Difficulty: Intermediate | Prerequisites: Structure, Bonding, and Electron Movement notes (Doc 1); Acid-Base and Molecular Properties notes (Doc 2)
This final set of notes covers how carbon skeletons behave in three dimensions. Hydrocarbons are the simplest organic molecules, but their conformational preferences, strain energies, and substituent effects set the stage for understanding reactivity in more complex systems. Chair conformations and axial/equatorial preferences are among the most heavily tested topics in organic chemistry I, and they require spatial reasoning that takes practice to develop. If Newman projections and chair flips feel abstract, build physical models or use a molecular modelling app.
Hydrocarbons come in four main flavours (alkanes, alkenes, alkynes, aromatics), each with distinct bonding and reactivity. Molecules experience strain when bonds are forced into unfavourable angles, eclipsed positions, or steric clashes. Cyclohexane's chair conformation is the most stable ring shape, and whether a substituent sits axial or equatorial determines a huge amount of the molecule's chemistry.
Alkane
A saturated hydrocarbon containing only carbon-carbon single bonds and carbon-hydrogen bonds. General formula: CnH(2n+2) for acyclic alkanes. In simple terms, the simplest and least reactive type of hydrocarbon.
Alkene
A hydrocarbon containing at least one carbon-carbon double bond. The double bond introduces a region of higher electron density and distinct reactivity (addition reactions).
Alkyne
A hydrocarbon containing at least one carbon-carbon triple bond. Even more electron-dense than alkenes, with its own set of reaction patterns.
Aromatic compound
A cyclic, planar molecule with a continuous ring of overlapping p orbitals containing 4n+2 pi electrons (Hückel's rule). Benzene is the classic example. Aromaticity confers special thermodynamic stability.
Think of it as: a molecule so stable from its electron arrangement that it resists the addition reactions that alkenes undergo.
Torsional strain (eclipsing strain)
The increase in energy when bonds on adjacent carbons are aligned (eclipsed) rather than staggered. Arises from electron-electron repulsion between the bonding electrons.
Steric strain (van der Waals strain)
The repulsion that occurs when atoms or groups are forced closer together than their van der Waals radii allow. Bulky groups in close proximity raise the energy of a conformation.
Angle strain (Baeyer strain)
The strain that results when bond angles are forced away from their ideal values. Particularly significant in small rings: cyclopropane's 60° angles are far from the ideal 109.5° for sp³ carbon.
Ring strain
The combined effect of angle strain and torsional strain in cyclic molecules. Most severe in three- and four-membered rings.
Newman projection
A way of viewing a molecule by looking straight down a carbon-carbon bond. The front carbon is shown as a dot, the back carbon as a circle. Useful for analysing torsional relationships between substituents.
Wedge-dash notation
A three-dimensional drawing convention where a wedge (solid triangle) indicates a bond coming toward the viewer and a dash (dashed line) indicates a bond going away.
Sawhorse projection
A perspective drawing of a molecule viewed from an angle, showing the spatial relationship between groups on adjacent carbons. Less commonly used than Newman projections.
Chair conformation
The most stable conformation of cyclohexane. All bond angles are close to 109.5°, and all adjacent C-H bonds are staggered, minimising both angle strain and torsional strain.
Boat conformation
A less stable conformation of cyclohexane with higher energy due to eclipsing interactions along the sides and steric clash between the two "flagpole" hydrogens.
Axial position
A bond on a cyclohexane chair that points straight up or straight down, parallel to the vertical axis of the ring. Three axial bonds point up and three point down, alternating around the ring.
Equatorial position
A bond on a cyclohexane chair that points roughly outward from the ring, slightly above or below the equator. Equatorial positions have more room, so bulky substituents prefer them.
Ring flip (chair-chair interconversion)
The process by which one chair conformation of cyclohexane converts to the other. Every axial substituent becomes equatorial, and every equatorial substituent becomes axial.
1,3-Diaxial interaction
The 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 positions.
Four classes of hydrocarbons, distinguished by their bonding:
Alkanes (C-C single bonds only): saturated, relatively unreactive under mild conditions. Their chemistry is dominated by combustion and radical halogenation.
Alkenes (at least one C=C double bond): the pi bond is a region of high electron density that reacts with electrophiles. Addition reactions are their signature.
Alkynes (at least one C≡C triple bond): two pi bonds, making them even more electron-rich. Terminal alkynes have a mildly acidic sp C-H bond (pKa ~25).
Aromatic compounds: cyclic, planar systems with 4n+2 pi electrons. Their special stability (aromaticity) means they undergo substitution rather than addition, preserving the aromatic ring.
The reactivity differences between these classes are a direct consequence of their bonding: more pi electrons means more electron density available for reaction, but aromaticity changes the game entirely.
Four kinds of strain raise a molecule's energy:
Torsional strain: caused by eclipsed bonds on adjacent atoms. In a Newman projection, eclipsed conformations are higher in energy than staggered ones. For ethane, the barrier to rotation is about 12 kJ/mol.
Steric strain: caused by atoms or groups being forced too close together. Think of two bulky tert-butyl groups trying to occupy neighbouring positions.
Angle strain: caused by bond angles deviating from ideal values. Cyclopropane (60°) and cyclobutane (~90°) have significant angle strain because sp³ carbons prefer 109.5°.
Ring strain: the combination of angle strain and torsional strain in cyclic molecules. Cyclopropane and cyclobutane are the most strained common rings; cyclopentane has moderate strain; cyclohexane has essentially none in its chair form.
Conformations are different spatial arrangements of the same molecule achieved by rotation around single bonds (no bonds are broken).
Three projection styles help visualise conformations:
Wedge-dash: shows 3D arrangement with wedges (toward viewer) and dashes (away).
Sawhorse: a perspective view along the C-C bond axis.
Newman projection: the key tool for conformational analysis. You look straight down the C-C bond. The front carbon is a dot; the back carbon is a circle.
For ethane and butane, the key conformations along a 360° rotation:
Staggered (dihedral angle 60°, 180°, 300°): lowest energy. All groups on adjacent carbons are as far apart as possible.
Eclipsed (dihedral angle 0°, 120°, 240°): highest energy. Groups on adjacent carbons are directly aligned.
For butane specifically:
Anti conformation (180° dihedral between the two methyl groups): the global energy minimum. No steric or torsional strain.
Gauche conformation (60° dihedral): slightly higher energy due to steric interaction between the methyl groups, but still staggered.
Eclipsed conformations: energy maxima, with the highest being the one where both methyl groups eclipse each other (0° dihedral).
Cyclohexane is the star of this section. It adopts two main conformations:
Chair: the most stable form. All C-C-C angles are ~111° (close to ideal tetrahedral), and all adjacent C-H bonds are perfectly staggered. Zero torsional strain, minimal angle strain.
Boat: higher energy. The "sides" of the boat have eclipsing interactions, and the two "flagpole" hydrogens (pointing inward at the top of the boat) clash sterically.
Twist-boat: a slightly more stable variant of the boat that reduces some of the flagpole strain by twisting.
Each carbon in the chair has one axial bond (pointing up or down) and one equatorial bond (pointing roughly outward). Axial and equatorial positions alternate: up, down, up, down around the ring.
When cyclohexane carries a substituent, the two chair conformations are no longer equal in energy:
A substituent in the equatorial position is more stable because it avoids 1,3-diaxial interactions with the axial hydrogens on the same face of the ring.
A substituent in the axial position experiences these 1,3-diaxial interactions, raising the energy.
Larger substituents have a stronger preference for equatorial. A tert-butyl group has such a large equatorial preference (~23 kJ/mol) that it effectively locks the ring in one chair conformation.
Ring flips interconvert the two chairs. Every axial group becomes equatorial, and every equatorial group becomes axial. For a monosubstituted cyclohexane, the more stable chair is the one with the substituent equatorial.
For disubstituted cyclohexanes, you need to consider both substituents and determine which chair places the larger group equatorial (or, if possible, both groups equatorial).
Conformational analysis is not just an academic exercise. In drug design, the three-dimensional shape of a molecule determines whether it fits into an enzyme's active site. A drug molecule locked in the wrong conformation will not bind. Cyclohexane chair analysis is used routinely in carbohydrate chemistry: every sugar ring (glucose, for instance) is a substituted cyclohexane or cyclohexane-like ring, and axial vs. equatorial preferences of hydroxyl groups determine the sugar's stability and biological behaviour.
Students often think that conformations are different molecules. They are not. Conformations interconvert freely by rotation around single bonds, with no bond breaking.
A common mistake is drawing a ring flip as flipping the entire molecule upside down. A ring flip is an internal rearrangement of the cyclohexane ring: what was axial becomes equatorial and vice versa, but the substituents remain on the same face (cis or trans relationship) of the ring.
Students sometimes believe the boat conformation of cyclohexane is a common, easily observed form. In practice, cyclohexane spends the vast majority of its time in chair conformations. The boat is a transition-state-like form, not a resting state.
Another error is forgetting that axial and equatorial positions alternate up and down around the ring. Not all axial bonds point the same direction: they alternate up-down-up-down.
⚠️ Drawing chair conformations with correct axial and equatorial positions is one of the most tested skills. Practise until you can draw both chairs of a substituted cyclohexane quickly and accurately.
⚠️ Expect questions asking you to identify the more stable chair of a mono- or disubstituted cyclohexane. The answer almost always comes down to which chair puts the larger group equatorial.
⚠️ Newman projection questions typically ask you to draw a specific conformation (anti, gauche, eclipsed) and identify it as a minimum or maximum on an energy diagram.
⚠️ Ring strain comparisons ("rank cyclopropane, cyclobutane, cyclopentane, cyclohexane by strain") are common short-answer or multiple-choice items.
⚠️ Knowing the four hydrocarbon classes (alkane, alkene, alkyne, aromatic) and their characteristic reactivity is foundational for every subsequent chapter.
True or False: Alkenes are saturated hydrocarbons. (Answer: False. Alkenes are unsaturated; they contain at least one double bond.)
Fill in the blank: The most stable conformation of cyclohexane is the ______ conformation. (Answer: chair.)
True or False: In a ring flip, a substituent that was axial becomes equatorial. (Answer: True.)
Fill in the blank: The strain caused by eclipsed bonds on adjacent carbons is called ______ strain. (Answer: torsional.)
True or False: Cyclopropane has less ring strain than cyclohexane. (Answer: 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. The front carbon shows CH3 at the top with H on either side. The back carbon shows CH3 at the bottom with H on either side. This is the lowest-energy conformation of butane.
Q: Rank the following cycloalkanes from most to least ring strain: cyclohexane, cyclopropane, cyclopentane, cyclobutane.
A: Cyclopropane (most strain) > cyclobutane > cyclopentane > cyclohexane (least strain, essentially zero in chair form).
Q: Methylcyclohexane has two chair conformations. In which chair is the methyl group more stable, and why?
A: The methyl group is more stable in the equatorial position. When axial, the methyl group experiences 1,3-diaxial interactions with the axial hydrogens on C3 and C5, raising the energy by about 7.6 kJ/mol. When equatorial, these interactions are avoided.
Q: Why are aromatic compounds unusually stable compared to similar non-aromatic cyclic alkenes?
A: Aromatic compounds have a continuous ring of overlapping p orbitals containing 4n+2 pi electrons (Hückel's rule). This arrangement leads to a large stabilisation energy (the resonance energy or aromatic stabilisation energy), making the ring much lower in energy than a hypothetical non-delocalised version would be.
Q: What is the energy relationship between the gauche and anti conformations of butane?
A: The anti conformation is lower in energy than gauche by about 3.8 kJ/mol. The gauche conformation has a steric interaction between the two methyl groups at a 60° dihedral angle, but both are staggered conformations and therefore energy minima (not maxima).
Conformational analysis connects directly to stereochemistry (Chapter 3). The spatial arrangement of substituents on a cyclohexane ring determines whether they are cis or trans to each other, which in turn affects physical and chemical properties.
The hydrocarbon functional groups introduced here set up the reactivity chapters that follow: alkene addition reactions, alkyne chemistry, and aromatic substitution all build on understanding the electron density and bonding patterns covered in these notes.
Strain energies reappear in discussions of reaction thermodynamics and ring-opening reactions of epoxides and small-ring compounds, where ring strain is the driving force for reactivity.
Alkane, alkene, alkyne, aromatic compound, aromaticity, Hückel's rule, 4n+2, benzene, saturated hydrocarbon, unsaturated hydrocarbon, torsional strain, eclipsing strain, steric strain, van der Waals strain, angle strain, Baeyer strain, ring strain, Newman projection, sawhorse projection, wedge-dash, staggered conformation, eclipsed conformation, anti conformation, gauche conformation, dihedral angle, cyclohexane chair, cyclohexane boat, twist-boat, axial position, equatorial position, ring flip, chair-chair interconversion, 1,3-diaxial interaction, A-value, conformational analysis, methylcyclohexane, tert-butylcyclohexane, disubstituted cyclohexane, cis and trans cyclohexane, organic chemistry I, ochem 1 study notes, CHEM 2301