Hydrolysis of esters
Anhydrous alkanoic acids react with alcohols in the presence of heated sulphuric acid (H2SO4) to form an ester and water. This process is known as esterification. This can be represented by the equation: Alkanoic acid + Alcohol (means reversible eq) ester + water Esters occur naturally in fruits and flowers and are described as oily, sweet smelling liquids. Vegetable oils and animal fats are esters of long-chain acids. Esters can undergo a range of reactions and this experiment focuses on one: the Hydrolysis reactions of esters.
Hydrolysis is the process whereby a substance is broken down by reacting it with water. Esters are hydrolysed by acid hydrolysis or alkaline hydrolysis. Acid hydrolysis involves boiling the ester with dilute aqueous acid (H2SO4 or HCl) while the latter involves boiling the ester with an aqueous alkali (KOH or NaOH). The products of acid hydrolysis are an alcohol and carboxylic acid. Alkaline hydrolysis yields a salt (called the salt of the acid) and the alcohol that formed the ester. Both types of hydrolysis were performed in this experiment.
First, the ester (ethyl ethanoate) was added to dilute HCl and boiled (acid hydrolysis). The pungent smell which was first observed indicated that the carboxylic acid was formed. This acid was ethanoic acid. The second smell, a sweet smell, marked the presence of the second product, the alcohol. The alcohol formed was ethanol. This reaction is represented by the following equation: CH3COOCH2CH3 (l) + 2HCl (aq) ? CH3COOCH (aq? ) + CH2CH3OH (aq? ) During the alkaline hydrolysis of esters, the first sweet smell indicated the presence of the alcohol ethanol.
A pungent smell then indicated that ethanoic acid was formed. However, due to the presence of excess sodium hydroxide, this acid was neutralized to form a salt (the salt of the acid). The salt formed was sodium ethanoate. The reaction is represented by this equation: CH3COOCH2CH3 (L) + NaOH (aq) ? CH3COONa (aq) + CH3CH2OH (aq) Alkaline hydrolysis is also known as saponification, and this is the process whereby soap is made. POSSIBLE ERRORS : Measuring the amount of materials to be used incorrectly Not heating the solution sufficiently Using expired or contaminated materials