Question

The following is a list of drugs. For this assignment, use any resources available to you to find the structures and the pKa of these drugs. Identify the functional groups on each drug that will readily ionize under the appropriate pH conditions. Finally, determine the degree of ionization of the drug at physiological pH (7.4).

• MDMA
• Amphetamine
• Butalbital
• Hydrocodone
• Carisoprodol

I understand how to find the functional groups. I need help understanding how to calculate the degree of ionization. With the explanation of at least one of the drugs I should be able to finish the other drugs on my own. Thank you in advance!

Answer 1

since you know functional group I am calculating degree of ionization

MDMA

MDMA pKa was estimated at 9.9

MDMA is basically a weak base

For a weak base, representing the base as B, the Henderson Hasselbach equation can be written as:

pH=pKa+log( [B]/[BH⁺] )

This can be written as:

pH = pKa + log((1-a)/a)

In this , 'a' represent the degree of ionization

at physiological pH 7.4 putting the values

7.4 = 9.9 + log((1-a)/a)

Solving we get:

a = 0.99 (approx)

(b) Amphetamine is also same as discussed above. Its PKa will be approx 10 hence its degree of ionizaton at this pH will also be 0.99

(c) Butalbital pKa is 12.36

pH=pKa+log( [A-]/[AH] )

This can be written as:

pH = pKa + log((a/(1-a)

In this , 'a' represent the degree of ionization

at physiological pH 7.4 putting the values

7.4 = 12.36 + log((a)/(1-a))

Solving we get:

a = a=0.000011(approx)

(d) Hydrocodone pKa is 7.95

at physiological pH 7.4 putting the values

7.4 = 7.95+ log((1-a)/a)

Solving we get:

a = 0.78013 (approx)