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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 13 of 14

Conformations of Ethane, Propane and Butane

Rotating a single bond changes a molecule's shape without breaking anything. This part draws conformations in sawhorse and Newman projections, ranks them by energy, and uses dipole moments to find the anti/gauche mix.

Builds on: Part 1 · Isomers: Chain, Position and Ring-Chain.

Conformations of Ethane, Propane and ButaneVideo coming soon
Representations

Sawhorse and Newman

The dihedral angle θ is the angle between a front and a back C–H. θ = 0°: eclipsed; 60°: staggered; in between: skew. Newman: front carbon a point, back carbon a circle.

Sawhorse and Newman projections of ethane.
Staggered ethane in both projections.
Energy profile of ethane.
Energy vs dihedral angle.
Ethane

A 12.5 kJ/mol barrier

Staggered is most stable and eclipsed least; the barrier is about 12.5 kJ/mol (3 kcal/mol), so the conformers interconvert at room temperature and can't be isolated. Propane: ≈ 14.2 kJ/mol.

n-Butane

anti > gauche > partly > fully eclipsed

About C2–C3: anti (180°) 0, gauche (60°, 300°) 3.8 kJ/mol, partially eclipsed (120°, 240°) 14.6, fully eclipsed (0°) 18.4.

Energy profile of n-butane.
Energy vs the CH₃–CH₃ dihedral angle.
Gauche ethane-1,2-diol.
The dashed line is the intramolecular H-bond.
Exception

When gauche wins

In ethane-1,2-diol the two OH groups can H-bond only in the gauche form, so gauche is the most stable conformation. 2-Fluoroethanol behaves the same way.

Dipole moment

Anti and gauche fractions

μ = μa xa + μg xg with xa + xg = 1 (the book's form). For 1,2-dichloroethane, μ = 1.0 D, μanti = 0 and μgauche = 5.55 D give xg = 0.18 and xa = 0.82.

Newman projections of anti and gauche dichloroethane.
Anti and gauche 1,2-dichloroethane.
Summary

Key formulas

Stability: anti > gauche > partially eclipsed > fully eclipsed n-Butane
Mole fraction of anti and gauche conformers
Mole fraction of anti and gauche conformers
JEE-style question

Your turn

The most stable conformation of ethane-1,2-diol is:

(a)anti
(b)gauche
(c)fully eclipsed
(d)partially eclipsed
Show the answer and the traps

Only in the gauche form are the two OH groups close enough for an intramolecular hydrogen bond. Option (b).

Anti wins for butane, but here the H-bond outweighs the crowding. The eclipsed forms are never the most stable.

Watch out

Common mistakes

Expecting anti to always win.Groups that attract (internal H-bond) make gauche the most stable.
Forgetting the anti term vanishes.A symmetric anti conformer has μ = 0.
Practice

Try these

1. Most stable conformation of n-butane?

Anti (180°).

2. μobs = 1.2 D, μgauche = 4.0 D, μanti = 0. Percentage of anti?

xg = 0.3, so 70 % anti.

3. Energy difference between staggered and eclipsed ethane?

About 12.5 kJ/mol (3 kcal/mol).

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