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Lactones- Meaning

Lactones are cyclic organic esters of hydroxycarboxylic acids, usually formed by the reaction of a halogen atom or hydroxyl group with a carboxylic acid group present in the same molecule. These are formed by intramolecular esterification of respective hydroxycarboxylic acids. It has a ring of two or more carbon atoms and one oxygen atom. Lactones with three or four-membered rings are highly reactive which makes it very difficult to isolate them.

Lactones 2

Name of lactone is derived from a compound called Lactide; that is generated by the dehydration of lactic acid. These are named with a Greek letter prefix and lactone suffix. Prefixes indicate the size of the lactone ring.

Naturally occurring Lactones are of two types; Saturated and unsaturated. Few commonly found lactones in nature are kavain, ascorbic acid, gluconolactone, neurotransmitters, and antibiotics.

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Many methods adopted for ester synthesis can be applied for Lactone synthesis. Methods include Shiina macro-lactonization, nucleophilic abstraction and Yamaguchi esterification. Lactones like γ-nonalactone,γ-octalactone, γ-undecalactone, γ-decalactone can be synthesised in a single step process.

Lactone- Reactions

The reactions of lactones are almost similar to the reactions of esters. Let’s have a look at some reactions.

  • Hydrolysis

When a lactone along with a base (Sodium hydroxide) is heated, it hydrolyses into its parent compound. Lactone’s hydrolysis condensation reaction is a reversible reaction with equilibrium.

  • Reduction

Lithium aluminium hydride reduces lactones to diols. The reaction involves the breaking of the ester bond and then the carboxylic acid group is reduced to the alcohol group.

  • Polymerisation

Polyesters are formed by lactones.

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  • Aminolysis

Alcohol and amide are formed when lactones react with ethanolic ammonia.

Lactones- Examples and Uses

Lactones play a vital role in providing flavour to  fruit and dairy products. Therefore, these are used as flavour and fragrances. Some lactones with their flavour are listed below.

  • γ-decalactone – Peach flavour
  • γ-dodecalactone – Coconut/fruit flavour
  • γ- octalactone – Coconut/fruit flavour
  • 0-decalactone – Creamy coconut/peach flavour

Some Examples of Lactones are Macrolides, Ellagic acid, Kavalactones, Lactide, Valoneic, Tergallic acid diactone.

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