Source: CHEM 2301 Learning Objectives, Ch. 3
Tags: conformational analysis, Newman projection, wedge-dash, sawhorse, staggered, eclipsed, gauche, anti, torsional strain, steric strain, angle strain, ring strain, cyclohexane, chair conformation, boat conformation, axial, equatorial, 1,3-diaxial interaction, ring flip
Difficulty: Intermediate | Prerequisites: Chapters 1-2 (Lewis structures, bond-line structures, hybridisation, sp³ geometry, tetrahedral bond angles).
Chapter 3 is about what molecules look like in three dimensions when you account for rotation around single bonds. Organic molecules are not rigid; their sigma bonds rotate freely, and each rotational arrangement (conformation) has a different energy. This chapter teaches you to draw and analyse these conformations for both open-chain alkanes and cycloalkanes, with special emphasis on cyclohexane chair conformations. Understanding conformational preferences is essential for predicting reactivity in substitution and elimination reactions later in the course.
Conformational analysis examines the 3D arrangements a molecule can adopt by rotating around single bonds. You need to draw Newman projections and chair conformations, identify sources of strain (torsional, steric, angle, ring), and predict which conformations are most stable. For cyclohexane, the chair is the most stable form, and larger substituents prefer the equatorial position.
Conformation
A specific 3D arrangement of atoms in a molecule that results from rotation around a single bond. Conformations interconvert rapidly at room temperature; they are not different molecules.
Staggered conformation
A conformation in which bonds on adjacent carbons are as far apart as possible (60° apart in a Newman projection). Lower energy than eclipsed.
Eclipsed conformation
A conformation in which bonds on adjacent carbons are directly aligned (0° dihedral angle in a Newman projection). Higher energy due to torsional strain.
Anti conformation
The staggered conformation where the two largest substituents on adjacent carbons are 180° apart. This is the lowest-energy conformation for a simple substituted ethane.
Gauche conformation
The staggered conformation where the two largest substituents on adjacent carbons are 60° apart. Higher in energy than anti due to steric strain, but lower than any eclipsed form.
Newman projection
A way of viewing a molecule by looking straight down a C-C bond. The front carbon is drawn as a point (with three bonds radiating out), and the back carbon as a circle (with three bonds sticking out from the circle's edge). Useful for analysing rotational conformations.
Torsional strain (eclipsing strain)
The increase in energy caused by bonds on adjacent atoms aligning in an eclipsed arrangement. Arises from repulsion between bonding electron pairs that are forced into close proximity.
Steric strain (van der Waals strain)
The increase in energy when atoms or groups are forced closer together than their van der Waals radii allow. Think of it as atoms physically bumping into each other.
Angle strain
The increase in energy when bond angles are compressed or expanded away from their ideal value. Significant in small rings: cyclopropane forces sp³ carbons into 60° angles instead of the ideal 109.5°.
Ring strain
The combined destabilisation of a cyclic molecule from angle strain and torsional strain. Cyclopropane and cyclobutane have the most ring strain; cyclohexane has essentially none.
Chair conformation
The most stable conformation of cyclohexane. All bond angles are close to 109.5° and all adjacent C-H bonds are staggered, so there is no angle strain and no torsional strain.
Boat conformation
A less stable conformation of cyclohexane with significant torsional strain (eclipsed bonds along the "sides" of the boat) and a flagpole interaction (steric strain between the two hydrogens that point inward at the "bow" and "stern").
Axial position
Bonds on a cyclohexane chair that point straight up or straight down, parallel to the axis of the ring. Three point up, three point down, alternating around the ring.
Equatorial position
Bonds on a cyclohexane chair that point roughly outward from the ring, slightly above or below the plane. Larger substituents prefer equatorial because it avoids 1,3-diaxial interactions.
1,3-Diaxial interaction
Steric strain between an axial substituent and the axial hydrogens (or other groups) on the same side of the ring, two carbons away. This is the main reason large groups prefer equatorial.
Ring flip (chair-chair interconversion)
The process by which a cyclohexane chair converts to the other chair form. Every axial bond becomes equatorial, and every equatorial bond becomes axial.
Four types of strain destabilise molecular conformations. Knowing which is which, and when each applies, is essential.
Torsional strain: caused by eclipsed bonds on adjacent carbons. Present in every eclipsed conformation. Even eclipsed H-H interactions contribute roughly 4 kJ/mol of strain each.
Steric strain: caused by non-bonded atoms being forced too close together. The gauche interaction in butane (~3.8 kJ/mol) is a classic example: the two methyl groups are close enough that their electron clouds repel. Steric strain increases sharply with substituent size.
Angle strain: caused by bond angles deviating from the ideal. Most relevant in small rings: cyclopropane has 60° angles instead of the sp³ ideal of 109.5°, creating about 115 kJ/mol of total ring strain.
Ring strain: the sum of angle strain and torsional strain in a cyclic molecule. Cyclopropane (115 kJ/mol) > cyclobutane (110 kJ/mol) > cyclopentane (26 kJ/mol) > cyclohexane (~0 kJ/mol). Cyclohexane adopts a chair conformation that eliminates both angle and torsional strain almost entirely.
These strain values are measured experimentally using heats of combustion. A ring with more strain releases more energy per CH₂ unit than the strain-free reference.
Three ways to draw the same conformation:
Wedge-dash notation: solid wedges come toward the viewer, dashed wedges go away. Works for any molecule but can be hard to read for conformational analysis.
Sawhorse projection: views the C-C bond from an oblique angle. The front carbon is lower-left, the back carbon is upper-right. Useful for seeing both staggered and eclipsed arrangements.
Newman projection: views the molecule by looking directly down the C-C bond axis. Front carbon = central dot, back carbon = circle. Bonds radiate out from each. This is the standard tool for conformational analysis of acyclic molecules.
You must be able to convert freely between all three representations. The key is identifying which groups are on the front carbon and which are on the back carbon, then placing them at the correct dihedral angles.
Ethane has the simplest conformational profile. As you rotate around the C-C bond:
Staggered (dihedral = 60°, 180°, 300°): lowest energy. All H atoms are as far apart as possible.
Eclipsed (dihedral = 0°, 120°, 240°): highest energy. Each eclipsed H-H pair adds ~4 kJ/mol of torsional strain.
The rotational barrier of ethane is about 12 kJ/mol (three eclipsed H-H pairs).
Butane introduces steric strain because of the two methyl groups. Looking down the C2-C3 bond:
Anti (180°): lowest energy. The two methyl groups are as far apart as possible.
Gauche (60°): ~3.8 kJ/mol higher than anti. The methyls are close enough to experience steric repulsion.
Eclipsed, methyls with H (120°): higher still. Torsional strain plus some steric strain.
Eclipsed, methyls with each other (0°): highest energy (~19 kJ/mol above anti). Maximum torsional and steric strain.
The energy diagram for butane rotation is a key figure to know. It is not symmetrical: the anti minimum is lower than the gauche minima, and the fully eclipsed maximum (methyls on top of each other) is higher than the partially eclipsed maxima.
Cyclohexane's chair is the most stable six-membered ring conformation. Drawing it correctly matters:
Start with a pair of parallel lines offset vertically (the "seat" of the chair). Add the footrest carbon below-left and the headrest carbon above-right (or vice versa).
Each carbon has one axial bond (pointing straight up or straight down) and one equatorial bond (pointing outward, slightly above or below the ring plane).
Axial bonds alternate up-down-up-down around the ring. Equatorial bonds also alternate, but point roughly in the direction opposite the axial bond on the same carbon.
The boat form of cyclohexane has both "bow" and "stern" carbons pointing up (or both down). It is less stable than the chair for two reasons:
The bonds along the sides of the boat are eclipsed, creating torsional strain.
The two flagpole hydrogens (pointing inward at the bow and stern) are forced close together, creating steric strain.
The twist-boat is a slightly lower-energy variant that partially relieves both problems, but it is still considerably less stable than the chair.
When a substituent is placed on cyclohexane, it can sit in either the axial or equatorial position. The equatorial position is preferred for any group larger than hydrogen, because an axial substituent experiences 1,3-diaxial interactions with the axial hydrogens on the same face of the ring two carbons away.
Methylcyclohexane: the methyl group prefers equatorial by about 7.6 kJ/mol. At room temperature, roughly 95% of the molecules have the methyl equatorial.
tert-Butylcyclohexane: the tert-butyl group is so large that it is almost exclusively equatorial. This effectively "locks" the ring in one chair conformation.
For disubstituted cyclohexanes, draw both chair conformations (ring flip), place each substituent in its correct position (axial or equatorial), and compare the total 1,3-diaxial strain in each. The chair with the larger group equatorial is usually preferred.
A ring flip converts one chair to the other. Every axial position becomes equatorial, and every equatorial position becomes axial. The ring flip is rapid at room temperature; you cannot isolate the two chairs of an unsubstituted cyclohexane because they interconvert too quickly.
Real-world note: Conformational preferences in six-membered rings govern the shape and reactivity of sugars (glucose adopts a chair with most substituents equatorial) and many drug molecules. Steroid rings, for example, are locked into specific chair conformations that determine their biological activity.
Students often think conformations are different molecules. They are not. Conformations interconvert by rotation around single bonds and are in rapid equilibrium at room temperature.
Students often draw cyclohexane chairs with incorrect axial/equatorial placement. A common error is making all bonds point up or all point outward. Remember: each carbon has one axial (up or down) and one equatorial (outward, angled), and they alternate around the ring.
Students often forget that a ring flip swaps all axial and equatorial positions simultaneously. You cannot flip just one substituent; the entire ring inverts.
Students often confuse gauche strain with eclipsed strain. Gauche is a staggered (lower-energy) conformation with groups at 60°. Eclipsed has groups at 0° and is always higher in energy than any staggered form.
⚠️ Drawing Newman projections and identifying the most/least stable conformation is one of the most commonly tested skills in this chapter.
⚠️ Drawing cyclohexane chairs with correct axial and equatorial bonds is tested on nearly every exam that covers Ch. 3. Practise until you can draw them quickly and correctly.
⚠️ Predicting the more stable chair of a disubstituted cyclohexane requires you to draw both chairs, assign each substituent to axial or equatorial, count up the 1,3-diaxial interactions, and compare. This is a multi-step problem that appears frequently.
⚠️ Be ready to explain your answer using the correct strain terminology (torsional, steric, angle, 1,3-diaxial). Naming the type of strain is often worth marks.
⚠️ Energy diagrams for rotation around the C-C bond in ethane and butane are classic exam figures. You should be able to sketch them from memory and label each minimum and maximum.
True or false: The anti conformation of butane is higher in energy than the gauche conformation. (False, anti is the lowest-energy staggered conformation.)
Fill in the blank: In a Newman projection, the front carbon is represented by a _______ and the back carbon by a _______. (Dot (or point); circle.)
True or false: A ring flip on cyclohexane converts all axial bonds to equatorial and all equatorial bonds to axial. (True.)
Fill in the blank: The type of strain caused by eclipsed bonds is called _______ strain. (Torsional.)
True or false: Cyclopropane has less ring strain than cyclopentane. (False, cyclopropane has far more ring strain.)
Q: Draw the Newman projection of butane looking down the C2-C3 bond in the anti conformation. Which groups are anti to each other?
A: In the anti conformation, the two methyl groups (CH₃) are 180° apart. In the Newman projection, place CH₃ at the top of the front carbon and CH₃ at the bottom of the back carbon (or equivalently, 12 o'clock front and 6 o'clock back). The remaining positions are filled by H atoms.
Q: Rank the following cyclohexane conformations from most to least stable: chair, boat, twist-boat.
A: Chair (most stable) > twist-boat > boat (least stable). The chair has no torsional or angle strain. The twist-boat partially relieves the eclipsing and flagpole interactions of the boat.
Q: For methylcyclohexane, draw both chair conformations and identify which is more stable. Explain.
A: In one chair, the methyl group is equatorial; in the other, it is axial. The equatorial chair is more stable by about 7.6 kJ/mol because the axial methyl experiences two 1,3-diaxial interactions with the axial hydrogens on C3 and C5.
Q: trans-1,4-Dimethylcyclohexane can adopt two chair conformations. In which conformation are both methyl groups equatorial?
A: In trans-1,4-dimethylcyclohexane, the two methyl groups are on opposite faces of the ring. In one chair, both are equatorial; in the other, both are axial. The diequatorial chair is strongly preferred.
Q: Explain why cyclopropane is much more reactive than cyclohexane, despite both being cycloalkanes.
A: Cyclopropane has severe angle strain (bond angles forced to 60° instead of 109.5°) and torsional strain (all C-H bonds are eclipsed). This stored strain energy makes the C-C bonds weaker and more reactive. Cyclohexane in its chair conformation has essentially no strain, so its C-C bonds are at their normal strength and unreactive.
Conformational analysis connects directly to stereochemistry (Ch. 4-5), where the 3D arrangement of groups determines whether molecules are chiral. The axial/equatorial preference in cyclohexane explains the reactivity differences in substitution and elimination reactions (Ch. 7-8): an axial leaving group undergoes E2 elimination much more readily than an equatorial one. Ring strain in small rings (cyclopropane) reappears in ring-opening reactions. The concept of steric effects introduced here also underpins steric arguments used throughout the course when explaining selectivity.
Conformational analysis, conformation, Newman projection, sawhorse projection, wedge-dash, staggered, eclipsed, anti, gauche, dihedral angle, torsional strain, eclipsing strain, steric strain, van der Waals strain, angle strain, ring strain, heat of combustion, cyclopropane, cyclobutane, cyclopentane, cyclohexane, chair conformation, boat conformation, twist-boat, half-chair, axial, equatorial, 1,3-diaxial interaction, ring flip, chair-chair interconversion, A-value, conformational energy, methylcyclohexane, tert-butylcyclohexane, disubstituted cyclohexane, cis, trans, CHEM 2301, organic chemistry I, UMN