Chloroprene

Chloroprene is the common name for the organic compound 2-chloro-1,3-butadiene, which has the chemical 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 DuPont gave it when the company first developed it and currently used by DuPont Dow.

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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 vinyl acetylene.^[2] In this process, acetylene is dimerized to give vinyl acetylene, which is then combine with hydrogen chloride to afford in succession vinyl acetylene, 3-chloro-1,2-butadiene, which, finally in the presence of cuprous chloride, rearranges to the targeted 2-chloro-1,3-butadiene.^[1] The conversion is shown here:

This process had disadvantages in that it 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