Introduction

α-Amino acids contain both amine and carboxyl groups attached to the same carbon atom, giving them distinctive chemical behaviour essential for biological function and synthetic transformations.

Structure of α-Amino Acids

α-Amino acids follow the general formula RCH(NH₂)COOH, where the α-carbon is bonded to four different groups: an amine group, a carboxyl group, a hydrogen atom, and a variable R group.

This diagram shows the general structure of an α-amino acid, highlighting the α-carbon bonded to an amine group, a carboxyl group, a hydrogen atom, and a variable R group. The R group differs between amino acids and determines their individual properties. Source

Functional Groups Present

• Amine group (–NH₂)

• Carboxyl group (–COOH)

• Side chain (R group), which determines the identity and properties of each amino acid

Key Terminology

Before continuing, it is essential to define the fundamental terms associated with α-amino acids.

The combination of an amine group and carboxyl group in the same molecule creates both acidic and basic behaviour, giving α-amino acids their characteristic amphoteric nature.

The α-Carbon

The α-carbon is positioned adjacent to the carboxyl group. For standard α-amino acids (except glycine, where R = H), this carbon is bonded to four different substituents, allowing chirality, although chirality is covered elsewhere in the specification.

Behaviour of the Carboxyl Group (–COOH)

The carboxyl group of an α-amino acid behaves similarly to all carboxylic acids: it is acidic, donates protons, and undergoes esterification and salt formation.

Reaction with Alcohols: Esterification

The specification states that the carboxyl group forms esters. In the presence of an alcohol and an acid catalyst (commonly concentrated H₂SO₄), esterification occurs.

• Produces esters and water

• The amine group must usually be protonated beforehand to prevent side reactions

This transformation is important because amino acid esters are used in peptide synthesis, protecting the carboxyl group during multi-step procedures.

Reaction with Alkalis

The specification emphasises that the carboxyl group reacts with alkalis. This reaction forms carboxylate salts, which are ionic and soluble in water.

• Reaction with NaOH yields RCH(NH₂)COO⁻Na⁺

• This process is typical of all carboxylic acids

This salt formation is useful for purification and increasing solubility of amino acids in aqueous systems.

Behaviour of the Amine Group (–NH₂)

Amino groups are basic because their nitrogen atom has a lone pair, enabling proton acceptance. This behaviour is central to how α-amino acids interact with acids.

Reaction with Acids: Salt Formation

The specification states that the amine group reacts with acids. When treated with dilute acids such as HCl, the amine group is protonated to form an ammonium salt.

The diagram illustrates how an amino acid’s functional groups respond to acidic and basic conditions. In acid solution, the amine group is protonated, while in base solution the carboxyl group loses a proton, demonstrating amphoteric behaviour. Source

• Produces RCH(NH₃⁺)COOH Cl⁻

• Reaction increases solubility in aqueous acidic environments

Protonating the amine group is important in peptide synthesis and in adjusting solubility for separation techniques.

Zwitterion Formation

α-Amino acids contain both acidic and basic groups, enabling them to form zwitterions, molecules with both positive and negative charges but overall electrical neutrality. This behaviour is related to, but not explicitly stated in, the specification; however, understanding functional group reactivity naturally leads to recognising zwitterionic structure.

• Carboxyl group donates a proton → COO⁻

• Amine group accepts a proton → NH₃⁺

• Internal transfer produces a dipolar ion

Amino acids in zwitterionic form display high melting points and good solubility in water due to strong ionic interactions.

Acid–Base Properties of α-Amino Acids

The dual presence of acidic and basic groups gives rise to amphoteric behaviour.

Amphoteric Nature

• Act as acids by donating a proton from the carboxyl group

• Act as bases by accepting a proton on the amine group

This dual reactivity underpins why amino acids interact differently under acidic, neutral, and basic conditions.

Reaction Summary

• Carboxyl group + alkali → carboxylate salt

• Carboxyl group + alcohol + acid catalyst → ester

• Amine group + acid → ammonium salt

This diagram shows the overall esterification reaction between a carboxylic acid and an alcohol, producing an ester and water. Although shown generically, the same transformation applies to the carboxyl group of amino acids under acidic conditions. Source

These reactions form the foundation of amino acid chemistry and link directly to synthetic applications.

Importance of R Group Variation

Although the specification focuses on core functional groups, it is important to recognise that R groups influence reactivity and physical properties.

Influence on Solubility and Reactivity

• Polar R groups increase solubility

• Non-polar R groups decrease solubility

• Acidic or basic R groups introduce additional reactive sites

This variation plays a major role in biochemical behaviour, though OCR A-Level primarily emphasises recognition of core reactions involving the amine and carboxyl groups.

Summary of Specification Links

The OCR specification for this subsubtopic requires clear understanding of the following:

• Structure of α-amino acids using RCH(NH₂)COOH

• Carboxyl group reactions: ester formation and reaction with alkalis

• Amine group reactions: salt formation with acids

These reactions define the chemical properties of α-amino acids and form the foundation for their behaviour in organic and biological systems.