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9.3 The acidic environment: 5. Esterification

Syllabus reference (October 2002 version)
5. Esterification is a naturally occurring process which can be performed in the laboratory	Students learn to: describe the differences between the alkanol and alkanoic acid functional groups in carbon compounds identify the IUPAC nomenclature for describing the esters produced by reactions of straight-chained alkanoic acids from C1 to C8 and straight-chained primary alkanols from C1 to C8 explain the difference in melting point and boiling point caused by straight-chained alkanoic acid and straight-chained primary alkanol structures identify esterification as the reaction between an acid and an alkanol and describe, using equations, examples of esterification describe the purpose of using acid in esterification for catalysis explain the need for refluxing during esterification outline some examples of the occurrence, production and uses of esters	Students: identify data, plan select equipment and perform a first-hand investigation to prepare an ester using reflux process information from secondary sources to identify and describe the uses of esters as flavours and perfumes in processed foods and cosmetics

Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 11Jun10]

Background: Living things store much of their energy as esters, known as fats or oils. Many of the flavours and odours of fruits are esters. The formation and breakdown of esters are reactions that occur frequently in living things.

describe the differences between the alkanol and alkanoic acid functional groups in carbon compounds

• The hydroxy functional group,-OH, in alkanols provides their characteristic properties, such as high melting points and boiling points.
  
  
• The carboxylic acid functional group, -COOH, in alkanoic acids can lose a hydrogen ion and behave as a weak acid.


• The diagram below shows a common way to illustrate the characteristic structure of the members of a homologous series. The R group represents a carbon chain of unspecified length. This is a skeletal formula as the carbon atom within the functional group (COOH) is not labelled with a C. In this type of formula the junction of bonds with no letter indicating the atom present is assumed to be a carbon and hydrogens bonded directly to carbon atoms are not shown. Note: it is best in HSC Chemistry to always draw full structural formulae but it is useful to be able to interpret other representations.

explain the difference in melting point and boiling point caused by straight-chained alkanoic acid and straight-chained primary alkanol structures

• The high melting points and boiling points in alkanols is due to hydrogen bonding between the O in one molecule and the H of an-OH in a nearby molecule.

  

• The ability of the-COOH group to be involved in two hydrogen bonds gives an alkanoic acid an even higher boiling point than that of a similar sized alkanol. Two hydrogen bonds can occur between a pair of alkanoic acid molecules.

  

identify data, plan, select equipment and perform a first-hand investigation to prepare an ester using reflux

• Use a chemical data book to identify the boiling points of the alkanol, alkanoic acid, ester and water.
  
  
• Plan the investigation by considering if the boiling points of the reactants and products are sufficiently different for you to easily separate the ester from the rest of the reaction mixture by fractional distillation. Consider the effect of using more than a few drops of concentrated sulfuric acid catalyst and how this might complicate the separation. Consider if you would be better off using differences in densities and water solubility of the components to separate the ester.
  
  
• Choose suitable reflux equipment:

  A large test tube fitted with a cork through which a pipette has been passed upside-down. The test tube could be heated in hot water; air circulating around the pipette acts as a condenser.

  Ground-glass joint equipment, including a water-cooled condenser, positioned vertically, and a reaction vessel heated by an electric heater.

• Perform the investigation, making sure you identify and use safe work practices. For instance, if using the first method above, make sure the pipette is not blocked by pieces of cork, otherwise you risk an explosion!

identify esterification as the reaction between an acid and an alkanol and describe, using equations, examples of esterification

• An acid, containing the-COOH functional group, can react with an alkanol, containing the-OH functional group, to produce an ester and water.
  R-OH     +     HOOC-R^!      R-OOC-R^!     +     H₂O
  alkanol            acid                     ester                water
  
  
• If an oxygen-18 isotope, O, is used in the alkanol only, it is found in the ester, but not in the water product. Use of this tracer shows that the O in water comes from the acid.
  R-OH     +     HOOC-R^!      R-OOC-R^!     +     H₂O
  alkanol             acid                     ester               water
  
  
• The reaction is reversible and comparable quantities of alkanol, acid, ester and water are present at equilibrium.
  
  
• Common names, rather than systematic names, are often used to obtain the ester name:
                   CH₃OH     +     HOOCCH₃      CH₃OOCCH₃     +     H₂O
  Common: methyl alcohol   acetic acid             methyl acetate       water
  Systematic: methanol     ethanoic acid         methyl ethanoate                 
  
                  CH₃CH₂OH     +     HOOCH      CH₃CH₂OOCH     +     H₂O
  Common: ethyl alcohol     formic acid                 ethyl formate            water
  Systematic: ethanol        methanoic acid         ethyl methanoate

identify the IUPAC nomenclature for describing the  esters produced by reactions of straight-chained alkanoic acids from C1 to C8 and straight-chained primary alkanols from C1 to C8

The naming of esters follows a straight forward pattern using IUPAC nomenclature.

The table below will give you a start. Copy it and attempt to complete it.

There is no need to learn all the ester names. Just remember the naming pattern you used.

Did you note that the alkanol always forms the first part of the ester's name having its ending changed from '...anol' to '...yl' and the alkanoic acid forms the second part of the ester's IUPAC name with its ending changing from '...oic acid' to '... oate'?

naming Alkanols AUS-e-TUTE

describe the purpose of using acid in esterification for catalysis

• Esterification is catalysed by the addition of a small amount of acid. Esterification is called a condensation reaction because a water molecule condenses out.
  
  
• Only a few drops of concentrated acid needs to be added to a mixture of alkanol and alkanoic acid to catalyse the reaction.
  
  
• If concentrated sulfuric acid is added in large amounts, say 5% to 10% of the reaction volume, it can have a significant effect on the position of equilibrium. Concentrated sulfuric acid is a dehydrating agent, that is, it has a strong affinity for water. If a significant amount of sulfuric acid is present, it will shift the equilibrium position to the right by absorbing water.

  alcohol + acid ester + water

  This increases the yield of ester. However using large amounts of sulfuric acid is wasteful, uneconomic and complicates the separation of ester from the reaction mixture.

explain the need for refluxing during esterification

• Esterification requires heat for the reaction to reach equilibrium within an hour, rather than after many days. When the reaction mixture is heated, volatile components, such as the reactant alcohol and the product ester, could escape. This problem is overcome by refluxing the reaction mixture.

  

• A condenser is placed on top of the reaction vessel so that any volatile components pass into the condenser. The condenser can be water or air-cooled and causes the volatile components to condense back to liquid and fall back into the reaction mixture.
  
  
• Refluxing also improves the safety of the operation, as the volatile components are flammable.

process information from secondary sources to identify and describe the uses of esters as flavours and perfumes in processed foods and cosmetics

• You can process information from the labels on processed food containers and cosmetics. You should be able to recognise and record the names of esters they contain. Remember the name of an ester is usually in two words:
  • the first word is derived from the alcohol and ends in -yl
  • the second word is derived from the acid and ends in -oate.
    
    
• Alternatively, you could look in a list of food additives or cosmetic ingredients and try to recognise ester names.
  
  
• Having identified the names of some esters, you should then use a chemical dictionary, chemical encyclopedia or Internet search engine to find uses of those esters.

outline some examples of the occurrence, production and uses of esters

• Use a chemical dictionary, chemical encyclopedia or Internet search engine to find occurrence, production and uses of esters.