SolutionsBonding Properties
1. Cyclohexane and cyclohexene
Do cyclohexane and cyclohexene have the same geometry?
Use the molecule panel: draw the two rings, convert them to 3D and observe the differences.
No. In cyclohexane the six carbons are sp3 and the ring adopts the chair conformation. In cyclohexene the carbons of the double bond and the two next to them lie in one plane, so the ring adopts a half-chair conformation: only the other two carbons lie above and below that plane.
2. Lengths and energies
Explain why the C=C bond (1.33 Å, 146 kcal/mol) is shorter and stronger than the C–C bond (1.54 Å, 83 kcal/mol), but not twice as strong.
The double bond has one σ bond and one π bond, and the two electron pairs bring the nuclei closer. But the π bond, formed by sideways overlap of two p orbitals, is weaker than the σ bond (about 63 kcal/mol versus 83), so the double bond is worth less than twice a single bond.
This is why alkenes react by breaking only the π bond: it is the one that costs least energy.
3. Hybridization
Give the hybridization of each carbon of propene, CH2=CH–CH3, and the approximate angle around each one.
The two carbons of the double bond are sp2, with angles close to 120°; the CH3 is sp3, with angles of about 109.5°.