Acidity and Basicity of Functional Groups

The functional groups in a molecule help to define its reactivity. As a first look (or for later reference), we have collected all of the functional groups that we will be studying this year, and shown Bronsted acid/base reactions for them.

You should note that these values are approximate. Other substitution on the molecules could have considerable effect on the values listed (depending on the substitution, the difference may be as much as 10-15, meaning a factor of 1010 or more in the equilibrium).

Try to look for trends. Remember the factors which affect the stability of molecules: resonance, charges on atoms with appropriate electronegativities, hybridization and charge, etc. If you forget how some of these work, look back at the stability rules in the resonance handout.

Compound or Functional Group

Conjugate Base

pKa

Conjugate Acid

pKa*

Carboxylic acid

4-5
-6

Ester

25
-6.5

Amide

25
-2

Nitrile

25
-10

Aldehyde

19
-8

Ketone

19-20
-7

Phenol

10
-5

Alcohol (R-OH)

R-O-
16-18
R-OH2+
-2

Amine (R-NH2)

R-NH-
33
R-NH3+
10

Ether (R-O-R)

R2O-H +
-3.5

Alkene

37
NA*

Alkyne

25
NA*

Alkane (R3C-H)

H3C: -
45
CH5 +
-20??
*These molecules undergo further reactions, making pKa impossible to measure. We might guess at values around 0 to -4.