Alkane reaction with chlorine

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

Chlorination of alkanes is less exothermic than fluonnation and bromination less exothermic than chlorination Iodine is unique among the halogens m that its reaction with alkanes is endothermic and alkyl iodides are never prepared by lodmation of alkanes... 

It has been proposed that this reaction intermediate could decompose to produce HCN and CH3 [55], Chemiluminescence from alkanes can be greatly enhanced by addition of HC1. The proposed explanation is that energy transfer from active nitrogen dissociates HC1 to produce chlorine atoms, which have rapid hydrogen-atom abstraction reactions with alkanes,... 

Table 6.4 summarizes the rate constants for the reactions of chlorine atoms with alkanes. Structure-reactivity relationships have again been developed for... 

The presence of chlorine atoms can be detected by their reaction with alkanes (RH) to give alkyl chlorides. The quantum yields for the photoreduction of copper(II) halide complexes in acetonitrile solution are given in Table 2.13. These values show that the neutral complex has a significantly larger quantum yield than the anionic complexes. 

In the laboratory it is more convenient to use light either visible or ultraviolet as the source of energy to initiate the reaction Reactions that occur when light energy IS absorbed by a molecule are called photochemical reactions Photochemical techniques permit the reaction of alkanes with chlorine to be performed at room temperature... 

In contrast with alkane chlorination, alkane bromination is usually much more selective. In its reaction with 2-methylpropane, for example, bromine abstracts the tertiary hydrogen with greater than 99% selectivity, as opposed to the 35 65 mixture observed in the corresponding chlorination. 

The enhanced selectivity of alkane bromination over chlorination can be explained by turning once again to the Hammond postulate. In comparing the abstractions of an alkane hydrogen by Cl- and Br- radicals, reaction with Br- is less exergonic. As a result, the transition state for bromination resembles the alkyl radical more closely than does the transition state for chlorination, and the stability of that radical is therefore more important for bromination than for chlorination. 

Halogenation reactions of alkanes provide good examples of radical processes, and may also be used to illustrate the steps constituting a radical chain reaction. Alkanes react with chlorine in the presence of light to give alkyl chlorides, e.g. for cyclohexane the product is cyclohexyl chloride. 

Reactions with halogens give boron halides. While reaction with chlorine can be explosive with diborane, it is slow with bromine. Diborane reacts with alkanes forming alkylboranes. Reactions with aromatics give arylboranes. 

The reaction with an alkane, for example, ethane, occurs at room temperature in the presence of UV hght. However, substitution can occur in the dark when the gaseous mixture of chlorine and ethane is at 100°C. 

For example, Figure 5.11 shows typical results from a relative rate experiment on the reaction of chlorine atoms with some simple alkanes (Beichert et al., 1995). The chlorine atoms in this case were produced by the... 

Beichert, P L. Wingen, J. Lee, R. Vogt, M. J. Ezell, M. Ragains, R. Neavyn, and B. J. Finlayson-Pitts, Rate Constants for the Reactions of Chlorine Atoms with Some Simple Alkanes at 298 K Measurement of a Self-Consistent Set Using both Absolute and Relative Rate Methods, J. Phys. Chem., 99, 13156-13162 (1995). 

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. 

The much higher yields of 1-chloropropane than 2-chIoropropane reported by Gol dshleger et al. (34) do not arise necessarily from preferred attack at the terminal carbon of the alkane, as the internal isomers are themselves oxidized faster than the terminal isomer. If 1-chlorohexane or a mixture of 2- and 3-chlorohexanes was used as the reactant, then, when the 2- and 3-isomers had been consumed, 75% of the 1-isomer still remained (84). The ultimate oxidation product, carbon dioxide, was not formed, and it is thought that the major product from alkane oxidation are polychlorinated carboxylic acids formed by chlorination and reaction with the solvent. These acids are difficult to find in the reaction mixture and despite strenuous efforts have not been identified. 

The free-radical-induced reaction of alkanes with sulfuryl chloride characteristically results in the chlorination of hydrocarbons. However, when pyridine is added to the irradiated reactants, sulfochlorination occurs in quite satisfactory yield. For example, the irradiated reaction of cyclohexane and sulfuryl chloride in the presence of pyridine resulted in a 54.8% yield of cyclohexanesulfonyl chloride and only 9.4% of chlorocyclohexane.161... 

Alkanes. The chlorination of ethane known to produce more 1,1-dichloroethane than 1,2-dichloroethane is explained by the so-called vicinal effect.115 One study revealed285 that this observation may be explained by the precursor 1,2-dichloroethane radical (the 11 2-chloroethyl radical) thermally dissociating into ethylene and a chlorine atom [Eq. (10.54)]. Indeed, this radical is the major source of ethylene under the conditions studied. At temperatures above 300°C, the dissociation dominates over the chlorination reaction [Eq. (10.55)], resulting in a high rate of ethylene formation with little 1,2-dichloroethane ... 

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