Practice MT2 Version 2 #12

[Zellmer, Robert]

Question 12 in practice MT2, V2 may be confusing,
particularly if you did EOCE 16.78.

It gives a pKa of 8.21 for codeine and treats it as an acid.
This is incorrect.  Codeine is a nitrogen base with a pKb of
5.79.  This is EOC problem 16.78 in the textbook.  This
problem is referenced in the solution.  The pKa of 8.21 is
actually for the conj. acid of codeine.  For whatever reason,
the pharmaceutical industry reports base strengths in terms
of the pKa of their conjugate acid (the higher the pKa of
this conj. acid the stronger the base of interest).  I have
a feeling whoever wrote this question in the distant past
saw codeine written with a pKa value and assumed it was
an acid and didn't realize what the pharma industry does
and that codeine is actually a base.

When you take codeine as a medicine it is generally in the
form of an acidic salt (reacted with HCl to make a salt). This
salt contains the conj. acid of codeine with Cl-.

Codeine + HCl -->  CodeineH+ + Cl-

On packages you see it as C18H22NO3*HCl which is
actually (HC18H22NO3)Cl, a salt, containing
(HC18H22NO3)+ with Cl-.

For the practice exam just go ahead and treat it as an
acid as it seems the exam and solutions do.  Just see
if you can get the right answer treating it as an acid.

Dr. Zellmer
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