2- Methyl-2-butene with hydrogen bromide

In the petroleum (qv) industry hydrogen bromide can serve as an alkylation catalyst. It is claimed as a catalyst in the controlled oxidation of aHphatic and ahcycHc hydrocarbons to ketones, acids, and peroxides (7,8). AppHcations of HBr with NH Br (9) or with H2S and HCl (10) as promoters for the dehydrogenation of butene to butadiene have been described, and either HBr or HCl can be used in the vapor-phase ortho methylation of phenol with methanol over alumina (11). Various patents dealing with catalytic activity of HCl also cover the use of HBr. An important reaction of HBr in organic syntheses is the replacement of aHphatic chlorine by bromine in the presence of an aluminum catalyst (12). Small quantities of hydrobromic acid are employed in analytical chemistry. 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

LAPIS INFERNALIS (7761-88-8) A powerful oxidizer. Forms friction- and shock-sensitive compounds with many materials, including acetylene, anhydrous ammonia (produces compounds that are explosive when dry), 1,3-butadiyne, buten-3-yne, calcium carbide, dicopper acetylide. Contact with hydrogen peroxide causes violent decomposition to oxygen gas. Violent reaction with chlorine trifluoride, metal powders, nitrous acid, phosphonium iodide, red or yellow phosphorus, sulfur. Incompatible with acetylides, acrylonitrile, alcohols, alkalis, ammonium hydroxide, arsenic, arsenites, bromides, carbonates, carbon materials, chlorides, chlorosulfonic acid, cocaine chloride, hypophosphites, iodides, iodoform, magnesium, methyl acetylene, phosphates, phosphine, salts of antimony or iron, sodium salicylate, tannic acid, tartrates, thiocyanates. Attacks chemically active metals and some plastics, rubber, and coatings. 

Cyclopentene is a symmetrical alkene, which takes on meaning when it is compared to the reaction of an unsymmetrical alkene such as 2-methyl-2-butene. An unsymmetrical alkene will have different atoms or groups attached to the carbons of the C=C unit. In 2-methyl-2-butene, one carbon of the C=C unit has a methyl and a hydrogen, and the other carbon has two methyl groups. If 2-methyl-2-butene reacts with HBr in the same way as cyclopentene, the intermediate is a carbocation, and subsequent reaction with the nucleophilic bromide ion will form an alkyl bromide. However, the reaction may generate two different products—4 and 5— via two different carbocation intermediates. When the percentage yield of products from this reaction is determined experimentally, it is clear that 5 is the mqjor product. Why An analysis of the mechanism for formation of 4 and also for formation of 5 will provide a prediction of the major product based on rmderstanding each carbocation intermediate. 

In Chapter 7 we discussed how haloalkanes (or alkyl sulfonates) in the presence of strong base can nndergo elimination of the elements of HX with simultaneons formation of a carbon-carbon donble bond. With many substrates, removal of a hydrogen can take place from more than one carbon atom in a molecule, giving rise to constitutional (donble-bond) isomers. In snch cases, can we control which hydrogen is removed—that is, the regio-selectivity of the reaction (Section 9-9) The answer is yes, to a limited extent. A simple example is the elimination of hydrogen bromide from 2-bromo-2-methylbutane. Reaction with sodinm ethoxide in hot ethanol fnmishes mainly 2-methyl-2-butene, but also some 2-methyl-1 -butene. 

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). 

Copper-catalyzed monoaddition of hydrogen cyanide to conjugated alkenes proceeded very conveniently with 1,3-butadiene, but not with its methyl-substituted derivatives. The most efficient catalytic system consisted of cupric bromide associated to trichloroacetic acid, in acetonitrile at 79 °C. Under these conditions, 1,3-butadiene was converted mainly to (Z )-l-cyano-2-butene, in 68% yield. A few percents of (Z)-l-cyano-2-butene and 3-cyano-1-butene (3% and 4%, respectively) were also observed. Polymerization of the olefinic products was almost absent. The very high regioselectivity in favor of 1,4-addition of hydrogen cyanide contrasted markedly with the very low regioselectivity of acetic acid addition (vide supra). Methyl substituents on 1,3-butadiene decreased significantly the efficiency of the reaction. With isoprene and piperylene, the mononitrile yields were reduced... 

Butadiene, Using butadiene as the model substrate, it has been found that the catalytic hydrogenation of dienes occurs in three steps (1) reversible addition of hydrido complex to butadiene to form a butenylcobalt complex (Reaction 3) (2) cleavage of the butenylcobalt complex by hydrido complex to form butenes and pentacyanocobaltate(II) (Reaction 4) and (3) hydrogen absorption by pentacyanocobaltate(II) to reform hydrido complex (Reaction 1) 20, 21), The butenylcobalt complex has the same properties as the complex obtained via reaction of y-methyl-allyl bromide with pentacyanocobaltate(II) (21, 22). 

The reaction is faster with electron-rich alkenes. The rate constants for addition of bromine to a series of alkenes were found to increase in the order ethene < propene < 2-butene isobutene < 2-methyl-2-butene. In other words, each methyl group that replaces a hydrogen atom on ethene increases reactivity. The addition of bromine to substituted ethenes in methanol with added sodium bromide can be correlated to the equation... 

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