Chapter 14 Sections 14.1-14.3, 14.5, 14.6:
Acids and Bases
In a reaction, how do I tell what is the
Acid and what is the Base?
Brønsted Acid – A proton (H+ ion) donor
(has a polar sigma H–X bond where X is usually groups 5-7 – N, O,
Cl, etc.)
In a reaction, how do I tell what is the
Acid and what is the Base?
Brønsted Base – A proton (H+ ion) acceptor
(anything with a pair of nonbonding electrons)
Most compounds have more than one acidic
proton and many have more than one
nonbonding electron pair that can act as a
base
pKa Tables can help us out:
The stronger the acid, the weaker that base.
We already know that the lower the pKa value
the stronger the acid. So the higher the pKa
of the conjugate acid, the stronger the base.
We already looked at comparing pKa values but let’s take
a more detailed look at everything in solution:
And let’s look at the Henderson-Hasselbalch
again:
[A! ]
pH = pKa + log
[HA]
What happens as the ratio of acid and base
change?
At close to a 1:1 ratio of acid and conjugate base
the reaction is buffered
Let’s look at acetic acid a few different pH values:
What is the ratio and structures of the species in pH 3.8 solution of acetic acid?
O
O
H
Acetic acid
pKa = 4.8
Let’s look at acetic acid a few different pH values:
What is the ratio and structures of the species in pH 2.8 solution of acetic acid?
O
O
H
Acetic acid
pKa = 4.8
Let’s look at acetic acid a few different pH values:
What is the ratio and structures of the species in pH 7.0 solution of acetic acid?
O
O
H
Acetic acid
pKa = 4.8
Polyprotic acids
What happens to
polyprotic acids at
different pH?
Polyprotic Acids – Amino Acids
O
•Protons are removed in order
of most acidic (lowest pKa) to
least acidic (highest pKa)
H
pKa = 8.00
S
pKa = 2.05
O
H
H
N H
H
pKa = 10.25
Cysteine
pKa = 2.05
pKa = 8.00
O
H
S
O
N H
H
H
H
O
O
H
S
O
H
N H
H
S
O
O
N H
H
H
Cysteine
pKa = 10.25
S
O
H
N
H
What happens to
polyprotic acids at
different pH?
Let’s practice:
What is the major species
present in a pH = 1.5
solution of Tyrosine?
What is the major species
present in a pH = 6.1
solution of Tyrosine?
What is the major species
present in a pH = 9.7
solution of Tyrosine?