Topic 19 · page 10 of 13

Amino acids as acids and bases

An amino acid has a basic –NH₂ group and an acidic –COOH group in the same molecule, so it exists as a zwitterion and reacts with both acids and alkalis. Its form depends on the pH.

12 min read19.11(i)

Read first: Names and formulae, Amines and water

The zwitterion 19.11(i)

An amino acid, RCH(NH₂)COOH, has two groups that react with H⁺ and OH⁻ in opposite ways:

the carboxylic acid group is acidic and can donate H⁺;
the amino group is basic and can accept H⁺.

In the solid and in solution near neutral pH, the acid group passes its H⁺ to the amino group inside the same molecule. The result is a zwitterion:

\ce{H2NCHRCOOH <=> H3N+CHRCOO-}

The equilibrium lies almost completely on the zwitterion side. The positive and negative ends of different molecules attract each other ionically. This explains why amino acids are crystalline solids with high melting temperatures and are soluble in water, unlike most organic compounds of their size.

Acid and alkali 19.11(i)

Because the molecule has both groups, an amino acid reacts with acids and with alkalis. A substance that can react as an acid or as a base is amphoteric.

Add acid (low pH). The –COO⁻ group accepts H⁺ and becomes $\ce{-COOH}$ . The amino acid is a cation.
Add alkali (high pH). The –NH₃⁺ group gives H⁺ to OH⁻ and becomes –NH₂. The amino acid is an anion.

The three forms of an amino acid RCH(NH2)COOH. In acid it is the cation, with NH3 plus and a carboxylic acid group. Near neutral pH it is the zwitterion, with NH3 plus and COO minus. In alkali it is the anion H2N CHR COO minus. Adding OH minus moves down the page and adding H plus moves up. — Cation, zwitterion and anion. The shaded groups are the ones that change.

For glycine:

\ce{H3N+CH2COO- + H+ -> H3N+CH2COOH}

\ce{H3N+CH2COO- + OH- -> H2NCH2COO- + H2O}

Experiment: acid and alkali with an amino acid 19.11(i)

PracticalGlycine with acid and alkali

Make up a solution of glycine, for example 0.1 mol dm⁻³, and put the same volume of distilled water in a second beaker.
Measure the pH of each with a calibrated pH meter.
Add 0.1 mol dm⁻³ hydrochloric acid, 1 cm³ at a time, to each beaker. Stir, and record the pH after every addition.
Repeat with fresh samples, using 0.1 mol dm⁻³ sodium hydroxide solution in place of the acid.

Observation. The glycine solution starts close to neutral, at about pH 6. When acid or alkali is added, the pH of the glycine solution changes by much less than the pH of the water does.

Explanation. The zwitterion removes added H⁺ (–COO⁻ becomes $\ce{-COOH}$ ) and removes added OH⁻ (–NH₃⁺ becomes –NH₂). Glycine has a group that reacts with each, so it resists changes in pH. This shows that it has both a basic and an acidic group.

Check yourself

Define the term zwitterion and draw the zwitterion of alanine, CH₃CH(NH₂)COOH.
Give two physical properties of amino acids that are explained by the zwitterion structure.
Draw the main form of glycine in strongly acidic solution and in strongly alkaline solution.
Write the ionic equation for glycine zwitterion reacting with H⁺ and with OH⁻.
Explain why an amino acid is described as amphoteric.
Describe an experiment to show that glycine reacts with both acid and alkali, and state the expected result.

Answers cover these until you have tried the questions

A zwitterion is an ion with both a positive and a negative charge and no overall charge. For alanine: CH₃CH(NH₃⁺)COO⁻.
Crystalline solid with a high melting temperature; soluble in water. The ionic attractions between zwitterions hold the lattice together and the ions interact with water.
Acid: H₃N⁺CH₂COOH (cation). Alkali: H₂NCH₂COO⁻ (anion).
$\ce{H3N+CH2COO- + H+ -> H3N+CH2COOH}$ and $\ce{H3N+CH2COO- + OH- -> H2NCH2COO- + H2O}$ .
It contains a group that can donate H⁺ ( $\ce{-COOH}$ , or –NH₃⁺ in the zwitterion) and a group that can accept H⁺ (–NH₂, or –COO⁻ in the zwitterion), so it reacts with both acids and alkalis.
Measure the pH of glycine solution with a pH meter and compare it with water. Add 0.1 mol dm⁻³ HCl, then (with fresh samples) 0.1 mol dm⁻³ NaOH, 1 cm³ at a time, noting the pH each time. The pH of glycine changes by much less than the water. The zwitterion removes added H⁺ (–COO⁻ → $\ce{-COOH}$ ) and added OH⁻ (–NH₃⁺ → –NH₂).