Halogen Derivatives of Aromatic Hydrocarbons

Iodine can be introduced into aromatic compounds by the action of the free halogen, if the reaction is carried out under conditions which bring about the removal of the hydriodic acid formed as the result of the substitution. For example, iodo-benzene is formed when benzene is heated with iodine and nitric acid, mercuric oxide, iodic acid, or other substances which react with hydriodic acid. Reactions of this kind are used in a limited number of cases only as a means of preparing iodo derivatives. Free iodine also converts aniline, C6H5.NH2, into substitution-products. In this case the hydriodic acid formed is removed, as the result of the formation of a salt of aniline, C6H5NH2.HI. 

Halogen derivatives of aromatic compounds may also be prepared from hydroxyl derivatives by a reaction which is analogous to that used in the case of aliphatic compounds. When phenol, CeHsOH, and similar substances are treated with the halides of phosphorus, the hydroxyl groups are replaced by halogen. The yield of halogen compound is small in most cases, however, and the reaction is seldom used as a means of preparing such compounds. When the hydroxyl group is situated in a side-chain, the reaction takes place, in the main, as in the case of aliphatic compounds. 

In this way the NH2 group may be replaced by chlorine, bromine, or iodine. As amino compounds are readily prepared from nitro compounds, which, in turn, are formed by the action of nitric acid on hydrocarbons, the reaction serves as a means of introducing halogen atoms into hydrocarbons. Although a number of steps are involved in the process, it is often used in the prepa- 

The number of halogen atoms introduced into the ring by direct chlorination or bromination is determined by the amount of the halogen used, and the temperature at which the reaction is carried out. The position taken by the entering atom is determined by the nature of the groups present. The conclusions which have been arrived at as the result of the study of many compounds, have been summarized in the rules which have been given (458). 

Magnesium also reacts with halogen compounds in which the halogen atom is in the ring. Bromobenzene and the metal yield the compound CeHsMgBr, which can be used in the preparation of a large number of substances by Grignard s method. 

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Traugott Sandmeyer (Wettingen, 15 September 1854-Zurich, 9 April 1922) was Victor Meyer s lecture assistant and participated in the discovery of thiophen (see p. 810). He discovered the Sandmeyer reaction for the preparation of halogen derivatives of aromatic hydrocarbons by warming a solution of a diazonium compound with cuprous chloride and hydrochloric acid, or cuprous bromide and hydrobromic acid, or with hydriodic acid or potassium... 

The halogen derivatives of aromatic hydrocarbons are divided into two classes namely, (a) aryl halides and (b) arylalkyl halides or aralkyl halides. 

A more convenient method for differentiation between the aromatic and paraffin hydrocarbons is the dimethyl sulfate solubility test (page 135). The paraffin hydrocarbons do not dissolve appreciably in this reagent, whereas ai-omatic hydrocarbons in general dissolve in all proportions, diu )robably to the formation of an addition product between fhe ester and f Ikj aromatic nuck us. The aromatic hydrocarbons may be r( ( .over( d from dimethyl sulfate by saponifying the latter with dilute alkali. Thia method of differentiation does not extend to the halogen derivatives of these hydrocarbons. 

The principal method of forming azo dyes involves diazotization of primary aromatic amines, followed by coupling with hydroxy or amino derivatives of aromatic hydrocarbons or with certain aliphatic keto compounds. Both the aromatic amine, which is diazotized, and the compound to which it is coupled may bear a variety of substituents, such as alkyl, alkoxyl, halogen, and sulfonic acid. Because of the large number of compounds that can be combined, often in more than one sequence, the number of possible azo dyes is almost infinite. 

Picrates. Some halogen derivatives of the higher aromatic hydrocarbons form picrates (for experimental details, see under Aromatic Hydrocarbons, Section IV,9, 1), for example, a-chloronaphthalene (m.p. 137°), a-bromonaphthalene (m.p. 134°), and p-bromonaphthalene (m.p. 86°). 

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. 

The lower members of the homologous series of 1. Alcohols 2. Aldehydes 3. Ketones 4. Acids 5. Esters 6. Phenols 7. Anhydrides 8. Amines 9. Nitriles 10. Polyhydroxy phenols 1. Polybasic acids and hydro-oxy acids. 2. Glycols, poly-hydric alcohols, polyhydroxy aldehydes and ketones (sugars) 3. Some amides, ammo acids, di-and polyamino compounds, amino alcohols 4. Sulphonic acids 5. Sulphinic acids 6. Salts 1. Acids 2. Phenols 3. Imides 4. Some primary and secondary nitro compounds oximes 5. Mercaptans and thiophenols 6. Sulphonic acids, sulphinic acids, sulphuric acids, and sul-phonamides 7. Some diketones and (3-keto esters 1. Primary amines 2. Secondary aliphatic and aryl-alkyl amines 3. Aliphatic and some aryl-alkyl tertiary amines 4. Hydrazines 1. Unsaturated hydrocarbons 2. Some poly-alkylated aromatic hydrocarbons 3. Alcohols 4. Aldehydes 5. Ketones 6. Esters 7. Anhydrides 8. Ethers and acetals 9. Lactones 10. Acyl halides 1. Saturated aliphatic hydrocarbons Cyclic paraffin hydrocarbons 3. Aromatic hydrocarbons 4. Halogen derivatives of 1, 2 and 3 5. Diaryl ethers 1. Nitro compounds (tertiary) 2. Amides and derivatives of aldehydes and ketones 3. Nitriles 4. Negatively substituted amines 5. Nitroso, azo, hy-drazo, and other intermediate reduction products of nitro com-pounds 6. Sulphones, sul-phonamides of secondary amines, sulphides, sulphates and other Sulphur compounds... 

C. M. Suter and A. W. Weston, Direct Sulfonation of Aromatic Hydrocarbons and Their Halogen Derivatives, Org. React. 1946, 3, 141-197. 

Considering what was said about stabilization energies in our previous discussion of thermochemical mimics, we now turn to aryl halides. There are several conceptual approaches to their thermochemistry one can take. The first is to consider halogenated derivatives of benzene, then of naphthalene, then of the isomeric anthracene and phenan-threne, etc. This approach, perhaps more appropriate for a study of generally substituted aromatic hydrocarbons, is immediately thwarted. Although there are many appropriate derivatives of benzene worthy of discussion, thermochemical data on halogenated naphthalenes are limited to the isomeric 1- and 2-monosubstituted derivatives, and halogenated derivatives of other aromatics remain thermochemically unstudied. 

Examples illustrating the application of DPSCA include the cleavage of the carbon-halogen bond in radical anions derived from aromatic compounds [215], the protonation of radical anions derived from aromatic hydrocarbons [90,213,216], the dimerization of radical anions [112,113,217], radical cations [218], and neutral radicals [219], and the conversion of the B form to the A form for 10,10 -dimethyl-9,9 -biacridylidene [220]. 

In order to overcome certain difficulties such as the dissipation of heat and the use of inflammable mixtures, certain liquid phase processes have been proposed for the oxidation of aromatic hydrocarbons and compounds. In such a process 100 the aromatic hydrocarbons or their halogenated derivatives are treated with air or gas containing free molecular oxygen in the liquid phase at temperatures above ISO0 C. and under pressure in the presence of a substantial quantity of liquid water. A small quantity of such oxidation catalysts as oxides or hydroxides of copper, nickel, cobalt, iron or oxides of manganese, cerium, osmium, uranium, vanadium, chromium and zinc is used. The formation of benzaldehyde from toluene is claimed for the process. 

Direct Sulfonation of Aromatic Hydrocarbons and Their Halogen Derivatives ... 

Grignard s Synthesis.—Magnesium reacts with halogen derivatives of the aromatic hydrocarbons as well as with those derived from the paraffins. The compounds so formed are decomposed by water, and hydrocarbons are formed. By the application of this method a halogen compound can be converted into the corresponding hydrocarbon —... 

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