B(OH)3 problem 6.13

[brennayoung]

How are we supposed to know that Boric acid B(OH)3 is a lewis acid? I woulve thought it would be a weak base b/c it has an OH in it. Also in terms of writing equations how do lewis acids/bases and bronsted acids/bases differ?

[Chem_Mod]

Hydroxides of Group 1 and 2 metal cations are strong bases; do not assume everything with a hydroxide group will be a strong base. B(OH)3 is a covalent compound so will not dissolve in water, thus OH- will not be released. Remember boron is an exception to the octet rule: having only 3 valence electrons, it can only form 3 bonds by itself and only have 6 electrons rather than an octet. However it can accept a lone pair from another molecule to achieve the full octet, making it a Lewis acid by definition. Lewis acid base reactions involve exchange of lone pair; Bronsted Lowry reactions must involve exchange of protons (which is also exchange of lone pair).

[Vincent Leong 2B]

Boron is one of those elements that have exception to the octect rule in that it doesn't fill it's complete octect. It's missing lone pair on Boron suggests that it's indeed an electron acceptor or in other terms a Lewis acid.