohexane

6 axial, 6 equatorial

In the chair every C–C bond is staggered. Axial bonds point straight up or down; equatorial bonds point around the ring. A ring flip swaps them.

Energy

Chair → boat → chair

The boat is ≈ 29.9 kJ/mol above the chair (eclipsed bonds, flagpole H 1.83 Å apart); the barrier is 37.7–46.0 kJ/mol. Stability: chair > twist boat > boat > half chair.

Energy profile of cyclohexane conformations.
Schematic energy profile.
Chair with two equatorial methyls and a table.
cis-1,3-Dimethylcyclohexane, both methyls equatorial.
Disubstituted

Can both be equatorial?

1,2: trans e,e · 1,3: cis e,e · 1,4: trans e,e. The diequatorial isomer is the stable one, so for 1,3 it is cis.

Summary

Key formulas

Baeyer strain theory
cyclopropane 24°44′, cyclobutane 9°44′, cyclopentane 0°44′, cyclohexane −5°16′ Baeyer strain theory
Stability: chair > twist boat > boat > half chair Chair and boat cyclohexane
1,2: trans = ee/aa, cis = ae Disubstituted cyclohexanes
1,3: cis = ee/aa, trans = ae Disubstituted cyclohexanes
1,4: trans = ee/aa, cis = ae Disubstituted cyclohexanes
JEE-style question

Your turn

The most stable form of 1,3-dimethylcyclohexane:

(a)cis, diequatorial
(b)trans, diequatorial
(c)cis, diaxial
(d)trans, axial-equatorial
Show the answer and the traps

For 1,3, cis puts both methyls equatorial, with no 1,3-diaxial clashes. Option (a).

(b) can't exist: trans-1,3 is always a,e. (c) is cis in its worse chair. (d) always has one axial methyl.

Watch out

Common mistakes

Concluding cyclohexane is strained.It puckers into a chair with 109°28′ angles.
Assuming trans is always the more stable disubstituted isomer.For 1,3-disubstitution, cis is diequatorial and more stable.
Practice

Try these

1. Which is more stable, cis- or trans-1,4-dimethylcyclohexane?

trans: it can be diequatorial.

2. Which ring has the greatest Baeyer angle strain?

Cyclopropane (d = 24°44′).

3. In the more stable chair of tert-butylcyclohexane, where is the tert-butyl group?

Equatorial; it is essentially never axial.

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