Acid/Base and pKa values
Major concepts
 A quantitative scale of the strength of an acid in protonating water gives us pKa values
 Because all pKa values are relative to water, they can be used to determine the relative strength
of acids to each other
 Compounds with lower pKa values are relatively stronger acids.
 Weaker acids are relatively more stable, less reactive, acids. Stronger acids are relatively less
stable, more reactive, acids.
 Stronger acids have weaker conjugate bases; weaker acids have stronger conjugate bases. This
can be restated that “Less stable acids have more stable conjugate bases,” or “More reactive
acids have less reactive conjugate bases.”
 The equilibrium of a reaction lies toward the weaker acid/base pair because they are more
stable.
Vocabulary
 pKa values
 stronger/weaker; less stable/more stable; more reactive/less reactive
Students should be able to:
 Know the pKa values of the protons of typical functional groups from memory
 Compare the relative acidity of two compounds using pKa values
 Use both pKa values and base stability principles to determine direction of equilibrium
 Given reagents, predict whether or not an acid/base reaction will happen and what the products
will be.
On the following page are a list of pKa values for functional groups and protonated functional groups.
Eventually, you will need to know these!
pKa chart of the functional groups: values to know
1. Protonated carbonyl pKa = -2 to -3
2. Protonated alcohol or ether pKa = -2 to -3
3. Carboxylic acid pKa = 4-5
4. Ammonium ion pKa = 9-10
5. Phenol pKa = 10
6. Thiol pKa = 10
7. Alcohol pKa = 16-18
8. Water pKa = 15.7
9. Amide pKa = 18
10. Alpha proton of ketone/aldehyde pKa = 20
11. Alpha proton of ester pKa = 25
12. Terminal alkyne pKa = 25
13. Amine pKa = 38-40
14. Aromatic: Aryl 43, benzylic 41
15. Alkene: vinyl 45-50; allylic 43
16. Alkane pKa = above 50
Examples:
Daily Problems and Cumulative problems Almost all of these acid/base problems are really cumulative
in nature. You need to understand relative stabilities, based on all the structural principles we have
learned in the course.
1. First, describe the protons in bold in terms of their functional group. Next, give the pKa of the
protons in bold. (Ideally, you can do this from memory. You don’t have to have it perfect right now, but
be working on it.)
amine 38
alcohol 16
alkane >50
phenol
10
protonated
amine 9
aryl 43
protonated
carboxylic acid
-2
ketone 20
hydroxide of a
carboxylic acid 5
alkyne 25
2. Which of the bold protons on each of these compounds would be the FIRST ONE removed by a base?
(Refer to problem 1. How does pKa value help you answer this question?)
The lower pKa is
the more acidic
proton or the
first one to be
removed by
base.
3. Which of these four compounds is the most acidic? Which is least acidic? Explain.
most acidic
molecule
least acidic
molecule
The most acidic molecule is determined by the molecule with the proton with the lowest pKa. The least
acidic molecule is determined by the molecule that is deprotonated last. The most acidic proton on each
molecule is compared and the molecule with the highest of those pKas is the least acidic molecule.
4. Use pKa values of the acid and conjugate acid to predict the direction of equilibrium for each of these
reactions. Are there any that are ambiguous based only on pKa values? Which of these reactions
proceeds as written based on pKa values?
“proceeds as
written” means
the equilibrium
lies to the right.
The equilibrium
lies to the higher
pKa value.
Tough to determine based
on pKas because they are
so close
5. Now solve the same problems a different way. Use stability principles to compare the base and
conjugate base to predict the equilibrium for each of these reactions.
Equilibrium
lies toward
the more
stable base.
Determine
stability of
base
through
ARIO
6. Compare problems 4 and 5. Generally, the two methods of determining direction of equilibrium
should give the same result. Is this true? Sometimes, though, there are exceptions. Which of these
problems give conflicting answers? If there is a conflict, which answer is correct, pKa values or base
stability trends?
We would hope that they would give the same result. Unfortunately, these methods are not always
complementary. The only way to truly know for sure where the equilibrium lies is to do experiments to
prove it. The problem with the CF3 group gave conflicting answers in the two methods. By pKa
standards, the equilibrium lies to the right. By ARIO technique, the equilibrium lies to the left.
7. The following problems are the most realistic, the most comprehensive, and require you to have the
most understanding of acid base chemistry because the minimum amount of information is given. In
these problems, you are given two reagents, and are asked A.) Does an acid/base reaction go to
completion? And B) If so, what are the products. Here are the steps:
A. Identify which compound acts as an acid and which acts as a base. Strategy—do you
recognize any strong acids or bases, or any compounds that can only act as an acid or base (ie. Is any
compound charged?) If both could possibly function as acids or bases, use pKa values to determine the
stronger acid. If that doesn’t work, draw the conjugate bases and use stability principles.
B. Find the most reactive lone pair on the base. Find the most acidic proton on the acid.
C. Draw a mechanism, and draw the products.
D. Determine the direction of equilibrium. If it lies to the right, the reaction does go to
completion. If it lies to the left, it does not occur. If it is near the middle of equilibrium, then reagents
and products are all present to varying degrees.
Extension problem
Instead of using a lone pair in an acid, base reaction, it is possible to use a pi bond. Predict the products
of this reaction, based on the arrows. Hint: the products will be ionic.
nucleophile
electrophile
For reactions that are not strictly Bronsted acid/base reactions, we have new terms. The compound
that is acting like an acid (electron poor) is called an “electrophile.” The compound that is acting like a
base (electron rich) is called the “nucleophile.” Label the two reactants in the reaction above as
“Nucleophile” and “Electrophile.”