2-Methyl-l,3-butadiene Isoprene

Simple conjugated dienes used in polymer synthesis include 1,3-butadiene, chloroprene (Z-chloro-l -butadiene), and isoprene (2-methyl-l,3-butadiene). Isoprene has been prepared industrially by several methods, including the acid-catalyzed double dehydration of S-methyl-l/S-butanediol. 

Auf analoge Weise erhalt man aus 2-Methyl-l,3-butadien (Isopren) mit Acetylnitrat unter Nachbehandlung mit Essigsaure 4-Acetoxy-2-methyl-l-nitro-2-buten (SO /o)1 ... 

Methyl-l,3-butadiene (Isoprene). Isoprene-acetone mixtures may form peroxides.7... 

Natural rubber is a polymer of 2-methyl-l,3-butadiene (isoprene Section 26.6). On average, a molecule of mbber contains 5000 isoprene units. All the double bonds in natural rubber are cis. Rubber is a waterproof material because it consists of a tangle of hydrocarbon chains that have no affinity for water. Charles Macintosh, a Scotsman, was the first to use mbber as a waterproof coating for raincoats. 

Scheme 6.39. A representation of acid-catalyzed cross polymerization of 2-methylpropene [wobutylene, (CH3)2C=CH2] with a trace of 2-methyl-l,3-butadiene [isoprene, CH2=C(CH3)-CH=CH2] followed by cross-linking chains with sulfur (part of the process of Vulcanization).

The polymerization of 2-methyl-l, 3-butadiene (isoprene [CH2=C(CH3)CH= CH2]) by radical processes produces a rubber-Uke material. Indeed, a synthetic rubber very close to natural rubber (all cis -l,4-polyisoprene. Figure 6.9) can be prepared from 2-methyl-l 3-butadiene (isoprene [CH2=C(CH3)CH=CH2]) with a Ziegler-Natta catalyst vide supra). 

Nonetheless, it is interesting to note that some of the dimethylallyl diphosphate formed in this way is converted to 2-methyl-l,3-butadiene ( isoprene ) and is directly released by plants into the atmosphere. It appears that those plants that... 

When 2-methyl-l,3-butadiene (isoprene) undergoes a 1,4 addition of hydrogen chloride, the major product that is formed is l-chloro-3-methyl-2-butene. Little or no l-chloro-2-methyl-2-butene is formed. How can you explain this ... 

In 2-methyl-l,3-butadiene (isoprene) hydrogenation, 2-methyl-2-butene resulting from 1,4 addition of H atoms is the main product on solid base hydrogenation catalysts. Deuteration, however, results in the formation of ( ) and (2) forms of 2-methyl-2-butene-d2- The ratio of ( ) form to (2) form varies with catalyst types, and informs us of the dynamic nature of the intermediates in the reaction. The dynamic... 

Terpenes are synthesized in the plant by the linkage of at least two molecular units, each containing five carbon atoms. The structure of these units is like that of 2-methyl-l,3-butadiene (isoprene), and so they are referred to as isoprene units. Depending on how many isoprene units are incorporated into the structure, terpenes are classified as mono- (Cio), sesqui- (C15), or diterpenes (C20). (The isoprene building units are shown in color in the examples given here.)... 

Like simple alkenes (Section 12-15), conjugated dienes can be polymerized. The elasticity of the resulting materials has led to their use as synthetic rubbers. The biochemical pathway to natural rubber features an activated form of the five-carbon unit 2-methyl-l,3-butadiene (isoprene, see Section 4-7), which is an important building block in nature. 

Polymerization of 2-methyl-l,3-butadiene (isoprene. Section 4-7) by a Ziegler-Natta catalyst (Section 12-15) results in a synthetic rubber (polyisoprene) of almost 100% Z configuration. Similarly, 2-chloro-1,3-butadiene furnishes an elastic, heat- and oxygen-resistant polymer called neoprene, with trans chain double bonds. Several million tons of synthetic rubber are... 

In Summary 1,3-Butadiene polymerizes in a 1,2 or 1,4 manner to give polybutadienes with various amounts of cross-linking and therefore variable elasticity. Synthetic rubber can be made from 2-methyl-l,3-butadiene and contains varied numbers of E and Z double bonds. Natural rubber is constructed by isomerization of 3-methyl-3-butenyl pyrophosphate to the 2-butenyl system, ionization, and electrophilic (step-by-step) polymerization. Similar mechanisms account for the incorporation of 2-methyl-l,3-butadiene (isoprene) units into the polycyclic structure of terpenes. 

A typical UV spectrum is that of 2-methyl-l,3-butadiene (isoprene), shown in Figure 14-15. The position of a peak is defined by the wavelength at its maximum, a Ani x value (in nanometers). Its intensity is reflected in the molar extinction coefficient or molar absorptivity, e, that is characteristic of the molecule. The value of e is calculated by dividing the measured peak height (absorbance, A) by the molar concentration, C, of the sample (assuming a standard cell length of 1 cm). 

The structure of the terpene limonene is shown below (see also Exercise 5-29). Identify the two 2-methyl-l,3-butadiene (isoprene) nnits in limonene. (a) Treatment of isoprene with catalytic amonnts of acid leads to a variety of oligomeric products, one of which is limonene. Devise a detailed mechanism for the acid-catalyzed conversion of two molecules of isoprene into limonene. Take care to nse sensible intermediates in each step, (b) Two molecnles of isoprene may also be converted into fimo-nene by a completely different mechanism, which takes place in the strict absence of catalysts of any kind. Describe this mechanism. What is the name of the reaction ... 

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