Bromination of methane

A new approach we found is based on the initial bromination of methane to methyl bromide, which can be effected with good selectivity, although still in relatively low yields. Methyl bromide is easily separated from exeess methane, whieh is readily recyeled. Hydrolysis of methyl bromide to methyl alcohol and its dehydration to dimethyl ether are readily achieved. Importantly, HBr formed as by produet ean be oxidatively reeycled into bromine, making the overall proeess cat-alytie in bromine. 

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... 

Chlorination or bromination of methane, ethylene, etc Maleic anhydride (from butane)... 

Molecular bromine is highly toxic, as is methyl bromide (CH3 Br), a dense gas used as an insecticide. Methyl bromide is produced by bromination of methane CH4 + Br2 CH3 Br + I Br Molecular bromine is also used in the synthesis of dyes and pharmaceuticals. 

Problem 4.22 Bromination of methane, like chlorination, is exothermic, but it proceeds at a slower rate under the same conditions. Explain in terms of the factors that affect the rate, assuming that the rate-controlling step is... 

Electrophilic chlorination and bromination of methane over supported noble metals9-11 (Pt on A1203, Pd on BaSC ) and solid superacid catalysts9-13 (e.g., TaOF3 on alumina, Nafion-H, zeolites, SbF5-graphite, sulfated zirconia) have been studied (see Section 3.4.2). Monosubstitution with selectivities better than... 

The bromination of methane proceeds through the following steps ... 

The literature references to this work are a little complicated, and are as follows preliminary note giving brief description of complete research, reference 8 study of the reaction CH3 + HBr -> CH4 + Br, reference 7 study of the bromination of methane, reference 266, and of ethane, reference 6. 

Methylene Bromide (CoU. Vol. i, 349) Prepared by the bromination of methane, along with methyl bromide. Peeelis, Ind. Eng. Chem. 25, 1160 (1933). 

Bromination of methane is slowed down by addition of fairly large amounts of HBr. (a) Suggest a possible explanation for this. (Hint See Sec. 2.17.) (b) Account for the fact that HCl does not have a similar effect upon chlorination, (c) Any reaction tends to slow down as reactants arc used up and their concentrations decrease. How do you account for the fact that bromination of methane slows down to an unusually great extent, more than, say, chlorination of methane ... 

Problem 3.20 Bromination of methane is slowed down by the addition of HBr (Problem 14, p. 71) this is attributed to the reaction... 

If the halogenation reaction is allowed to continue, the alkyl halide formed may react with other halogen atoms. When this happens, a mixture of products may be formed. For instance, bromination of methane will produce bromomethane (CHgBr), dibromomethane (CH,Br2), tribromomethane (CHBrg), and tetrabromomethane (CBr4). 

Explain why the rate of bromination of methane is decreased if HBr is added to the reaction mixture. 

TABLE 8. Chlorination and bromination of methane over suported acid catalysts ... 

PROBLEM 2.19 Write the names and structures of all possible products for the bromination of methane. 

Activation energies of endothermic reactions that involve both bond formation and bond rupture will be greater than the heat of reaction, AH°. Two examples illustrate this principle, namely, the first chain-propagating step in the chlorination of methane and the corresponding step in the bromination of methane ... 

Lin, R., Ding, Y., Gong, L., et al. (2009). Oxidative Bromination of Methane on SUiea-Supported Non-Noble Metal Oxide Catalysts, Appl. Catal. A Gen., 353, pp. 87-92. 

When we examine the enthalpy changes for the propagation steps in the bromination of methane, we find that the first step is now even more endothermic than the comparable step for chlorination. The overall process is exothermic only because the second step is exothermic. 

The first propagation step for the bromination of methane is strongly endothermic, so the transition state is more product-like. The C—H bond is broken to a significant extent, and the H—Br bond is more strongly formed (Figure 4.20). 

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