Alkanes chlorine atom reactions

Depending on the relative amounts of the two reactants and on the time allowed, a sequential substitution of the alkane hydrogen atoms by chlorine occurs, leading to a mixture of chlorinated products. Methane, for instance, reacts with CI2 to yield a mixture of CH3CI, CH2CI2, CHCI3, and CCI4. We ll look at this reaction in more detail in Section 5.3. 

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,... 

Finally, when we are running out of cyclohexane, the process terminates by the interaction of two radical species, e.g. two chlorine atoms, two cyclohexyl radicals, or one of each species. The combination of two chlorine atoms is probably the least likely of the termination steps, since the Cl-Cl bond would be the weakest of those possible, and it was light-induced fission of this bond that started off the radical reaction. Of course, once we have formed cyclohexyl chloride, there is no reason why this should not itself get drawn into the radical propagation steps, resulting in various dichlorocyclohexane products, or indeed polychlorinated compounds. Chlorination of an alkane will give many different products, even when the amount of chlorine used is limited to molar ratios, and in the laboratory it is not going to be a particularly useful process. 

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). 

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

Alkane chlorination is a stoichiometric reaction [such as Eq. (11)], and a Pt(IV) atom is lost each time a chloroalkane molecule is produced. Efforts to make it catalytic, by converting the Pt(II) formed back to Pt(IV), have so far been totally unsuccessful (86) added K3Fe(CN)6, RuCls, and Fe(OH)(OAc)2 are without effect. Certainly a very subtle... 

Bromine trifluoride is used to selectively substitute fluorine for bromine in brominated alkanes and esters. The reactions are carried out by gradual addition of bromine trifluoride to a solution of the substrate in CFC-113 or CFC-112 at 10-20nC. The bromine-fluorine exchange in mono-bromohaloalkanes is nonstereoselective and accompanied, in some eases, by skeletal rearrangements, hydride shifts, and halogen migrations. All three fluorine atoms in bromine trifluoride are involved in the fluorination reaction. Chlorine atoms in the substrate molecules remain intact.109... 

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 ... 

Nanocrystalline MgO and CaO with high surface area are able to absorb large amounts of chlorine, which undergo dissociative chemisorption. These can serve as rather selective, catalytic alkane chlorination reagents, which suggests that trapped Cl atoms are involved in the reaction.295 Liquid-phase low-temperature chlorination of alkanes is also possible in the presence of various alkenes as inductors and AIBN [azobis(isobutyronitrile)] 296... 

Hydroxylation of alkanes preferentially occurs at the more nucleophilic C—H bonds, with a relatively low isotope effect (fcH/fcD = 2.8 for cyclohexane) and a significant amount of epimerization at the hydroxylated carbon atom. Radical carbon intermediates were revealed in this reaction by trapping experiments with chlorine atoms coming from CC14. 

In addition to combustion, alkanes undergo substitution reactions in which one or more H atoms on an alkane are replaced by atoms of another element. The most common such reaction is the replacement of H by chlorine, to yield organochlorine compounds. For example, methane reacts with chlorine to give chloromethane. This reaction begins with the dissociation of molecular chlorine, usually initiated by ultraviolet electromagnetic radiation ... 

In its attack on alkanes, the bromine atom is much more selective than the chlorine atom (with relative rate factors of 1600 82 1 as compared with 5.0 3.8 1). It is also much less reactive than the chlorine atom (only 1/250,000 as reactive toward methane, for example, as we saw in Sec. 2.19). This is just one example of a general relationship in a set of similar reactions, the less reactive the reagent, the more selective it is in its attack. 

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. 

How can different compounds containing functional groups be made Alkanes are not very reactive, so it can be difficult to react them directly to form substituted molecrdes. You read earlier of one reaction used to substitute chlorine atoms for hydrogen atoms in methane. However, that reaction is not practical because a mixture of four different products is formed. Each must be separated and purified before it can be used. 

At decreasing monomer concentration, reactions 1 and 2 should therefore be more favored. This can e.g. be shown by the ratio between 1,2-dichloro alkane end groups and internal double bonds. It can be assumed that each macromolecule contains 1 end group of this type (6. 15). For the 65°C series, the molecular weight data in Table 1 indicates that this ratio decreases from 8.4 at P/PQ to 4.5 at P/PQ = 0.61. Furthermore, the rate of formation of chlorine atoms will increase at decreasing monomer concentration (see below). The observed increase in the content of internal double bonds is therefore easy to understand. 

The chlorination of alkanes, such as methane, leads to the substitution of hydrogen atoms for chlorine atoms in a radical chain reaction (see Section 5.2.1). 

Nanocrystalline MgO has demonstrated some unusual catalytic properties when chlorine gas is absorbed into the nanocrystals (Sun and Klabunde, 1999b). Contacts of this adduct with alkanes results in their catalytic chlorination see reaction (17.10). It appears that the chlorine is behaving in a manner more consistent with chlorine atoms being formed on the surface of the MgO by dissociative chemisorption. 

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