# Is BBr3 Lewis acid or base?

A Lewis acid is an electron pair acceptor. Boron halides, $B {X}_{3}$, are excellent acceptors of electron density to form $\left\{{X}_{3} B \cdot D\right\}$. It turns out that $B B {r}_{3}$ is a stronger Lewis acid than $B {F}_{3}$ or $B C {l}_{3}$ inasmuch as the lone pairs on the lower halide fluorine and chorine are the right size and the right shape to donate electron density to the boron, and moderate its (the boron's) Lewis acidity.