Caprolactam (CPL) is an
organic compound
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...
(CH
2)
5C(O)NH. This colourless solid is a
lactam
A lactam is a Cyclic compound, cyclic amide, formally derived from an amino alkanoic acid through cyclization reactions. The term is a portmanteau of the words ''lactone'' + ''amide''.
Nomenclature
Greek_alphabet#Letters, Greek prefixes in alpha ...
(a cyclic
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...
) of
caproic acid. Global demand for this compound is approximately five million tons per year, and the vast majority is used to make
Nylon 6
Nylon 6 or polycaprolactam is a polymer, in particular semicrystalline polyamide. Unlike most other nylons, nylon 6 is not a condensation polymer, but instead is formed by ring-opening polymerization; this makes it a special case in the compari ...
filament, fiber, and plastics.
Synthesis and production
Caprolactam was first described in the late 1800s when it was prepared by the cyclization of ε-
aminocaproic acid
Aminocaproic acid (also known as ε-aminocaproic acid, ε-Ahx, or 6-aminohexanoic acid) is a derivative and analogue of the amino acid lysine, which makes it an effective inhibitor for enzymes that bind that particular residue. Such enzymes inc ...
, the product of the hydrolysis of caprolactam. World demand for caprolactam was estimated to reach five million tons per year for 2015. 90% of caprolactam produced is used to make filament and fiber, 10% for plastics, and a small amount is used as a chemical intermediate.
[ Due to its commercial significance, many methods have been developed for the production of caprolactam. It was estimated that 90% of all caprolactam is synthesised from ]cyclohexanone
Cyclohexanone is the organic compound with the formula (CH2)5CO. The molecule consists of six-carbon cyclic molecule with a ketone functional group. This colorless oily liquid has a sweet odor reminiscent of benzaldehyde. Over time, samples of ...
(1), which is first converted to its oxime
In organic chemistry, an oxime is an organic compound belonging to the imines, with the general Chemical formula, formula , where R is an organic Side chain, side-chain and R' may be hydrogen, forming an aldoxime, or another organic functional g ...
(2). Treatment of this oxime with acid induces the Beckmann rearrangement
The Beckmann rearrangement, named after the German chemist Ernst Otto Beckmann (1853–1923), is a rearrangement reaction, rearrangement of an oxime functional group to substituted amides. The rearrangement has also been successfully performed on ...
to give caprolactam (3):[
:
The immediate product of the acid-induced rearrangement is the bisulfate salt of caprolactam. This salt is neutralized with ]ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
to release the free lactam and cogenerate ammonium sulfate
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...
. In optimizing the industrial practices, much attention is directed toward minimizing the production of ammonium salts.[
The other major industrial route involves formation of the oxime from ]cyclohexane
Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...
using nitrosyl chloride
Nitrosyl chloride is the chemical compound with the formula NOCl. It is a yellow gas that is commonly encountered as a component of aqua regia, a mixture of 3 parts concentrated hydrochloric acid and 1 part of concentrated nitric acid. It is a str ...
, and this method accounts for 10% of world production.[ The advantage of this method is that cyclohexane is less expensive than ]cyclohexanone
Cyclohexanone is the organic compound with the formula (CH2)5CO. The molecule consists of six-carbon cyclic molecule with a ketone functional group. This colorless oily liquid has a sweet odor reminiscent of benzaldehyde. Over time, samples of ...
.
A minor industrial route involves the treatment of cyclohexanecarboxylic acid
Cyclohexanecarboxylic acid is the organic compound with the formula C6H11CO2H. It is the carboxylic acid of cyclohexane. It is a colorless oil that crystallizes near room temperature..
Preparation and reactions
It is prepared by hydrogenation of ...
with nitrosylsulfuric acid
Nitrosylsulfuric acid is the chemical compound with the formula . It is a colourless solid that is used industrially in the production of caprolactam, and was formerly part of the lead chamber process for producing sulfuric acid. The compound is t ...
(the Snia Viscosa process). This is thought to proceed via a ketene
In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary valence (chemistry), monovalent functional group, chemical groups (or two separate Substituent, substitution sites in the same molecule). The na ...
.
:
Other paths to caprolactam include the depolymerization of waste Nylon 6, and the reaction of caprolactone
ε-Caprolactone or simply caprolactone is a lactone (a cyclic ester) possessing a seven-membered ring. Its name is derived from caproic acid. This colorless liquid is miscible with most organic solvents and water. It was once produced on a large ...
with ammonia.[ At bench scale, the reaction between cyclohexanone with ]hydrazoic acid
Hydrazoic acid, also known as hydrogen azide, azic acid or azoimide, This also contains a detailed description of the contemporaneous production process. is a compound with the chemical formula . It is a colorless, volatile, and explosive liquid ...
to give caprolactam in the Schmidt reaction
In organic chemistry, the Schmidt reaction is an organic reaction in which an azide reacts with a carbonyl derivative, usually an aldehyde, ketone, or carboxylic acid, under acidic conditions to give an amine or amide, with expulsion of nitrogen. ...
has been reported.
:
Uses
Almost all caprolactam produced goes into the manufacture of Nylon 6
Nylon 6 or polycaprolactam is a polymer, in particular semicrystalline polyamide. Unlike most other nylons, nylon 6 is not a condensation polymer, but instead is formed by ring-opening polymerization; this makes it a special case in the compari ...
. The conversion entails a ring-opening polymerization
In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer (see figure). The reactive center can be radical, anion ...
:
:
Nylon 6 is widely used in fiber
Fiber (spelled fibre in British English; from ) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often inco ...
s and plastic
Plastics are a wide range of synthetic polymers, synthetic or Semisynthesis, semisynthetic materials composed primarily of Polymer, polymers. Their defining characteristic, Plasticity (physics), plasticity, allows them to be Injection moulding ...
s.
In situ anionic polymerization is employed for cast nylon production where conversion from ε-caprolactam to Nylon 6 takes place inside a mold. In conjunction with endless fiber processing the term thermoplastic resin transfer molding (T-RTM) is often used.
Caprolactam is also used in the synthesis of several pharmaceutical drugs including pentylenetetrazol, meptazinol, and laurocapram.
Safety
Caprolactam is an irritant and is mildly toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
, with an of 1.1 g/kg (rat, oral). In 1991, it was included on the list of hazardous air pollutants by the U.S. Clean Air Act of 1990. It was subsequently removed from the list in 1996 at the request of the manufacturers. In water, caprolactam hydrolyzes to aminocaproic acid
Aminocaproic acid (also known as ε-aminocaproic acid, ε-Ahx, or 6-aminohexanoic acid) is a derivative and analogue of the amino acid lysine, which makes it an effective inhibitor for enzymes that bind that particular residue. Such enzymes inc ...
, which is used medicinally.
As of 2016 caprolactam had the unusual status of being the only chemical in the International Agency for Research on Cancer
The International Agency for Research on Cancer (IARC; ) is an intergovernmental agency forming part of the World Health Organization of the United Nations.
Its role is to conduct and coordinate research into the causes of cancer. It also cance ...
's lowest hazard category, Group 4: "probably not carcinogenic to humans".
Currently, there is no official permissible exposure limit
The permissible exposure limit (PEL or OSHA PEL) is a legal limit in the United States for exposure of an employee to a chemical substance or physical agents such as high level noise. Permissible exposure limits were established by the Occupational ...
set for workers handling caprolactam in the United States. The recommended exposure limit
A recommended exposure limit (REL) is an occupational exposure limit that has been recommended by the United States National Institute for Occupational Safety and Health. The REL is a level that NIOSH believes would be protective of worker safety ...
is set at 1 mg/m3 over an eight-hour work shift for caprolactam dusts and vapors. The short-term exposure limit A short-term exposure limit (STEL) is the acceptable average exposure over a short period of time, usually 15 minutes as long as the time-weighted average is not exceeded.
STEL is a term used in exposure assessment, occupational health, industrial ...
is set at 3 mg/m3 for caprolactam dusts and vapors.
Climate impact
The production of caprolactam can produce nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...
as a by-product, a highly potent greenhouse gas. Emissions differ significantly due to different production processes and inconsistent use of emission abatement technology. A study commissioned by the German Federal Ministry for Economic Affairs and Climate Action
The Federal Ministry for Economic Affairs and Energy (, ; abbreviated BMWE, formerly BMWi) is a Cabinet of Germany, cabinet-level ministry of the Federal Republic of Germany. It was previously known as the "Ministry of Economy". It was recreate ...
estimates emissions between 9 kg of nitrous oxide per ton of caprolactam and almost zero.
Nitrous oxide emissions from caprolactam production are unregulated in most countries. Unlike other chemical production processes, nitrous oxide emissions from caprolactam production are not included in the European Union Emissions Trading System
The European Union Emissions Trading System (EU ETS) is a carbon emission trading scheme (or ''cap and trade'' scheme) that began in 2005 and is intended to lower greenhouse gas emissions in the EU. Cap and trade schemes limit emissions of spec ...
.
References
{{Carcinogen, state=collapsed
Epsilon-lactams
Monomers
Commodity chemicals