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Organic Chemistry by Brown, Iverson, Anslyn, Foote | Mayya's Study Guide| Chapter 4

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


The Position of Equilibrium in Acid-Base Reactions
The Position of Equilibrium in Acid-Base Reactions

How to calculate the Equilibrium Constants for Acid-Base Reactions

  1. Look up the pKa values of the acid and conjugate acid in the reactants and the products.

  2. Subtract the pKa of the conjugate acid (product) from the pKa of the acid (reactant).

  3. Do 10^-pka to find Keq (equilibrium constant)

Molecular Structure and Acidity


pKa Values Table

pKa values table
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:

  1. 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

  2. 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

  3. The more resonance stabilized the conjugate base is, the stronger is the acid. For example, carboxylic acid is a stronger acid than alcohol.

  4. Inductive effect. Electronegative groups adjacent to the carbon that has the acidic hydrogen make the molecule more acidic.

  5. % 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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