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

  • investigate the production, in a school laboratory, of simple esters – Fisher esterification – concentrated H2SO4 as a catalyst and dehydrating agent – reflux, isolation and purification of esters

Esters: Structure, Nomenclature, Boiling Point and Solubility

     

    Esters

    Structure and Nomenclature

    • 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

    • 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

    • 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

    • 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.

    Production of Simple Esters, Reflux, Isolation and Purification

     

    Production of Esters

    • The reaction between an alcohol and a carboxylic acid functional group produces an ester and water.
    • The reaction is considered a condensation reaction due to the production of a small molecule i.e. water alongside the main organic product.
      • The water molecule consists of –OH group from the carboxylic acid and proton (H+) from the alcohol.
      • The oxygen atom in ester (C–O) originates from the alcohol.
    • Concentrated H2SO4 is an important reagent for esterification due to two reasons:
      • Catalyst: increases reaction rate by lowering the activation energy
      • Dehydrating agent: by removing water from the reaction, the equilibrium position
      • constantly shifts to the product side to produce more esters and water (Le Chatelier’s principle). Thus, concentrated sulfuric acid increases reaction rate and yield.
      • Concentrated H2SO4 must be added slowly to the reaction mixture (dropwise) because the addition of sulfuric acid to an aqueous solution is exothermic.

     

     

    • Esterification is
      • Slow
      • Reversible because H2SO4 catalyses esterification and also hydrolysis of the ester (reverse reaction)
      • Exothermic because more energy is released in the formation of bonds in the ester and water molecule than the energy absorbed to break bonds.

     

    • Water in the presence of acid can easily hydrolyse an ester into a carboxylic acid and an alcohol.

     

    Reaction Conditions

    • Esterification is conducted under heat with reflux at 140 – 180ºC in a round-bottom flask.
      • Heat is required to meet the activation energy of the reaction and increase reaction rate.
      • Reflux is the process of condensing gaseous products back into liquid form, allowing them to return to the reaction mixture.
      • Use of a round-bottom flask promotes even heating of reaction mixture.

     

    Reflux is essential for two reasons:

    1. Allows for an open system reaction to be conducted at high temperatures by constantly releasing pressure from inside the reaction chamber.
    2. Prevents the loss of volatile substances e.g. alcohol, carboxylic acid and sulfuric acid.

     

    • Reflux is achieved through the use of a vertical column condenser connected to cool water supply. Cool tap water flows into the condenser at the bottom, absorbs heat while travelling upwards through the column, and finally exits at the type as warm water.
      • Water enters at the bottom to allow for more efficient heat absorption as water spends long time travelling upwards against gravity.
      • Cool water lowers the temperature inside the condenser column which in turn turns gaseous substances back into liquids so they remain inside the reaction apparatus.

     

    • Boiling chips are commonly used to avoid superheating and achieve safe practice in school laboratories.
      • Superheating is a phenomenon in which a liquid is heated to a temperature above its boiling point, without boiling.
      • Superheated solutions can flash boil, causing a sudden increase in pressure and often results in flask breakage.

     

    Figure: reflux set-up

     

     

     

     

     

    Isolation of Ester

    • NaHCO3 or Na2CO3 (weak bases) is added to the reaction mixture at the end of esterification to neutralise any remaining, unreacted carboxylic acid. This reaction produces soluble salts.

     

    • Separating funnel separates substances based on their solubility and density.

     

    • Unreacted alcohol and salts formed from neutralisation are soluble in water whereas esters are typically sparingly soluble at best but more commonly miscible with water.

     

    • When Na2CO3-treated mixture is added to the separating funnel, two distinct layers will form:
      • Organic layer which contains the ester is formed at the top
      • Clear aqueous layer which contains the alcohol and soluble salts is formed below the organic layer due to its higher density.

     

    • The aqueous layer can be discarded by opening the stopcock of the separating funnel, leaving behind the organic layer. This step is repeated several times until only the organic layer remains in the funnel.

     

     

     

    Figure: separating funnel with a non-aqueous organic layer and an aqueous inorganic layer.

     

     

    Purification of Ester

     

    • Distillation purifies esters by making use of its lower boiling point compared to other substances in the mixture.
    • Temperature of the mixture is raised just above the ester’s boiling to allow for its evaporation.
    • Gaseous ester is returned to liquid state as it goes through a condenser (similar to reflux). Liquid ester is then collected in a separate vessel as the distillate.
    • The distillate can be confirmed to be an ester by smelling its aroma from a safe distance.

     

    Figure: distillation set-up used to purify esters