Methyl bromide, reaction

Ca—H(D) stretching vibrations is slightly larger than the decrease in the contribution due to the C —H(D) bending vibrations. A typical relationship is shown for the chloride ion-methyl bromide reaction in Fig. 2. 

Consider the hydrolyses of methyl bromide (Reaction 1 in Table 13.2) and ethyl acetate (reaction 5 in Table 13.2) at 25°C and pH 7.0 in a contaminated groundwater containing 100 mM Cl and 1 mM Br. Assume that neither the pH nor the inorganic species concentrations change significantly during the reaction. Also assume that the activity coefficients of all species are 1. To what extent do the two compounds hydrolyze under these conditions ... 

The methyl bromide reaction is said to follow second-order kinetics, since its rate is dependent upon the concentrations of two substances. The rer/-butyl bromide reaction is said to follow first-order kinetics its rate depends upon the concentration of only one substance. 

To understand how structure does influence the rate, let us compare transition state and reactants with regard to shape starting with the methyl bromide reaction. The carbon in reactant and product is tetrahedral, whereas carbon in the transition state is bonded to five atoms. As indicated before, the C—H bonds are arranged like the spokes of a wheel, with the C--OH and C—Br bonds lying along the axle (Fig. 14.2). 

Representative s values are collected in Table 24 and n values in Table 25. The order of nucleophhcity (n) in the Swain-Scott approach bears no relationship to the pK of the conjugate acid of the nucleophile. Since in a given constant series of nucleophiles basicity and nucleophiUcity are related there have been attempts to correlate nucleophiUcity with one or more parameters. If the relative importance of these parameters is the same as in the methyl bromide reaction then the Swain-Scott equation will hold. 

Bromination of methane is exothermic but less exothermic than chlorination The value calculated from bond dissociation energies is AH° = -30 kJ Al though bromination of methane is energetically fa vorable economic considerations cause most of the methyl bromide prepared commercially to be made from methanol by reaction with hydrogen bromide... 

Recall that the term kinetics refers to how the rate of a reaction varies with changes m concentration Consider the nucleophilic substitution m which sodium hydroxide reacts with methyl bromide to form methyl alcohol and sodium bromide... 

The rate of this reaction is observed to be directly proportional to the concentration of both methyl bromide and sodium hydroxide It is first order m each reactant or second order overall... 

Methyl bromide slowly hydrolyzes in water, forming methanol and hydrobromic acid. The bromine atom of methyl bromide is an excellent leaving group in nucleophilic substitution reactions and is displaced by a variety of nucleophiles. Thus methyl bromide is useful in a variety of methylation reactions, such as the syntheses of ethers, sulfides, esters, and amines. Tertiary amines are methylated by methyl bromide to form quaternary ammonium bromides, some of which are active as microbicides. 

Commercial manufacture of methyl bromide is generally based on the reaction of hydrogen bromide with methanol. For laboratory preparation, the addition of sulfuric acid to sodium bromide and methanol has been used (80). Another method involves the treatment of bromine with a reducing agent, such as phosphoms or sulfur dioxide, to generate hydrogen bromide (81). 

In the late 1980s, however, the discovery of a noble metal catalyst that could tolerate and destroy halogenated hydrocarbons such as methyl bromide in a fixed-bed system was reported (52,53). The products of the reaction were water, carbon dioxide, hydrogen bromide, and bromine. Generally, a scmbber would be needed to prevent downstream equipment corrosion. However, if the focus of the control is the VOCs and the CO rather than the methyl bromide, a modified catalyst formulation can be used that is able to tolerate the methyl bromide, but not destroy it. In this case the methyl bromide passes through the bed unaffected, and designing the system to avoid downstream effects is not necessary. Destmction efficiencies of hydrocarbons and CO of better than 95% have been reported, and methyl bromide destmctions between 0 and 85% (52). 

Although ethereal solutions of methyl lithium may be prepared by the reaction of lithium wire with either methyl iodide or methyl bromide in ether solution, the molar equivalent of lithium iodide or lithium bromide formed in these reactions remains in solution and forms, in part, a complex with the methyllithium. Certain of the ethereal solutions of methyl 1ithium currently marketed by several suppliers including Alfa Products, Morton/Thiokol, Inc., Aldrich Chemical Company, and Lithium Corporation of America, Inc., have been prepared from methyl bromide and contain a full molar equivalent of lithium bromide. In several applications such as the use of methyllithium to prepare lithium dimethyl cuprate or the use of methyllithium in 1,2-dimethyoxyethane to prepare lithium enolates from enol acetates or triraethyl silyl enol ethers, the presence of this lithium salt interferes with the titration and use of methyllithium. There is also evidence which indicates that the stereochemistry observed during addition of methyllithium to carbonyl compounds may be influenced significantly by the presence of a lithium salt in the reaction solution. For these reasons it is often desirable to have ethereal solutions... 

There are relatively few kinetic data on the Friedel-Crafts reaction. Alkylation of benzene or toluene with methyl bromide or ethyl bromide with gallium bromide as catalyst is first-order in each reactant and in catalyst. With aluminum bromide as catalyst, the rate of reaction changes with time, apparently because of heterogeneity of the reaction mixture. The initial rate data fit the kinetic expression ... 

With magnesium methyl bromide tuberostemonine gives a product, which on treatment with ammonium chloride solution yields a substance (a), C24H4JO4N, m.p. 110-2 , but with dilute sulphuric acid, furnishes the dehydrated compound, (b) C24H3903N, m.p. 164 . The results of this reaction are represented as follows —... 

Another approach is therefore to adopt a model process that is very similar to the reactions of interest. Swain and Scott ° selected as a standard reaction the nucleophilic substitution reaction of methyl bromide in water at 25°C. 

The real world of Sn reactions is not quite as simple as the discussion has so far suggested. The preceding treatment in terms of two clearly distinct mechanisms, SnI and Sn2, implies that all substitution reactions will follow one or the other of these mechanisms. This is an oversimplification. The strength of the dual mechanism hypothesis and its limitations are revealed by these relative rates of solvolysis of alkyl bromides in 80% ethanol methyl bromide, 2.51 ethyl bromide, 1.00 isopropyl bromide, 1.70 /er/-butyl bromide, 8600. Addition of lyate ions increases the rate for the methyl, ethyl, and isopropyl bromides, whereas the tert-butyl bromide solvolysis rate is unchanged. The reaction with lyate ions is overall second-order for methyl and ethyl, first-order for tert-butyl, and first- or second-order for the isopropyl member, depending upon the concentrations. Similar results are found in other solvents. These data show that the methyl and ethyl bromides solvolyze by the Sn2 mechanism, and tert-butyl bromide by the SnI mech-... 

Other possible ambident nucleophiles include cyanii anion (CN ), methyl sulfinate anion (CH3SO2 ), ar acetone enolate (CH3COCH2 ). Identify the most electro rich atom(s) in each anion (based on charges alone), ar indicate the major product that should result from an S, reaction with methyl bromide at this atom(s). 

Another way to assess nucleophilic reactivity is to examii the shape of the nucleophile s electron-donor orbital (th is the highest-occupied molecular orbital or HOMC Examine the shape of each anion s HOMO. At which ato would an electrophile, like methyl bromide, find the be orbital overlap (Note This would involve overlap of tl the HOMO of the nucleophile and the lowest-unoccupif molecular orbital or LUMO of CH3Br.) Draw all of tl products that might result from an Sn2 reaction wi CHaBr at these atoms. 

Sn2 reactions proceed through transition states in whicl the central carbon has five neighbors instead of the usua four, e.g., for reaction of bromide and methyl bromide. 

Calculate activation barriers for bromide addition t( methyl bromide, ethyl bromide, 2-propyl bromide an( 2-methyl-2-propyl bromide using energies for Sn transition states bromide+methyI bromide, bromide- ethyl bromide, bromide+2-propyl bromide and bromides 2-methyl-2-propyl bromide) and Br (at left). Whicl reaction is fastest Slowest ... 

Swain and Eddy have queried the wide applicability of the S l and Sif2 mechanisms and favored a push-pull termolecular process for the reaction of pyridine with methyl bromide in benzene solution for example, they have suggested that the effects observed on the addition of methanol, phenol, p-nitrophenol, and mercuric bromide to the reaction mixture can be explained by an intermediate of type 168. ... 

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