Questions: Alkane Structure and Conformational Analysis

Question 1

A chemist claims to have isolated both the gauche and anti conformations of butane as separate, stable compounds at room temperature. What is wrong with this claim?

• A. The gauche conformation is too high in energy to exist at room temperature
• B. Conformations interconvert continuously via bond rotation at room temperature and cannot be isolated as separate compounds
• C. Only the anti conformation exists; gauche is purely a theoretical energy state
• D. Butane is a gas at room temperature, so neither conformation can be characterized structurally

Answer: (B)

This is the key distinction: conformations are NOT isomers. They interconvert by rotating around single bonds, with an energy barrier of only ~12 kJ/mol for eclipsed transitions — far less than the thermal energy available at room temperature (~2.5 kJ/mol per degree of freedom). Both gauche and anti conformations exist and interconvert millions of times per second. Only constitutional isomers and stereoisomers (which require bond-breaking to interconvert) can be isolated separately.

Question 2

In a Newman projection of butane viewed along the C2–C3 bond, which arrangement is at the LOWEST energy?

• A. Eclipsed, with the two methyl groups aligned directly behind each other (0° dihedral)
• B. Gauche, with the two methyl groups 60° apart
• C. Anti, with the two methyl groups 180° apart
• D. Eclipsed, with the two methyl groups offset by 120°

Answer: (C)

Anti places the two methyl groups at maximum separation (180°), minimizing both torsional strain (staggered arrangement) and steric strain (no van der Waals repulsion between methyls). This is the global energy minimum on the butane rotational potential energy diagram. The gauche conformation is also staggered but has methyls only 60° apart, introducing steric strain (~3.8 kJ/mol above anti). Eclipsed conformations are always energy maxima.

Question 3

The terms 'staggered' and 'anti' are synonymous in conformational analysis — both describe the lowest-energy arrangement of a molecule.

• T. True
• F. False

Answer: False

'Staggered' describes a class of conformations where the bonds on the front and back carbons are maximally offset (60° between them), minimizing torsional strain. Both anti and gauche are staggered conformations. Anti is ONE specific staggered arrangement — the one where the two largest groups are 180° apart. Gauche is another staggered arrangement where they are 60° apart. Anti is the most stable staggered conformation, but not all staggered conformations are anti.

Question 4

The energy difference between eclipsed and staggered conformations of ethane arises from repulsion between adjacent bond electron clouds (torsional strain), not from steric clashes between large groups.

• T. True
• F. False

Answer: True

Ethane has only hydrogen atoms attached to both carbons — there are no large groups to clash sterically. Yet eclipsed ethane is about 12 kJ/mol higher in energy than staggered ethane. This energy cost is torsional (Pitzer) strain: adjacent C–H bonds forced into the same plane experience repulsion between their electron clouds. Steric strain (van der Waals repulsion between bulky groups) becomes important in butane and larger alkanes, where methyl groups can be close enough to repel each other.

Question 5

Explain why gauche and anti are both classified as 'staggered' conformations, yet anti is lower in energy than gauche.

Model answer: Both gauche and anti have the front and back substituents offset by 60° relative to each other (staggered pattern), so both minimize torsional strain — neither has bonds eclipsing each other. The energy difference between them comes from steric strain: in gauche butane, the two methyl groups are only 60° apart, close enough that their van der Waals radii overlap, creating repulsion (~3.8 kJ/mol). In anti butane, the methyls are 180° apart — maximally separated — so there is no steric repulsion between them. Staggered describes the bond arrangement; anti vs gauche describes which groups are doing the staggering.

This distinction matters throughout organic chemistry: when a molecule is described as 'in a staggered conformation,' that does not mean it is in the lowest-energy state. You must specify whether it is anti or gauche. The preference for anti over gauche conformations influences ring conformations, stereochemical outcomes of reactions, and protein backbone geometry.