Isoprene 2-methylbuta-l,3-diene

The UV spectrum of isoprene (2-methylbuta-l,3-diene) is shown in Figure 15-27. This spectrum could be summarized as follows ... 

In 1887, German chemist Otto Wallach determined the structures of several terpenes and discovered that all of them are formally composed of two or more five-carbon units of isoprene (2-methylbuta-l,3-diene). The isoprene unit maintains its isopentyl structure in a terpene, usually with modification of the isoprene double bonds. 

You will notice that they are all aliphatic compounds with a scattering of double bonds and rings, few functional groups, and an abundance of methyl groups. A better definition (that is, a bio synthetically based definition) arose when it was noticed that all these compounds have 5n carbon atoms. Pinene and camphor are Cto compounds, humulene is Cl5> and phytol is C20 It seemed obvious that terpenes were made from a C5 precursor and the favourite candidate was isoprene (2-methylbuta-l,3-diene) as all these structures can be drawn by joining together 2-, 3-, or 4-isoprene skeletons end to end. Humulene illustrates this idea. 

Unlike polyethylene and other simple alkene polymers, natural rubber is a polymer of a conjugated diene, isoprene (2-methylbuta-l,3-diene). The polymerization takes place by 1,4-addition of isoprene monomer units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds have Z stereochemistry ... 

Nickel(0)-catalyzed dimerization of isoprene (3 2-methylbuta-l.3-diene) has been used as the key step in the synthesis of (+)-grandisol (8), a male boll weevil pheromone. It is interesting to note that the cyclobutane 4 was formed by unsymmetrical and head-to-head cyclodimerization and that cycloadduets 5-7 were also formed in this transformation.9... 

Isoprene The basic building unit of the group of chemicals called the terpenes (2-methylbuta-l,3-diene), molecular formula C5Hg. 

Earlier in the chapter you saw the epoxidation of a diene to give a monoepoxide only one of the double bonds reacted. This is quite a usual observation dienes are more nucleophilic than isolated alkenes. This is easy to explain by looking at the relative energy of the HOMO of an alkene and a diene—this discussion is on p. 138 of Chapter 7. Dienes are therefore very susceptible to protonation by add to give a cation. This is what happens when 2-methylbuta-l,3-diene (isoprene) is treated with add. Protonation gives a stable delocalized allylic cation. 

Isoprene A colourless liquid diene, CH2 C(CH3)CH CH2. The systematic name is 2-methylbuta-l,3-diene. It is the structural unit in terpenes and natural rubber, and is used in maldng synthetic rubbers. 

Isoamyl acetate see 3-Methylbutyl ethanoate Isoamyl alcohol see 3-Methylbutan-l-ol heta.-Isoamylene see 2-Methylbut-2-ene Isohutane see 2-Methylpropane Isohutanoic acid see 2-Methylpropanoic acid Isohutanol see 2-Methylpropan-l-ol Isobutene see 2-Methylpropene Isobutyl acetate see 2-Methylpropyl ethanoate Isobutyl alcohol see 2-Methylpropan-l-ol Isobutylbenzene see (2-Methylpropyl)benzene Isobutyl chloride see l-Chloro-2-methylpropane Isobutylene see 2-Methylpropene Isobutylene oxide see 2,2-Dimethyloxirane Isobutyl ethanoate see 2-Methylpropyl ethanoate Isobutyl methyl ketone see 4-Methylpentan-2-one Isobutyraldehyde see 2-Methylpropanal Isobutyric acid see 2-Methylpropanoic acid Isocyanic acid butyl ester see Butyl isocyanate Isohexane see 2-Methylpentane Isooctane see 2,2,4-Tiimethylpentane Isopentane see 2-Methylbutane Isopentanol see 3-Methylbutan-l-ol Isopentyl alcohol see 3-Methylbutan-l-ol Isoprene see 2-Methylbuta-l,3-diene Isopropanol see Propan-2-ol Isopropenylbenzene... 

Isoprene is the common name for the chemical compound 2-methylbuta-l,3-diene. Conventionally, all natural compounds built up from isoprene subunits, (Cj) , are denoted as terpenes, therefore terpenes are also denoted as isoprenoids. Terpenes cover most of the range of natural products about 55000 terpenes are known at present in the literature. In Nature, terpenes occur predominantly as hydrocarbons, alcohols, and their glycosides, ethers, aldehydes, ketones, carboxylic acids, and esters [103]. 

Isobutyric acid see 2-Methylpropanoic acid Isobutyron see 2,4-Dimethylpentan-3-one Isohexane see 2-Methylpentane Isohexyl alcohol see 2-Ethylbutan-l-ol Isooctane see 2,2,4-Trimethylpentane Isopentane see 2-Methylbutane Isopentanol see 3-Methylbutan-l-ol Isopentyl alcohol see 3-Methylbutan-l-ol Isoprene see 2-Methylbuta-1,3-diene Isopropanol see Propan-2-ol Isopropenylbenzene... 

Natural rubber (abbreviation NR) is a polyisoprene that can be considered as originating from the polymerization of 2-methylbuta-1,3-diene (called isoprene) and contains more than 99% of l,4-c monomeric units as well as traces of 1,2 and 3,4 units ... 

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