Question

Relative strengths of acids:

Can an Organic Chemistry I and II MASTER explain to me some rules of thumb in regards to the relative strengths of acids.

1) List the functional groups (alcohols, carboxylic acids, phenols, anhydrides, amides, etc.) in order of DECREASING ACIDITY.

2) In regards to substituents on aromatic rings, which substituents would act as stronger bases, why, and in what relative position (ortho, meta, or para); and if the number of substituents affects basicity?

The list will be most helpful, so please list as many functional groups as possible. Thank you in advance.

Answer 1

1) The acidity of organic functional groups can be judged from a number of points:

a) Bronsted acids are proton donors while Bronsted bases are proton acceptors.

b) Lewis acids are electron pair acceptors while Lewis bases are electron pair donors.

c) A conjugate base is formed from a Bronsted acid by loss of a proton. Conversely, a conjugate base is a Bronsted base which must accept proton.

D) A substituent or effect which stabilizes the Bronsted base will make the loss of proton easier and consequently increase the acidity.

The given functional groups display resonance or conjugation effects in addition to the above mentioned factors and hence the acidity will be guided by considering the contributions from all factors. We look at those below:

A somewhat comprehensive list of acidity of functional groups can be seen here. Remember the higher the pKa, the less acidic is the compound.

http://www.chemistry2011.org/ResourceFiles/pka.pdf