Bromine reaction with methane

Figure 10.3 Potential energy diagrams (a) for the reaction of a chlorine atom with methane and (b) for the reaction of a bromine atom with methane.

The destruction of 03 by chlorine and bromine can be short-circuited by removing either Cl and Br or, alternatively, CIO and BrO. For chlorine atoms, this occurs by reaction with methane that has been transported from the troposphere ... 

Bromine is potentially able to interact with stratospheric ozone in the same manner as chlorine (Wofsy et al., 1975). The catalytic cycle for bromine is expected to be quite efficient, because its reaction with methane is slower than that of Cl atoms in addition, the reaction of OH with HBr is faster than that of OH with HC1. The major bromine compound in the troposphere is methyl bromide, which has a natural origin and occurs with a mixing ratio of about 10 pptv (see Table 6-14). This seems small enough to neglect bromine to a first approximation. 

Although chlorine and bromine react with methane and other saturated hydrocarbons, iodine enters into combination only under exceptional circumstances. Chlorine and bromine react with hydrogen with the evolution of heat, whereas under the same conditions (400 C) hydrogen iodide is formed with the absorption of heat. Since the heat of formation is a measure of the strength of the bond, then, compared with the halogens of lower molecular weight, iodine exhibits a stronger tendency to combine only loosely and to enter into reversible reactions. 

In Summary Fluorine, chlorine, and bromine react with methane to give halomethanes. All three reactions follow the radical chain mechanism described for chlorination. In these processes, the first propagation step is always the slower of the two. It becomes more exo-... 

Methane reacts with bromine to give bromomethane and hydrogen bromide. The mechanism forthe reaction is called free-radical substitution and involves homolytic fission of chemical bonds. The reaction proceeds via initiation, propagation and termination steps, a By what mechanism does bromine react with methane b Write a balanced symbol equation forthis reaction, c Bonds break in this reaction. What type of bond breaking is involved d What essential conditions are required forthis reaction Why e Forthis reaction, write down an equation for an initiation step a propagation step... 

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... 

Classify each of the following reactions as addition or substitution and write its chemical equation (a) chlorine reacts with methane when exposed to light (b) bromine reacts with ethene in the absence of light. 

The bromoallene (-)-kumausallene (62) was isolated in 1983 from the red alga Laurencia nipponica Yamada [64a], The synthesis of the racemic natural product by Overman and co-workers once again employed the SN2 -substitution of a propargyl mesylate with lithium dibromocuprate (Scheme 18.22) [79]. Thus, starting from the unsymmetrically substituted 2,6-dioxabicyclo[3.3.0]octane derivative 69, the first side chain was introduced by Swern oxidation and subsequent Sakurai reaction with the allylsilane 70. The resulting alcohol 71 was protected and the second side chain was attached via diastereoselective addition of a titanium acetylide. The synthesis was concluded by the introduction of two bromine atoms anti-selective S -substitution of the bulky propargyl mesylate 72 was followed by Appel bromination (tetrabromo-methane-triphenylphosphine) of the alcohol derived from deprotection of the bromoallene 73. 

How is the course of halogen substitution in the benzene nucleus to be explained It is not at all probable that direct replacement of hydrogen occurs, such as we must assume in the formation of benzyl chloride and in the reaction between methane and chlorine, since the hydrogen attached to the doubly bound carbon atom of olefines exhibits no special reactivity. However, various facts which will be considered later (p. 164) indicate that benzene reacts with halogen in fundamentally the same way as does ethylene. The behaviour of ethylene towards bromine is the subject of the next preparation. 

Biedermann and Jacobson, who first prepared thieno[2,3-6]-thiophene (1) in 1886, characterized it as a 2,3,4,5-tetrabromo derivative with m.p. 172°. Later Capelle reported the isolation of a dibromo derivative of thienothiophene 1 with m.p. 122.5°, which was shown by Challenger and Harrison to be 2,3,5-tribromothieno[2,3-6]thiophene (m.p. 123°-124°). Capelle also obtained a tetrabromide, m.p. 223°, by bromination of the product of reaction of acetylene with sulfur. The tetrabromide seems to be identical with that prepared from the product of reaction of methane, acetylene, and hydrogen sulfide, m.p. 229°-230°, and is evidently 2,3,5,6-tetrabromothieno[3,2-6j-thiophene. ... 

Chlorine or bromine reacts with alkanes in the presence of light (hv) or high temperatures to give alkyl halides. Usually, this method gives mixtures of halogenated compounds containing mono-, di-, tri- and tetra-halides. However, this reaction is an important reaction of alkanes as it is the only way to convert inert alkanes to reactive alkyl halides. The simplest example is the reaction of methane with CI2 to yield a mixture of chlorinated methane derivatives. 

Bromofluoromenthane has been prepared in high yield using a one-step reaction with bromine, hydrogen fluoride and methane in the presence of an aluminum trifluoride fluidized bed or fluorinated aluminum catalysts optionally containing cobalt or nickel between 450 and 500°C.20... 

Alternatively, quaternisation of bisphosphinomethanes with bromine, followed by treatment with a primary amine and subsequent reaction with a base affords the corresponding methanes as a mixture of bis-imino and imino-amine tautomers the latter is favoured by /V-alkyl substituents, Scheme l.14 Irrespective of which valence tautomer is isolated, or whether a mixture is obtained, both afford the same methanide and methandiide following deprotonation so the Kirsanov method provides a valuable alternative to the preferred Phospha-Staudinger method when the required azide is unavailable or too unstable to be used. 

Practice Problem 20.1 Write equations for the reaction of methane with (a) excess bromine and (b) excess oxygen. ... 

Preparation of Alkyl Halides.—We have spoken of the formation of the alkyl halides by the direct action of the halogen upon the saturated hydrocarbon. In the case of chlorine this action takes place at ordinary temperatures as in the reaction between methane and chlorine in the sunlight. Bromine, however, does not act directly at ordinary temperatures but by heating in a sealed tube. Iodine does not act directly with the hydrocarbons. In any case the result is a mixture of several substitution products, and the method is not, therefore, of practical value. Where direct action does not occur the presence of iodine chloride or antimony chloride, which act as carriers, is necessary. The two reactions of most importance in the preparation of these compounds are those involving either alcohols or unsaturated hydrocarbons. These will be taken up when these compounds are studied. 

It should be understood that when we compare reactivities we compare rates of reaction. When we say that chlorine is more reactive than bromine toward methane, we mean that under the same conditions (same concentration, same temperature, etc.) chlorine reacts with methane faster than does bromine. From another point of view, we mean that the bromine reaction must be carried out under... 

The nature of the probability factor is very poorly understood. Since our. two reactions are quite similar, however, we might expect them to have similar probability factors. Experiment has shown this to be true whether chlorine or bromine atoms are involved, about one in every eight collisions with methane has the proper orientation for reaction. In general, where closely related reactions are concerned, we may assume that a difference in probability factor is not likely to be the cause of a large difference in reactivity. 

We are left with a consideration of the energy factor. At a given temperature, the fraction of collisions that possess the amount of energy required for reaction depends upon how large that amount is, that is, depends upon the In our example act is 4 kcal for the chlorine reaction, 18 kcal for the bromine reaction. As we have seen, a difference of this size in the - act causes an enormous difference in the energy factor, and hence in the rate. At 275 , of every 10 million collisions, 250,000 are sufficiently energetic when chlorine atoms are involved, and only one when bromine atoms are involved. Because of the difference in act alone, then, chlorine atoms are 250,000 times as reactive as bromine atoms toward methane. 

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