Organic Chemistry I

Approach
: If H
+
is the acid as in previous examples, it is rather easy to predict how the reaction will
90 | 3.2 Organic Acids and Bases and Organic Reaction Mechanism

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proceed. However, if there is no obvious acid (or base) as in this example, how do you determine which
is the acid, and which is the base?
Methanol CH
3
OH is neutral, and the other reactant, NH
2
–
, is a negatively charged amide. The amide
with a negative charge has higher electron density than the neutral methanol, therefore amide
NH
2
–
should act as base, and CH
3
OH is the acid that donates H
+
.
Solution:
Exercises 3.1
Predict and draw the products of following reaction and use curved arrow to show the mechanism.
Answers to Practice Questions Chapter 3
3.2 Organic Acids and Bases and Organic Reaction Mechanism | 91

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3.3 pKa of Organic Acids and Application of pKa to
Predict Acid-Base Reaction Outcome
As we mentioned before, all organic compounds could be acids, because they all have hydrogen atoms that could
potentially be donated. Most organic acids are weak acids with a small
K
a.
For example, acetic acid CH
3
COOH has a
K
a
of 1.8×10
-5
. Lots of other organic acids are even weaker than acetic acid, and it is this weak acidity that makes it difficult
to realize that some organic compounds are actually acids.
However, this weak acidity is very important in Organic Chemistry. Since it is not that very convenient to say or to
remember
K
a
values like 1.8×10
-5
,
p
K
a
is used more often in Organic Chemistry to refer to the relative acidity of different
acids. The definition of p
K
a is:
p
K
a
= -log
K
a