Chemistry · Chapter 8

Multiple Bonds: Why a Double Bond Locks

Grab the twist slider and try to turn the right hand carbon. The pi overlap and the energy bill update as you go. Then switch to a single bond and watch it spin for nothing.

The molecule teal and gold are the two halves of each p orbital

cis both methyl groups on the same side

What you are seeing

Move the slider to begin.

Controls

0degree twist locked

Live numbers

Pi overlap left100%
Energy cost so far0 kJ/mol
Cost compared with RT at 298 K0 times
Highest bill on the way round267 kJ/mol
Can room heat pay it?no

Carbon bond data

BondLengthEnergyParts
C–C154 pm347 kJ/mol1 σ
C=C134 pm614 kJ/mol1 σ + 1 π
C≡C120 pm839 kJ/mol1 σ + 2 π

Subtracting gives the pi bonds: 614 − 347 = 267 kJ/mol for the first, and 839 − 614 = 225 kJ/mol for the second. RT at 298 K is only 2.48 kJ/mol.