Chlorine atoms alkanes, reactions with

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,... 

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... 

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... 

Atom-radical reactions have recently been investigated using the CS time-resolved instrument (with interleaved sampling) in Leone s laboratory [51,86]. Infrared emission from HC1 was observed in the chain chlorination of C2H6, initiated by 351-nm photolysis of Cl2 in the presence of the alkane [86]. Interferograms were recorded at 34-/is time intervals (fixed by the sampling rate of the HeNe crossings) at a resolution of 0.36 cm -1 as shown in... 

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 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. 

A functional group is the structural unit responsible for a given molecule s reactivity under a particular set of conditions. It can be as small as a single hydrogen atom, or it can encompass several atoms. The functional group of an alkane is any one of its hydrogen substituents. A reaction that we shall discuss in Chapter 4 is one in which an alkane reacts with chlorine. For example ... 

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. 

Alkanes are called saturated hydrocarbons because they do not contain any double or triple bonds. Since they also have only strong cr bonds and atoms with no partial charges, alkanes are very umeactive. Alkanes do undergo radical substitution reactions with chlorine (Cl 2) or bromine (Br2) at high temperatures or in the presence of light, to form alkyl chlorides or alkyl bromides. The substitution reaction is a radical chain reaction with initiation, propagation, and termination steps. Unwanted radical reactions are prevented by radical inhibitors—compounds that destroy reactive radicals by creating umeactive radicals or compounds with only paired electrons. 

Typical chlorinations of alkanes or alkenes with (dichloroiodo)benzene proceed via a radical mechanism and generally require photochemical conditions or the presence of radical initiators in solvents of low polarity, such as chloroform or carbon tetrachloride. However, the alternative ionic pathways are also possible due to the electrophilic properties of the iodine atom in PhICH or electrophilic addition of CI2 generated by the dissociation of the reagent. An alternative synchronous molecular addition mechanism in the reactions of PhICl2 with alkenes has also been discussed and was found to be theoretically feasible [44]. The general reactivity patterns of ArICh were discussed in detail in several earlier reviews [8, 45, 46]. 

---