Back to the on-screen lesson ·
A conformer's energy is the sum of its torsional, steric and angle strain; comparing totals ranks conformers, orders cyclohexane's shapes, and shows which chair of a substituted ring is favoured and by how much.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
You will name the three kinds of strain, rank cyclohexane's shapes, choose the favoured chair of a disubstituted ring, measure angle strain, and calculate the energy gap between chairs.
You can price eclipsed pairs, gauche interactions and 1,3-diaxial interactions. This lesson adds them up to compare whole conformers.
Torsional strain: lined-up bonds. Steric strain: groups crowding each other. Angle strain: bond angles forced away from the hybridization's ideal. A conformer is a conformation at an energy minimum.
A conformer's energy is the sum of its strains:
| Strain | Cause | Example |
|---|---|---|
| torsional | eclipsed bonds | eclipsed ethane, 12 kJ/mol |
| steric | crowded groups | gauche butane, 3.8; axial methyl, 7.6 |
| angle | angles far from 109.5° | cyclopropane, bent 49.5° |
To choose between two conformers, total each and compare: the lower total is favoured, and the bigger the gap the more completely.
Cyclohexane shows all three. The chair has none. The boat has eclipsed bonds along its sides and two flagpole hydrogens crowding each other across the ring: about 30 kJ/mol. The twist-boat eases that to 23. The half-chair, partly flattened, has angle strain too: 45 kJ/mol, the top of the barrier the ring crosses when it flips.
Another way: steps
To compare the two chairs of a substituted ring:
In a ring with two groups and one axial in each chair, the chair that puts the costlier group equatorial wins. A tert-butyl group's 1,3-diaxial cost, about 23 kJ/mol, dwarfs every other common group's, so it effectively locks the ring with itself equatorial whatever else is on it. Chemists use exactly this to hold a ring in one chair while they study how a reaction depends on axial or equatorial position — which is where conformation stops being geometry and starts deciding reactivity (unit 7).
The most symmetrical-looking conformer is always lowest in energy. Add the strains; symmetry decides nothing.
Count axial groups, not their costs. Costs differ widely.
A boat avoids strain. It has more than the chair.
All strain is the same kind. Torsional, steric and angle strain have different causes.
Chair A: methyl axial, 7.6 kJ/mol. Chair B: –OH axial, 3.8 kJ/mol.
Price each chair.
Chair B is 3.8 kJ/mol lower.
Compare totals.
So the methyl is equatorial in most molecules, and the –OH axial.
The costlier group goes equatorial.
Isopropyl axial costs?
9.2 kJ/mol.
Chlorine axial costs?
2.0 kJ/mol, so the favoured chair has chlorine axial, 7.2 kJ/mol lower.
Match each situation to the kind of strain it causes.
| torsional strain | steric strain | angle strain | |
|---|---|---|---|
| the C–H bonds of eclipsed ethane | |||
| the two methyls of gauche butane | |||
| the 60° angles of cyclopropane | |||
| an axial methyl on cyclohexane |
Rank cyclohexane's shapes from lowest energy to highest: chair (0 kJ/mol), twist-boat (23), boat (30), half-chair (45).
Number the steps in order (write the number in the box):
A cyclohexane ring carries an ethyl group, –CH2CH3 on C1 and a chlorine atom, –Cl on C4, arranged so that in each chair exactly one of them is axial. Each 1,3-diaxial interaction costs $4$ kJ/mol for the first group and $1$ kJ/mol for the second. Which chair is favoured?
The C–C–C angle in cyclopropane is 60°. By how many degrees is it bent away from the 109.5° an sp3 carbon prefers?
Answer:
cis-1-tert-butyl-4-methylcyclohexane has one axial group in each chair. An axial tert-butyl group costs about 23 kJ/mol; an axial methyl, 7.6 kJ/mol. Which chair does almost every molecule adopt?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A cyclohexane carries a hydroxyl group, –OH and a chlorine atom, –Cl, arranged so that each chair has exactly one of them axial. Each 1,3-diaxial interaction costs $1.9$ kJ/mol for the first group and $1$ kJ/mol for the second. By how many kJ/mol is the favoured chair lower?
The favoured chair is lower in energy by a kJ/mol.
You can compare conformers by their total strain. Tell someone why a tert-butyl group locks a ring. Next unit: organic acids and bases, and what makes a proton easy to give up.
8. Your turn: isopropyl (4.6 per interaction) and chlorine (1.0 per interaction), one axial per chair, step 3