Betel contains a spectrum of phenylpropene compounds which makes up a good part of the smell from the leaves. Eugenol is one of them and it causes your mouth to go numb. Eugenol is also found in other plants such as basil. There's also other chemicals such as chavicol, isoeugenol, and chavibetol.

Interestingly these chemicals can (theoretically) be used in the synthesis of psychedelic phenethylamines. A good example is safrole in the synthesis of MDMA. Plants are chemical factories.