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Acids and Bases

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Aromatic Bases

By way of introduction, let us first consider the simplest aromatic base, aniline.

Aliphatic Bases – Amides

If we attach electron withdrawing groups to an amine, we can significantly reduce its basicity by removing the tendency for the electron pair of the nitrogen to attack a proton.

Aliphatic Bases – Amines

The trends of bases of the type NR3, where R is an alkyl group or hydrogen, can be explained quite well, and quite simply.

Heterocyclic Bases

Two very important bases in organic chemistry are pyrrole and pyridine. These are heterocyclic bases. Firstly, what is a heterocycle? A heterocycle is a cyclic compound with a heteratom in the ring. A heteroatom in this context is any atom other than carbon.

Acid Catalysis

Catalysis provides a reaction a lower energy pathway, so that the reaction proceeds at a greater rate. Acids can act as catalysts in certain situations. There are two types of acid catalysis; Specific Acid Catalysis and General Acid Catalysis.

Ester Hydrolysis

Esters can be hydrolysed by either aqueous solutions of acid or base. The products in both cases are an alcohol and a carboxylic acid.

Base Catalysis

As with acids, there are two types of base catalysis; specific and general.

Base Strength

Just as with acid strength, base strength can be determined by considering an equilibrium.

Temperature Dependence of pKa

We know from previous discussions that Ka is not an absolute value for a particular substance, because it depends intimately on the surroundings; such as solvent.

Strength of the HA bond

One critical factor in determining the strength of acids can be the strength of the HA bond. Clearly if the proton is very tightly held by the rest of the molecule, it will require a lot of energy to remove it, and hence the acid will be weak.

Acid Strength

Organic acids have a wide range of strengths. The following discussion will explain some of the trends observed, by reference to Brønsted acids.

pKa and pKb

The strength of an acid can be determined by considering the extent to which it dissociates in a given solvent (i.e. to which side this equilibrium lies).

Solvent Effects

Water is the most common solvent with respect to acids and bases. Often, the critical factor in determining the strength of an acid is played by the solvent, especially when that solvent is water.

Other Factors Affecting Acid/Base Strength

This discussion will focus on factors that are internal to the acid which affect its strength.

Dicarboxylic Acids

A carboxyl group is inductively electron withdrawing. As we have already shown, the presence of electron withdrawing groups can be expected to increase the acidity of compounds.

Phenols

The effect of substituents on phenols is not as straightforward as with aliphatic acids. Phenol itself is weakly acidic, with a pKa of 9.95.

Introduction to Acids and Bases

There are several definitions of acids and bases. The two most useful and most commonly encountered are the Lewis and Brønsted definitions.