Ch.2
Acids and Bases
There are many good reasons for understanding the concepts of acid/base chemistry, but for us, in
organic chemistry, the concepts of Lewis acids and base and electron movement will be central.
2.1
Arrhenius definition
Arrhenius definition of acids: substances produce H+ or increase the H3O+ ion concentration in water
E.g
Arrhenius definition of bases: substances produce –OH or increase the -OH ion concentration in water
2.2
Bronsted definition
Bronsted definition of an acid: a proton donor (no reference to the solvent)
E.g:
HCl, H2SO4, CH3CO2H, HNO3, HCN, H2CO3, HClO4, H2O, etc…
Bronsted definition of a base: a proton acceptor (no reference to the solvent)
E.g:
-OH,
HSO4-, CH3CO2-, NO3-, NC-, HCO3-, H-, NH3, etc..
Chm 201 – Dang1
Neutralization reaction (various solvents)
Acid
Base
Salt
Solvent
HCl(aq)
+
NaOH(aq)
NaCl(aq)
+
H2O(l)
solvent = H2O
NH4Cl(aq)
+
NaNH2(aq)
NaCl(aq)
+
2NH3(aq)
solvent = NH3
HCl(aq)
+
CH3CO2Na(aq)
NaCl(aq)
+ CH3CO2H (aq)
E.g
solvent = CH3CO2H
Indicate which of the following can act as a Bronsted acid, base or both.
2.3 and 2.4 Acid-base strength and position of equilibrium
* Two acids are in completion: forward and reverse reactions are in equilibrium
** Equilibrium lies in the direction of the ……………….......acid/base pair**
When a strong acid is dissolved in water, almost all the molecules dissociate, which means the products
are favored at equilibrium.
When a much weaker acid is dissolved in water, very few molecules dissociate, so the reactants are
favored at equilibrium
Chm 201 – Dang2
Acid dissociation constant is defined as the degree to which an acid (HA) dissociate in water
For simplicity, pKa is used instead of Ka to
describe the characteristic of a particular
compound
pKa = -log (Ka)
p Ka < 1
very strong acid
p Ka = 1 – 3
moderately strong acid
p Ka = 3 – 5
weak acid
p Ka = 5 – 15
very weak acid
p Ka > 15
extremely weak acid
The acidity of an acid solution can be measured
from the concentration of H3O+ ion
pH = log [H3O+]
pH < 7
acidic
pH > 7
Basic
Chm 201 – Dang3
E.g
If the pKa of an acid is 4.5 then what is its Ka value?
E.g
Predict the products then use the pKa value table and quantitatively predict whether the equilibria
below favor the reactants or the products
Organic acids and Bases
Common organic acids are carboxylic acids (pKa = 3-5)
Amine (R-NH2) is an organic molecule that can behave as an acid or a base
Chm 201 – Dang4
E.g
Write an equation for the acid-base reaction of the following acid-base reaction. Be sure to use
arrows to show the movement of electrons.
2.5
How the structure of an acid affects its pKa?
Recall: The weaker the base, the stronger its conjugated acid is and the more stable the base, the
stronger its conjugated acid is.
Three factors:
For acids of element in the same row of the periodic table, changes in H—A bond strength are smaller and
the polarity of H-A bond is the most important determinant of acid strength
Electronegativities
A more electronegative element bears a negative charge more easily, giving more stable conjugated base
and a stronger acid.
(least)
C <
N <
O < F
(most)
Relative acidities
(weakest acid)
CH4 < NH3 < H2O < HF (strongest acid)
Stabilities
(strongest conjugated base)
-CH3
> -NH2 > -OH > F- (weakest conjugated base)
Conclusion: the stronger acid is the one with the most stable (less reactive, weaker) conjugate
base!
Chm 201 – Dang5
The size
When atoms are different in size, the strongest acid will have its hydrogen attached to the largest atom
pKa
3
-7
-9
-10
Relative stabilities
(least stable)
F- < Cl- < Br- < I- (most stable)
(weakest acid)
HF < HCl < HBr < HI (strongest acid)
Why? The degree of orbital overlap between the small Hydrogen 1s orbital with the halogen orbital is
sometimes used to rationalize the given bond energies. Fluorine (n = 2 valence shell) is small with
concentrated electron density, which can efficiently overlap with the small hydrogen 1s orbital iodine (n
= 5 valence shell), has very larger and diffuse valence orbitals, which lead to poor overlap with the small
hydrogen 1s orbital. We can therefore argue that the H-F requires a large amount of energy to break its
bond, while the H-I bond is broken more easily.
Chm 201 – Dang6
E.g
Which species is a stronger acid?
a)
H2S or HCl
E.g
Which species is a stronger base?
a)
E.g
HO- or HS-
b)
+PH4
b)
or +NH4
CH3CN: or CH3O-
Which is the most acidic hydrogen in each of these compounds?
Inductive effects: is a donation of electron density through the sigma bond that is a group can withdraw
(pull) or donate (push) electrons through sigma framework, based on relative electronegativities.
Chm 201 – Dang7
Donating electron density will stabilize a positive or partial positive center and destabilize partial
negative center. In the opposite sense, electron withdrawal will stabilize a negative center and
destabilize positive center.
E.g
Which is the stronger acid, CH3OH vs. CF3OH
Draw the conj. Base
Resonance effect
The Effect of pH on the structure of an Organic Compound
A compound will exist primarily in its acidic form (with its proton) in solution that are more acidic than
the pKa value of the group that undergoes dissociation and primarily in its basic form (without its
proton) in solution that are more basic than the pKa value of the group that undergoes dissociation.
Chm 201 – Dang8

If the pKa of an acid is greater than the pH of a solution, then it will exist in the basic form (without
its proton)

If the pKa of an acid is smaller than the pH of a solution, then it will exist in the acid form (without
its proton)
E.g
Draw the form in which the following compounds will predominate in a solution of pH 6.5
a.
HNO2 (pKa = 3.4)
b.
+NH4 (pKa
2.6
Lewis Acids and Bases
= 9.4)
The Lewis definitions provide a very useful way of viewing acids and base in chemistry. In organic
chemistry, Lewis acid will also be called electrophiles (electron or negative loving) and Lewis base will
also be called nucleophiles (nucleus or positive loving).
Lewis base = Nucleophile = electron pair donor (curve arrows start here)
Lewis acid = Electrophile = electron pair acceptor (curve arrows point here)
Chm 201 – Dang9