HSC Chemistry: Esters
This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.
HSC Chemistry Syllabus
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investigate the structural formulae, properties and functional group of esters
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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 |
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- 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 |
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Propyl propanoate |
Propan-1-ol |
Propanoic acid |
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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:
- Allows for an open system reaction to be conducted at high temperatures by constantly releasing pressure from inside the reaction chamber.
- Prevents the loss of volatile substances e.g. alcohol, carboxylic acid and sulfuric acid.
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Figure: reflux set-up
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Isolation of Ester
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Figure: separating funnel with a non-aqueous organic layer and an aqueous inorganic layer.
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Purification of Ester
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Figure: distillation set-up used to purify esters |