Difficulty: Intermediate | Prerequisites: Lewis structures, sigma bonds, sp3 hybridisation, basic stereochemistry.
This topic sits at the start of the stereochemistry sequence in organic chemistry. Understanding how molecules rotate around single bonds, and why some rotational arrangements are more stable than others, is the foundation for everything that follows: ring conformations, reaction selectivity, and even protein folding. You should already be comfortable drawing Lewis structures and know what a sigma bond is.
Conformational analysis is the study of how molecules arrange themselves in three-dimensional space through rotation around sigma bonds, and how those arrangements affect the molecule's energy. Staggered conformations (where substituents are spread apart) are lower in energy than eclipsed ones (where substituents line up). For butane, the lowest-energy arrangement is anti (methyl groups 180° apart) and the highest is totally eclipsed (methyl groups 0° apart, called syn).
Conformation
The different spatial arrangements of atoms in a molecule that result from rotation about sigma (σ) bonds. No bonds are broken or formed. Think of it as: different poses the same molecule can strike by twisting around its single bonds.
Conformer
A specific conformation of a molecule, considered as a stereoisomer related to other conformers by facile σ-bond rotation. Not isolable under ambient conditions because interconversion is fast. In simple terms, these are the individual "snapshots" you can draw as a molecule rotates.
Configuration
The permanent spatial arrangement of atoms that can only be changed by breaking and re-forming bonds. Conformations change freely at room temperature; configurations do not.
Newman projection
A way of drawing a molecule looking straight down a C-C bond. The front carbon is shown as a dot (centre of the circle) and the rear carbon as a circle. This projection makes it easy to see the dihedral angle between substituents.
Sawhorse projection
A perspective drawing showing the C-C bond at an angle, with all substituents visible. Less commonly used than Newman projections for conformational analysis, but useful for seeing the three-dimensional shape.
Dihedral angle (torsion angle)
The angle between two substituents on adjacent carbons when viewed along the C-C bond axis (as in a Newman projection). Measured from 0° (eclipsed) to 180° (anti).
Torsional strain (eclipsing strain)
The increase in potential energy that occurs when bonds on adjacent carbons are eclipsed (aligned, dihedral angle = 0°, 120°, or 240°). In simple terms, this is the energy penalty for having bonds line up with each other.
Steric strain (van der Waals strain)
Repulsion caused by the overlapping electron clouds of two substituents that are too close together. Larger groups cause more steric strain. Think of it as: bulky groups bumping into each other.
Eclipsed conformation
A conformation in which substituents on adjacent carbons are aligned (directly behind one another in a Newman projection). Higher energy due to torsional strain.
Staggered conformation
A conformation in which substituents on adjacent carbons are offset by 60° in a Newman projection. Lower energy because torsional strain is minimised.
Anti conformation
The staggered conformation in which the two largest substituents are 180° apart (opposite sides of the Newman projection). This is the global energy minimum for butane.
Gauche conformation
The staggered conformation in which the two largest substituents are 60° apart. Higher in energy than anti because of steric strain between the nearby groups, but still a local energy minimum.
Syn (totally eclipsed) conformation
The eclipsed conformation in which the two largest substituents are 0° apart (directly behind each other). This is the global energy maximum for butane, with both torsional and steric strain at their worst.
Three drawing styles are used to show conformations:
Wedge and dash: Standard 3D representation. Wedges come towards you, dashes go away.
Sawhorse projection: Shows the C-C bond at an angle with all substituents drawn out.
Newman projection: The most useful for conformational analysis. You look directly down the C-C bond. The front carbon is a dot, the rear carbon is a circle. Substituents on the front carbon radiate from the centre; substituents on the rear carbon appear at the edges of the circle.
Ethane (CH3-CH3) has two extreme conformations when viewed along the C-C bond:
Staggered: All H atoms on the front carbon sit between the H atoms on the rear carbon (60° dihedral angles). This is the energy minimum.
Eclipsed: All H atoms on the front carbon line up directly behind the H atoms on the rear carbon (0° dihedral angles). This is the energy maximum.
The energy difference between eclipsed and staggered ethane is 12.6 kJ/mol (3.0 kcal/mol). This barrier comes entirely from torsional strain.
This barrier is small enough that rotation is essentially free at room temperature. Ethane does not "lock" into one conformation.
As you rotate from 0° to 360°, the energy rises and falls in a smooth wave: eclipsed at 0°, 120°, 240°, 360° (energy maxima); staggered at 60°, 180°, 300° (energy minima). All three staggered conformations of ethane are equivalent, and all three eclipsed conformations are equivalent.
Butane (CH3-CH2-CH2-CH3) is more interesting because the two methyl groups on C2 and C3 create different types of staggered and eclipsed conformations. Looking along the C2-C3 bond:
Energy minima (staggered conformations):
Anti (180°): Methyl groups are 180° apart, as far from each other as possible. No torsional strain, no steric strain. This is the global energy minimum. Energy: 0 kJ/mol (reference point, lowest).
Gauche (60° and 300°): Methyl groups are 60° apart. No torsional strain (still staggered), but steric strain from the proximity of the two methyl groups. Energy: 3.8 kJ/mol (0.9 kcal/mol) above anti. This is a local energy minimum.
Energy maxima (eclipsed conformations):
Eclipsed (120° and 240°): One methyl-hydrogen eclipsing interaction plus two H-H eclipsing interactions. Energy: 15 kJ/mol (3.6 kcal/mol) above anti. Local energy maximum.
Totally eclipsed / syn (0° and 360°): Both methyl groups directly behind each other. Maximum torsional strain plus maximum steric strain. Energy: ~21 kJ/mol (5 kcal/mol) above anti. This is the global energy maximum.
The key lesson from butane: steric strain adds on top of torsional strain. The totally eclipsed conformation is higher in energy than a regular eclipsed one because the methyl groups are larger than hydrogen atoms and their electron clouds repel each other more strongly.
Molecule | Conformation | Dihedral Angle | Energy Above Minimum | Strain Type |
|---|---|---|---|---|
Ethane | Staggered | 60° | 0 kJ/mol | None |
Ethane | Eclipsed | 0° | 12.6 kJ/mol (3.0 kcal/mol) | Torsional |
Butane | Anti | 180° | 0 kJ/mol | None |
Butane | Gauche | 60° | 3.8 kJ/mol (0.9 kcal/mol) | Steric |
Butane | Eclipsed | 120° | 15 kJ/mol (3.6 kcal/mol) | Torsional + steric |
Butane | Totally eclipsed (syn) | 0° | ~21 kJ/mol (5 kcal/mol) | Torsional + steric |
A useful rule of thumb: each H-H eclipsing interaction in ethane contributes about 4.2 kJ/mol of torsional strain (12.6 / 3 pairs).
Muscle contraction is driven by conformational changes in proteins. Myosin (a motor protein) undergoes a series of shape changes powered by ATP hydrolysis, pulling actin filaments and shortening the muscle fibre. The same principles of σ-bond rotation and energy barriers that govern small molecules like butane scale up to govern the function of biological macromolecules.
Drug design also relies on conformational analysis. Many drugs must adopt a specific three-dimensional shape to fit into their target receptor, and understanding which conformations are energetically favoured helps chemists design molecules that spend more time in the "active" shape.
Students often confuse conformations with configurations. Conformations interconvert freely through bond rotation at room temperature. Configurations require bond breaking to change. If you can get from one arrangement to the other just by rotating a single bond, they are conformers.
Students sometimes think the gauche conformation has torsional strain. It does not. Gauche is staggered, so it has zero torsional strain. Its higher energy relative to anti comes entirely from steric strain (the methyl groups are close together).
Students often assume that eclipsed always means the highest energy. In butane, a regular eclipsed conformation (120°) is lower in energy than the totally eclipsed (syn) conformation (0°). The distinction matters.
Students sometimes think molecules "prefer" one conformation and stay locked in it. At room temperature, rotation around C-C single bonds is rapid and continuous. The molecule samples all conformations; it simply spends more time in the lower-energy ones.
⚠️ You will almost certainly be asked to draw Newman projections for a given molecule and rank the conformers from most stable to least stable. Practise this until it is automatic.
⚠️ Know the difference between torsional strain and steric strain, and be able to identify which type(s) are present in any given conformation.
⚠️ Be able to reproduce the butane energy diagram from memory, including approximate energy values at each dihedral angle and the names of each conformation.
⚠️ When ranking conformers with different substituents (e.g. a molecule with both a Cl and a CH3), remember that steric strain depends on the size of the substituent. Cl is smaller than CH3 for steric purposes.
True or false: Conformers can be isolated and separated at room temperature.
Fill in the blank: The energy barrier to rotation in ethane is ______ kJ/mol.
True or false: The gauche conformation of butane has torsional strain.
Fill in the blank: In a Newman projection, the front carbon is represented by a ______ and the rear carbon by a ______.
True or false: The anti conformation of butane is higher in energy than the gauche conformation.
Answers: 1. False (they interconvert too rapidly). 2. 12.6 kJ/mol. 3. False (gauche is staggered; its extra energy is from steric strain). 4. Dot; circle. 5. False (anti is the global minimum, lower than gauche).
Q: Draw the Newman projection of butane looking along the C2-C3 bond in its most stable conformation. What is this conformation called?
A: The most stable conformation is anti. In the Newman projection, the two methyl groups are at 180° to each other (one at the top of the front carbon, one at the bottom of the rear carbon, or equivalent positions). All substituents are staggered.
Q: Rank the following butane conformations from most stable to least stable: gauche, anti, eclipsed, totally eclipsed.
A: Anti (most stable) > gauche > eclipsed > totally eclipsed (least stable).
Q: A molecule of 2-chlorobutane is viewed along the C2-C3 bond. How many distinct staggered conformations are there, and how many distinct eclipsed conformations?
A: Three distinct staggered conformations and three distinct eclipsed conformations, for a total of six. Because the substituents on C2 are not all identical (H, Cl, CH3), each 60° rotation produces a different arrangement.
Q: Explain why the totally eclipsed (syn) conformation of butane is higher in energy than a regular eclipsed conformation.
A: In the totally eclipsed conformation, both methyl groups are directly behind each other (0° dihedral angle). This produces torsional strain (from eclipsing) plus severe steric strain (from the electron cloud overlap of two bulky methyl groups). In a regular eclipsed conformation (120°), a methyl eclipses a hydrogen, which causes less steric strain because hydrogen is much smaller.
Q: What type(s) of strain are present in the gauche conformation of butane?
A: Steric strain only. The gauche conformation is staggered, so there is no torsional strain. The 3.8 kJ/mol energy cost relative to anti comes from the proximity of the two methyl groups at 60°.
This material leads directly into the conformational analysis of cycloalkanes (Part 2), where the same types of strain (torsional, steric, and angle strain) determine ring stability and chair conformations. The gauche interaction you learn here reappears as the 1,3-diaxial interaction in cyclohexane.
Conformational preferences also matter in reaction mechanisms: nucleophilic substitution and elimination reactions are sensitive to the geometry of the reacting bonds, which depends on conformation.
Conformational analysis, Newman projection, sawhorse projection, wedge and dash, dihedral angle, torsion angle, torsional strain, eclipsing strain, steric strain, van der Waals strain, staggered conformation, eclipsed conformation, anti conformation, gauche conformation, syn conformation, totally eclipsed, sigma bond rotation, free rotation, ethane conformations, butane conformations, energy diagram, potential energy vs dihedral angle, conformers vs configurations, Klein Ch. 4, organic chemistry CHEM 101