Acids and Bases
Arrhenius
Bronsted-Lowry
Lewis
Definitions of Acids/Bases
+
3O
Arrhenius acid - forms H in H2O
-
in
Arrhenius base - forms OH H2O
+
Bronsted-Lowry acid - donates a H (proton)
+
Bronsted-Lowry base - accepts a H (proton)
Lewis acid - accepts an electron pair to form a new bond
Lewis base - donates an electron pair to form a new bond
Measuring Acid Strength
Ka
Acid Strength defined by pKa
HCl + H2O H3O + Cl
[H3O ][Cl ]
Keq =
[HCl][H2O]
[H3O ][Cl ] 7
Ka = K [H2O] =
eq = 10
[HCl]
pKa = -log(K ) = -7
a
Dissociation in H2O
Arrhenius Acid forms H3O+
Bronsted-Lowry Acid donates a H+
Resonance Stabilization
H O
H O
H C C H
+ NaHCO3
O H C C
O
H
H
pKa 4.7
H2CO3 + Na
O
H
Na H O
H C C
O H C C
H O
H
resonance hybrid
Stronger Acid Controls
Equilibrium
HCl + H O
2 H3O + Cl
acid base conjugate conjugate
acid base
pKa = -7 -1.7
stronger weaker
Reaction Described with Arrows
HCl donates a proton
H2O accepts a proton
H
O
H Cl + O + Cl
H H
H H
Equilibrium Reactions
Draw the Conjugate
Acid and Base
CH 3CH 2CH 2OH + H2SO4
Propanol is a Base
H
CH3CH2CH2OH + H2SO4 CH3CH2CH2OH
base acid
+ HSO4
Draw the Conjugate
Acid and Base
CH 3CH 2CH 2OH + Na NH 2
Propanol is an Acid
CH3CH2CH2O H + Na NH2 CH3CH2CH2O Na
acid base c.b.
+ NH 3
c.a.
Some Acid-Base Reactions
The Effect of Resonance on pKa
Identify the Acid and Base
O O
CH 3OH + OCOH CH 3O + HOCOH
Equilibrium Favors Reactants
O O
CH3OH + OCOH CH3O + HOCOH
acid base conj. base conj. ac
pKa 15.5 6.5
Carboxylic Acids can be
Separated from non-acidic
Compounds O
O
OCCH3 O OCCH3 O
C
C H O Na
O
+ Na OH
+ H2O
acid base conj. base conj. acid
Aspirin O
OCCH3 O Na
HCl C
O
Lewis Acids and Bases
B H B H
base acid complex
arrow always goes from the base to the acid
always originate your arrow at the e- donor
Formation of a new bond
H H
BH 3 + NH3 H B N H
acid base
H H
complex
AlCl3
O O
CH3CH + AlCl 3 CH3CH
base acid complex
Lewis Base = Nucleophile
Lewis Acid = Electrophile
Draw the Acid-Base Complexes
CH OCH3 + FeCl3
3
+
CH 3 + H 2O
HOC(CH 3)3 + AlCl 3
H H H
a) H
K H + C C C + H2
C
H C O H C O K
H H
H
O O
H
C H
C
b) CH3 C H O CH3 C H + H2O
H H H
H
O O Li
Li H H
N
c) N
+ H +
H H H
a) H
K H + C C C + H2
C
H C O H C O K
H H
H
O O
H
C H
C
b) CH3 C H O CH3 C H + H2O
H H H
H
O O Li
Li H H
N
c) N
+ H +
Draw Arrows
H H H H
a) H3N + C C C
H3N C
H C O H C O
H H
Br H H
b) CH3O H H C C + CH3OH + Br
C C H H CH3
H H
H H
Cl H CH3 CH3
c)
Cl B N BCl 3 + HN
Cl CH3 CH3
H H H H
a) C C C
H3N H3N C
H C O H C O
H H
Br H H
b) CH3O H H C C + CH3OH + Br
C C H H CH3
H H
H H
Cl H
c)
Cl B N BCl 3 + HN
Cl
Identify the Conjugate Base
(i.e. Which H is most acidic?)
Ha O
H C H
e
Hb + Na NH2 NH3 + C6H7ONa
H Hd
Hc H
Ha O
H C H
e
Hb + Na NH2 NH3 + C6H7ONa
H Hd
Hc H By deprotonating one of the Hb protons, one can
distribute the charge onto three atoms and give a
major resonance structure with the charge on the
Oxygen (the third structure).
Ha O Ha O Ha O
C H H C H H C H
e b e e
Hb Hb
Hd Hd H Hd
H H
Hc H Hc H Hc H