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Summary ALevel Chemistry - Carboxylic Acids and Esters Notes
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Carboxylic Acids and Esters Notes
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Carboxylic Acids and EstersCarboxylic acids have a carbonyl functional group -COOH. Two carboxylic groups is a dioic
acid.
They are weak acids and only partially dissociate in water.
The weak dissociation and partial ionisation means that carboxylic acid is a weak acid.
In aqueous solution they are only slightly ionised, give low concentrations of hydronium ions
and alkanoate ions (often called carboxylate ions).
CH3CO2H <---- CH3CO2- + H+ In water they partially dissociate into a carboxylate ion and H+.
The reaction is reversible but equilibrium lies to the left and most molecules don't disassociate.
Small carboxylic acids partially dissolve in water but after this solubility decreases. They
dissolve as they hydrogen bond to water molecules O. Solubility decreases for larger
compounds as the longer hydrocarbon tails get in between in hydrogen bonding and this
hydrogen bonding is replaced by van der waals forces.
Strength of carboxylic acids decreases with chain length - increased chain length pushes
electron density on the COO- making it more negative less stable so the acid is less strong.
acid + metal salt + hydrogen
acid + alkali salt + water
acid + carbonate salt + water + carbon dioxide
Esterification Reactions
When heating a carboxylic acid with an alcohol in the presence of a strong acid catalyst an
ester is produced - esterification.
Concentrated acid (sulphuric or hydrochloric) is used as a catalyst but the reaction is still slow
and reversible.
Esterification is one example of a condensation reaction as water is eliminated from the acid
and alcohol reacting together.
, Esters are a carboxylic acid derivative which contains the ester group, -COO-.
The nomenclature of esters follows the pattern:
Remove the -oic acid suffix from the parent carboxylic acid and replace with -oate
The alkyl chain attached to the oxygen atom of the -COO- group is then added as the first
word in the name
Esters can also be produced when reacting an alcohol and acid anhydride.
Uses of Esters
Sweet smelling compounds - perfumes and flavourings. Esters are non toxic, soluble in
solvents and do not react with water.
Plasticisers form polymers to be more flexible. Adding plasticiser allows chains to move
more easily so that the polymer becomes more flexible.
Solvents as they are polar substances - glues/inks. Dissolve in solvent and do not form
hydrogen bonds although polar. Very volatile due to low boiling point.
Testing for alcohols:
Unknown substance should be warmed with the carboxylic acid + sulphuric acid.
Excess acid can be removed by adding warm aqueous sodium carbonate solution.
If the remaining compound has a sweet smell of an ester this indicates the presence
of an alcohol.
Warmth of the solution causes the ester to evaporate so the smell can be easily
detected.
Naturally occurring esters
Animal Fats and Vegetable Oils are esters of glycerol and fatty acids.
Fats have mainly saturated hydrocarbons so fit together neatly - this increases the Van Der
Waals forces so higher temperatures are needed to melt.
Oil have unsaturated hydrocarbons. The double bonds create bends and kinks decreasing the
Van Der Waals forces so lower temperatures are needed.
Acid Hydrolysis of Esters
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