Chapter 4: Acids and Bases
Table of contents of main topics:
Arrhenius Acids and Bases Brønsted-Lowry Acids and Bases
Acid Dissociation Constants, pKa, and the Relative Strengths of Acids and Bases
The Position of Equilibrium in Acid-Base Reactions
Molecular Structure and Acidity
Lewis Acids and Base
Arrhenius Acids and Bases Brønsted-Lowry Acids and Bases
Arrhenius Acid = gives H+ ions when dissociated in water
Arrhenius Base = gives OH- ions when dissociated in water
Bronsted Lowry Acid = proton( H+) donor
Bronsted Lowry Base = proton( H+) acceptor
Conjugate Acid Base Pair
Conjugate Acid Base pair differ in one hydrogen only. Acid will have one more hydrogen than base.
For example, HCl and Cl- are conjugate acid base pair. They differ in on hydrogen only. HCl has one more hydrogen than Cl-.
Acid Dissociation Constants, pKa, and the Relative Strengths of Acids and Bases
pKa tells us how strong the acid is. The smaller the pKa value, the stronger the acid is. The stronger the acid, the weaker its conjugate base and vice versa.
pKa = -log Ka
Some students in organic chemistry classes are asked to memorize pka values of common molecules and functional groups for the test.
The Position of Equilibrium in Acid-Base Reactions
In an acid-base reaction, equilibrium always favors the formation of weaker acid or base
How to calculate the Equilibrium Constants for Acid-Base Reactions
Look up the pKa values of the acid and conjugate acid in the reactants and the products.
Subtract the pKa of the conjugate acid (product) from the pKa of the acid (reactant).
Do 10^-pka to find Keq (equilibrium constant)
Molecular Structure and Acidity
pKa Values Table
It is very common for organic chemistry students to be asked to arrange molecules in order of acidity. One way is memorizing the pka values for the common molecules and functional groups
Another way to compare acidities is by looking at the molecular structure.
Take off the hydrogen to get the conjugate base. The more stable the conjugate base, the more acidic was the molecule.
Stability Of Conjugate Base:
Having a negative charge on the more electronegative atom, makes the base more stable. As we go from left to right on periodic table, acidity increases. CH3OH>CH3NH2
Acidity increases down the group on the periodic table (size increases). As the size of an atom increases, negative charge is spread over a greater volume of space and is more stable. CH3SH>CH3OH
The more resonance stabilized the conjugate base is, the stronger is the acid. For example, carboxylic acid is a stronger acid than alcohol.
Inductive effect. Electronegative groups adjacent to the carbon that has the acidic hydrogen make the molecule more acidic.
% S Character. The more s character in a hybrid orbital the more acidic the hydrogen bonded to it. Alkyne is more acidic than alkene and alkene is more acidic than alkane.
Lewis Acids and Base
Lewis acid is an electron pair acceptor.
Lewis base is an electron pair donor.
In reactions, nucleophile (the attacker) is analogues to Lewis base and electrophile
(the molecule being attacked) is analogoues to Lewis Acid.
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