Isoprene

2- methylbuta -1 ,3-diene

Colorless, highly flammable liquid with a characteristic odor

Liquid

0.68 g · cm -3

-146 ° C

34 ° C.

604 hPa ( 20 ° C)

• Very poor in water ( 0.7 g · l-1 at 20 ° C)
• Well in ethanol and diethyl ether

1.4218

Risk

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Isoprene is the common name for unsaturated hydrocarbon 2- methylbuta -1 ,3-diene. It is a derivative of 1,3-butadiene. Isoprene is the basic unit of terpenes, is itself but usually not counted among these. Isoprene is produced by many plants and released into the atmosphere, it is in addition to methane, the hydrocarbon with the highest emission rate.

Synthesis

Ethene reacts with propene to 2-methyl. 2 -methylbutene is dehydrated to chromia-alumina catalysts isoprene.

A further possibility is the dimerization of propene to form 2- methyl pentene and the subsequent loss of methane:

Importance and occurrence

Isoprenproduktion by plants

From isoprene can be formally many natural substances derived that are combined to form the isoprenoid natural products. An example of this are the steroids and terpenes. However, the direct precursor and starting point for the biosynthesis of isoprenoids is not isoprene, but the biochemically activated isopentenyl pyrophosphate ( IPP) and its isomer dimethylallyl pyrophosphate ( DMAPP ). Isoprene itself is produced by many trees and phytoplankton; the reason is not clear. According to one hypothesis, the gaseous isoprene is to protect the plant against oxidation by ground-level ozone.

Isoprene is linked in repeating units in membranes of Archaea ago. There they form the hydrophobic portions of the lipids and are linked via ether with glycerol molecules. The isoprene units of the outer and inner layers can be combined so that a single-layer lipid layer is formed.

Isoprene in the atmosphere

Isoprene discharged from plants in the atmosphere. The annual emission of about 600 Tg, half of which comes from tropical trees. This is about the annual methane emissions. Isoprene is degraded by reaction with OH radicals and ozone, thereby arise aldehydes, peroxides and organic nitrates, which can dissolve in droplets or form particles.

Use

For the production of isoprene rubber (IR ), in addition also for synthesis of natural products, eg for α -terpineol.

Safety and Toxicology

Isoprene is of low acute toxicity: The oral LD50 for rats is > 2000 mg / kg. Isoprene can be taken by inhalation and if swallowed. Contact with eyes and skin produces redness and pain, an inhalation causes cough, nausea, burning and shallow breathing forth. In animal experiments, isoprene acts carcinogenic and mutagenic.

Reactions

• Isoprene dimerization in a Diels -Alder reaction when heated to limonene; this reaction could be the first time Gustave Bouchardat in 1878.

• Isoprene is reacted with methyl acrylate in a Diels -Alder reaction to give racemic 4- methylcyclohex -3-ene -1-carboxylic acid methyl ester. This results in a Grignard reaction with methyl magnesium bromide ( CH3MgBr, Grignard reagent ) to the hydrolysis α -terpineol as a racemic mixture: