HSC Chemistry: Esters

 

This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.

HSC Chemistry Syllabus

  • investigate the structural formulae, properties and functional group of esters

Esters: Structure, Nomenclature, Boiling Point and Solubility

     

    Esters

    Structure and Nomenclature of Esters

    • Esters are derivatives of carboxylic acids because they share a common carbonyl (C=O) group and can be produced from carboxylic acids using various reaction mechanisms.

    System name

    Generic structure

    Example

    -yl -oate

     

    methyl ethanoate
    • Esters are formed through condensations reactions between a carboxylic acid and an alcohol. This is called esterification.

     

     

    • Naming convention of esters is always split into two parts
      • First part (-yl) of the name indicates the alkyl group originated from the alcohol.
      • Second part (-oate) of the name indicates the carboxylic acid derivative.

     

    Ester

    Alcohol

    Carboxylic acid

    Propyl propanoate

    Propan-1-ol

    Propanoic acid

    Butyl ethanoate

    Butan-1-ol

    Ethanoic acid

     

     

     

    Properties of Esters

    Boiling and melting points of esters

    • Esters cannot form hydrogen bonds within themselves due to the lack of a hydrogen (connected to either O, N or F).
      • Esters have lower boiling and melting points than alcohols and carboxylic acids.

     

    Table: comparison of boiling points of compounds of similar mass in different functional groups.

    • Esters have permanent dipoles due to the presence of electronegative oxygen atoms. Molecules of ester are also attracted by dispersion forces.
      • Esters have stronger intermolecular forces, higher boiling and melting points than non-polar hydrocarbons e.g. alkanes, alkenes and alkynes.
      • Esters have slightly weaker dipole-dipole interactions than aldehydes of similar molar mass because the carbonyl group is always located at the end of a carbon chain in aldehydes. However, this difference is small thus, the difference in boiling & melting point between aldehyde and ester is small.

     

    Solubility in Water of Esters

    • Esters can accept hydrogen bonds from water molecules.
      • This allows them to be soluble in water.

     

    • As esters increase in molar mass, they become more non-polar and their permanent dipole decreases in magnitude.
      • Esters become less soluble in water as they increase in size. Typically, only small esters are soluble in water at 25ºC.
    • Esters are less soluble in water than aldehydes and ketones of similar molar mass. This is because aldehydes and ketones form stronger dipole-dipole forces in addition to their hydrogen bonds.
    • In general, esters are less soluble than alcohols and carboxylic acids of similar molar mass. This is because esters can only accept hydrogen bonds from water using electron lone pairs of oxygen whereas alcohols and carboxylic acids can donate and accept hydrogen bonds.

     

    Aroma of Esters

    • Esters are known to give off a fruity odour. Each unique structure of an ester provides a different scent.
    • These esters are found naturally in fruits, vegetables and artificially used in perfumes.
    • Identification of esters: esters’ characteristic scents are the main way to identify their presence or formation.

     

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