Lewis acids and bases

Draw the bonding in a hydronium ion Notice that another name for a dative bond is “coordinate bond”

In this reaction the H+ ion acts as an acid The H2O acts as a base (it accepts a proton) A dative bond is formed It could be considered that the base (water) donates an electron pair to the acid (proton) This is the definition that is used for a Lewis acid or base

Lewis acid = a substance that accepts an electron pair Lewis base = a substance that donates an electron pair This definition actually applies to ALL acids and bases. Arrhenius and Bronsted- Lowry are just special cases!

The oxygen donates an electron pair to the proton of the hydrochloric acid

The big advantage of this definition of acids and bases is that we can extend our understanding to molecules which don’t contain a proton! The classic example of a Lewis acid is boron trifluoride, BF3 Draw a Lewis diagram for BF3 The boron only has 6 electrons in its outer shell so it can accept a pair of electrons

The boron trifluoride can react with ammonia.

A dative bond is formed when the lone pair of electrons on the nitrogen atom form a dative bond by completing the outer shell of the boron

Show the bonding that occurs when BF3 reacts with F- Show the shape of the resulting molecule

Another common example is AlCl3 This behaves in a similar way to BF3, but in addition it forms a dimer

Other good examples involve metal ions. HL students may remember this from transition metal complexes!

Lewis theory can also explain the AMPHOTERIC nature of some metal hydroxides. Al(OH)3(s) + 3 H+ --> Al3+ + 3 H2O Here Al(OH)3 is a Brønsted-Lowry base. Al(OH)3(s) + OH- --> Al(OH)4- Here Al(OH)3 is a Lewis acid. (it accepts a pair of protons from the hydroxide ion)

Question For each of the following species, state whether it is more likely to behave as a Lewis acid or a Lewis base. Explain your answer

PH3 BCl3 H2S SF4 Cu2+

PH3 P has 5 electrons in its outer shell (2,8,5) 3 of these are used in making bonds to Hydrogen The remaining 2 electrons are a lone pair Thus it is a Lewis base

BCl3 B has 3 electrons in its outer shell (2,3) All of these are used in bonding It still only has 6 electrons in the outer shell So it has room to accept another pair of electrons It is a Lewis acid

H2S S has 6 electrons in outer shell Uses 2 of these in bonding Still has lone pair electrons left It is a Lewis base

SF4 S still has a lone pair of electrons It is still a base

Cu2+ Easy! It can accept a pair of electrons So it is a Lewis acid