# Is "BCl"_3 a lewis acid?

Boron trichloride is probably the most potent Lewis acid known, and will form adducts with ethers, halides, and amines. While boron trifluoride is also Lewis-acidic, counter-intuitively, the lone pairs on the fluoride, are the right size and shape to donate electron density to the electron deficient boron atom, and the Lewis-acidity of $B {F}_{3}$ is somewhat moderated in comparison.