Difficulty: Introductory-Intermediate | Prerequisites: General chemistry (bonding, molecular geometry, energy diagrams). Familiarity with Lewis structures and sp3 hybridisation helps.
Conformational analysis is the study of how rotating around single (sigma) bonds changes a molecule's shape, energy, and stability. This chapter sits right at the start of organic chemistry's 3D thinking: you already know how to draw structures flat, and now you learn why the same molecule can look and behave differently depending on which rotational arrangement it adopts. The ideas here, particularly staggered vs eclipsed forms and axial vs equatorial positions, come back constantly in reaction mechanisms and stereochemistry later in the course. If you are comfortable with sp3 hybridisation and tetrahedral geometry from general chemistry, you have everything you need to start.
Molecules with single bonds can rotate into different spatial arrangements called conformations. Some conformations are lower in energy (staggered, anti) and some are higher (eclipsed, gauche). The energy differences come from torsional strain (eclipsing of bonds) and steric strain (atoms bumping into each other), and these same principles explain why bigger substituents prefer equatorial positions on cyclohexane rings.
Conformation
The different spatial arrangements a molecule can adopt through rotation about single bonds, without breaking any bonds.
In simple terms, think of it as the different poses a molecule can strike just by spinning parts of itself around.
Conformational analysis
The study of how a molecule's shape (conformation) affects its energy, structure, and properties.
In simple terms, it is the process of figuring out which pose is most comfortable for the molecule.
Torsion angle (dihedral angle)
The angle between two bonds on adjacent atoms, measured as the difference between the planes of an H-C-C-H (or C-C-C-C) unit. Ranges from 0 to 360 degrees.
Think of it as how far you have twisted one end of the molecule relative to the other, like wringing out a cloth.
Staggered conformation
A conformation in which the bonds on one carbon bisect the bond angles on the adjacent carbon when viewed end-on (Newman projection). Torsion angles are 60 or 180 degrees.
In simple terms, the groups are as far apart as they can get, so this is the low-energy, comfortable arrangement.
Eclipsed conformation
A conformation in which the bonds on adjacent carbons are aligned directly behind one another (torsion angle = 0 degrees). This is the highest-energy arrangement.
Think of it as everyone trying to stand in the same spot, which is crowded and unstable.
Anti conformation
A staggered conformation where two particular substituents (e.g. the two methyl groups in butane) are 180 degrees apart. The lowest-energy staggered form.
Gauche conformation
A staggered conformation where two particular substituents are 60 degrees apart. Higher in energy than anti because of mild steric crowding.
Torsional strain
The increase in energy that results when bonds on adjacent atoms eclipse one another (torsion angle deviates from 60-degree multiples). In ethane, the total torsional strain of the eclipsed form is 12 kJ/mol across three bond pairs, so each eclipsed H-H pair contributes roughly 4 kJ/mol.
Think of it as the energetic cost of forcing bonds to line up rather than stagger.
Steric strain (steric energy)
A general term for all the destabilising energy in a molecule from atoms or groups being too close together. Includes contributions from bond stretching, angle bending, torsion, and nonbonded repulsions.
Van der Waals strain (steric hindrance)
The repulsive force between atoms or groups that are not bonded to each other but are close enough that their electron clouds overlap. In butane's gauche conformation, for example, the methyl groups experience 3.8 kJ/mol of van der Waals strain relative to anti.
Newman projection
A way of drawing a molecule looking straight down a C-C bond. The front carbon is a dot, the back carbon is a circle. Bonds radiate outward from each. This representation makes it easy to see torsion angles and eclipsing at a glance.
Staggered vs eclipsed
In the staggered form, each C-H bond on one carbon bisects the H-C-H angle on the adjacent carbon. Maximum separation between bonds.
In the eclipsed form, the C-H bonds on adjacent carbons line up directly behind one another. Minimum separation.
The staggered form is 12 kJ/mol (2.9 kcal/mol) more stable than the eclipsed form.
Why staggered wins
Repulsions between bonding electrons on adjacent atoms destabilise the eclipsed conformation.
Better electron delocalisation (hyperconjugation) stabilises the staggered conformation.
Torsional strain in ethane
The 12 kJ/mol energy difference is spread across three eclipsed bond pairs, so each pair contributes about 4 kJ/mol of torsional strain.
Any conformation that is not perfectly staggered (torsion angle not a multiple of 60 degrees) carries some torsional strain. Staggered = zero torsional strain. Eclipsed = maximum.
Potential energy diagram
The energy plotted against torsion angle is a sine curve with a period of 120 degrees.
Rotating 120 degrees from one staggered position brings you to the next staggered position, passing through an eclipsed maximum halfway.
Twice the amplitude of the curve = 12 kJ/mol.
The eclipsed form is the transition state for converting between staggered conformations.
Practical consequence
12 kJ/mol is a small barrier. At room temperature, rotation is very fast.
At any instant, virtually all ethane molecules are in or near a staggered conformation.
Butane introduces a new complication: the substituents on the central C-C bond are no longer all hydrogens. The two methyl groups can be gauche or anti to each other.
Two staggered conformations
Anti: methyl groups are 180 degrees apart. This is the most stable conformation of butane.
Gauche: methyl groups are 60 degrees apart. 3.8 kJ/mol less stable than anti.
Why anti is more stable
In the gauche form, hydrogens on the two methyl groups are close enough to repel each other. This is van der Waals strain (also called steric hindrance).
The anti form places the bulky methyl groups as far apart as possible.
Eclipsed conformations in butane
Butane has two distinct eclipsed forms. The highest-energy eclipsed form has the two methyl groups directly aligned (torsion angle = 0 degrees). A lower-energy eclipsed form has each methyl eclipsed with a hydrogen (torsion angle = 120 or 240 degrees).
Potential energy diagram
Similar sine-curve shape to ethane, but the peaks and valleys are no longer equal.
The global minimum is anti (180 degrees). Local minima are gauche (60 and 300 degrees).
The highest barrier occurs when the two methyls eclipse each other.
Longer unbranched carbon chains are most stable in their all-anti conformation, where every consecutive pair of carbons is anti. In practice, appreciable amounts of gauche conformations are present at room temperature because the energy differences are modest.
Steric energy equation
The total steric energy of a molecule is the sum of four components:
Bond stretching: strain from bond lengths deviating from their ideal values.
Angle bending: strain from bond angles expanding or contracting away from ideal (e.g. 109.5 degrees for sp3).
Torsional: strain from dihedral angles deviating from ideal staggered values.
Nonbonded: attractive or repulsive forces between atoms that are not directly bonded to each other.
All four contribute to what is broadly called steric strain or strain energy. Computational methods (molecular mechanics) add these terms together to predict which conformation of a large molecule is most stable.
Steric energy equation
E(steric) = E(bond stretch) + E(angle bend) + E(torsion) + E(nonbonded)
Key energy values to know
Ethane: eclipsed is 12 kJ/mol (2.9 kcal/mol) above staggered.
Per eclipsed H-H pair in ethane: approximately 4 kJ/mol.
Butane: gauche is 3.8 kJ/mol above anti.
Butane: the highest eclipsed form (CH3 eclipsing CH3) is the global maximum on the energy diagram.
Potential energy diagrams
Ethane: symmetric sine curve, period 120 degrees. All three maxima are equal, all three minima are equal.
Butane: asymmetric curve. One global minimum (anti, 180 degrees), two local minima (gauche, 60 and 300 degrees), one global maximum (methyl-methyl eclipsed, 0 degrees), two smaller maxima (methyl-hydrogen eclipsed, 120 and 240 degrees).
Conformational preferences directly affect the physical properties of polymers (plastics, fibres). Polyethylene, for example, is most stable in an all-anti, zigzag chain, which is why it packs into crystalline sheets that give the material its strength. Pharmaceutical chemists use conformational analysis to predict how drug molecules will fold and bind to receptor sites, because a molecule locked in the wrong conformation simply will not fit.
Students often think eclipsed and staggered are two fixed structures. They are not. They are points on a continuous rotation, and the molecule passes through every angle in between. Eclipsed and staggered are just the energy extremes.
Students sometimes assume gauche and eclipsed are the same thing. Gauche is a type of staggered conformation (60-degree torsion angle) and is a local energy minimum. Eclipsed is an energy maximum.
A common mistake is thinking that van der Waals strain and torsional strain are the same. Torsional strain comes from bond-bond repulsion in eclipsed conformations. Van der Waals strain comes from nonbonded atoms being too close, which can happen even in staggered forms (e.g. gauche butane).
Some students believe rotation around single bonds is restricted at room temperature. For ethane and butane, the barriers are low enough that rotation is essentially free at 25 degrees Celsius.
⚠️ Be able to draw Newman projections for ethane and butane in all key conformations (anti, gauche, eclipsed) and label the torsion angles.
⚠️ Know the energy difference between staggered and eclipsed ethane (12 kJ/mol) and between anti and gauche butane (3.8 kJ/mol). These numbers appear frequently.
⚠️ Be able to sketch a potential energy diagram for rotation about the C2-C3 bond of butane, label all maxima and minima, and rank them by energy.
⚠️ Understand which type of strain (torsional vs van der Waals) is responsible for each energy difference.
⚠️ Know the four components of steric energy (bond stretch, angle bend, torsion, nonbonded). Exams sometimes ask you to identify which component is at play in a given scenario.
True or False: The eclipsed conformation of ethane is a transition state, not a stable intermediate. (True)
Fill in the blank: In butane, the anti conformation is ______ kJ/mol more stable than the gauche conformation. (3.8)
True or False: Gauche is a type of eclipsed conformation. (False, gauche is staggered)
Fill in the blank: Each eclipsed H-H pair in ethane contributes approximately ______ kJ/mol of torsional strain. (4)
True or False: At room temperature, rotation about the C-C bond in ethane is essentially free. (True)
Q: Draw the Newman projection of butane looking down the C2-C3 bond in its most stable conformation. What is the torsion angle between the two methyl groups?
A: The most stable conformation is anti. The two methyl groups are 180 degrees apart. In the Newman projection, the front carbon has CH3 at the top with H at 4 o'clock and H at 8 o'clock. The back carbon has CH3 at the bottom with H at 10 o'clock and H at 2 o'clock.
Q: Explain why the eclipsed conformation of ethane is higher in energy than the staggered conformation. Name the specific type of strain involved.
A: The eclipsed form is higher in energy because of torsional strain, which arises from repulsions between the bonding electrons of C-H bonds on adjacent carbons when they are aligned. The staggered form also benefits from better electron delocalisation (hyperconjugation).
Q: Butane has two different eclipsed conformations. Which one is higher in energy, and why?
A: The eclipsed conformation where the two methyl groups are directly aligned (torsion angle = 0 degrees) is higher in energy. It has both torsional strain and significant van der Waals strain from the bulky methyl groups being forced into close proximity.
Q: What are the four components of steric energy? Give a one-sentence description of each.
A: Bond stretching (strain from bonds being longer or shorter than ideal), angle bending (strain from bond angles deviating from ideal, e.g. 109.5 degrees), torsional (strain from dihedral angles that are not at ideal staggered values), and nonbonded (attractive or repulsive forces between atoms not directly bonded to each other).
Q: At room temperature, roughly what fraction of ethane molecules are in the staggered conformation at any given instant?
A: Virtually all of them. The 12 kJ/mol barrier is significant enough that the eclipsed form is energetically unfavourable, so the vast majority of molecules sit in or near the staggered form at any moment.
The idea that molecules prefer low-energy conformations connects directly to Chapter 4's discussion of stereochemistry and chirality: the 3D arrangement of atoms matters, and conformational preferences determine which stereoisomers are more stable. Torsional and steric strain reappear in the cycloalkane sections of this same chapter (Parts 2 and 3 of these notes), where they explain why cyclohexane adopts the chair form and why substituents prefer equatorial positions. Later in the course, conformational analysis is essential for understanding reaction mechanisms, because reactants must adopt specific conformations for elimination reactions (E2) and substitution reactions to proceed.
Conformation, conformational analysis, conformer, rotational isomer, rotamer, Newman projection, sawhorse projection, wedge-dash projection, torsion angle, dihedral angle, staggered, eclipsed, anti, gauche, torsional strain, steric strain, steric energy, van der Waals strain, steric hindrance, ethane conformation, butane conformation, higher alkanes, potential energy diagram, activation energy, hyperconjugation, bond rotation, sigma bond rotation, organic chemistry chapter 3