Bromopropane reaction order

Elimination to give propene competes with substitution to give ethyl isopropyl ether. Furthermore, the rate of elimination, like the rate of substitution, is proportional to the concentrations of 2-bromopropane and ethoxide ion. Thus elimination here is a second-order reaction (it may be helpful to review Section 8-4 at this point) ... 

The E2 mechanism is a concerted mechanism and involves both the alkyl halide and the nucleophile. Due to this, the reaction rate depends on the concentration of both reagents and is called second order (E2 = Elimination second order). To illustrate the mechanism, we shall look at the reaction of 2-bromopropane with a hydroxide ion given below ... 

Benzimidazoles, too, are readily alkylated either in neutral or basic medium. As with the uncondensed compounds, the former conditions are complicated by salt formation, but to a lesser extent since benzimidazoles are less basic. It is often valuable to vary the initial amounts of alkali and alkyl halide in order to improve yields. The best yields of 1-alkylbenzimidazole (76-83% for primary and 50-60% for secondary alkyl and aralkyl bromides) result with two moles of bromide and 1.5 moles of alkali per mole of benzimidazole (66RCRI22). There is severe steric hindrance to the alkylation of 2-arylben-zimidazoles in alkaline medium, but reactions with the silver salts seem more successful. The rapid and almost quantitative reaction between a benzimidazole and dimethylphenyl-alkylammonium chlorides in aqueous sodium hydroxide provides a very convenient method of introducing a primary aralkyl group (benzyl, a-naphthylmethyl). There has been little systematic study of the dkylation of unsymmetrical benzimidazoles, though much the same criteria should apply as in the uncondensed compounds. In this respect the observation that 1-bromopropane reacts with the anion of 2,6-dimethyl-4-nitrobenzimidazole to give... 

Under conditions of maximal inhibition the decomposition behaviour often closely resembles that which is characteristic of a unimolecular reaction. For example Agius and MaccolP have shown that thermal decomposition of bromopropane alone has an order of 1.5, but when maximally inhibited by propene the decomposition becomes first-order and the Arrhenius parameters are A = 7.94 x 10 sec E - 50.7 kcal.mole It has been argued that, in such cases, decomposition in the absence of inhibitor proceeds by simultaneous radical-chain and unimolecular processes and that under conditions of maximal inhibition, the radical-chain process is suppressed and the residual unimolecular process is predominant. Wojciechowski and Laidler have argued against this proposition, maintaining that the maximally inhibited reaction may still represent a residual chain process, and that the lack of inhibitor action for compounds in Class 1 cannot be taken as an indication that the mechanism is molecular. (See also the comments of Benson and Bose .)... 

Later, Morris Kharasch found that, if the reaction occurs in the presence of air, oxygen or a small amount of a peroxide, then the reaction goes in the reverse order H goes to the C with the smaller number of H atoms and the Br to the other, so forming 1-bromopropane. A summary of the mechanism of what is going on here is given in Section 6.2.4 as an extension topic at the end of this unit. 

Despite the diversity of identified reaction paths, it is not yet possible to predict just when any of them will operate. Thus, the addition of bromopropane to rans-[RhBr(CO)(P p-EtC6H4 3)2] is first order in both reactants but is not catalyzed by free bromide (196). 

In many reactions, two or more processes may contribute to the disappearance of a reactant. These processes may have different kinetic dependence on the reagent concentrations. For example, when 2-bromo-propane is hydrolysed to propan-2-ol by sodium hydroxide in aqueous ethanol, both SN1 and SN2 processes take place at the same time, and each contributes to the loss of the 2-bromopropane, as shown in reactions (2.29) and (2.30). Both processes involve [Me2CH-Br], but the SN2 process also depends on [OH] whereas the SN1 process does not. The overall rate of loss of 2-bromopropane is given by expression (2.31), showing a mixed kinetic dependence of zero and first order with respect to the hydroxide ion. 

In order to ascertain the sensitivity of the process to changes in the experimental parameters, the reaction of 1-bromopropane with butanal in the presence of lithium was studied in some detail. 

The relative rates of reaction of 2-bromopropane and 2-bromo-2-methylpropane with water to give the corresponding alcohols are shown in Table 7-1 and are compared with the corresponding rates of hydrolysis of their unbranched counterparts. Although the process gives the products expected from an Sn2 reaction, the order of reactivity is reversed from that found under typical Sn2 conditions. Thus, primary halides are very slow in their reactions with water, secondary halides are more reactive, and tertiary halides are about 1 million times as fast as primary ones. 

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