ε-Caprolactone or simply caprolactone is a cyclic ester

Ester

Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

, a member of the lactone

Lactone

In chemistry, a lactone is a cyclic ester which can be seen as the condensation product of an alcohol group -OH and a carboxylic acid group -COOH in the same molecule...

 family, with a seven-membered ring with the formula (CH₂)₅CO₂. This colorless liquid is miscible with most organic solvents. It is produced on a very large scale as a precursor to caprolactam

Caprolactam

Caprolactam is an organic compound with the formula 5CNH. This colourless solid is a lactam or a cyclic amide of caproic acid. Approximately 2 billion kilograms are produced annually...

.

Production and uses

The great majority of caprolactone is consumed, often in situ, as a precursor to caprolactam. It is also a monomer used in the manufacture of highly specialised polymer

Polymer

A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s. Ring-opening polymerization

Ring-opening polymerization

In polymer chemistry, ring-opening polymerization is a form of chain-growth polymerization, in which the terminal end of a polymer acts as a reactive center, where further cyclic monomers join to form a larger polymer chain through ionic propagation...

, for example, gives polycaprolactone

Polycaprolactone

Polycaprolactone is a biodegradable polyester with a low melting point of around 60°C and a glass transition temperature of about −60°C. PCL is prepared by ring opening polymerization of ε-caprolactone using a catalyst such as stannous octoate. Recently a wide range of catalysts for the ring...

. Another polymer is polyglecaprone, used as suture material in surgery.

Caprolactone is prepared industrially by Baeyer-Villiger oxidation

Baeyer-Villiger oxidation

The Baeyer–Villiger oxidation is an organic reaction in which a ketone is oxidized to an ester by treatment with peroxy acids or hydrogen peroxide. Key features of the Baeyer–Villiger oxidation are its stereospecificity and predictable regiochemistry...

 of cyclohexanone

Cyclohexanone

Cyclohexanone is the organic compound with the formula 5CO. The molecule consists of six-carbon cyclic molecule with a ketone functional group. This colorless oil has an odor reminiscent of peardrop sweets as well as acetone. Over time, samples assume a yellow color due to oxidation...

 with peracetic acid. The three main manufacturers are BASF in the USA, Daicel in Japan and the largest Perstorp in the UK.

Reactions

The dominant reaction for caprolactone is its conversion to caprolactam, billions of kilograms of which are produced annually. Caprolactone is treated with ammonia at elevated temperatures to give the lactam:₅CO₂ + NH₃ → (CH₂)₅C(O)NH + H₂O

Carbonylation

Carbonylation

Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry.-Organic chemistry:...

 of caprolactone gives, after hydrolysis, pimelic acid

Pimelic acid

Pimelic acid is the organic compound with the formula HO2C5CO2H. Derivatives of pimelic acid are involved in the biosynthesis of the amino acid called lysine. Pimelic acid is one methylene longer than a related dicarboxylic acid, adipic acid, a precursor to many polyesters and polyamides...

. The lactone ring is easily opened with nucleophiles including alcohols and water to give polylactones and eventually the 6-hydroxyadipic acid.

Related compounds

Several other caprolactones are known, although none approaches the technological importance of ε-caprolactone. These isomers include alpha-, beta-, gamma-, delta-caprolactones. All are chiral. Gamma-caprolactone is component of flower

Flower

A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

 aromas and an insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

 pheromone

Pheromone

A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

. delta-Caprolactone is found in heated milk fat.

Safety

Caprolactone hydrolyses rapidly and the resulting hydroxycarboxylic acid displays unexceptional toxicity, as is common for the other hydroxycarboxylic acids. It is known to cause severe eye irritation. Exposure may result in corneal injury.