Lactones

Lactones Definition and Nomenclature of Lactones

The name of lactone is derived from a compound called as Lactide. The dehydration of lactic acid generates lactones. Lactones get their names according to the precursor acid molecule, with lactone as a suffix and a Greek letter as a prefix. For example, macrocyclic lactones are named as macrolactones. Lactones are cyclic organic esters of hydroxycarboxylic acid, usually formed when hydrogen or halogen atoms react with any carboxylic group present in the same molecule. All this process is done by intramolecular esterification of hydroxycarboxylic acid. There is a ring containing two or more carbon atoms and just one oxygen atom. 

Lactones with three- or four-membered rings (α-lactones and β-lactones) are highly reactive, because of which isolation becomes difficult. Unique laboratory methods are used for both small ring lactones, and for lactones that contain rings larger than six-membered, for a reaction.

Two types of lactones occur naturally; saturated and unsaturated. Some commonly found lactones in nature are kavain, gluconolactone, neurotransmitters, ascorbic acid, and antibiotics.

Lactone Synthesis

Different methods of ester synthesis are applied to lactones. In oxandrolone synthesis, the last forming step is the organic reaction, known as esterification.  

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However, in the halolactonization, halogen attacks an alkene through electrophilic addition along with the cationic intermediate. It is then captured intramolecularly by adjacent carboxylic acid.

Lactone Reactions

5-membered γ-lactones and 6-membered δ-lactones are the most stable structure because, as in every organic cycle, 5 and 6 membered rings minimize the pressure of bond angles. At room temperature, γ-lactones, along with the presence of dilute acid, are very stable. Thus, 4-hydroxy acids (R-CH(OH)-(CH2)2-COOH) instantly undergo natural esterification and cyclization to the lactone. β-lactones do prevail, but can only be made by unique methods. α-lactones can be detected as temporary species in mass spectrometry experiments.

Lactones reaction and ester reaction are very similar, and the methods applied for ester synthesis can be used here. For example, Shiina macro-lactonization, Yamaguchi esterification, and nucleophilic abstraction. Given below are some of the lactones reactions: 

Hydrolysis

When a lactone with a base is heated, it hydrolyzes the lactones into its parent compound, a straight-chained bifunctional compound. The hydrolysis-condensation reaction of lactones is reversible, just like the ester reaction.  After hydrolysis, lactones offer only a single product.

Reduction

By the reduction reaction, lactones get reduced to dios with the help of lithium hydride in the presence of dry ether. At first, the reaction will break the ester bond of the lactone, and later it will reduce the aldehyde group (-CHO) into the alcohol group (-OH). For example, gamma lactones get  reduced to:

butane-1,4-diol, (CH2(OH)-(CH2)2-CH2(OH).

Polymerization

Lactones from polyesters, according to the below-given formula:

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Aminolysis

Lactones even react with ethanolic ammonia. First, it will break the ester bond, then will respond with the acidic -COOH group. This happens because of the fundamental properties of ammonia of forming a dual functional group, i.e., alcohol and amide. Gamma-lactones will react to produce CH2(OH)-(CH2)2-CO-NH2.

Michael Reaction

Some lactones like Sesquiterpene lactones, which are found in several plants, react with different molecules by Michael's reaction.

(Michael reaction is nucleophilic addition of a carbon or another nucleophile, with an α,β-unsaturated carbonyl compound which contains an electron-withdrawing group)

Uses of Lactones

Lactones are used for adding flavors and fragrances to fruits and dairy products. It is used for adding flavors in both fermented and unfermented dairy products. Some of the examples of lactones are;

  • γ-decalactone also named as 4-decanolide has a natural peach flavor 

  • δ-decalactone also called as 5-decanolide has a creamy coconut as well as peach flavor 

  • γ-dodecalactone also named as 4-dodecanolide has coconut and fruity flavor 

  • γ-octalactone even named as 4-octanolide has a herbaceous character, although it's not a herb

  • γ-nonalactone has an intense coconut flavor of this series, but it is not present in coconut 

  • Macrocyclic lactone has the same odor as of animal origin

  • Polycaprolactone is a vital plastic

Other examples of lactones are Macrolides, Ellagic acid, Kavalactones, Tergallic acid diacetone, Lactide, and Valoneic.


FAQ (Frequently Asked Questions)

1. What are the Applications of Lactones?

Lactones are very important because of the sensory properties and it is widely distributed in the food and beverage industry, as it acts as a flavour additive. The presence of the aroma of lactone is in more than 120 foodstuffs, like, food, meat, dairy products and beverages. The odour quality and the intensity of lactones is the primary reason behind its popularity. Also, a type of lactone is used as tablets for the treatment of heart failure and high blood pressure. The polycaprolactone is formed by lactones, and it is a biodegradable polyester,  which is essential in food and drug applications (FDA).

2. What is the Difference Between a Lactone and a Lactide?

Although many do get confused between lactone and lactides, as they both sound similar, there's just a key difference between both. A lactide is simply a form of lactone, which can be derived from lactic acid by heating. The chemical formula of a lactide is C6H8O4. In contrast, the lactone is a cyclic ester derived from hydroxy acid.  Lactones are usually used for adding flavors to fruits and fermented as well as unfermented dairy products, whereas lactides are used as a precursor for polymer materials, like, polystyrene. In the process of polymerisation, lactide becomes polylactide, and lactones lead to form plastic named as, "polycaprolactone".

3. What are the Natural Sources of Lactones?

Lactones that occur naturally are primarily saturated and unsaturated γ- and δ-lactones, but to a lesser extent, macrocyclic lactones. The γ-lactones and δ-lactones are intramolecular esters of the related hydroxy fatty acids. They provide the aroma of fruits, butter, cheese, and other food products. Cyclopentadecanolide is behind the musk-like odor of angelica root oil. Most of the naturally occurring bicyclic lactones, phthalides are responsible for the fragrances of celery and lovage oils, and coumarin for woodruff. Lactones are even present in oak wood, and they highly contribute to Barrel-aged beers' flavor profile.