Lecture 4 (9-17-17) Chapters 3 and 4
Goals:
Become familiar with major and minor resonance contributors
Revisit acid/base reactions in the context of organic chemistry
Discuss factors affecting acid strength
Problems: (for all formal charge problems, use the table where possible rather than calculating)
Chapter 1: 6, 7, 10, 13, 14, 20, 22, 23, 26, 29, 32, 33, 39, 51-53, 55, 56, 61
Chapter 2: 5-8, 10-14, 17, 18, 23-28, 32-34, 36, 44-47, 49, 51, 53, 55, 72, 76 AND hybridization
practice and resonance problems on website
Chapter 3 1-11, 13-15, 18-26, 28-31, 34, 35, 44, 45, 47-51, 56, 60
Next time: Begin chapter 4
Warm up problem: Draw all resonance structures (having the same number of formal charges for
each of the following anions. Then draw the resonance hybrid. Which is more stable (lower in
energy)? Explain.
Examples of reactions of organic acids with bases:
(predict the direction of the equilibrium)

carboxylic acids, pKa ≈ 5
H
H
N
H
H
O
O
O
pKa = 4.8
O
H
N
H
H
H
pKa = 10
(weaker acid is ammonium ion, so equilibrium lies to RIGHT)

alcohols, pKa = 15-18
H
H
H
N
H
O
O
pKa = 16
H
N
H
H
H
pKa = 10
(weaker acid is the alcohol, so equilibrium lies to LEFT)

carbonyl compounds (C=O), pKa = 20-30
O
O
H
O
pKa = 20
H
O
pKa = 16
(weaker acid is carbonyl compound, so equilibrium lies to LEFT)

alkynes, pKa ≈ 25
H
NH2
pKa = 25
H
N H
H
pKa = 38
(weaker acid is ammonia, so equilibrium lies to RIGHT)
Factors that Determine Acid Strength
Stabilizing the conjugate base makes the acid stronger

Conjugate base stability increases with increasing electronegativity (across a period)
H
H
C
H
pKa = 50
(weakest acid)
H
N
H
+
Na
OH
H
O
H
H
+
C
Na
H
least stable conjugate base:
anion is least electronegative element
H
N
+
Na
OH
H
O
Na+
H
pKa = 40
Na+
O
O
H
+
Na
OH
H
O
H
most stable conjugate base:
anion is most electronegative element
pKa = 17
(strongest acid)

Conjugate base stability increases with increasing anion size
o This is often more important than electronegativity
S
+
Na
H
+
Na
OH
H
O
S
more stable conjugate base:
anion is on larger element
H
pKa = 8
(stronger acid)
+
O
Na
H
pKa = 10
(weaker acid)
+
Na
OH
H
O
O
H
less stable conjugate base:
anion is on smaller element
o Another set of examples:
Na+
H F
pKa = 3.2
H Cl
Na+
OH
OH
+
Na
H
OH
H
O
H
pKa = -7
H Br
pKa = -9
O
H
H
O
Na+ F
Na+ Cl
+
H
least stable conjugate base:
anion is on smallest element
Na
Br
morst stable conjugate base:
anion is on largest element
Conjugate bases are stabilized by inductive effects
o An inductive effect is an electron withdrawing effect of an electronegative atom through
a  (single bond).
o For each pair, circle the more acidic compound
O
O
F
OH
O
F
O
H
H
O
H
H
O
O
H
H
O
OR
H
H
H
O
H
O
OH
O
H
F
O
More stable base (so stronger acid)
because anion is stabilized by inductive
effect from F.
O
No F means no inductive effect and no added
stabilization. Stronger base, so weaker acid.
O
H
O
O
OR
OH
OH
F F
H
O
O
H
O
H
O
H
F
H
O
O
H
F F
F F
O
H
O
O
H
H
O
O
H
H
More stable base (so stronger acid)
More Fs mean more inductive effect,
and greater anion stabilization.
O
H
F
F
O
O
OH
OR
F
OH
F
O
O
H
H
O
H
H
H
O
O
O
H
F
F
F
O
H
O
O
H
H
O
H
H
O
H
F
O
More stable base (so stronger acid)
F is closer to anion, so stronger inductive effect,
and greater anion stabilization.
O
O
OH
OR
OH
F
Cl
O
O
H
H
O
H
H
H
O
O
F
F
O
O
Cl
O
H
H
H
O
H
H
H
O
More stable base (so stronger acid)
F is more electronegative, so stronger inductive effect,
and greater anion stabilization.
O
O
H
Cl
Summary of Inductive Effects and pKa


The greater the inductive effect, the more stable the conjugate base.
Inductive effects depend on the number and location of electronegative atom. They are
increased when:
o A greater number of electronegative atoms is present
o The electronegative atoms are closer to the negative charge
o An atom with greater electronegativity is present (ex. F vs. Cl)

Conjugate bases are stabilized by resonance (so resonance opportunities affect pKa)
o
o
o
Delocalization of negative charge is stabilizing
The more resonance structures, the more stable the base, the stronger the acid
For each pair, circle the more acidic compound
O
O
OR
H
H
O
O
+
Na
OH
H
O
H
+
Na
H
O
O
+
Na
H
conjugate base has one
additional resonance structure
OH
H
O
H
+
Na
conjugate base has no
additional resonance structures
Conjugate bases may be stabilized by the orbitals that the lone pairs occupy:
o Closer the charge is to the nucleus, the more stable it is.
least stable
most stable
sp2
sp3
25% s character

sp
33% s character
50% s character
Rank the following in order of increasing acidity (1 = weakest, 3 = strongest):
2
3
1
H
H
H
Summary of 4 factors affecting acid strength (Figure 2.5, p. 71):
Lewis Acid / Base theory (more broad than Bronsted-Lowry)
acid = electron pair acceptor = electrophile
base = electron pair donor = nucleophile
Types of Lewis acids:
1. Bronsted acids (polar covalent bonds to hydrogen)
Curved arrow (reaction mechanism) example: H—O—H +
H O
H
H Cl
H—Cl
H O
H
H
Cl
2. Atoms with vacant, low energy orbitals (may or may not have positive charge)
an empty p orbital (boron, aluminum, or carbocation)
b. Empty d orbital (transition metals)
Cl
H N
H
H
Ti
Cl
Cl
H
Cl
+
H
N
H
Cl
Ti
Cl
c. s orbital (group I or group II metal cations Li+, Mg2+)
H O
H
Li
H O
H
Li
Cl
Cl
Types of Lewis bases:
Any compound with at least one lone pair of electrons OR a multiple bond (carbonyl, alcohol, sulfide,
amine, alkene, etc.)
*
*
O
C
carbonyl
*
*
N
O
H
S
sulfide
alcohol
amine
* Lewis basic site
Another Example:
Cl
Cl
Al Cl
Cl
*
Cl
Al
Cl
alkene