Halogen Substituted Hydrocarbon Radicals

Halogen-Substituted Hydrocarbon w-Radicals Generated in Photochemical Systems3... 

SALT. A compound formed by replacement of part or all of the hydrogen of an acid by one (or more) element(s) or radrcal(s) that are essentially inorganic. Alkaloids, amines, pyridines, and other basic organic substances may be regarded as substituted ammonias in this connection. The characteristic properties of salts are the ionic lattice in the solid state and the ability to dissociate completely in solution. The halogen derivatives of hydrocarbon radicals and esters are not regarded as salts in the strict definition of the term,... 

Alkyl.—The general name for a radical of this series is alkyl. A halogen alkyl or alkyl halide is thus a halogen substitution product of any paraffin hydrocarbon, or it is composed of a paraffin or alkyl radical linked to a halogen atom. The general formula for an alkyl radical is (CnH2 +i). 

Hydroxyl Substitution Products.— All of the preceding facts lead to the conclusion that alcohol is a compound in which the ethyl radical is linked to the hydroxyl radicalj i.e, it is the hydroxyl substiltUion product of ethane or hydroxy ethane. Alcohols thus belong to the same general class of compounds as the halogen substitution products. The relationship between the hydrocarbons, the halogen substitution products, alkyl halides and the hydroxyl substitution products, alcohols, may be shown as follows ... 

Allyl Chloride.—The halogen substitution products of propene and the higher hydrocarbons of the ethene series, when the substitution is in a carbon group not doubly linked, are of importance in the synthesis of derivatives in the same way as are the alkyl halides. 3-Chlor propene or propenyl chloride, CH2 = CH—CH2CI, is known also as allyl chloride, the radical (CH2 = CH—CH2—) being known as allyl. 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

R organic group, R = organic group, may be functionally substituted, X halogen, Y = divalent hydrocarbon radical. 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

The initial step in the oxidation of a hydrocarbon is substitution. For saturated hydrocarbons such as open- and closed-chain paraffins and the alkyl groups of other hydrocarbons, the substitution reaction moves by a free radical mechanism, and sets up a chain reaction. Such a reaction Ls best illustrated by halogenation. 

This bromination reaction results exclusively in alpha substitution and therefore is limited to carboxylic acids with a hydrogens. Chlorine with a trace of phosphorus reacts similarily but with less overall specificity, because concurrent free-radical chlorination can occur at all positions along the chain (as in hydrocarbon halogenation see Section 4-6A). 

Halogenated hydrocarbons often bring about substitution at silicon (see Section III,C), but in the case shown in entry 4, Si-Ti (but not Si-Si) bonds are cleaved, and rearrangement to a cyclic Si5 derivative occurs. A radical process is postulated, similar to that observed in Ti-Ge (422) and H-Mn-CO (72) systems. Silicon-zirconium bonds are cleaved in an analogous way (entry 5). 

It says that the substitution product R—X is produced at a rate that is determined by two constants and two concentration terms. For given initial concentrations of the substrate R—H and the halogen and for a given reaction temperature, the rate of formation of the substitution product is directly proportional to the rate constant ky k being the rate constant of the propagation step in which the radical R is produced from the hydrocarbon R—H. 

The hydrocarbon portion of an aliphatic acid can undergo the free-radical halogenation characteristic of alkanes, but because of the random nature of the substitution it is seldom used. The presence of a small amount of phosphorus, however, causes halogenation (by an ionic mechanism) to take place exclusively at the alpha position. T his reaction is known as the Hell-Volhard-Zelinsky reaction, and it is of great value in synthesis. 

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