Acid and bases Flashcards
Theory of acid and bases
- Arhennius theory
- Bronsted-lowry theory
- Lewis theory
Arhennius theory
- Acid produces H+/H3O+ ions
- Bases produces OH- ions
Bronsted-lowry theory
- An acid is a proton (hydrogen ion) donor.
- A base is a proton (hydrogen ion) acceptor.
Lewis theory
- An acid is an electron pair acceptor.
- A base is an electron pair donor.
There isn’t an empty orbital anywhere on the HCl which can accept a pair of electrons.
Why, then, is the HCl a Lewis acid?
- HCl is polar. Chlorine is more electronegative than hydrogen, leaving the hydrogen slightly positive and the chlorine slightly negative.
- Lone pair on the nitrogen of an ammonia molecule is attracted to the slightly positive hydrogen atom
- e- are slowly pushed towards chlorine, HCl bond breaks and dative/coordinate bond is formed
pKa table
Acid-Base reaction overview
- The species which loses H+ is the acid
- The species which gains H+ is the base
- The conjugate base is what becomes of the acid after it loses H+
- The conjugate acid is what becomes of the base after it gains H+
What determines acidity?
- any factor which stabilizes the conjugate base will increase the acidity.
- any factor which destabilizes the conjugate base will decrease the acidity.
- a roughly equivalent word for “stability” is “basicity
What determines basicity?
- Stabilizing a lone pair lowers the basicity;
- Destabilizing the lone pair increases basicity.
Acidity trend
- the lesser the charge, the better the acidity
All else being equal, the lower the charge density, the more stable a species is.
Acidity trend
Polarizability
Acidity trends
- Electron withdrawing groups
orbitals
Acidity trend
aromacity
What makes a good leaving group?
Good leaving groups are weak bases.
How do we know which are weak bases?
- The stronger the acid, the weaker the conjugate base
- the weaker the conjugate base, the better the leaving group
pka table, handy guide to leaving group
A leaving group is a nucleophile in reverse
OH- is a bad leaving group
OH- IN sn1 example
OH- in SN2 example
conjugate acid will always be a better leaving group.
Acids helps facillitates cleavage of ether
