Methane reaction with halogens

Alkenes can undergo free radical substitution reactions with halogens. Which of the following best represents a chain propagation step during the free radical chlorination of methane ... 

The reaction of methane tricarboxylate with indoline 342 gave the tricyclic derivative 361 which can be transformed to the amide derivatives 362 (97JHC969). Alkylation of the iV-benzyl indoline 360 with pentafluor-oacetone gave 363 which upon debenzylation and subsequent acylation with diketene followed by cyclization gave 364. Other haloacetones were used to prepare different halogenated derivatives (79BEP872311) (Scheme 63). 

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. 

Difluorobis(fluoroxy)methane reacts with tetrafluoroethene at low temperature to give up to 90% of the two-to-one adduct 6,6 while the formation of halogenated 2,2-difluoro-l,3-dioxo-lanes 7 takes place in reactions with various halo-substituted ethencs.7... 

The atmospheric fate of a halocarbon molecule depends upon whether or not it contains a hydrogen atom. Hydrohalomethanes are oxidized by a series of reactions with radicals prominant in the troposphere, predominantly hydroxyl OH. Fully halogenated methanes are unreactive towards these radicals and consequently are transported up through the troposphere into the stratosphere, where their oxidation is initiated by UV photolysis of a carbon-halogen bond. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

Kuznetsov, A.M., German, E.D., Masliy, A.N. and Korshin, G.V. (2004) A density functional study of dissociative electron transfer reactions with partidpation of halogenated methanes. J. Elec-troanal. Chem. 573, 315-325. 

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. 

The first type of reaction is called the substitution reaction. In this reaction a hydrogen atom from an alkane is replaced by a halogen. For example, when methane reacts with diatomic chlorine gas, the major organic product is chloromethane CH4 + Cl2 — CH3C1 + HCl. 

As saturated hydrocarbons, the alkanes have the lowest chemical reactivity of organic compounds. However, under certain conditions these compounds can undergo substitution reactions, especially with the halogens. An example of such a reaction is that between an alkane, such as methane, and a halogen, such as chlorine. In this substitution reaction, a chlorine atom replaces a hydrogen atom on the methane molecule. 

Reactions of halogen-stabilized carbenoids with imines have been carried out using prefoimed lithium species (e.g. equation 36), or via a carbenoid generated from diiodomethane utilizing zinc-copper couple (Simmons-Smith conditions). A stereospecific ring closure is observed after the addition of lithiodichloromethane to a benzaldimine (equation 37).The addition of lithiochloro(phenylsulfon-yl)methane to aromatic imines affords 2-phenylsulfonyl-substituted aziridines, which can be deproton-ated and alkylated in excellent yield (Scheme 20). 

According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere, bromotrichloromethane, which has a measured vapor pressure of 39mmHg at 25°C, is expected to exist solely as a vapor in the ambient atmosphere. Based on bromotrichloromethane s structural similarity to bromotrifluoromethane, it is expected to slowly degrade in the atmosphere by reaction with photochemically produced hydroxyl radicals the half-life for bromotrichloromethane s reaction in air is estimated to be greater than 44 years. Photolysis may occur based on bromotrichloromethane s structural similarity to other halogenated methane compounds but not at an environmentally relevant rate. 

However redox reactions of alkanes, in particular with oxygen and the halogens, are possible as the carbon atoms are in a strongly reduced condition in the case of methane, the lowest possible oxidation state for carbon (-4) is reached. Reaction with oxygen leads to combustion without any smoke with halogens, substitution. In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes. 

Advanced technologies now under consideration for the conversion of methane to more useful materials involve halogenation [1], oxychlorina-tion [2], oxidation, Including oxidative coupling and metal oxide reactions 3], reaction with superacids [4], and various other methods [5],... 

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