Methane chlorine reaction with

This cycle is terrninated by the reaction of chlorine atoms with methane Cl + CH — HCl + CH3. The importance of this cycle depends on the avadabiLity of oxygen atoms and varies with altitude as well as the time of year it accounts for only 5% of the halogen-controUed loss at 15 km, but increases to 25% at 21 km. 

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. 

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 ... 

TABLE 4. Room temperature rate constants and preexponential A factors (in cm3 molecule 1 s 1), as well as activation energies and reaction enthalpies (in kJ mol 1) for reactions of chlorine atoms with methane and halomethanes. 

The most effective wavelength of light is a blue color that is strongly absorbed by chlorine gas. This finding implies that light is absorbed by the chlorine molecule, activating chlorine so that it initiates the reaction with methane. 

Transition states have high energies because bonds must begin to break before other bonds can form. The following equation shows the reaction of a chlorine radical with methane. The transition state shows the C — H bond partially broken and the H—Cl bond partially formed. Transition states are often enclosed by brackets to emphasize their transient nature. 

We now apply what we know about rates to the reaction of methane with halogens. The rate-limiting step for chlorination is the endothermic reaction of the chlorine atom with methane to form a methyl radical and a molecule of HC1. 

A chlorine atom may go through this cycle many thousands of times before it is removed in the form of HC1 following reaction with methane ... 

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... 

There are one or more chain-propagating steps, each of which consumes a reactive particle and generates another here they are the reaction of chlorine atoms with methane (step 2), and of methyl radicals with chlorine (step 3). 

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. 

Kandel S A and Zare R N 1998 Reaction dynamics of atomic chlorine with methane importance of methane bending and tortional excitation in controlling reactivity J. Chem. Phys. 109 9719-27... 

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... 

Chlorine reacts with saturated hydrocarbons either by substitution or by addition to form chlorinated hydrocarbons and HCl. Thus methanol or methane is chlorinated to produce CH Cl, which can be further chlorinated to form methylene chloride, chloroform, and carbon tetrachloride. Reaction of CI2 with unsaturated hydrocarbons results in the destmction of the double or triple bond. This is a very important reaction during the production of ethylene dichloride, which is an intermediate in the manufacture of vinyl chloride ... 

Chemical initiation generates organic radicals, usually by decomposition of a2o (11) or peroxide compounds (12), to form radicals which then react with chlorine to initiate the radical-chain chlorination reaction (see Initiators). Chlorination of methane yields all four possible chlorinated derivatives methyl chloride, methylene chloride, chloroform, and carbon tetrachloride (13). The reaction proceeds by a radical-chain mechanism, as shown in equations 1 through. Chain initiation... 

Thermal chlorination of methane was first put on an industrial scale by Hoechst in Germany in 1923. At that time, high pressure methanol synthesis from hydrogen and carbon monoxide provided a new source of methanol for production of methyl chloride by reaction with hydrogen chloride. Prior to 1914 attempts were made to estabHsh an industrial process for methanol by hydrolysis of methyl chloride obtained by chlorinating methane. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Chloroform can be manufactured from a number of starting materials. Methane, methyl chloride, or methylene chloride can be further chlorinated to chloroform, or carbon tetrachloride can be reduced, ie, hydrodechlorinated, to chloroform. Methane can be oxychlorinated with HCl and oxygen to form a mixture of chlorinated methanes. Many compounds containing either the acetyl (CH CO) or CH2CH(OH) group yield chloroform on reaction with chlorine and alkali or hypochlorite. Methyl chloride chlorination is now the most common commercial method of producing chloroform. Many years ago chloroform was almost exclusively produced from acetone or ethyl alcohol by reaction with chlorine and alkali. 

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