Title: An Introduction to Organic Reactions and Their Mechanism
1Chapter 3
- An Introduction to Organic Reactions and Their
Mechanism - Acids and Bases
23.4 Acid Strength
- Strong acids completely ionized or completely
dissociated - Forward reaction is predominated
- Most of HA is dissociated
- Conjugated base is weak and has low attraction
for proton - HA(aq) H2O(l) ? H3O(aq) A-(aq)
3Acids Strength
- Weak acids partially ionized or dissolved
- Reverse reactions is predominated
- Most of HA is undissociate
- Conjugated base is strong and has high attraction
for proton - HC2H3O4(aq) H2O(l) H3O(aq)
C2H3O4-(aq)
4Acid Strength
53.5 The Strength of Acids and Bases Ka and pKa
- The acidity constant, Ka
- a quantitative measure of the strength of an acid
in solution. - It is known as the equilibrium constant for a
chemical reaction known as dissociation in the
context of acid-base reactions - Larger Ka, stronger the acid
6Acidity and pKa
- Logarithm scale of Ka with the base of 10
- pKa - log Ka
- The larger pKa, the smaller extent of
dissociation - Weaker acids
7Relative Strength of selected acids and their
conjugated bases
8Examples
- An acid (HA) has Ka 10-7, what is its pKa?
- Another acid (HB) has Ka 5, what is its pKa?
- Which is the stronger acid?
9Predicting the Strength of Bases
- The strength of acids will decide the how weak or
strong its conjugated bases - Stronger the acids, the weaker will be its
conjugated base - After losing a proton, acid will become a
conjugated base - When a base accepts a proton, the resulting
chemical is called the conjugate acid of that
original base
10Acid and its conjugated base
11Examples
- The pKa of anilinium ion (C6H5NH3) is equal to
4.6. On the basis of this fact, decide whether
aniline (C6H5NH2) is a stronger or weak base than
methylamine
123.6 Predicting the Outcome of Acid-Base Reactions
- Acid-base reactions always favor the formation of
the weaker acid and weaker base - Equilibrium favor the formation of the most
stable (lowest potential energy) species - Stronger acid stronger base ? weaker base
weaker acid
13Example
- Consider the mixing of an aqueous solution of
acetic acid, CH3CO2H (see Table 3.1), and NaOH.
What acid base reaction, if any would take place - H2SO4(aq) NH3(aq) ? NH4(aq) HSO4-(aq)
143.6 Water Solubility as the Result of Salt
Formation
- Recall Longer chain of carbon (4 or more) will
be less soluble - Higher molecular weight of carboxylic compounds
do not dissolve in water - Instead, they dissolved in a strong base to form
a water-soluble salt
15Water Solubility
- We can predict that an amine will react with
aqueous hydrochloric acid
16Water Solubility
- Methylamine and most amines of low molecular
weight are very soluble in water - Formed a water-soluble salt from hydrochloric acid
173.7 Relationships between structure and Acidity
- The strength of a Bronsted-Lowry acid depends on
the extend to which a proton can be separated
from it and transferred to a base. - Removing H ? breaking a bond
- Making the conjugated base more electronegative
- Accepting H ? forming a bond
- Making the conjugated acid less electronegative
183.7 Relationships between structure and Acidity
- Bond strength to proton decreases as we move down
to column - Decreasing effectiveness of orbital overlap
between the hydrogen 1s orbital and the orbitals
of successively larger element in the column - Less effective the orbital overlap, the weaker
bond, stronger acid - Acidity increases from left to right
193.7 Relationships between structure and Acidity
203.7 Relationships between structure and Acidity
213.7 The Effect of Hybridization
223.7 The Effect of Hybridization
- Electrons of 2s orbtials have lower energy than
those of 2p orbitals because these electrons are
much closer to the nucleus - Having more s orbitals means that electrons of
the anion will be lower in energy and more stable
233.7 The Effect of Hybridization
- sp C sH hybridization
- Contain 50 s character
- 1s 1p
- More electronegative spC
243.7 The Effect of Hybridization
- sp2C sH hybridization
- Contains 33.3 s character
- 1s 2p
- Less electronegative than spC
253.7 The Effect of Hybridization
- sp3C-sH hybridization
- Contains 25 s character
- 1s 3p
- Least electronegative C
263.7 The Effect of Hybridization
273.7 The Effect of Hybridization
- Relative basicity of the carbanions
- Ethynide ion is the weakest base
- The more electronegative carbon, the more stable
the anion