The relative rates at which dichlorocarbene (an electrophilic carbene) adds to alkenes increase with alkyl substitution of the double bond. Which alkene reacts fastest?

An electrophilic carbene attacks the pi electrons, and alkyl groups (+I and hyperconjugation) raise the electron density of the double bond. The reactivity order is tetrasubstituted > trisubstituted > disubstituted > monosubstituted > unsubstituted. 2,3-Dimethylbut-2-ene is tetrasubstituted, while ethene is unsubstituted and propene is monosubstituted.