Methyl isoprene

During World War I German chemists, whose coxmoy was cut off from its sources of natural rubber by the British blockade, polymerized 3-methyl-isoprene (2,3-dimethyl-1,3-butadiene) units, (CH2=C(CH3)C(CH3)=CH2), obtained from acetone, to form an inferior substitute called methyl rubber. By the end of the war Germany was producing 15 tons (13.6 metric tons) of this rubber per month. The USSR (Union of Soviet Socialist Republics), which built a pilot plant at Leningrad (now St. Petersburg) in 1930 and three factories in 1932 and 1933, was the first country to institute a full-scale synthetic rubber industry. 

The term vitamin K2 was applied to 2-methyl-3-difarnesyl-l,4-naphthoquinone, m.p. 54 C, isolated from putrefied fish meal. It now includes a group of related natural compounds ( menaquinones ), differing in the number of isoprene units in the side chain and in their degree of unsaturation. These quinones also appear to be involved in the electron transport chain and oxidative phosphorylation. 

Cholesterol was isolated m the eighteenth century but its structure is so complex that Its correct constitution was not determined until 1932 and its stereochemistry not verified until 1955 Steroids are characterized by the tetracyclic ring system shown m Figure 26 9a As shown m Figure 26 9b cholesterol contains this tetracyclic skeleton modified to include an alcohol function at C 3 a double bond at C 5 methyl groups at C 10 and C 13 and a C Hn side chain at C 17 Isoprene units may be discerned m var lous portions of the cholesterol molecule but the overall correspondence with the iso prene rule is far from perfect Indeed cholesterol has only 27 carbon atoms three too few for It to be classed as a tnterpene... 

Gutta percha is a natural polymer of isoprene (3-methyl-l,3-butadiene) in which the configuration around each double bond is trans. It is hard and horny and has the following formula ... 

Aliphatic C-5—C-6. Aliphatic feedstreams are typically composed of C-5 and C-6 paraffins, olefins, and diolefins, the main reactive components being piperylenes cis-[1574-41 -0] and /n j -l,3-pentadiene [2004-70-8f). Other main compounds iaclude substituted C-5 and C-6 olefins such as cyclopentene [142-29-OJ, 2-methyl-2-butene [513-35-9] and 2-methyl-2-pentene [625-27-4J. Isoprene and cyclopentadiene maybe present ia small to moderate quaatities (2—10%). Most steam cracking operatioas are desigaed to remove and purify isoprene from the C-5—C-6 fraction for applications ia mbbers and thermoplastic elastomers. Cyclopentadiene is typically dimerized to dicyclopentadiene (DCPD) and removed from C-5 olefin—diolefin feedstreams duriag fractionation (19). 

G-5—G-9 Aromatic Modified Aliphatic Petroleum Resins. Compatibihty with base polymers is an essential aspect of hydrocarbon resins in whatever appHcation they are used. As an example, piperylene—2-methyl-2-butene based resins are substantially inadequate in enhancing the tack of 1,3-butadiene—styrene based random and block copolymers in pressure sensitive adhesive appHcations. The copolymerization of a-methylstyrene with piperylenes effectively enhances the tack properties of styrene—butadiene copolymers and styrene—isoprene copolymers in adhesive appHcations (40,41). Introduction of aromaticity into hydrocarbon resins serves to increase the solubiHty parameter of resins, resulting in improved compatibiHty with base polymers. However, the nature of the aromatic monomer also serves as a handle for molecular weight and softening point control. 

Isoprene [78-79-5] (2-methyl-1,3-butadiene) is a colorless, volatile Hquid that is soluble in most hydrocarbons but is practically insoluble in water. Isoprene forms binary azeotropes with water, methanol, methylamine, acetonitrile, methyl formate, bromoethane, ethyl alcohol, methyl sulfide, acetone, propylene oxide, ethyl formate, isopropyl nitrate, methyla1 (dimethoxymethane), ethyl ether, and / -pentane. Ternary azeotropes form with water—acetone, water—acetonitrile, and methyl formate—ethyl bromide (8). Typical properties of isoprene are Hsted in Table 1. 

The rates of these two reactions have been studied for the attack of trifluoromethyl (51) and methyl radicals (52) in isoprene that has been dissolved in 2,3-dimethylbutane and isooctane, respectively. The rate constants for the reactions with isoprene are much greater than those for the attack on the solvent. The ratio between the two rates for the attack of trifluoromethyl radicals varies from 1090 at 65°C to 233 at 180°C. For the corresponding reaction involving methyl radicals, the ratio is 2090 at 65°C. 

Bromination of isoprene using Br2 at —5 ° C in chloroform yields only /n j -l,4-dibromo-2-methyl-2-butene (59). Dry hydrogen chloride reacts with one-third excess of isoprene at —15 ° C to form the 1,2-addition product, 2-chloro-2-methyl-3-butene (60). When an equimolar amount of HCl is used, the principal product is the 1,4-addition product, l-chloro-3-methyl-2-butene (61). The mechanism of addition is essentially all 1,2 with a subsequent isomerization step which is catalyzed by HCl and is responsible for the formation of the 1,4-product (60). The 3,4-product, 3-bromo-2-methyl-1-butene, is obtained by the reaction of isoprene with 50% HBr in the presence of cuprous bromide (59). Isoprene reacts with the reactive halogen of 3-chlorocyclopentene (62). 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

Isoprene reacts with a-chloroalkyl ethers in the presence of ZnCl in diethyl ether from 0—10°C. For example,a-chloromethyl methyl ether at 10°C gives a 6 1 ratio of the 1,4-adduct, (F)4-chloro-l-methoxy-2-methyl-2-butene, to the 1,2-adduct, 2-chloro-l-methoxy-2-methyl-3-butene. Other a-chloroalkyl ethers react in a similar manner to give predominately the 1,4-addition product. A wide variety of aHyUc chlorides and bromides and a-chloroethers and esters add primarily 1,4- to isoprene in the presence of acid catalysts (8). 

Propylene Dimer. The synthesis of isoprene from propjiene (109,110) is a three-step process. The propjiene is dimeri2ed to 2-methyl-1-pentene, which is then isomeri2ed to 2-methyl-2-pentene in the vapor phase over siUca alumina catalyst. The last step is the pyrolysis of 2-methyi-2-pentene in a cracking furnace in the presence of (NH 2 (111,112). Isoprene is recovered from the resulting mixture by conventional distillation. 

Dehydrogenation of Tertiary Amylenes, The staiting material here is a fiaction which is cut from catal57tic clacking of petroleum. Two of the tertiary amylene isomers, 2-methyl-l-butene and 2-methyl-2-butene, are recovered in high purity by formation of methyl tertiary butyl ether and cracking of this to produce primarily 2-methyl-2-butene. The amylenes are mixed with steam and dehydrogenated over a catalyst. The cmde isoprene can be purified by conventional or extractive distillation. 

Methyl Isopropyl Ketone. Methyl isopropyl ketone [563-80-4] (3-methyl-2-butanone) is a colorless Hquid with a characteristic odor of lower ketones. It can be produced by hydrating isoprene over an acidic catalyst at 200—300°C (150,151) or by acid-catalyzed condensation of methyl ethyl ketone and formaldehyde to 2-methyl-l-buten-3-one, foUowed by hydrogenation to the product (152). Other patented preparations are known (155,156). Methyl isopropyl ketone is used as an intermediate in the production of pharmaceuticals and fragrances (see Perfumes), and as a solvent (157). It is domestically available from Eastman (Longview, Texas) (47). 

Such copolymers of oxygen have been prepared from styrene, a-methylstyrene, indene, ketenes, butadiene, isoprene, l,l-diphen5iethylene, methyl methacrjiate, methyl acrylate, acrylonitrile, and vinyl chloride (44,66,109). 1,3-Dienes, such as butadiene, yield randomly distributed 1,2- and 1,4-copolymers. Oxygen pressure and olefin stmcture are important factors in these reactions for example, other products, eg, carbonyl compounds, epoxides, etc, can form at low oxygen pressures. Polymers possessing dialkyl peroxide moieties in the polymer backbone have also been prepared by base-catalyzed condensations of di(hydroxy-/ f2 -alkyl) peroxides with dibasic acid chlorides or bis(chloroformates) (110). 

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