Ethane radical chlorination

As a result, free-radical chlorination of alkanes is a nonselective process. Except when only one type of replaceable hydrogen is present (methane, ethane, neopentane, unsubstituted cycloalkanes), all possible monochlorinated isomers are usually formed. Although alkyl chlorides are somewhat less reactive than alkanes, di- and polychlorinations always occur. The presence of a chlorine atom on a carbon atom tends to hinder further substitution at that carbon. The one exception is ethane that yields more 1,1-dichloroethane than 1,2-dichloroethane. The reason for this is that chlorination of an alkyl chloride occurs extremely slowly on the carbon atom adjacent to the one bearing the chlorine atom (vicinal effect).115... 

Since free-radical chlorination is a nonselective process, overchlorination may be a problem in the manufacture of ethyl chloride. Temperature-induced pyrolysis to yield ethylene and hydrogen chloride may occur, too. A fluidized-bed thermal chlorination reactor may be used to overcome these problems. The best selectivity achieved in the temperature range of 400-450°C is 95.5% with a chlorine to ethane ratio of 1 5. 

Chloroacetic Acid (ClCHiCOOHf. [CAS 79-11-8 J. Chloroacelic acid can be synthesized by the radical chlorination of acetic acid, treatment of trichloroethylene with concentrated H S04. oxidation of 1.2-dichloro ethane or chloroaceialdehyde. amine displacement from glycine, or chlorination of ketene. It behaves as a very strong monobasic acid and is used as a strong acid catalyst for diverse reactions. The Cl function can be displaced in base-catalyzed reactions. For example, it condenses with alkoxides to yield alkoxyacetic acids CICH COOH... 

A reaction of this type is of little use unless you happen to need the four products in the ratios found and can manage to separate them (their boiling points are very similar). But industrially the radical chlorination of methane and ethane are important reactions and the products can be separated... 

A second characteristic feature of chain propagation steps is that, when added together, they give the observed stoichiometry of the reaction. Adding Steps 2 and 3 and canceling structures that appear on both sides of the equation gives the balanced equation for the radical chlorination of ethane (Figure 8.1). 

When this type of thermodynamic analysis is appHed to alkanes other than methane, similar results are obtained. For example, ethane will undergo both radical chlorination and radical bromination ... 

Energy diagrams for the two propagation steps of radical chlorination and radical bromination of ethane. 

The reaction between hydrogen and chlorine is probably also of this type and many organic free radical reactions (e.g. the decomposition of ethanal) proceed via chain mechanisms. 

Most chlorofluorocarbons are hydrolytically stable, CCI2F2 being considerably more stable than either CCl F or CHCI2F. Chlorofluoromethanes and ethanes disproportionate in the presence of aluminum chloride. For example, CCl F and CCI2F2 give CCIF and CCl CHCIF2 disproportionates to CHF and CHCl. The carbon—chlorine bond in most chlorofluorocarbons can be homolyticaHy cleaved under photolytic conditions (185—225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

Thermal chlorination of ethane is generally carried out at 250—500°C. At ca 400°C, a free-radical chain reaction takes place ... 

In a chain reaction, the step that determines what the product will be is most often an abstraction step. What is abstracted by a free radical is almost never a tetra- or tervalent atom (except in strained systems, see p. 989) and seldom a divalent one. Nearly always it is univalent, and so, for organic compounds, it is hydrogen or halogen. For example, a reaction between a chlorine atom and ethane gives an ethyl radical, not a hydrogen atom ... 

Miller passed fluorine over liquid chloroform, pentachloroethane, 1,1,2,2-tetrachloro-ethane, tetrachloroethene, and trichloroethene. He was able to control the reactions, though mixtures of products resulted. Replacements of hydrogen and of chlorine, additions to C = C bonds, dimerizations (particularly at C = C bonds) and chlorination (by displaced chlorine) all occurred radical mechanisms were thought to prevail. 

A highly economical production of ethyl chloride combines radical ethane chlorination and ethylene hydrochlorination.185 186 Called the Shell integrated process, it uses the hydrogen chloride produced in the first reaction to carry out the second addition step ... 

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 ... 

Addition. Vinyl chloride undergoes a wide variety of addition reactions. Chlorine adds to vinyl chloride to form 1,1.2-tnchloroethane by either an ionic or a radical path. Hydrogen halides add to vinyl chloride, usually to yield the 1.1-adduct. Many other vinyl chlonde adducts can be formed under acid-catalyzed Fnedel-Crafts conditions. Vinyl chloride can be hydrogenated to ethyl chloride and ethane over a platinum on alumina catalyst. 

Decomposition of benzoyl peroxide in hexamethyldisilane at 80° C gives, as major products, benzene, benzoic acid, l,2-bis(pentamethyldisilanyl)-ethane and benzylpentamethyldisilane (151). The reaction of hexamethyldisilane in carbon tetrachloride with benzoyl peroxide (at reflux temperature) and with di-tert-butyl peroxide (in a sealed tube at 129° C) gives (chloro-methyl)pentamethyldisilane as the main product arising from the silane (150). In no case are rearrangement products formed. Therefore, in solution at relatively low temperature, the pentamethyldisilanylmethyl radical does not undergo rearrangement as in the thermolysis. The main fate of this free radical is dimerization in the absence of solvent or chlorine atom abstraction when carbon tetrachloride is present. 

Chlorinated ethanes could be divided into two types, those that could carry the chlorine atom chain and those that could not. 1,2-Dichloroethane 1, 1,1,2-trichloroethane 10, 1,1,1-trichloroethane 11, 1,1,2,2-tetrachloroethane 12, 1,1,1,2-tetra-chloroethane 13, and 1,1,1,2,2-pentachloroethane 14 all decomposed with enhanced rates by a chlorine atom chain mechanism. Ethyl chloride and 1,1-dichloroethane 4 did not. The reason for the latter has been explained. Ethyl chloride gave likewise the radical 15 which could not carry the chain. In 1949, in a paper with the late Professor P. F. Onyon,7 the observations made up to that time were correlated and a number of predictions were made (Table 1). In later work all the predictions were shown to be true. 

The chlorination of ethane illustrates the three distinct parts of radical halogenation (Mechanism 15.1) ... 

Thus, the Cl-Cl bond AH° = 58 kcal/mol), which is weaker than either the C-C or C- H bond in ethane (AH° = 88 and 98 kcal/mol, respectively), is broken to form two chlorine radicals. 

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