Esters are the derivatives of the carboxylic acids in which the -OH part of the carboxylic group has been replaced by -OR group where, R may be alkyl or aryl group.
SOURCES, PREPARATION AND PROPERTIES OFALKANOATES
Alkyl alkanoates are found widely in nature. Short carbon chain simple alkyl alkanoates exist as liquids and have a characteristic pleasant odour. They occur in essential oils, many fruits and flowers and are sometimes called fruit essences because of their pleasant odours.
More complex alkyl alkanoates are found in fats and oils and some waxes.
Fats and oils contain one, two or three alkanoate groups which generally do not have pleasant odours. It is worth noting that there is a mixture of different alkanoate derivates of propane-1, 2, 3-triol and long-chain fatty acids in fats and oils. Table 48.1 tabulates common and direct sources of some alkyl alkanoates which give pleasant or fruity smells. It includes some waxes which do not have a fruity smell and are formed from long carbon chain fatty acids. Esters and their mixtures are prepared and used as artificial scents in many foods, drinks and perfumes.
Table 48.1 Examples of Sources and Uses of Alkyl Alkanoates
PREPARATION OF ALKANOATES
1. 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
2. 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
3. By Tischenko Reaction. When aldehydes containing a-hydrogen atoms are treated with aluminium ethoxide. They undergo condensation to produce esters.
PHYSICAL PROPERTIES OF ALKANOATES
1. Physical state. Esters are colourless, oily liquids with a characteristic fruity odour. The odour of the most of the flowers and fruits is due to esters present in them. The characteristic tastes and odours of different esters find applications in the manufacture of artificial flavouring and perfuming agents. The flavours of some of the esters are given below:
Ester Flavour Ester Flavour
n-Pentyl ethanoate Banana Amyl but)’rate Apricot
Octyl ethanoate Orange Isobutyl Raspberry
Ethyl butanoate Pineapple Benzyl ethanoate Jasmine
2. 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.
3. Boiling points. The boiling points of esters are always less than the corresponding carboxylic acids because esters do not form hydrogen bonds.
1. 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-C 17) give soap on hydrolysis with a base. Soaps are sodium or potassium salts of carboxylic acids with high molecular mass (C12-C17).
2. Reduction. Esters are reduced to alcohols by the reducing agents like (sodium/ethanol) or (lithium aluminium hydride).
3. Reaction with Ammonia. Esters on treatment with alcoholic ammonia yield acid amides. This reaction is known as ammonolysis of esters.
4. 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
5. 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.
6. Reaction with Grignard’s Reagent. All esters except the esters of formic acid react with Grignard’s reagent to give tertiary alcohols. However, the esters of formic acid, i: e., alkyl formates form secondary alcohols.
Similarly, if we start with ethyl formate (HCOOC2H5) and methyl magnesium iodide, the product will be isopropyl alcohol.