CHEM 121 glossary

Conjugation

π bonds are conjugated when they alternate with single bonds, each π bond separated from the next by exactly one single bond, so every atom along the chain has a p orbital that overlaps its neighbours'. The π electrons then spread over the whole system, and longer conjugation means a smaller HOMO–LUMO gap and absorption at longer wavelengths.

1,3-Butadiene: two C=C separated by one single bond.

Example

2-Butenal, $\ce{CH3-CH=CH-CH=O}$ , is conjugated: the C=C and the C=O are separated by one single bond, giving 4 conjugated π electrons. The $\ce{CH3}$ carbon is $sp^3$ and sits outside the system.