Unsaturated hydrocarbons halogen reactions

Preparation of Alkyl Halides from Olefins. There are two general methods for the synthesis of alkyl halides (1) by the interaction of an alcohol with a halogen hydride—a procedure that may reasonably be discussed under esterification or halogenation and, also, under the Friedel-Crafts synthesis when a metal halide is used to catalyze the reactions— and (2) by the addition of a halogen hydride to an unsaturated hydrocarbon. This reaction may be catalyzed by metal halides and by sulfuric acid. In the latter instance, the ethylsulfuric acid first formed is converted to the halide by gaseous chlorine or chlorine liberated in situ by action of sulfuric acid on a halide. 

In presence of unsaturated hydrocarbons, chain reaction can be generated by halogen radical. This process is termed as photo halogenation. 

Reaction with Organic Compounds. Aluminum is not attacked by saturated or unsaturated, aUphatic or aromatic hydrocarbons. Halogenated derivatives of hydrocarbons do not generally react with aluminum except in the presence of water, which leads to the forma tion of halogen acids. The chemical stabiUty of aluminum in the presence of alcohols is very good and stabiUty is excellent in the presence of aldehydes, ketones, and quinones. 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

Such a molecule, containing alternating single and double bonds, would be expected to be quite reactive. Actually, benzene is quite unreactive, and its chemical properties resemble those of the alkanes much more than those of the unsaturated hydrocarbons. For example, the characteristic reaction of benzene with halogens resembles that of the reaction of the alkanes ... 

The great chemical resistibility of aliphatic hydrocarbons and the aggregate1 state of their members poor in carbon make them appear us unsuitable material for eleotrolytical experiments. Only the addition-reactions of unsaturated hydrocarbons offer an experimental field. This has not yet been developed. These reactions are cathodic in the addition of hydrogen, and anodic in the addition of halogens, etc. The fact that such hydrocarbons occur in the decomposition of aliphatic acids gives us an indication as to their behavior, which will he mentioned at the proper place. 

Halogenation of saturated hydrocarbon polymers can hardly be controlled and is frequently assodated with chain degradation phenomena In contrast, the presence of randomly distributed olefinic unsaturations, allows selective halogenation reactions by adopting appropriate conditions. For instance, butyl rubber can be chiorinated or brominated in allylic positions and chloro-butyl or bromo-butyl rubber results The latter polymers are very interesting since they exhibit fast curing rates when sulfur and ZnO are introduced in the formulations. 

Preparation of Alkyl Halides.—We have spoken of the formation of the alkyl halides by the direct action of the halogen upon the saturated hydrocarbon. In the case of chlorine this action takes place at ordinary temperatures as in the reaction between methane and chlorine in the sunlight. Bromine, however, does not act directly at ordinary temperatures but by heating in a sealed tube. Iodine does not act directly with the hydrocarbons. In any case the result is a mixture of several substitution products, and the method is not, therefore, of practical value. Where direct action does not occur the presence of iodine chloride or antimony chloride, which act as carriers, is necessary. The two reactions of most importance in the preparation of these compounds are those involving either alcohols or unsaturated hydrocarbons. These will be taken up when these compounds are studied. 

The ethylene halides may be prepared by direct halogenation of ethane, but this is not a practical method as it yields a mixture of the two isomeric compounds as in the further halogenation of the monohalogen ethanes. The best method of preparation is from the unsaturated hydrocarbon, ethylene. This reaction has been fully considered already (p. 154) and need not be discussed again. 

Unsaturated Hydrocarbons.—In the case of beta-halogen acids the reaction takes place differently. When these are heated with water and alkali they lose carbon dioxide and the halogen-hydrogen acid, an unsaturated hydrocarbon resulting. 

Relatively little is known of the chemistry of antimony rings compared to arsenic rings. The reactions of arsenic homocycles have been reviewed several times " . They include cleavage of the As—As bonds with halogens and insertion of chalcogens or unsaturated hydrocarbons and reductive cleavage with potassium metal. Representative examples are given in equations 15-18. 

G. Reactions of Halogen Atoms with Unsaturated Hydrocarbons 296... 

Whereas alkanes undergo substitution reactions, alkenes and alkynes undergo addition reactions. The principal addition reactions of the unsaturated hydrocarbons are halogenation, hydration, hydrohalogenation, and hydrogenation. Polymers can be made from alkenes or substituted alkenes. 

Esters can be prepared in many cases by the action of alcohols on acids in the presence of sulphuric acid. Certain alcohols are very readily converted into unsaturated hydrocarbons by concentrated sulphuric acid. For this and other reasons hydrogen chloride is frequently used instead of sulphuric acid. The part played by the halogen acid in effecting the elimination of water is not definitely understood. One explanation offered is that a small quantity of the acyl chloride is formed, which then reacts with the alcohol present to form the ester. In the case of the formation of ethyl acetate the reactions according to this hypothesis are,—... 

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