Aryl halide

In organic chemistry, an aryl halide (also known as haloarene) is an aromatic compound in which one or more hydrogen atoms, directly bonded to an aromatic ring are replaced by a halide. Haloarenes are different from haloalkanes because they exhibit many differences in methods of preparation and properties. The most important members are the aryl chlorides, but the class of compounds is so broad that there are many derivatives and applications.

Classification according to halide

Aryl fluorides

Aryl fluorides are used as synthetic intermediates, e.g. for the preparation of pharmaceuticals, pesticides, and liquid crystals. The conversion of diazonium salts is a well established route to aryl fluorides. Thus, anilines are precursors to aryl fluorides. In the classic Schiemann reaction, tetrafluoroborate is the fluoride donor:

[C₆H₅N+2]BF−4 → C₆H₅F + N₂ + BF₃

In some cases, the fluoride salt is used:

[C₆H₅N+2]F⁻ → C₆H₅F + N₂

Many commercial aryl fluorides are produced from aryl chlorides by the Halex process. The method is often used for aryl chlorides also bearing electron-withdrawing groups. Illustrative is the synthesis of 2-fluoronitrobenzene from 2-nitrochlorobenzene:

O₂NC₆H₄Cl + KF → O₂NC₆H₄F + KCl

Aryl chlorides

Aryl chlorides are the aryl halides produced on the largest scale commercially: 150,000 tons/y in the US alone (1994). Production levels are decreasing owing to environmental concerns. Chlorobenzenes are used mainly as solvents.

Friedel-Crafts halogenation or "direct chlorination" is the main synthesis route. Lewis acids, e.g. iron(III) chloride, catalyze the reactions. The most abundantly produced aryl halide, chlorobenzene, is produced by this route:

C₆H₆ + Cl₂ → C₆H₅Cl + HCl

Monochlorination of benzene is accompanied by formation of the dichlorobenzene derivatives. Arenes with electron donating groups react with halogens even in the absence of Lewis acids. For example, phenols and anilines react quickly with chlorine and bromine water to give multihalogenated products. Many detailed laboratory procedures are available. For alkylbenzene derivatives, e.g. toluene, the alkyl positions tend to be halogenated by free radical conditions, whereas ring halogenation is favored in the presence of Lewis acids. The decolouration of bromine water by electron-rich arenes is used in the bromine test.

Reaction between benzene and halogen to form an halogenobenzene

The oxychlorination of benzene has been well investigated, motivated by the avoidance of HCl as a coproduct in the direct halogenation:

4 C₆H₆ + 4 HCl + O₂ → 4 C₆H₅Cl + H₂O

This technology is not widely used however.

The Gatterman reaction can also be used to convert diazonium salts to chlorobenzenes using copper-based reagents. Owing to high cost of diazonium salts, this method is reserved for specialty chlorides.

Aryl bromides

The main aryl bromides produced commercially are tetrabromophthalic anhydride, decabromodiphenyl ether, and tetrabromobisphenol-A. These materials are used as flame retardants. They are produced by direct bromination of phenols and aryl ethers. Phthalic anhydride is poorly reactive toward bromine, necessitating the use of acidic media.

The Gatterman reaction can also be used to convert diazonium salts to bromobenzenes using using copper-based reagents. Owing to high cost of diazonium salts, this method is reserved for specialty bromides.

Aryl iodides

Synthetic aryl iodides are used as X-ray contrast agents, but otherwise these compounds are not produced on a large scale. Aryl iodides are "easy" substrates for many reactions such as cross-coupling reactions and conversion to Grignard reagents, but they are much more expensive than the lighter, less reactive aryl chlorides and bromides.

Aryl iodides can be prepared by treating diazonium salts with iodide salts. Electron-rich arenes such as anilines and dimethoxy derivatives react directly with iodine.

Aryl lithium and aryl Grignard reagents react with iodine to give the aryl halide:

ArLi + I₂ → ArI + LiI

This method is applicable to the preparation of all aryl halides. One limitation is that most, but not all, aryl lithium and Grignard reagents are produced from aryl halides.

Classification according to aryl group

Halobenzenes and halobenzene derivatives

Main article: Halobenzene

Although the term aryl halide includes halogenated derivatives of any aromatic compound, it commonly refers to halobenzenes, which are specifically halogenated derivatives of benzene. Groups of halobenzenes include fluorobenzenes, chlorobenzenes, bromobenzenes, and iodobenzenes, as well as mixed halobenzenes containing at least two different types of halogens bonded to the same benzene ring. There are also many halobenzene derivatives.

Halopyridines

Halopyridines are based on the aromatic compound pyridine. This includes chloropyridines and bromopyridines. Chloropyridines are important intermediates to pharmaceuticals and agrochemicals.

Halogenated naphthalenes

Halogenated naphthalenes are based on naphthalene. Polychlorinated naphthalenes were used extensively from the 1930s to 1950s in cable and capacitor production, due to their insulating, hydrophobic, and flame retardant properties, but they have since been phased out for this use due to toxicity, environmental persistence, and introduction of new materials.

Aryl halides in nature

The thyroid hormones triiodothyronine (T₃) and thyroxine (T₄) are aryl iodides. A tetraiodide, T₄ is biosynthesised by electrophilic iodination of tyrosine derivative. Synthetic T₄ is one of the most heavily prescribed medicines in the U.S.

Many chlorinated and brominated aromatic compounds are produced by marine organisms. The chloride and bromide in ocean waters are the source of the halogens. Various peroxidase enzymes (e.g., bromoperoxidase) catalyze the reactions. Numerous are derivatives of electron-rich rings found in tyrosine, tryptophan, and various pyrroles. Some of these natural aryl halides exhibit useful medicinal properties.

Vancomycin, an important antibiotic, is an aryl chloride isolated from soil fungi.

The chemical structure of 6,6′-dibromoindigo, the main component of Tyrian Purple.

Structural trends

The C-X distances for aryl halides follow the expected trend. These distances for fluorobenzene, chlorobenzene, bromobenzene, and methyl 4-iodobenzoate are 135.6(4), 173.90(23), 189.8(1), and 209.9 pm, respectively.

Reactions

Substitution

Unlike typical alkyl halides, aryl halides typically do not participate in conventional substitution reactions. Aryl halides with electron-withdrawing groups in the ortho and para positions, can undergo S_NAr reactions. For example, 2,4-dinitrochlorobenzene reacts in basic solution to give a phenol.

Unlike in most other substitution reactions, fluoride is the best leaving group, and iodide the worst. A 2018 paper indicates that this situation may actually be rather common, occurring in systems that were previously assumed to proceed via S_NAr mechanisms.

Benzyne

Aryl halides often react via the intermediacy of benzynes. Chlorobenzene and sodium amide react in liquid ammonia to give aniline by this pathway.

Organometallic reagent formation

Aryl halides react with metals, generally lithium or magnesium, to give organometallic derivatives that function as sources of aryl anions. By the metal-halogen exchange reaction, aryl halides are converted to aryl lithium compounds. Illustrative is the preparation of phenyllithium from bromobenzene using n-butyllithium (n-BuLi):

C₆H₅Br + BuLi → C₆H₅Li + BuBr

Direct formation of Grignard reagents, by adding the magnesium to the aryl halide in an ethereal solution, works well if the aromatic ring is not significantly deactivated by electron-withdrawing groups.

Other reactions

The halides can be displaced by strong nucleophiles via reactions involving radical anions. Alternatively aryl halides, especially the bromides and iodides, undergo oxidative addition, and thus are subject to Buchwald–Hartwig amination-type reactions.

Chlorobenzene was once the precursor to phenol, which is now made by oxidation of cumene. At high temperatures, aryl groups react with ammonia to give anilines.

Biodegradation

Rhodococcus phenolicus is a bacterium that degrades dichlorobenzene as sole carbon sources.

Applications

The aryl halides produced on the largest scale are chlorobenzene and the isomers of dichlorobenzene. One major but discontinued application was the use of chlorobenzene as a solvent for dispersing the herbicide Lasso. Overall, production of aryl chlorides (also naphthyl derivatives) has been declining since the 1980s, in part due to environmental concerns. Triphenylphosphine is produced from chlorobenzene:

3 C₆H₅Cl + PCl₃ + 6 Na → P(C₆H₅)₃ + 6 NaCl

Aryl bromides are widely used as fire-retardants. The most prominent member is tetrabromobisphenol-A, which is prepared by direct bromination of the diphenol.