Staggered and eclipsed

Each eclipsed pair costs energy — H/H 4, CH3/H 6, CH3/CH3 11 kJ/mol — so staggered conformations are lower than eclipsed ones; butane's anti form is lowest, gauche 3.8 kJ/mol higher, and molecules spend most time in the valleys.

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.

1. What you will learn

You will price a conformation from its eclipsed pairs, explain torsional strain, plot ethane's energy curve, rank butane's conformations, and tabulate their energies.

2. What you already have

You can read a Newman projection and count the eclipsed pairs in it. Each pair has an energy cost, and adding them gives the conformation's energy.

3. Words for this lesson

Torsional strain is the energy of lined-up (eclipsed) bonds, whose electron pairs repel. Steric strain is the energy of bulky groups crowding each other. The barrier to rotation is the energy of the highest conformation above the lowest.

4. Price the pairs, add them up

Measured costs of each eclipsed pair:

Pair	Cost
H/H	4 kJ/mol
CH3/H	6 kJ/mol
CH3/CH3	11 kJ/mol

Ethane: eclipsed has three H/H pairs, $3 \times 4 = 12$ kJ/mol above staggered. That is the barrier to rotation — small enough that ethane turns freely at room temperature, about a hundred billion times a second.

Butane along C2–C3 has more to count. Staggered forms have nothing eclipsed, but in gauche the two methyls are neighbours, costing 3.8 kJ/mol of steric strain; anti costs nothing. Eclipsed at 120°: two CH3/H and one H/H, 16 kJ/mol. Fully eclipsed at 0°: one CH3/CH3 and two H/H, 19 kJ/mol.

Another way: table

Butane along C2–C3.

Angle	Conformation	Energy, kJ/mol
180°	anti	0
60°, 300°	gauche	3.8
120°, 240°	eclipsed	16
0°	fully eclipsed	19

5. The energy curve

Plot energy against dihedral angle and rotation becomes a landscape: valleys at the staggered angles, hills at the eclipsed ones. Molecules spend most of their time in the valleys and pass quickly over the hills, so at any moment most butane molecules are anti, fewer gauche, and almost none eclipsed. Long alkane chains are therefore drawn as zigzags: every C–C bond anti. The cost of eclipsing in ethane is torsional — bonds lined up — while in butane the bulky methyls add steric strain on top.

6. Where this goes wrong

Eclipsed conformations are more stable because groups overlap. They are higher in energy.

Free rotation means every angle is equally likely. The valleys are favoured.

Ethane's barrier comes from hydrogens bumping. It is mostly torsional.

Gauche is eclipsed. It is staggered, with a steric cost.

7. Eclipsed propane along C1–C2

Count the pairs: one CH3/H and two H/H.
From the Newman projection.
$6 + 4 + 4 = 14$ kJ/mol.
Price each pair and add.
So propane's barrier is 14 kJ/mol, a little above ethane's 12.
The methyl adds 2.

8. Your turn: butane eclipsed at 240°

It mirrors 120°. What pairs are lined up?
Two CH3/H and one H/H.
So its energy is?
Your turn: work this step out. Its working is at the end of the packet.
$6 + 6 + 4 = 16$ kJ/mol, the same as at 120°.

9. Guided practice

How much higher in energy, in kJ/mol, is butane with its CH3 groups at 120°, viewed along C2–C3 than the staggered conformation? (Costs of each eclipsed pair: H/H 4 kJ/mol, CH3/H 6 kJ/mol, CH3/CH3 11 kJ/mol.)

Answer:

10. Guided practice

Why is eclipsed ethane about 12 kJ/mol higher in energy than staggered ethane?

Lined-up bonds hold their electron pairs close together, and the pairs repel (torsional strain).
Eclipsed conformations are more stable, because the groups overlap and bond.
The C–C bond is partly broken in the eclipsed form.
The hydrogen atoms collide because they are large.

11. Practice

Plot ethane's energy, in kJ/mol above the staggered form, at dihedral angles of 0°, 60°, 120°, 180° and 240°. The eclipsed form is 12 kJ/mol higher.

Plot your answer on the grid:

12. Practice

Rank butane's conformations, viewed along C2–C3, from lowest energy to highest. (Costs of each eclipsed pair: H/H 4 kJ/mol, CH3/H 6 kJ/mol, CH3/CH3 11 kJ/mol.) A gauche CH3/CH3 pair costs 3.8 kJ/mol.

Number the steps in order (write the number in the box):

anti, 180°
gauche, 60°
eclipsed, 120°
fully eclipsed, 0°

13. Somewhere new

A sample of hexane is at room temperature. Along its middle C3–C4 bond, which conformation will most molecules be found in at any moment?

Anti, with the two chains 180° apart
Fully eclipsed, because the chains attract each other
Gauche, because it is a compromise
All conformations equally, because rotation is free

14. Lesson test

Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.

15. Test question

Fill in butane's energy above the anti conformation, in kJ/mol, at each dihedral angle along C2–C3. (Costs of each eclipsed pair: H/H 4 kJ/mol, CH3/H 6 kJ/mol, CH3/CH3 11 kJ/mol.) A gauche CH3/CH3 pair costs 3.8 kJ/mol.

	energy, kJ/mol
180°
60°
120°
0°

16. What you can do now

You can put a number on a conformation's energy. Tell someone why long alkane chains are drawn as zigzags. Next: the six-membered ring that has no eclipsed bonds at all — the cyclohexane chair.

Working for the steps left to you

8. Your turn: butane eclipsed at 240°, step 3