(A) ESTERS, FATS AND OILS

HYDROLYSIS OF ESTERS

HCCHEMISTRY

HCCHEMISTRY

HYDROYLSIS ESTERS

(A) ESTERS, FATS AND OILS

HCCHEMISTRY

HCCHEMISTRY

After completing this lesson you should be able to :

(A) ESTERS, FATS AND OILS

HYDROLYSIS OF ESTERS

HCCHEMISTRY

HCCHEMISTRY

• Esters can be hydrolysed to produce the parent carboxylic acid and parent alcohol.

• Given the name of an ester or its structural formula, the hydrolysis products can be named and their structural formulae drawn.

• In a hydrolysis reaction, an ester is split apart by a water molecule.

• Describe the difference between dilute acid catalysed hydrolysis of an ester and using a strong alkali, such as NaOH(aq) or KOH(aq).

HYDROLYSIS OF ESTERS

• Breakdown of an ester by water.• Process sped up by catalysis• Can use a dilute acid to catalyse (H2SO4)• Alkali catalysts (e.g. sodium hydroxide) can

also be used but instead of producing carboxylic acid a carboxylate salt is formed.

• Alkaline hydrolysis goes to completion & hence is usually preferred.

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Hydrolysing EstersAlcohol + Carboxylic Acid Ester + Water

Alcohol + Carboxylic Acid Ester + Water

Condensation

Hydrolysis

The ester is split up by the chemical action of water, hydrolysis.The hydrolysis and formation of an ester is a reversible reaction.

R C

O

O R H

O

H+

Bonds broken

Ester + Water

+R C

O

HO

R

H

O

Bonds formed

Carboxylic Acid + Alcohol

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HEATING UNDER REFLUX• Anti-bumping granules ensure even boil and provide uneven surface for bubbles of

gas to form. • you heat reactants under reflux if the organic compound you are heating is volatile

(that is it goes to the gaseous state very easily under heat). • When you heat reactants, you turn them back into a liquid so they don’t escape

from your reaction vessel. Reactants are turned back into a liquid by having a condenser at the top of the apparatus. So you’re 'fluxing' between the two states, gaseous and liquid.

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Alkaline hydrolysis

Alkaline hydrolysis of an ester has two main advantages over acid-catalysed hydrolysis.

1) Alkaline hydrolysis is not reversible and hence there should be a higher yield.

2) It is usually easier to separate the products when alkaline hydrolysis has been used.

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If sodium hydroxide is used a sodium salt of the acid is formed. If this is reacted with a strong acid (eg. Hydrochloric acid) the acid can be displaced from the salt

RCOO- Na+ + HCl RCOOH + NaCl

Carboxylate + hydrochloric + sodiumSalt acid acid chloride

…After separating your carboxylate salt via distillation you can displace the sodium using a strong acid to obtain the parent acid of the ester.

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In hydrolysis reactions large molecules are broken down into smaller molecules by water.

Esters can be hydrolysed to produce the parent ................... .......... and the parent .................... .

ESTER + WATER ⇌ ACID + ALCOHOL

An alkali is often used to catalyse the reaction and the mixture heated under reflux.

If sodium hydroxide is used a sodium salt of the acid is formed. If this is reacted with a strong acid (eg. Hydrochloric acid) the acid can be displaced from the salt

HCOO- Na+ + HCl HCOOH + NaCl

Sodium + hydrochloric methanoic + sodiummethanoate acid acid chloride

Past paper question 2015

Esters hydrolysisEster hydrolysis, especially by alkali, is an important process in the manufacture of soap from fats and oils. This will be looked at in the next subtopic (B).

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COPYPrevention of ester hydrolysis

• Prevention of ester hydrolysis is also important in the food, cosmetic and pharmaceutical industries.

• As esters are found in these products, if they hydrolyse easily then the property of the products will change.

• Expiry dates are predicted, along with other factors, to take account how quickly the product is likely to hydrolyse.

• When the acid is produced upon ester hydrolysis it can have an undesirable smell.

Extension material - Higher Properties of Carboxylic Acids

Carboxylate salts are often used as preservatives and flavor enhancers in soups and seasonings.

Sodium propionate, a preservative, is added to cheeses, bread, and other bakery items to inhibit the spoilage of the food by microorganisms.

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Polarity of Carboxylic Acids

Carboxylic acids are strongly polar because they have two polar groups:• hydroxyl group (—OH)• carbonyl group (C O)

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Solubility in WaterCarboxylic acids • form hydrogen bonds with

many water molecules• with one to four carbon

atoms are very soluble in water

As the number of carbons increases, the solubility of the carboxylic acid in water is reduced.

Acetic acid forms hydrogen bonds with water molecules.

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Solubility in Water

Acidity of Carboxylic AcidsCarboxylic acids • are weak acids• ionize in water to produce carboxylate ions and hydronium ions • can lose a proton because two oxygen atoms in the

carboxylate ion stabilize negative charge

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Study Check

Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion.

Solution

Write the balanced equation for the ionization of butanoic acid in water and identify the carboxylate ion.

carboxylate ion

(butanoate)

+ H3O+

-

Butanoic acid

Neutralization of Carboxylic Acids

Carboxylic acid salts • are a product of the neutralization of a

carboxylic acid with a strong base, such as NaOH or KOH

(carboxylic acid salt)

• are used as preservatives and flavor enhancers

Carboxylic Acid Salts

The carboxylate ion is named by replacing the ic acid ending of the acid name with ate.

Carboxylic Acid Salts

Carboxylic acid salts • are ionic compounds with strong attractive

forces between ions• are solids at room temperature• have high melting points• are usually soluble in water

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Carboxylic Acid SaltsCarboxylic acid salts are used as preservatives andflavor enhancers such as

• sodium propionate, which is used in cheese and breads

• sodium benzoate, which inhibits growth of mold and bacteria and is added to fruit juices, margarine, relishes, salads, and jams

• monosodium glutamate, MSG, which is added to meats, fish, vegetables, and baked goods to enhance flavor

Study Check

Write the equation for the reaction of propanoic acid with NaOH.

Solution

Write the equation for the reaction of propanoic acid with NaOH.