Condensation and Hydrolysis Reactions

A condensation reaction joins two molecules into a larger one with the loss of a small molecule such as water, and hydrolysis is the reverse: a larger molecule is split by the addition of water. In brief: two glycine molecules (75.07 g/mol each) condense to the dipeptide glycylglycine (132.12 g/mol) and one water molecule, and a protein of 100 amino acids is formed with the loss of 99 molecules of water.

Condensation and hydrolysis are the main ways in which biological molecules are built up and broken down: sugars become polysaccharides, amino acids become proteins and fats are formed and digested. They also lie behind polyesters and nylon. This article defines the two reactions, lists the main types in a table, gives worked examples of the mass balance, and compares acid, base and enzyme-catalyzed hydrolysis.

What are condensation and hydrolysis?

In a condensation reaction, two molecules, each with a functional group, react to form a new bond, and a small molecule is released as a by-product. The by-product is most often water (in which case it is also called dehydration synthesis), but it can also be HCl, ammonia or methanol. In a hydrolysis reaction, a molecule of water splits a bond, adding –H to one fragment and –OH to the other. The two processes are the reverse of each other, and in an equilibrium, such as RCOOH + R′OH ⇌ RCOOR′ + H₂O, the forward reaction is condensation and the reverse reaction is hydrolysis. The direction is controlled by the concentration of water: excess water favors hydrolysis, and removal of water favors condensation.

Main types of condensation reactions

How do esters form and break?

Esterification is a condensation: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. Hydrolysis with dilute acid reverses it, and hydrolysis with alkali (saponification) goes to completion because the carboxylic acid is converted to its salt. Triglycerides are formed by condensation of three fatty acids with glycerol (three molecules of water lost) and digested by lipase enzymes with the addition of three water molecules.

What happens with amides and peptides?

Two amino acids join when the carboxyl group of one reacts with the amino group of the other, losing water, to form a peptide (amide) bond: H₂N–CHR–COOH + H₂N–CHR′–COOH → H₂N–CHR–CO–NH–CHR′–COOH + H₂O. A chain of n amino acids has n − 1 peptide bonds, formed with the loss of n − 1 water molecules. Peptide bonds are very stable in water at neutral pH; a half-life of hundreds of years at 25 °C is estimated. Without a catalyst they are hydrolyzed only by boiling in strong acid (6 M HCl, 110 °C, 24 hours), which is the standard method for the analysis of the amino acid composition of a protein. In the body, proteases such as trypsin and pepsin do the same at 37 °C in seconds.

What happens with sugars?

Two monosaccharides link through a glycosidic bond with the loss of water. Glucose + glucose → maltose + H₂O; glucose + fructose → sucrose + H₂O. Starch, glycogen and cellulose are polymers of glucose. Hydrolysis of sucrose in acid or by the enzyme invertase gives an equimolar mixture of glucose and fructose, called invert sugar. The name comes from the fact that the optical rotation of the solution changes from positive (sucrose, +66.5°) to negative (the mixture is levorotatory, since fructose rotates more strongly in the negative direction than glucose does in the positive direction).

How is hydrolysis catalyzed?

• Acid: protonation of the carbonyl oxygen makes the carbon more electrophilic, so that water can attack. The reaction is reversible.
• Base: hydroxide is a stronger nucleophile than water. For esters, it is irreversible.
• Enzymes: hydrolases such as amylase, lipase, proteases and nucleases speed up hydrolysis by factors of 10⁶ to 10¹⁷ under mild conditions, with high specificity.
• Heat and pressure: industrial hydrolysis of fats uses superheated steam, and the hydrolysis of PET plastic in chemical recycling uses high temperature and a catalyst.

Worked examples

Example 1: mass balance of a dipeptide

Glycine (C₂H₅NO₂, M = 75.07 g/mol) condenses to give glycylglycine (C₄H₈N₂O₃) and water. M(dipeptide) = 2 × 75.07 − 18.02 = 132.12 g/mol. From 15.0 g of glycine (0.200 mol), the maximum amount of dipeptide is 0.100 mol, or 13.2 g, and 0.100 mol of water (1.80 g) is released.

Example 2: hydrolysis of sucrose

C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose). The molar mass of sucrose is 342.30 g/mol, and of water 18.02 g/mol, so 342.30 g of sucrose gives 360.32 g of monosaccharides, with 2 × 180.16 = 360.32 g. The mass increases by 5.3 %, which shows that water has been taken up.

Example 3: number of water molecules in a polypeptide

A polypeptide of 10 amino acids has 9 peptide bonds, and was formed with the loss of 9 water molecules. Complete hydrolysis requires the addition of 9 molecules of water.

Example 4: nylon-6,6

Hexane-1,6-diamine (116.21 g/mol) and hexanedioic acid (146.14 g/mol) condense repeatedly to give nylon-6,6; each repeat unit (C₁₂H₂₂N₂O₂, 226.32 g/mol) forms with the loss of two molecules of water: 116.21 + 146.14 − 2 × 18.02 = 226.31 g/mol. For 1.00 kg of nylon, 1000 / 226.32 = 4.42 mol of repeat units, so 8.84 mol or 159 g of water is released.

Example 5: the hydrolysis of an ester in base

CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH. Starting with 8.81 g of ethyl ethanoate (M = 88.11 g/mol, 0.100 mol), 0.100 mol of NaOH (4.00 g) is needed to convert it completely to sodium ethanoate (8.20 g).

How did the idea develop?

The hydrolysis of sucrose was studied by Ludwig Wilhelmy in 1850, who followed the change in optical rotation with time and showed that the rate was proportional to the concentration of sugar, which was the first quantitative study of the rate of a reaction. The structure of fats as esters of glycerol was explained by Michel Eugène Chevreul in the early 19th century. Emil Fischer proposed in 1902 that proteins are chains of amino acids linked by peptide bonds, formed by condensation, and synthesized the first dipeptide in the same year. Wallace Carothers used condensation reactions systematically to design the first synthetic polyamides and polyesters in the 1930s.

Which mistakes are common?

• Forgetting the small molecule: condensation always produces one, usually water.
• Miscounting the water in polymers: a chain of n units has n − 1 bonds.
• Treating hydrolysis as a simple dissolving: bonds are broken and water is incorporated.
• Assuming that hydrolysis is always irreversible: only in base for esters.
• Confusing dehydration with hydrolysis: they are opposite processes.