CHEM 121 glossary

Polyene

A hydrocarbon with several C=C double bonds; in this course, a linear conjugated polyene such as 1,3-butadiene or 1,3,5-hexatriene, with double and single bonds alternating along the chain. Its π electrons are modelled as a particle in a box of length $L$ = (number of conjugated carbons) × 0.140 nm, with one π electron per carbon.

1,3,5,7-Octatetraene: 8 conjugated carbons, box length 1.12 nm.

Example

1,3,5,7,9-Decapentaene has 10 conjugated carbons and 10 π electrons: $L$ = 1.40 nm, HOMO $n = 5$ , LUMO $n = 6$ . The box model predicts λ ≈ 587 nm, but the measured $\lambda_{\max}$ is 334 nm, still in the UV, so it is colourless.