chloroprene

Chloroprene
IUPAC name	2-Chloro-1,3-butadiene
Other names	Chloroprene
Identifiers
CAS number	[126-99-8]
RTECS number	EL9625000
SMILES	C=C(Cl)C=C
Properties
Molecular formula	C4H5Cl
Molar mass	88.5365 g/mol
Appearance	Colorless liquid.
Density	0.9598 g/cm3, liquid.
Melting point	-130 °C, 143 K, -202 °F
Boiling point	59.4 °C, 333 K, 139 °F
Solubility in water	0.026 g/100 mL, liquid.
Hazards
R-phrases	R45, R11, R20/22, R36/37/38, R48/20
S-phrases	S53, S45
NFPA 704	3 2 0
Flash point	-15.6°C
Related compounds
Related Dienes	Butadiene Isoprene
Related compounds	Vinyl chloride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Chloroprene is the common name for the organic compound 2-chloro-1,3-butadiene, which has the formula CH₂=CCl-CH=CH₂. This colorless liquid is the monomer for the production of the polymer polychloroprene, a type of synthetic rubber. Polychloroprene is better known to the public as Neoprene, the trade name given by DuPont.

Production of chloroprene

Chloroprene is produced in three steps from 1,3-butadiene: (i) chlorination, (ii) isomerization of part of the product stream, and (iii) dehydrochlorination of 3,4-dichloro-1-butene.

Chlorine adds to 1,3-butadiene to afford a mixture of 3,4-dichloro-1-butene and 2,3-dichloro-2-butene. The 2,3-chloro isomer is subsequently isomerized to 3,4 isomer, which in turn is treated with base to induce dehydrochlorination to 2-chloro-1,3-butadiene. This dehydrohalogenation entails loss of a hydrogen atom in the 3 position and the chlorine atom in the 4 position thereby forming a double bond between carbons 3 and 4. In 1983, approximately 2,000,000 kg were produced in this manner.^[1] The chief impurity in chloroprene prepared in this way is 1-chloro-1,3-butadiene, which is usually separated by distillation.

Acetylene process

Until the 1960s, chloroprene production was dominated by the “acetylene process,” which was modeled after the original synthesis of vinylacetylene.^[2] In this process, acetylene is dimerized to give vinyl acetylene, which is then combined with hydrogen chloride to afford 4-chloro-1,2-butadiene (an allene derivative), which in the presence of cuprous chloride, rearranges to the targeted 2-chloro-1,3-butadiene:^[1]

HC≡C-CH=CH₂ + HCl → H₂C=C=CH-CH₂Cl

H₂C=C=CH-CH₂Cl → H₂C=CCl-CH=CH₂

This process is very energy-intensive and has high investment costs. Furthermore, the intermediate vinyl acetylene is unstable.

References

1. ^ ^{a b} Manfred Rossberg, Wilhelm Lendle, Gerhard Pfleiderer, Adolf Tögel, Eberhard-Ludwig Dreher, Ernst Langer, Heinz Rassaerts, Peter Kleinschmidt, Heinz Strack, Richard Cook, Uwe Beck, Karl-August Lipper, Theodore R. Torkelson, Eckhard Löser, Klaus K. Beutel, “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry, 2006 John Wiley-VCH: Weinheim.DOI: 10.1002/14356007.a06_233.pub2
2. ^ Wallace H. Carothers, Ira Williams, Arnold M. Collins, and James E. Kirby (1937). "Acetylene Polymers and their Derivatives. II. A New Synthetic Rubber: Chloroprene and its Polymers". J. Am. Chem. Soc. 53: 4203–4225. doi:10.1021/ja01362a042. 

• Polychloroprene (CR), chloroprene rubber

External links

• International Chemical Safety Card 0133
• NIOSH Pocket Guide to Chemical Hazards 0133
• European Chemicals Bureau
• IARC Monograph "Chloroprene."