Amino acids, amides and chirality (6.2.2)

Amino acids have both an amino and carboxylic acid group.

Amino groups are Brønsted–Lowry bases; they accept protons to form ions.

Carboxylic acids are Brønsted–Lowry acids, and donate protons to form ions.

Amino acids that are used in protein formation are known as alpha () amino acids, or 2-amino acids.

The carbon after the carboxylic acid group carbon, (carbon 2), is the carbon and also holds the amino group, a hydrogen atom and the variable group, .

At values close to seven, the amino acid contains both a group and group.

A molecule that contains both positive and negative ions, whilst remaining neutral overall, is called a zwitterion.

As they contain both positive and negative ions, the zwitterions can form a giant ionic lattice; therefore, pure amino acids form crystalline solids.

In acidic conditions, the basic part of the amino acid is protonated.

The amino group is basic and the lone pair of electrons on the amino nitrogen can be donated to a proton in a dative covalent bond. The product is an ammonium salt made charge neutral by the anion from the acid used.

Note that the carboxylic acid group also remains protonated in acidic conditions.

In alkaline conditions, there is a high concentration of ions, and so the acidic part of the amino acid is deprotonated.

The carboxylic acid group acts as a Brønsted–Lowry acid, and donates a ion to the solution. The products are a carboxylate salt with the cation from the base, and water: a neutralisation reaction.

Note that the amine group remains deprotonated and uncharged in alkaline conditions.

The carboxylic acid group of an amino acid, can undergo a condensation reaction with an alcohol in the presence of a strong acid catalyst, such as sulfuric acid, to form an ester and water.

The reaction is reversible and slow.

Although the conditions remain acidic, the amine group will be protonated.

Amides are functional groups also based on amines, with a hydrogen atom substituted for an acyl group, .

A primary amide has the general formula , with the nitrogen only bonded to one carbon atom, retaining two hydrogen atoms.

The acyl group in an amide can originate from acyl chlorides, acid anhydrides, or carboxylic acids.

A secondary amide has the structure .

In a secondary amide the nitrogen is bonded to two carbon atoms and only one hydrogen.

A chiral carbon atom that bonds to four different atoms or groups has no plane of symmetry. This is known as a chiral centre.

A chiral carbon atom can exist as optical isomers.

Optical isomers are based on the tetrahedral geometry.

In most cases, they are drawn with two bonds on the plane of the paper (lines), one bond emerging (bold wedge) and one bond receding (dashed wedge).