Amino acids (A-level only)

What you'll learn

What an amino acid is, and which functional groups it contains.
Why amino acids can behave as both acids and bases.
How zwitterions form.
How to draw amino acids in acid solution and alkaline solution.

The starting point: functional groups

Amino acids are organic compounds that contain two important functional groups:

an amino group, –NH₂
a carboxyl group, –COOH

A functional group is the atom or group of atoms in an organic molecule that gives it its characteristic chemical reactions.

Most amino acids you meet in this topic are α-amino acids. This means the amino group and carboxyl group are attached to the same carbon atom, called the alpha carbon. The general structure is:

H2N−CH(R)−COOH\mathrm{H_2N{-}CH(R){-}COOH}

The R group is the side chain. It changes from one amino acid to another. For example, in glycine, R is H; in alanine, R is CH₃.

Definition

Amino acid

An amino acid is an organic compound containing both an amino group, –NH₂, and a carboxyl group, –COOH.

Key Idea

The two important groups

For this sub-topic, most of the chemistry comes from just two groups: –COOH can donate a proton, and –NH₂ can accept a proton.

Acids, bases and protons

In this topic, use the Brønsted–Lowry definitions:

an acid is a proton donor
a base is a proton acceptor

A proton is written as $H+\mathrm{H^+}$ . Strictly, in water it is usually present as $H3O+\mathrm{H_3O^+}$ , but A-Level organic chemistry commonly writes $H+\mathrm{H^+}$ as shorthand.

The carboxyl group can act as an acid:

RCOOH⇌RCOO−+H+\mathrm{RCOOH \rightleftharpoons RCOO^- + H^+}

The amino group can act as a base:

RNH2+H+⇌RNH3+\mathrm{RNH_2 + H^+ \rightleftharpoons RNH_3^+}

Because amino acids contain both groups, they can show both acidic and basic behaviour.

Definition

Amphoteric

A substance is amphoteric if it can behave as both an acid and a base. Amino acids are amphoteric because they contain both –COOH and –NH₂.

Zwitterions

Amino acids often exist as zwitterions. A zwitterion is a species with both a positive charge and a negative charge in the same molecule, but with no overall charge.

For a simple α-amino acid, the carboxyl group donates a proton to the amino group:

H2N−CH(R)−COOH⇌H3N+−CH(R)−COO−\mathrm{H_2N{-}CH(R){-}COOH \rightleftharpoons H_3N^+{-}CH(R){-}COO^-}

So the amino acid has:

–NH₃⁺, a positively charged ammonium group
–COO⁻, a negatively charged carboxylate group

The total charge is zero overall, because +1 and −1 cancel.

Diagram showing the zwitterion, acid solution form, and alkaline solution form of a general alpha-amino acid

Example

Drawing the zwitterion of alanine

Alanine has the structure $H2N−CH(CH3)−COOH\mathrm{H_2N{-}CH(CH_3){-}COOH}$ . Draw its zwitterion.

Identify the side chain: in alanine, the R group is CH₃, so the carbon skeleton stays as $−CH(CH3)−\mathrm{{-}CH(CH_3){-}}$ .
Change the carboxyl group from –COOH to –COO⁻, because it has donated a proton.
Change the amino group from –NH₂ to –NH₃⁺, because it has accepted that proton.
Combine the unchanged side chain with the new charged groups:
$H3N+−CH(CH3)−COO−\mathrm{H_3N^+{-}CH(CH_3){-}COO^-}$

Common Mistake

Forgetting the molecule is neutral overall

A zwitterion is not a positive ion and it is not a negative ion overall. It has internal charges, but the total charge is zero.

Amino acids in acid solution

An acid solution contains an excess of protons, $H+\mathrm{H^+}$ .

In acid solution, the amino acid becomes more protonated. Protonated means a species has gained a proton.

For a simple amino acid:

the amino group is protonated as –NH₃⁺
the carboxylate group is also protonated, becoming –COOH

So the acid solution form is:

H3N+−CH(R)−COOH\mathrm{H_3N^+{-}CH(R){-}COOH}

This has an overall charge of +1, so it is a cation. A cation is a positively charged ion.

Key Idea

Acid solution form

In acid solution, a simple amino acid is fully protonated: –NH₃⁺ and –COOH.

Amino acids in alkaline solution

An alkaline solution contains hydroxide ions, $OH−\mathrm{OH^-}$ .

In alkaline solution, protons are removed. Deprotonated means a species has lost a proton.

For a simple amino acid:

the carboxyl group is deprotonated to –COO⁻
the ammonium group loses a proton, becoming –NH₂

So the alkaline solution form is:

H2N−CH(R)−COO−\mathrm{H_2N{-}CH(R){-}COO^-}

This has an overall charge of −1, so it is an anion. An anion is a negatively charged ion.

Key Idea

Alkaline solution form

In alkaline solution, a simple amino acid is fully deprotonated: –NH₂ and –COO⁻.

Example

Drawing acid and alkaline forms of glycine

Glycine has the structure $H2N−CH2−COOH\mathrm{H_2N{-}CH_2{-}COOH}$ . Draw the ions formed in acid solution and alkaline solution.

Identify the two reactive groups: glycine contains an amino group, –NH₂, and a carboxyl group, –COOH.
In acid solution, add a proton to the amino group and keep the carboxyl group protonated. The structure is:
$H3N+−CH2−COOH\mathrm{H_3N^+{-}CH_2{-}COOH}$
Check the charge in acid solution: –NH₃⁺ gives +1, while –COOH is neutral, so the ion has overall charge +1.
In alkaline solution, remove a proton from the carboxyl group and keep the amino group as –NH₂. The structure is:
$H2N−CH2−COO−\mathrm{H_2N{-}CH_2{-}COO^-}$
Check the charge in alkaline solution: –NH₂ is neutral, while –COO⁻ gives −1, so the ion has overall charge −1.

A quick decision method

For the simple amino acids in this part of the specification, think in terms of protonation level.

Acid solution: add protons

The acid form is the most protonated form:

H3N+−CH(R)−COOH\mathrm{H_3N^+{-}CH(R){-}COOH}

Zwitterion: one internal proton transfer

The zwitterion has one positive and one negative charge:

H3N+−CH(R)−COO−\mathrm{H_3N^+{-}CH(R){-}COO^-}

Alkaline solution: remove protons

The alkaline form is the most deprotonated form:

H2N−CH(R)−COO−\mathrm{H_2N{-}CH(R){-}COO^-}

Tip

Memory shortcut

Acid adds/keeps H: –NH₃⁺ and –COOH. Alkali removes H: –NH₂ and –COO⁻.

Common Mistake

Changing the carbon skeleton

When converting between forms, do not change the R group or the carbon chain. Only change the hydrogens and charges on the amino and carboxyl groups.

Why zwitterions matter

Zwitterions help explain why amino acids often behave more like ionic compounds than typical small covalent organic molecules.

Because zwitterions contain charged groups, amino acids often have:

relatively high melting points
good solubility in water
poor solubility in non-polar solvents

This comes from strong attractions between oppositely charged groups and interactions with polar water molecules.

Common Mistake

Side chains can complicate things

Some amino acids have acidic or basic side chains, but for this specification point you are usually expected to focus on the amino group and carboxyl group in the main amino acid structure.

Exam technique

In the exam

If asked for the zwitterion, draw both charges: –NH₃⁺ and –COO⁻, with no overall charge.
If asked for the ion in acid solution, draw the fully protonated form: –NH₃⁺ and –COOH.
If asked for the ion in alkaline solution, draw the fully deprotonated form: –NH₂ and –COO⁻.

Self review

Check yourself

What two functional groups are present in an amino acid?
Why does a zwitterion have no overall charge?
What are the structures of glycine in acid solution and alkaline solution?

What you'll learn

What an amino acid is, and which functional groups it contains.
Why amino acids can behave as both acids and bases.
How zwitterions form.
How to draw amino acids in acid solution and alkaline solution.

The starting point: functional groups

Amino acids are organic compounds that contain two important functional groups:

an amino group, –NH₂
a carboxyl group, –COOH

A functional group is the atom or group of atoms in an organic molecule that gives it its characteristic chemical reactions.

H2N−CH(R)−COOH\mathrm{H_2N{-}CH(R){-}COOH}

The R group is the side chain. It changes from one amino acid to another. For example, in glycine, R is H; in alanine, R is CH₃.

Definition

Amino acid

An amino acid is an organic compound containing both an amino group, –NH₂, and a carboxyl group, –COOH.

Key Idea

The two important groups

For this sub-topic, most of the chemistry comes from just two groups: –COOH can donate a proton, and –NH₂ can accept a proton.

Acids, bases and protons

In this topic, use the Brønsted–Lowry definitions:

an acid is a proton donor
a base is a proton acceptor

A proton is written as $H+\mathrm{H^+}$ . Strictly, in water it is usually present as $H3O+\mathrm{H_3O^+}$ , but A-Level organic chemistry commonly writes $H+\mathrm{H^+}$ as shorthand.

The carboxyl group can act as an acid:

RCOOH⇌RCOO−+H+\mathrm{RCOOH \rightleftharpoons RCOO^- + H^+}

The amino group can act as a base:

RNH2+H+⇌RNH3+\mathrm{RNH_2 + H^+ \rightleftharpoons RNH_3^+}

Because amino acids contain both groups, they can show both acidic and basic behaviour.

Definition

Amphoteric

A substance is amphoteric if it can behave as both an acid and a base. Amino acids are amphoteric because they contain both –COOH and –NH₂.

Zwitterions

Amino acids often exist as zwitterions. A zwitterion is a species with both a positive charge and a negative charge in the same molecule, but with no overall charge.

For a simple α-amino acid, the carboxyl group donates a proton to the amino group:

H2N−CH(R)−COOH⇌H3N+−CH(R)−COO−\mathrm{H_2N{-}CH(R){-}COOH \rightleftharpoons H_3N^+{-}CH(R){-}COO^-}

So the amino acid has:

–NH₃⁺, a positively charged ammonium group
–COO⁻, a negatively charged carboxylate group

The total charge is zero overall, because +1 and −1 cancel.

Diagram showing the zwitterion, acid solution form, and alkaline solution form of a general alpha-amino acid

Example

Drawing the zwitterion of alanine

Alanine has the structure $H2N−CH(CH3)−COOH\mathrm{H_2N{-}CH(CH_3){-}COOH}$ . Draw its zwitterion.

Identify the side chain: in alanine, the R group is CH₃, so the carbon skeleton stays as $−CH(CH3)−\mathrm{{-}CH(CH_3){-}}$ .
Change the carboxyl group from –COOH to –COO⁻, because it has donated a proton.
Change the amino group from –NH₂ to –NH₃⁺, because it has accepted that proton.
Combine the unchanged side chain with the new charged groups:
$H3N+−CH(CH3)−COO−\mathrm{H_3N^+{-}CH(CH_3){-}COO^-}$

Common Mistake

Forgetting the molecule is neutral overall

A zwitterion is not a positive ion and it is not a negative ion overall. It has internal charges, but the total charge is zero.

Amino acids in acid solution

An acid solution contains an excess of protons, $H+\mathrm{H^+}$ .

In acid solution, the amino acid becomes more protonated. Protonated means a species has gained a proton.

For a simple amino acid:

the amino group is protonated as –NH₃⁺
the carboxylate group is also protonated, becoming –COOH

So the acid solution form is:

H3N+−CH(R)−COOH\mathrm{H_3N^+{-}CH(R){-}COOH}

This has an overall charge of +1, so it is a cation. A cation is a positively charged ion.

Key Idea

Acid solution form

In acid solution, a simple amino acid is fully protonated: –NH₃⁺ and –COOH.

Amino acids in alkaline solution

An alkaline solution contains hydroxide ions, $OH−\mathrm{OH^-}$ .

In alkaline solution, protons are removed. Deprotonated means a species has lost a proton.

For a simple amino acid:

the carboxyl group is deprotonated to –COO⁻
the ammonium group loses a proton, becoming –NH₂

So the alkaline solution form is:

H2N−CH(R)−COO−\mathrm{H_2N{-}CH(R){-}COO^-}

This has an overall charge of −1, so it is an anion. An anion is a negatively charged ion.

Key Idea

Alkaline solution form

In alkaline solution, a simple amino acid is fully deprotonated: –NH₂ and –COO⁻.

Example

Drawing acid and alkaline forms of glycine

Glycine has the structure $H2N−CH2−COOH\mathrm{H_2N{-}CH_2{-}COOH}$ . Draw the ions formed in acid solution and alkaline solution.

Identify the two reactive groups: glycine contains an amino group, –NH₂, and a carboxyl group, –COOH.
In acid solution, add a proton to the amino group and keep the carboxyl group protonated. The structure is:
$H3N+−CH2−COOH\mathrm{H_3N^+{-}CH_2{-}COOH}$
Check the charge in acid solution: –NH₃⁺ gives +1, while –COOH is neutral, so the ion has overall charge +1.
In alkaline solution, remove a proton from the carboxyl group and keep the amino group as –NH₂. The structure is:
$H2N−CH2−COO−\mathrm{H_2N{-}CH_2{-}COO^-}$
Check the charge in alkaline solution: –NH₂ is neutral, while –COO⁻ gives −1, so the ion has overall charge −1.

A quick decision method

For the simple amino acids in this part of the specification, think in terms of protonation level.

Acid solution: add protons

The acid form is the most protonated form:

H3N+−CH(R)−COOH\mathrm{H_3N^+{-}CH(R){-}COOH}

Zwitterion: one internal proton transfer

The zwitterion has one positive and one negative charge:

H3N+−CH(R)−COO−\mathrm{H_3N^+{-}CH(R){-}COO^-}

Alkaline solution: remove protons

The alkaline form is the most deprotonated form:

H2N−CH(R)−COO−\mathrm{H_2N{-}CH(R){-}COO^-}

Tip

Memory shortcut

Acid adds/keeps H: –NH₃⁺ and –COOH. Alkali removes H: –NH₂ and –COO⁻.

Common Mistake

Changing the carbon skeleton

When converting between forms, do not change the R group or the carbon chain. Only change the hydrogens and charges on the amino and carboxyl groups.

Why zwitterions matter

Zwitterions help explain why amino acids often behave more like ionic compounds than typical small covalent organic molecules.

Because zwitterions contain charged groups, amino acids often have:

relatively high melting points
good solubility in water
poor solubility in non-polar solvents

This comes from strong attractions between oppositely charged groups and interactions with polar water molecules.

Common Mistake

Side chains can complicate things

Some amino acids have acidic or basic side chains, but for this specification point you are usually expected to focus on the amino group and carboxyl group in the main amino acid structure.

Exam technique

In the exam

If asked for the zwitterion, draw both charges: –NH₃⁺ and –COO⁻, with no overall charge.
If asked for the ion in acid solution, draw the fully protonated form: –NH₃⁺ and –COOH.
If asked for the ion in alkaline solution, draw the fully deprotonated form: –NH₂ and –COO⁻.

Self review

Check yourself

What two functional groups are present in an amino acid?
Why does a zwitterion have no overall charge?
What are the structures of glycine in acid solution and alkaline solution?

What you'll learn

The starting point: functional groups

Acids, bases and protons

Zwitterions

Amino acids in acid solution

Amino acids in alkaline solution

A quick decision method

Acid solution: add protons

Zwitterion: one internal proton transfer

Alkaline solution: remove protons

Why zwitterions matter

Review Amino acids (A-level only) by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Amino acids (A-level only)

What you'll learn

The starting point: functional groups

Acids, bases and protons

Zwitterions

Amino acids in acid solution

Amino acids in alkaline solution

A quick decision method

Acid solution: add protons

Zwitterion: one internal proton transfer

Alkaline solution: remove protons

Why zwitterions matter

Review Amino acids (A-level only) by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Amino acids (A-level only)

What you'll learn

The starting point: functional groups

Acids, bases and protons

Zwitterions

Amino acids in acid solution

Amino acids in alkaline solution

A quick decision method

Acid solution: add protons

Zwitterion: one internal proton transfer

Alkaline solution: remove protons

Why zwitterions matter

Review Amino acids (A-level only) by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Amino acids (A-level only) Revision Guide

Amino acids (A-level only)

What you'll learn

The starting point: functional groups

Acids, bases and protons

Zwitterions

Amino acids in acid solution

Amino acids in alkaline solution

A quick decision method

Acid solution: add protons

Zwitterion: one internal proton transfer

Alkaline solution: remove protons

Why zwitterions matter

Review Amino acids (A-level only) by teaching Genie

Recap your knowledge with an interactive lesson

Remember key concepts with flashcards

Amino acids (A-level only) Revision Guide