Alkanoates or Esters
All esters are similar chemically although they may vary in degree of reactivity. Ethyl ethanoate is one of the simple esters. Its molecular formula is CH3COOC2H2.
Ethyl ethanoate is prepared by the esterification between ethanol and glacial ethanoic acid at 150oC in the presence of concentrated tetraoxosulphate (vi) acid.
C2H5OH(aq) + CH3COOH(l) = CH3COOC2H5(l) + H2O(l)
Preparation of Alkanoates
- Acylation. Esters are prepared by the acylation of alcohols or phenols. The acylating agents can be any of the following:
Carboxylic acid / H 2SO 4 or Acyl chloride or Acid anhydride
(i) Condensation of alcohols with carboxylic acid This reaction involves esterification of alkanols by alkanoic acid
(ii) Esterification through acid derivatives
- By Reaction of Acids with Diazomethane. Acids on being treated with ethereal solution of diazomethane yield methyl esters.
RCOOH + CH2N2 à RCOOCH3 + N2
Acid Diazo Ester
- By Tischenko Reaction. When aldehydes containing a-hydrogen atoms are treated with aluminium ethoxide. They undergo condensation to produce esters.
Physical Properties of Alkanoates
- Physical state. Esters are colourless, volatile and oily liquids with a characteristic fruity smell. The smell of the most of the flowers and fruits is due to esters present in them. The characteristic tastes and smells of different esters find applications in the manufacture of artificial flavouring and perfuming agents.
- Solubility. Esters are sparingly soluble in water but are quite miscible in organic solvents like alcohols and ethers. In fact, most of the esters are themselves very good solvents for plastics and nitrocellulose.
- Boiling points. The boiling points of esters are always less than the corresponding carboxylic acids because esters do not form hydrogen bonds.
- Hydrolysis.Esters are hydrolysed slowly by water at boiling temperature. The reaction is catalysed by small amount of acid or base. The basic hydrolysis is also known as saponification. It is because of the fact that the esters with high molecular mass acids (C12-C17) give soap on hydrolysis with a base. Soaps are sodium or potassium salts of carboxylic acids with high molecular mass (C12-C17).
Ethyl ethanoate can be hydrolyzed by water into its component acid and alkanol again. The reaction is catalyzed by hydrogen or hydroxide ions i.e dilute acid or alkaliCH3COOC2H5(l) + H2O(l) → C2H5OH(aq) + CH3COOH(l)
- Reduction. Esters are reduced to alcohols by the reducing agents like (sodium/ethanol) or (lithium aluminium hydride).
- Reaction with Ammonia. Esters on treatment with alcoholic ammonia yield acid amides. This reaction is known as ammonolysis of esters.
- Reaction with Phosphorus Pentachloride. Esters are converted into acid chlorides and alkyl halides by heating with phosphorus pentachloride.
RCOOR’ + PC15 → RCOCl + R’Cl + POC13
C6H5COOC2H5 + PCl → C6H5 COCl + C2H2OH
Ethyl benzoate Benzoyl chloride
- Alcoholysis. An ester on refluxing with a large excess of an alcohol in the presence of a little acid or alkali, undergoes exchange of alcohol residues, i.e., alkoxy parts as shown below:
This reaction is known as alcoholysis or trans-esterification.
Uses of Esters
They are mainly used as solvents for cellulose nitrate and quick drying substances like paints, nail varnishes, lacquer and adhesives. The commonly known thinner water is a mixture of esters. Esters are used in perfumes and cosmetics and artificial flavouring for foods. Certain volatile esters are used as solvents for lacquers, paints, and varnishes; for this purpose, large quantities of ethyl acetate and butyl acetate are commercially produced.
- Esters that are have fragrant odours are used as a constituent of perfumes, essential oils, food flavourings, cosmetics, etc
- Esters are used as an organic solvent
- Natural esters are found in pheromones
- Naturally occurring fats and oils are fatty acid esters of glycerol
- Phospoesters form the backbone of DNA molecules
- Nitrate esters, such as nitroglycerin, are known for their explosive properties
- Polyesters are used to make plastics
- Esters are used to make surfactants E.g. soap, detergents
ASSESSMENT (POST ANSWERS BELOW USING THE BOX)
- How is Ethyl ethanoate prepared?
- What are some of the uses of Esters?
- What are the chemical properties of Esters?
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