Chapter 4 Acid - Base Chemistry
4-
Brønsted-Lowry and Lewis Acids/Bases Acid Dissociation Constants, pKa, the Relative strength of Acids and Bases. [electron pushing, arrows, electronic view rxs.]
Equilibrium in Acid-Base Reactions Molecular structure and acidity [periodicity: electronegativity & size of atom hybridization, inductive effects, resonance]
See the Chapter Summary
H
H
CH3
F H3C
H
O
HO
H3C F H3C
4-
OH C
C
C
H
H
O
+
+
C
H
Chapter 4
δ+ Br Mg
H
H
δ− CH3 LiCu
OH
C
H
O
CH2
I C
H3C D
acid-base strength
CH3
general, Bronsted Lowry, Lewis Acid-Base
h t n
CH3
H
s i s e
2
H
(-)
I
(-):CN:
O
O
NC D H3C
H
H3C
X
Cl
H
H 3C O
H2 / M
O
sy
structure effects on acidity
b5
O OH
ab
H
O
O
b2
b4 OH
oad
OH
j
b4
ma
p
k i
c3
b1
OR
Br
OH O
is r
OH
Br
4-
hes
OR
b7
b6
O
syn t
road trip started
Cl
O
c4
b1 b
u
OR
v
OR
h z
Nuc(Anionic)
b3 g
r
t
Br
c a
Br
OH
l
OH
m
o
f
Br
n
1 other product OHOH
e
2
R
a7 Br O Br
R
NH2
Li
CuLi
OH
O
a2 y
OH
w
u
OR
u
a8
b
c
x
H O H
OH OH
2 other product
HO
H
H
CH3
OH C
H3C F H3C
F H3C
H
4-
O
HO C
C H
H O
+
C
H
Chapter 4
δ+ Br Mg
H
OH
C
H
O
CH2
O
O
H
δ− CH3
I C
H3C D
acid-base strength
general, Bronsted Lowry, Lewis Acid-Base
th n y
CH3
H
s i s e
2
H
(-)
I
(-):CN:
H
LiCu
NC D H3C
+
H3C
X
Cl
H
H 3C O
H2 / M
O
CH3
s
structure effects on acidity
Arrhenius Acids and Bases
4-
acid: substance that produces H3O+ ions aqueous solution
H+(aq) + H2O(I)
H3O+(aq) hydronium ion
base: substance that produces OH- ions in aqueous solution
KOH
H 2O
K+ + -OH(aq)
Brønsted-Lowry Definitions Acid: a proton donor Base: a proton acceptor
4-
Conjugate Acids & Bases Brønsted-Lowry does not require water only H+ transfer
4-
Pi Electrons As Basic Sites
4-
pi electrons - base - donate a pair of e-s
Result: formation of a carbocation, C has 6-e-s, ∴+1charge
Lewis Acids and Bases Lewis acid: molecule/ion that accepts a pair of electrons Lewis base: molecule/ion that donates a pair of electrons
forms a new covalent bond
4-
Lewis Acids and Bases examples
carbocations very strong Lewis acid
4-
H
H
CH3
OH C
H3C F H3C
F H3C
H
4-
O
HO C
C H
H O
+
C
H
Chapter 4
δ+ Br Mg
H
OH
C
H
O
CH2
O
O
H
δ− CH3
I C
H3C D
acid-base strength
general, Bronsted Lowry, Lewis Acid-Base
th n y
CH3
H
s i s e
2
H
(-)
I
(-):CN:
H
LiCu
NC D H3C
+
H3C
X
Cl
H
H 3C O
H2 / M
O
CH3
s
structure effects on acidity
Acids & Base Strengths
4-
How is acid [base] strength expressed/compared?
By equilibrium constant e.g.: dissociation (ionization) of acetic acid O
+ H3C C O H
H O H
pKa = -log Ka
O H3C C O
+
H
O H
H
4-
3.18 HF
Acid-Base Equilibria
4-
What favors the direction of acid - base reactions?
Strong acid - completely to product(s) Weak - incomplete, gives an Equilibrium direction favors reaction of stronger acid-base pair
pKa 4.76
pKa 9.24
Acid-Base Equilibria
4-
acetic acid + sodium bicarbonate (omit Na+ ) CO2 + H 2O pKa= 4.76
6.36
H
H
CH3
OH C
H3C F H3C
F H3C
H
4-
O
HO C
C H
H O
+
C
H
Chapter 4
δ+ Br Mg
H
OH
C
H
O
CH2
O
O
H
δ− CH3
I C
H3C D
acid-base strength
general, Bronsted Lowry, Lewis Acid-Base
th n y
CH3
H
s i s e
2
H
(-)
I
(-):CN:
H
LiCu
NC D H3C
+
H3C
X
Cl
H
H 3C O
H2 / M
O
CH3
s
structure effects on acidity
Acid-Base Theories
4-
Brønsted-Lowry: acid-proton donor, base-proton acceptor
Lewis: acid-accept pair electrons, base-donate pair electrons
How does Structure effect Acidity?
4-
relative acidities the more A:(-) stable, greater the acidity of H-A
Ways to stabilize A:(-) the negative charge ON a more electronegative atom ON larger atom RESONANCE delocalized STABILIZED by inductive effect IN an orbital with more s character
electronegativity
4-
within a period - the greater the electronegativity of A:(-) the more A:(-) is stablized ∴ the stronger the acid incerasing acidity Conjugate base
H3C N H H H pKa 51 H3C C H H
pKa 38
CH3 O
CH3 N
H
CH3
H C H
increasing basicity
pKa 16 H3C O H
increasing acidity
Acid
4-
Size of A:(-)
Within a column -
the larger the atom bearing the (-), the greater its stability H3C S H pKa 7.0
+ CH3 O
CH3 S
+
H3C O H pKa 16
4-
inductive effect, electron-withdrawing -covalent bonds transmit electronegativity (polarizing) effects -push or pull shared e(-)s of adjacent atoms
decreases w/ distance
4-
inductive effect: O OH pKa 4.82
Cl
butanoic and chlorobutanoic acids Cl
O OH pKa 4.52
O
O OH
pKa 3.98
OH Cl pKa 2.83
4-
Hybridization greater the % s character with (-) the more stable the anion Conjugate pKa (s-character) Base
Weak Acid
H C C H H
H
C C H H H H H C C H H H
HO-H
water
H C C
25
(50%)
44
(33%)
H
C C H H H H C C H H H
HO–
51 15.7
(25%)
Conjugate Acids & Bases
4-
curved arrows show the flow of electrons in an acid-base reaction
recall resonance - also uses curved arrows
4-
Resonance delocalized of charge on A:
(-)
Compare alcohol and carboxylic acid acidity H H H C + H C O O HH H H
CH3
O C O
+
H
H O H
H H C H C O HH
CH3
CH3
O C O
+ H O
+ H O
H
pKa = 15.9
H
H H
pKa = 4.76
O C O
resonance stabilization (of A:-)
Use of resonance theory molecules may have 2 or more sites that can accept a H+ e.g. carboxylic acids, esters, and amides protonation favored where the charge is more delocalized which oxygen is protonated?
4-
Use of resonance theory resonance octets greater contribution even with plus charge on O
4-
Use of resonance theory
4-
H+ on the hydroxyl
∴ H+ on the carbonyl
can “write” contributing structures create & separate charge