Chemistry · JEE Advanced only

Conformations of Butane (JEE Advanced)

Not in the JEE Main syllabus, but it is in JEE Advanced. Study time: about 2-3 hours. If you already understand ethane conformations well, butane adds one new concept (gauche strain) and requires practice drawing Newman projections with larger substituents. Spend time on energy diagrams and Boltzmann calculations..

Last updated: 26 October 2026

Direct answer

Conformations of butane refer to the different spatial arrangements that arise from rotation around the C2-C3 bond.

Conformations of butane refer to the different spatial arrangements that arise from rotation around the C2-C3 bond. Unlike ethane, butane has two bulky methyl groups that create additional steric interactions, leading to multiple distinct conformers with different energies. Understanding these conformations helps predict molecular stability and reactivity patterns.

Official syllabus

What the JEE Advanced syllabus says

“Conformations of ethane and butane (Newman projections only)”

Source: JEE Advanced 2026 Information Bulletin, Annexure-I. Closest JEE Main topic: Conformations: Sawhorse and Newman projections (of ethane).

JEE Main only covers ethane conformations, which has identical hydrogen substituents on both carbons. Butane introduces the concept of gauche interactions between methyl groups, a crucial idea for understanding stereochemistry and organic reaction mechanisms that Advanced tests.

Key ideas

What you must understand

Butane has four key conformations: anti, gauche, eclipsed, and fully eclipsed (syn)
Anti conformation is most stable because the two methyl groups are 180° apart, minimizing steric strain
Gauche conformation has methyl groups 60° apart, causing steric strain of about 3.8 kJ/mol relative to anti
Eclipsed conformations occur at 0°, 120°, and 240° dihedral angles and are energy maxima
Fully eclipsed (syn) conformation at 0° is the highest energy state due to direct methyl-methyl eclipsing
Energy difference between conformers determines their population ratio at equilibrium using Boltzmann distribution

Formulas

Formulas and key facts

Dihedral angle (θ): angle between C1 and C4 when viewed along C2-C3 bond
Anti: θ = 180°, most stable, energy = 0 (reference)
Gauche: θ = 60° or 300°, energy ≈ 3.8 kJ/mol above anti
Eclipsed (CH₃-H): θ = 120° or 240°, energy ≈ 16 kJ/mol above anti
Fully eclipsed (syn): θ = 0°, energy ≈ 19 kJ/mol above anti
Population ratio: N₁/N₂ = exp(−ΔE/RT)
Gauche strain per interaction ≈ 3.8 kJ/mol
H-H eclipsing strain ≈ 4 kJ/mol, CH₃-H eclipsing strain ≈ 6 kJ/mol

Avoid

Common mistakes

Confusing gauche (staggered, 60°) with eclipsed conformations. Gauche is a local minimum, eclipsed is a maximum.
Forgetting that butane has TWO gauche conformations (at 60° and 300°) that are mirror images and equal in energy.
Assuming anti is always the only populated conformer. At room temperature, gauche conformers also exist in significant amounts.
Drawing Newman projections incorrectly by placing the wrong groups on front vs back carbon. Always label which carbon you are viewing from.

Worked example

A JEE Advanced-level example

Question. For n-butane, the energy of the gauche conformation is 3.8 kJ/mol higher than the anti conformation. At 300 K, calculate the ratio of molecules in anti conformation to those in gauche conformation. (R = 8.314 J/mol·K)

Show solution

Use Boltzmann distribution: N_anti/N_gauche = exp(ΔE/RT), but we must account for the fact that there are TWO equivalent gauche conformations.
For one gauche vs anti: N_anti/N_one_gauche = exp(3800/(8.314 × 300)) = exp(1.52) = 4.57
Since there are two gauche conformations, total gauche population is doubled.
N_anti/N_total_gauche = 4.57/2 = 2.29
This means for every 2.29 molecules in anti, there is 1 molecule in gauche (combined).

Answer: N_anti : N_gauche(total) ≈ 2.3 : 1 or approximately 70% anti and 30% gauche at 300 K

Solved twice independently; both solutions agree.

FAQs

Frequently asked questions

Is Conformations of Butane in the JEE Main syllabus?

No. It is in the JEE Advanced 2026 syllabus only. The closest JEE Main topic is Conformations: Sawhorse and Newman projections (of ethane).

How long does Conformations of Butane take to prepare for JEE Advanced?

About 2-3 hours. If you already understand ethane conformations well, butane adds one new concept (gauche strain) and requires practice drawing Newman projections with larger substituents. Spend time on energy diagrams and Boltzmann calculations., if you already know the related JEE Main topic.