Question

L-dopa is used in treatment of Parkinson's disease. Using Table 3.1 (pka values of common compounds), identify the four most acidic protons in the compound and then arrange them in order of increasing acidity (two of the protons will be very similar in acidity and difficult to distinguish at this point in time). --Organic Chemistry (2nd Edition) Chapter 3, Problem 6S

Answer 1

1 is most acidic and 4 is least acidic. 2 and 3 are of comparable acidity. The increasing order of acidity is

Carboxylic proton is more acidic than proton of phenolic -OH group. Which in turn is more acidic than proton of amino group. In fact amino group is a base.

Note: Carboxylic acids are weak acids and phenols are very weak acids.

Acetic acid has pKa around 4.8, phenol has pKa around 9 and diisopropyl amine has pKa of 40. Higher is the pKa, lower is the acidic nature and higher is the basic character.