M12Q6: Condensation and Hydrolysis Reactions

Learning Objectives

| Key Concepts and Summary | Glossary |

Condensation reactions are reactions between two molecules that create one large and one small (usually H2O) molecule. For this module, we will be learning about two condensation reactions: one which creates esters and one which creates amides. Hydrolysis reactions are just the opposite of condensation reactions. In a hydrolysis reaction, we take one large and one small molecule to create two new molecules.

Condensation to Create Esters

Esters are produced by the reaction of carboxylic acids with alcohols with an acid catalyst (usually H2SO4). In these reactions, the -OH group from the carboxylic acid combines with the -H from the alcohol to produce water. The resulting -OR group from the alcohol combines with the carboxylic acid remnant to create an ester group, as can be seen in Figure 1.

This figure is a condensation reaction, with two reactants and two products. In the reactants, there is a central carbon with a R group to left, double bonded oxygen (with two lone pairs) to the top, and a single bond to oxygen to the right, which has two lone pairs and a single bond to hydrogen. Underneath is carboxylic acid. This structure is added to a central oxygen with two lone pairs, a single bond to hydrogen to the left, and a single bond to R' group to the right. Underneath is written alcohol. There is a blue box around the single bonded oxygen and hydrogen of the carboxylic acid and the hydrogen of the alcohol. Then there are equilibrium arrows with H 2 S O 4 above the arrow. In the products, there is a central carbon with a single bond to a R group to the left, a double bond to oxygen (with two lone pairs) to the top, and a single bond to oxygen to the right, which has two lone pairs and a single bond to a R' group. The second structure is water, a central oxygen with two lone pairs and two single bonds to hydrogen, one to the left and one to the right. Under the first structure is written ester and the under the second is written water.
Figure 1: Reactions between carboxylic acids and alcohols result in an ester and water formation.

For example, the ester ethyl acetate, CH3CO2CH2CH3, is formed when acetic acid reacts with ethanol:

This figure has four structures, two reactants and two products. The first reactant has a central carbon attached to a C H 3 group to the left, a double bonded oxygen (with two lone pairs) to the top, and a single bond to oxygen to the right, which has two lone pairs and is single bonded to a hydrogen. Underneath is written acetic acid. The second structure has a central oxygen has two lone pairs with a single bond to hydrogen to the left and a C H 2 C H 3 group to the right. Underneath is written ethanol. Then there are equilibrium arrows with H 2 S O 4 written above it. The first product has a central carbon with a single bond to a C H 3 group, a double bond to oxygen (which has two lone pairs) and a single bond to an oxygen, which has two lone pairs which is single bonded to a C H 2 C H 3 group. Underneath is written ethyl acetate. The second product is H 2 O, underneath is written water.

Hydrolysis of Esters

Ester functional groups can also be broken back into carboxylic acids and alcohols with the opposite, hydrolysis reaction. In Figure 2, we can see that the ester bond is broken between the C-O bond. The carbonyl (C=O) carbon forms a covalent bond with -OH from water to create a carboxylic acid group, and the -OR group that broke off of the ester combines with the -H of water to create an alcohol.

This figure has four structures, two reactants and two products. The first reactant has a central carbon with an R group to the left and a double bonded oxygen (with two lone pairs) to the top and a single bond to oxygen to the right, which has two lone pairs and a single bond to a R’ group. There is an arrow pointing to the bond between the central carbon and single bonded oxygen which says “Ester bond broken”. Underneath is written ester. The second structure has a central oxygen with two lone pairs and two single bonds to hydrogen. Underneath is written hydrogen. Then there are equilibrium arrows with H 2 S O 4 written above it. The first product has a central carbon with a single bond to a R group, a double bond to an oxygen (which has two lone pairs) and a single bond to oxygen, which has two lone pairs and a single bond to hydrogen. Underneath is written carboxylic acid. The second product has a central oxygen with two lone pairs, a single bond to hydrogen to the left and a single bond to an R’ group to the right. Underneath is written alcohol.
Figure 2: Reactions between an ester and water result in carboxylic acid and alcohol formation.

Condensation to Create Amides

Amides can be produced when carboxylic acids react with amines or ammonia through a condensation reaction (Figure 3). A water molecule is eliminated from the reaction, and the amide is formed from the remaining pieces of the carboxylic acid and the amine (note the similarity to formation of an ester from a carboxylic acid and an alcohol discussed previously):

This figure has four structures, two reactants and two products. The first reactant has a central carbon with a single bond to a R group to the left, a double bond to oxygen (which has two lone pairs) to the top, and a single bond to oxygen to the right, which has two lone pairs and a single bond to hydrogen. Underneath is written carboxylic acid. The second structure has a central nitrogen with a lone pair, a single bond to hydrogen, and two single bonds to a R’ and a R” group, one of which is a dash and one of which is a wedge. Underneath is written “primary amine, secondary amine, or ammonia”. There is a single headed arrow pointing towards products with “Heat” written above it. The first product has a central carbon with a single bond to a R group to the left, a double bond to oxygen to the top (which has two lone pairs), a single bond to nitrogen with a lone pair and two single bonds, one to R’ and another to R”. Underneath is written carboxamide (amide). The second product has a central oxygen with two lone pairs and two single bonds to hydrogen. Underneath is written water.
Figure 3: Reactions between carboxylic acids and amines result in an amide and water formation.

The reaction between amines and carboxylic acids to form amides is biologically important. It is through this reaction that amino acids (molecules containing both amine and carboxylic acid substituents) link together in a polymer to form proteins.

Hydrolysis of Amides

Amide functional groups can also be broken back into carboxylic acids and amines with the opposite, hydrolysis reaction. In Figure 4, we can see that the amide bond is broken between the C-N bond. The carbonyl (C=O) carbon forms a covalent bond with -OH from water to create a carboxylic acid group, and the -NRR” group that broke off of the amide combines with the -H of water to create an amine.

Figure 4: Reactions between an amide and water result in carboxylic acid and amine formation.

Chemistry in Real Life: Proteins and Enzymes

Proteins are large biological molecules made up of long chains of smaller molecules called amino acids. Organisms rely on proteins for a variety of functions—proteins transport molecules across cell membranes, replicate DNA, and catalyze metabolic reactions, to name only a few of their functions. The properties of proteins are functions of the combination of amino acids that compose them and can vary greatly. Interactions between amino acid sequences in the chains of proteins result in the folding of the chain into specific, three-dimensional structures that determine the protein’s activity.

Amino acids are organic molecules that contain an amine functional group (–NH2), a carboxylic acid functional group (–COOH), and a side chain (that is specific to each individual amino acid). Most living things build proteins from the same 20 different amino acids. Amino acids connect by the formation of a peptide bond, which is a covalent bond formed between two amino acids when the carboxylic acid group of one amino acid reacts with the amine group of the other amino acid. The formation of the bond results in the production of a molecule of water (in general, reactions that result in the production of water when two other molecules combine are referred to as condensation reactions). The resulting bond—between the carbonyl group carbon atom and the amine nitrogen atom–is called a peptide link or peptide bond. Since each of the original amino acids has an unreacted group (one has an unreacted amine and the other an unreacted carboxylic acid), more peptide bonds can form to other amino acids, extending the structure. (Figure 5) A chain of connected amino acids is called a polypeptide. Proteins contain at least one long polypeptide chain.

This figure shows two amino acid molecules. These molecules have two singly bonded carbon atoms to which an amino group is bonded on the left and the C atom to the right is a component of a carboxyl group. The C atom at the center has an R group bonded below and an H atom bonded above. The amino acid at the top left has an amino group identified and enclosed in a green dashed rectangle. This group is comprised of an N atom with two bonded H atoms. The amino acid at the right has a carboxyl group identified in a green dashed rectangle. This group has a C atom to which an O H group and a doubly bonded O atom are bonded. The amino acid to the left has the O H group to the lower right in red. The amino acid on the right has an H atom that is bonded to the N atom in red. An arrow points downward and is labeled condensation reaction. A curved arrow extends down and to the right off of the downward arrow, pointing to H subscript 2 O, which is in red. A single, larger molecule appears beneath the downward arrow. At the locations of the red O H group and H atom, the amino acid molecules are bonded together. This bond is labeled as a peptide bond and the larger molecule formed is labeled as a polypeptide chain.
Figure 5. This condensation reaction forms a dipeptide from two amino acids and leads to the formation of water.

Enzymes are large biological molecules, mostly composed of proteins, which are responsible for the thousands of metabolic processes that occur in living organisms. Enzymes are highly specific catalysts; they speed up the rates of certain reactions. Enzymes function by lowering the activation energy of the reaction they are catalyzing, which can dramatically increase the rate of the reaction. Most reactions catalyzed by enzymes have rates that are millions of times faster than the noncatalyzed version. Like all catalysts, enzymes are not consumed during the reactions that they catalyze. Enzymes do differ from other catalysts in how specific they are for their substrates (the molecules that an enzyme will convert into a different product). Each enzyme is only capable of speeding up one or a few very specific reactions or types of reactions. Since the function of enzymes is so specific, the lack or malfunctioning of an enzyme can lead to serious health consequences. One disease that is the result of an enzyme malfunction is phenylketonuria. In this disease, the enzyme that catalyzes the first step in the degradation of the amino acid phenylalanine is not functional (Figure 6). Untreated, this can lead to an accumulation of phenylalanine, which can lead to intellectual disabilities.

This figure includes a computer generated image of an enzyme molecule showing string and curled ribbon-like structural components in purple, green, and yellow hues.
Figure 6. A computer rendering shows the three-dimensional structure of the enzyme phenylalanine hydroxylase. In the disease phenylketonuria, a defect in the shape of phenylalanine hydroxylase causes it to lose its function in breaking down phenylalanine.

Chemistry in Real Life: Kevlar

Kevlar (Figure 7) is a synthetic polymer made from two monomers 1,4-phenylene-diamine and terephthaloyl chloride (Kevlar is a registered trademark of DuPont). Kevlar’s first commercial use was as a replacement for steel in racing tires. Kevlar is typically spun into ropes or fibers. The material has a high tensile strength-to-weight ratio (it is about 5 times stronger than an equal weight of steel), making it useful for many applications from bicycle tires to sails to body armor.

A structural formula is shown for the polymer Kevlar. The structure appears inside brackets which have single dashes extending from them at the left and right ends. Outside the lower right corner of the brackets, an italicized n appears. The structure inside the brackets includes a C atom forming a double bond with an O atom and a bond with a benzene ring. The benzene ring forms a bond with another C atom which has a double bond with an O atom. The C atom is bonded to an N atom. The N atom is bonded to an H atom and a benzene ring. The benzene ring bonds with another N atom which is also bonded to an H atom.
Figure 7. This illustration shows the formula for polymeric Kevlar.

The material owes much of its strength to hydrogen bonds between polymer chains (refer back to the chapter on intermolecular interactions). These bonds form between the carbonyl group oxygen atom (which has a partial negative charge due to oxygen’s electronegativity) on one monomer and the partially positively charged hydrogen atom in the N–H bond of an adjacent monomer in the polymer structure (see dashed line in Figure 8). There is additional strength derived from the interaction between the unhybridized p orbitals in the six-membered rings, called aromatic stacking.

This diagram shows the repeating, interlinked units that exist in Kevlar, taking on a sheet-like appearance. Dashed line segments are indicated between units. Individual units are composed of nitrogen, hydrogen, oxygen and carbon atoms. The repeating structural units include benzene rings and double bonds.
Figure 8. The diagram shows the polymer structure of Kevlar, with hydrogen bonds between polymer chains represented by dotted lines.

Kevlar may be best known as a component of body armor, combat helmets, and face masks. Since the 1980s, the US military has used Kevlar as a component of the PASGT (personal armor system for ground troops) helmet and vest. Kevlar is also used to protect armored fighting vehicles and aircraft carriers. Civilian applications include protective gear for emergency service personnel such as body armor for police officers and heat-resistant clothing for fire fighters. Kevlar based clothing is considerably lighter and thinner than equivalent gear made from other materials (Figure 9).

Three photos are shown. In the first, two male soldiers are shown sorting through green brown material on a table. In the second, two people are shown paddling a canoe. In the third, heavy white rope is being manipulated with a hand tool.
Figure 9. (a) These soldiers are sorting through pieces of a Kevlar helmet that helped absorb a grenade blast. Kevlar is also used to make (b) canoes and (c) marine mooring lines. (credit a: modification of work by “Cla68”/Wikimedia Commons; credit b: modification of work by “OakleyOriginals”/Flickr; credit c: modification of work by Casey H. Kyhl)

In addition to its better-known uses, Kevlar is also often used in cryogenics for its very low thermal conductivity (along with its high strength). Kevlar maintains its high strength when cooled to the temperature of liquid nitrogen (–196 °C).

Key Concepts and Summary

Each functional group is used to create other functional groups and created by a variety of reactions. Two types of reaction, condensation and hydrolysis, are introduced in this section. For simplicity, condensation reactions come in two types:

carboxylic acid  +  alcohol  ⇌  ester  +  H2O (catalyst: H2SO4)

carboxylic acid  +  amine  ⇌  amide  +  H2O (catalyst: heat)

Hydrolysis reactions are the opposite of condensation reactions:

ester  +  H2O  ⇌  carboxylic acid  +  alcohol (catalyst: H2SO4)

amide  +  H2O  ⇌  carboxylic acid  +  amine (catalyst: H2SO4)

Glossary

condensation reaction
a reaction between two molecules that create one large and one small (usually H2O) molecule

hydrolysis reaction
a reaction between two molecules (one usually H2O) to create two different molecules; the opposite of a condensation reaction

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Chem 103/104 Resource Book by crlandis and Chem 104 Textbook Team is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.