Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

6.8 Esters - preparation, physical and chemical properties and uses including transesterification (interesterification)

Sub-index for this page

6.8.1 Two methods for preparing of esters - esterification

6.8.2 Physical properties of esters

6.8.3 Hydrolysis of esters with acid or alkali

6.8.4 Transesterification of esters (swapping the acid or alcohol component)

6.8.5 Uses of esters

INDEX of all carboxylic acids and derivatives notes

All Advanced A Level Organic Chemistry Notes

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6.8.1 Methods of esters

In principle the reaction equations for the two methods described here are:

(a) alcohol/phenol  +  carboxylic acid  ===>  ester  +  water

(b) alcohol/phenol  +  acid chloride  ===>  ester  +  hydrogen chloride

 

The first part of the ester name is derived from the alcohol or phenol e.g. methyl, ethyl, phenyl.

 The second part of the name is derived from the carboxylic acid - examples in table below in 6.8.2.

The reverse is 'usually' true when writing out the abbreviated formula, so take care!

See Structure & naming of carboxylic acids and derivatives

 

Brief descriptions of the methods with equations

(a) Esters from carboxylic acid plus alcohol using an acid catalyst

Concentrated sulphuric acid acts as a catalyst in this reaction.

General word equation for esterification:

carboxylic acid  +  alcohol  == acid catalyst ==>  ester  +  water

e.g. ethanoic acid  +  ethanol    ethyl ethanoate  +  water

+ + H₂O

sometimes more simply written as

CH₃COOH + CH₃CH₂OH CH₃COOCH₂CH₃ + H₂O

The reaction is reversible and the mixture reaches equilibrium, and only about ²/₃rds of the carboxylic acid and alcohol have been converted to the ester.

Without a strong acid catalyst e.g. conc. sulfuric acid, the reaction is very slow and the mixture is heated to further increase the rate of reaction (see details of the method below).

See another page for a full description of making an ester like ethyl ethanoate

Its easy to do a simple laboratory demonstration

Mix equal volumes of small quantities of butanoic acid (pungent rancid small) and methanol with an even smaller volume of concentrated sulfuric acid.

The mixture gently warmed in beaker of warm water for 5-10 minutes.

butanoic acid  +  methanol    methyl butanoate  +  water

CH₃CH₂CH₂COOH  +  CH₃OH  CH₃CH₂CH₂COOCH₃  +  H₂O

The mixture is then poured into a beaker of sodium hydrogencarbonate solution.

The sodium hydrogencarbonate neutralises the acid catalyst and any unreacted 'smelly' butanoic  acid. The excess methanol dissolves in the water too.

You should get some drops of methyl butanoate ester left on the surface which can be carefully smelled to appreciate the aroma of the ester, which in this case should smell of apples of pineapple!

You can do this is as a nice class experiment with various alcohols and ethanoic acid - far less rancid than butanoic acid, as long as you don't mind a super vinegar smell !!!

Notes what students think the esters smells like (likely to be 'fruity') alongside appreciating its molecular structure too!

If you repeat the experiment with ethanol you get ethyl butanoate which also smells of pineapples and it is used as a flavour enhancer in processed orange juices.

CH₃CH₂CH₂COOH  +  CH₃CH₂OH  CH₃CH₂CH₂COOCH₂CH₃  +  H₂O

 

(b) Reaction between an acid chloride and an alcohol

The acyl chloride and alcohol usually readily react at room temperature, especially if both are aliphatic.

Examples of nucleophilic addition of an alcohol to acid/acyl chlorides,  followed by elimination to give the ester and hydrogen chloride.

(i) ethanoyl chloride  +  ethanol ===>  ethyl ethanoate  +  hydrogen chloride

  + CH₃-CH₂-OH  ===>    +  HCl

(ii) ethanoyl chloride  +  phenol  ===>  phenyl ethanoate  +  hydrogen chloride

  +    ===>    +  HCl

(iii) pentanoyl chloride  +  propan-1-ol  ===>  propyl pentanoate  +  hydrogen chloride

  +    ===>    +  HCl

 

Unlike the acid catalysed esterification described in method (a) with ~67% yield, this reaction gives a very high yield >90%.

For more details see 6.7 Preparation & reactions of acid chlorides

6.8.2 Physical properties of esters

The lower members of the aliphatic acid esters are colourless liquids at room temperature with relatively low boiling points.

A few of the lower members are quite soluble in water, but most are ~insoluble or sparingly soluble.

Table of abbreviated structural formula and boiling point in ^oC, plus comments on water solubility and isomerism.

Notes and further comments on the information table above.

(i) The isomers tend to have similar boiling points and solubility and have similar polarity.

With equal numbers of electrons in the molecule, they have similar intermolecular forces - mainly instantaneous dipole - induced dipole plus permanent dipole - permanent dipole attractive forces.

(ii) Despite being polar molecules, they generally have a low solubility in water because they cannot hydrogen bond with water.

The first few lower members are quite or very soluble in water, but, with increasing length of the molecule, too many hydrogen bonds between water molecules are disrupted without compensation intermolecular attractions, so the solubility rapidly decreases.

(iii) Alternative names

propyl butanoate can also be called 1-propyl butanoate, to distinguish it from structural isomer

CH₃CH₂CH₂COOCH(CH₃)₂, which can called 2-propyl butanoate (1-methylethyl butanoate)

 

Odour

Esters tend to have pleasant odours and are used in flavourings and perfumes and they occur naturally in many plants.

6.8.3 Hydrolysis with acid or alkali - the opposite of esterification

Esters can be hydrolysed by refluxing with strong mineral acids or alkali.

The reaction is the opposite of esterification.

(a) Hydrolysis of an ester with dilute hydrochloric or sulfuric acid

RCOOR'  +  H₂O   RCOOH  +  R'OH

This reaction cannot go to completion because it is a reversible reaction, despite the use of a catalyst and heating the mixture under reflux.

You can go to a high % yield to the right using an aqueous solution with lots of excess water.

e.g. butyl propanoate  +  water    propanoic acid  +  butan-1-ol

CH₃CH₂COOCH₂CH₂CH₂CH₃  +  H₂O    CH₃CH₂COOH  +  CH₂CH₂CH₂CH₃OH

 

(b) Hydrolysis of an ester with aqueous sodium hydroxide

Hydrolysing an ester with strong alkali e.g. aqueous or ethanolic sodium/potassium hydroxide is called saponification, i.e. its a specific name for a particular type of hydrolysis reaction.

RCOOR'  +  NaOH ===>  RCOONa  +  R'OH

RCOOR'  +  OH^- ===>  RCOO^-  +  R'OH

e.g. using the same ester as in (a) for this non-reversible reaction ...

e.g. butyl propanoate  +  sodium hydroxide  ===>  sodium propanoate  +  butan-1-ol

CH₃CH₂COOCH₂CH₂CH₂CH₃  +  NaOH  ===>  CH₃CH₂COONa  +  CH₂CH₂CH₂CH₃OH

CH₃CH₂COOCH₂CH₂CH₂CH₃  +  OH^-  ===>  CH₃CH₂COO^-  +  CH₂CH₂CH₂CH₃OH

This reaction is not reversible and goes to 100% completion forming the sodium salt of the acid.

The carboxylic acid is freed by adding excess of a strong hydrochloric or sulfuric mineral acid.

e.g. RCOONa  +  HCl  ===> RCOOH  +  NaCl  or   RCOO^-  +  H⁺  ===>  RCOOH

CH₃CH₂COONa  +  HCl  ===>  CH₃CH₂COOH  +  NaCl

CH₃CH₂COONa  +  H⁺  ===>  CH₃CH₂COOH

Note on hydrolysis and SOAP making

What is it? How is it made? and it is another use of esters!

'Traditional' soap is a product of the hydrolysis of fats from animals and vegetable oils from plants.

You reflux the fat or vegetable oil with aqueous sodium hydroxide.

A saponification reaction

'Soapy' soaps (not modern 'soapless' detergents) are the sodium salts of long chain fatty acids formed by heating fatty oils with concentrated alkalis like sodium hydroxide or potassium hydroxide to hydrolyse them.

This is known as a saponification reaction and a typical equation is illustrated above and the general word equation quoted below.

vegetable oil/animal fat + sodium hydroxide ==> soap molecule + glycerol

In the example illustrated above, the saponification (hydrolysis) product is the soap sodium palmitate.

The original ester is a saturated fat found in olive oil, palm oil, and body lipids.

This reaction breaks the fat molecule down into one glycerol molecule (a triol alcohol) and three sodium salts of the long chain carboxylic fatty acids that formed part of the original oil/fat ester.

Sodium stearate is another soap molecule - it is actually a salt of a long chain fatty acid like stearic acid shown below.

6.8.4 Transesterification of esters (swapping the acid or alcohol component - interesterification)

Since the acid catalysed esterification reaction is reversible, you can actually swap the alcohol or carboxylic acid derived component in the ester molecule.

You gently reflux the ester with another alcohol or another carboxylic acid to effect the transesterification.

So, there are two possibilities e.g.

 

(a) Swapping the alcohol derived component of the ester

RCOOR'  +   R"OH  RCOOR"  +  R'OH

ester 1  +  alcohol 1    ester 2  +   alcohol 2 

So this is effectively exchanging the alcohol reactant in the esterification.

Examples

(i)  ethyl ethanoate  +  propan-1-ol    propyl ethanoate  +  ethanol

  +        + 

(ii)  butyl pentanoate  +  ethanol    ethyl pentanoate  +  butan-1-ol

  +        + 

The first ester can be named 1-butyl pentanoate.

 

(b) Swapping the carboxylic derived component of the ester

RCOOR'  +   R"COOH  R"COOR'  +  RCOOH 

ester 1  +  carboxylic acid 1    ester 2  +   carboxylic acid 2 

So this is effectively exchanging the carboxylic acid reactant in the esterification.

Examples

(i)  propyl pentanoate  +  butanoic acid    propyl butanoate  +  pentanoic acid

  +        + 

(ii)  methyl ethanoate  +  benzoic acid    methyl benzoate  +  ethanoic acid

  +        + 

 

A transesterification reaction is used in the production of margarine and biodiesel

See 6.9 Natural esters - triglyceride fats and oils, manufacture of margarine and biodiesel

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6.8.5 The uses of esters

Esters are used in perfumes and flavourings.

Perfumes are complex mixtures of esters and ketones - blended to a give a particular volatility and aroma.

Many esters are used as flavour enhancers in fruit drink.

Solvents may contain esters e.g. nail varnish remover includes some ethyl ethanoate.

Biofuels are made from the production of methyl esters of fatty acids using transesterification.

Esters crop up in all sorts of pharmaceutical products!

 Aspirin (right) is an ester of ethanoic acid - but the 'alcohol' is actually a phenol called 2-hydroxybenzoic acid.

 Some anaesthetics like benzocaine and procaine are esters.

 Benzocaine (left) is the ethyl ester of 4-aminobenzoic acid.

 

Many anaesthetics ('painkillers') are derived from natural products. Synthetic codeine is made from natural morphine, but neither is an ester. However, the related molecule, natural heroin, is an ester, I think you can you spot the two ester linkages (and an ether link and tertiary amine group too!!!).

I've already written a lot on the uses of esters on another webpage

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Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of all carboxylic acids and derivatives notes

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