(g) Esters

4:39 (Triple only) know that ethyl ethanoate is the ester produced when ethanol and ethanoic acid react in the presence of an acid catalyst

Ethyl ethanoate is the ester produced when ethanol and ethanoic acid react in the presence of an acid catalyst.

ethanoic acid           +           ethanol           ⇋           ethyl ethanoate           +           water

CH₃COOH (l)         +         CH₃CH₂OH (l)         ⇋         CH₃COOCH₂CH₃ (l)         +         H₂O (l)

The ethyl ethanoate produced is an ester.

The reaction is called esterification.

The reaction can also be described as a condensation reaction because water is made when two molecules join together.

4:41 (Triple only) understand how to write the structural and displayed formulae of an ester, given the name or formula of the alcohol and carboxylic acid from which it is formed and vice versa

To work out the structure of the ester formed when an alcohol reacts with a carboxylic acid, it is easiest to first draw the structures of alcohol and acid and then remove the H₂O to see what is left when the molecules join.

For example, propan-1-ol and ethanoic acid react together to form propyl ethanoate and water.

         propan-1-ol            +            ethanoic acid            →            propyl ethanoate            +            water

         CH₃CH₂CH₂OH (l)         +         CH₃COOH (l)         →         CH₃COOCH₂CH₂CH₃ (l)          +         H₂O (l)

 

The displayed formula for esters is typically written to show the part which came from the carboxylic acid on the left:

This is still called propyl ethanoate: the alcohol bit of the name (propyl) comes first and then the carboxylic acid bit (ethanoate), even though the displayed formula is typically written the other way around. Notice that the structural formulae for esters is also typically written with the carboxylic acid bit first, so propyl ethanoate is CH₃COOCH₂CH₂CH₃.

4:42 (Triple only) know that esters are volatile compounds with distinctive smells and are used as food flavourings and in perfumes

Esters are volatile compounds with distinctive smells. A volatile liquid is one that turns into to a vapour easily.

Esters are used as food flavourings and in perfumes.

Esters are often described as having a sweet, fruity smell. They typically smell of bananas, raspberries, pears or other fruit because esters occur in all these natural products.

4:43 (Triple only) practical: prepare a sample of an ester such as ethyl ethanoate

Heating a mixture of ethanoic acid and ethanol produces a liquid called ethyl ethanoate. A few drops of concentrated sulfuric acid must be added for the reaction to work. The sulfuric acid acts as a catalyst.

ethanoic acid           +           ethanol           ⇋           ethyl ethanoate           +           water

CH₃COOH (l)         +         CH₃CH₂OH (l)         ⇋         CH₃COOCH₂CH₃ (l)         +         H₂O (l)

The ethyl ethanoate produced is an ester.

The reaction is called esterification.

The reaction can also be described as a condensation reaction because water is made when two molecules join together.

Notice that the reaction is reversible. Pure reactants are used to maximise the yield of ethyl ethanoate. Pure ethanoic acid is called glacial ethanoic acid.

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Section 1: Principles of chemistry

      a) States of matter

      b) Atoms

      c) Atomic structure

     d) Relative formula masses and molar volumes of gases

     e) Chemical formulae and chemical equations

     f) Ionic compounds

     g) Covalent substances

     h) Metallic crystals

     i) Electrolysis

 Section 2: Chemistry of the elements

     a) The Periodic Table

     b) Group 1 elements: lithium, sodium and potassium

     c) Group 7 elements: chlorine, bromine and iodine

     d) Oxygen and oxides

     e) Hydrogen and water

     f) Reactivity series

     g) Tests for ions and gases

Section 3: Organic chemistry

     a) Introduction

     b) Alkanes

     c) Alkenes

     d) Ethanol

Section 4: Physical chemistry

     a) Acids, alkalis and salts

     b) Energetics

     c) Rates of reaction

     d) Equilibria

Section 5: Chemistry in industry

     a) Extraction and uses of metals

     b) Crude oil

     c) Synthetic polymers

     d) The industrial manufacture of chemicals