Exam 2- Lewis Acids & Bases

Lewis acids vs. Lewis bases

acids are electron acceptors; bases are electron donors

Deeper definitions:

Lewis bases: Anything with a lone pair, Brønsted bases, Nucleophiles, Usually neutral or anionic, Filled non-bonding orbitals (lone pairs)

Lewis acids: Not necessarily Brønsted acids, Electrophiles, Usually neutral or cationic, Empty non-bonding orbitals that are energetically accessible

Bronsted Acids vs. Bronsted bases

acids are proton (H+ ) donors bases are proton (H+ ) acceptors

How to identify strongest Lewis acid

1.) Choose things with no lone pairs on central atom

2.) Choose electronegative atoms that withdraw electron density away from central atom making the central atom electron-deficient

The strongly electronegative F atoms withdraw electron density from B, making it even more electron-deficient. The CH3 groups are electron-donating, adding electron density to B and making it less Lewis acidic.

Cationic Lewis Pairs (Metals in water)

Electrostatic → negative lone pair of e- stabilizing positive Na+

(non-directional, # of waters dictated by size)

Orbital : LUMO of Na+ is the 3s orbital (non-directional bc s orbitals are spherical)

Bonding Interactions

1.) Can have perfect all s orbitals together (most stable at bottom norm)

2.) Can have pi orbital and 1 H different

3.) Can have 2 different H and 1 H not accounted for with corresponding pi orbital and different H sides

Non-bonding interactions

Only 1 true thing just 2 different perspectives

Pz orbital only! No overlap with hydrogens (normally LUMO)

Acid vs. base in MO diagram

For Acid: LUMO (Pz) is empty orbital → can easily accept e- (has no e- to donate easily)

If BF3 instead of BH3 → better lewis acid bc higher Pz energy

For Base: Electron in HOMO are easily accessible (Pz orbital) (non-bonding and high in energy)

How to identify if mixing is occurring

3 lines all going to HOMO (Can see with base example of MO diagram)

How to distinguish stronger acid and bases

Strong acids → more e- withdrawing groups

Strong bases → more e- donating groups

Donor ability

Donor ability increases with lewis basicity (more electrons to donate)