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Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group [R−N≡N]X where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. The parent compound where R is hydrogen, is diazenylium.

Structure and general properties

Arenediazonium cations and related species

According to X-ray crystallography the C−N≡N linkage is linear in typical diazonium salts. The N≡N bond distance in benzenediazonium tetrafluoroborate is 1.083(3) Å, which is almost identical to that for dinitrogen molecule (N≡N).

The linear free energy constants σ_m and σ_p indicate that the diazonium group is strongly electron-withdrawing. Thus, the diazonio-substituted phenols and benzoic acids have greatly reduced pK_a values compared to their unsubstituted counterparts. The pK_a of phenolic proton of 4-hydroxybenzenediazonium is 3.4, versus 9.9 for phenol itself. In other words, the diazonium group raises the ionization constant K_a (enhances the acidity) by a million-fold. This also causes arenediazonium salts to have decreased reactivity when electron-donating groups are present on the aromatic ring.

The stability of arenediazonium salts is highly sensitive to the counterion. Phenyldiazonium chloride is dangerously explosive, but benzenediazonium tetrafluoroborate is easily handled on the bench.

Alkanediazonium cations and related species

Alkanediazonium salts are synthetically unimportant due to their extreme and uncontrolled reactivity toward S_N2/S_N1/E1 substitution. These cations are however of theoretical interest. Furthermore, methyldiazonium carboxylate is believed to be an intermediate in the methylation of carboxylic acids by diazomethane, a common transformation.

Loss of N₂ is both enthalpically and entropically favorable:

[CH₃N₂] → [CH₃] + N₂, ΔH = −43 kcal/mol

[CH₃CH₂N₂] → [CH₃CH₂] + N₂, ΔH = −11 kcal/mol

For secondary and tertiary alkanediazonium species, the enthalpic change is calculated to be close to zero or negative, with minimal activation barrier. Hence, secondary and (especially) tertiary alkanediazonium species are either unbound, nonexistent species or, at best, extremely fleeting intermediates.

The aqueous pK_a of methyldiazonium ([CH₃N₂]) is estimated to be <10.

Preparation

The process of forming diazonium compounds is called "diazotation", "diazoniation", or "diazotization". The reaction was first reported by Peter Griess in 1858, who subsequently discovered several reactions of this new class of compounds. Most commonly, diazonium salts are prepared by treatment of aromatic amines with nitrous acid and additional acid. Usually the nitrous acid is generated in situ (in the same flask) from sodium nitrite and the excess mineral acid (usually aqueous HCl, H₂SO₄, p-H₃CC₆H₄SO₃H, or H[BF₄]):

ArNH₂ + HNO₂ + HX → [ArN₂]X + 2 H₂O

Chloride salts of diazonium cation, traditionally prepared from the aniline, sodium nitrite, and hydrochloric acid, are unstable at room temperature and are classically prepared at 0 – 5 °C. However, one can isolate diazonium compounds as tetrafluoroborate or tosylate salts, which are stable solids at room temperature. It is often preferred that the diazonium salt remain in solution, but they do tend to supersaturate. Operators have been injured or even killed by an unexpected crystallization of the salt followed by its detonation.

Due to these hazards, diazonium compounds are often not isolated(not necessarily). Instead they are used in situ. This approach is illustrated in the preparation of an arenesulfonyl compound:

Reactions

Diazo coupling reactions

The first and still main use of diazonium salts is azo coupling, which is exploited in the production of azo dyes. In some cases water-fast dyed fabrics are simply immersed in an aqueous solution of the diazonium compound, followed by immersion in a solution of the coupler (the electron-rich ring that undergoes electrophilic substitution). In this process, the diazonium compound is attacked by, i.e., coupled to, electron-rich substrates. When the coupling partners are arenes such as anilines and phenols, the process is an example of electrophilic aromatic substitution:

[ArN₂] + Ar'H → ArN₂Ar' + H

The deep colors of the dyes reflects their extended conjugation. A popular azo dye is aniline yellow, produced from aniline. Naphthalen-2-ol (beta-naphthol) gives an intensely orange-red dye. Methyl orange is an example of an azo dye that is used in the laboratory as a pH indicator..

Another commercially important class of coupling partners are acetoacetic amides, as illustrated by the preparation of Pigment Yellow 12, a diarylide pigment.

Displacement of the N₂ group

Arenediazonium cations undergo several reactions in which the N₂ group is replaced by another group or ion.

Sandmeyer reaction

Benzenediazonium chloride heated with cuprous chloride or cuprous bromide respectively dissolved in HCl or HBr yield chlorobenzene or bromobenzene, respectively.

[C₆H₅N₂] + CuCl → C₆H₅Cl + N₂ + Cu

In the Gattermann reaction (there are other "Gattermann reactions"), benzenediazonium chloride is warmed with copper powder and HCl or HBr to produce chlorobenzene and bromobenzene respectively.

2 Cu + 2 [C₆H₅N₂] → 2 Cu + (C₆H₅)₂ + 2 N₂ (initiation)

[C₆H₅N₂] + HX → C₆H₅X + N₂ + H (Cu catalysis)

Replacement by iodide

Arenediazonium cations react with potassium iodide to give the aryl iodide:

[C₆H₅N₂] + KI → C₆H₅I + K + N₂

Replacement by fluoride

Fluorobenzene is produced by thermal decomposition of benzenediazonium tetrafluoroborate. The conversion is called the Balz–Schiemann reaction.

[C₆H₅N₂][BF₄] → C₆H₅F + BF₃ + N₂

The traditional Balz–Schiemann reaction has been the subject of many motivations, e.g. using hexafluorophosphate(V) ([PF₆]) and hexafluoroantimonate(V) ([SbF₆]) in place of tetrafluoroborate ([BF₄]). The diazotization can be effected with nitrosonium salts such as nitrosonium hexafluoroantimonate(V) [NO][SbF₆].

Biaryl coupling

A pair of diazonium cations can be coupled to give biaryls. This conversion is illustrated by the coupling of the diazonium salt derived from anthranilic acid to give diphenic acid ((C₆H₄CO₂H)₂). In a related reaction, the same diazonium salt undergoes loss of N₂ and CO₂ to give benzyne.

Replacement by hydrogen

Arenediazonium cations reduced by hypophosphorous acid, ethanol, sodium stannite or alkaline sodium thiosulphate gives benzene:

[C₆H₅N₂]Cl + H₃PO₂ + H₂O → C₆H₆ + N₂ + H₃PO₃ + HCl

[C₆H₅N₂]Cl + CH₃CH₂OH → C₆H₆ + N₂ + CH₃CHO + HCl

[C₆H₅N₂]Cl + NaOH + Na₂SnO₂ → C₆H₆ + N₂ + Na₂SnO₃ + NaCl

An alternative way suggested by Baeyer & Pfitzinger is to replace the diazo group with H is: first to convert it into hydrazine by treating with SnCl₂ then to oxidize it into hydrocarbon by boiling with cupric sulphate solution.

Replacement by a hydroxyl group

Phenols are produced by heating aqueous solutions of arenediazonium salts:

[C₆H₅N₂] + H₂O → C₆H₅OH + N₂ + H

This reaction goes by the German name Phenolverkochung ("cooking down to yield phenols"). The phenol formed may react with the diazonium salt and hence the reaction is carried in the presence of an acid which suppresses this further reaction. A Sandmeyer-type hydroxylation is also possible using Cu₂O and Cu in water.

Replacement by a nitro group

Nitrobenzene can be obtained by treating benzenediazonium fluoroborate with sodium nitrite in presence of copper. Alternatively, the diazotisation of the aniline can be conducted in presence of cuprous oxide, which generates cuprous nitrite in situ:

[C₆H₅N₂] + CuNO₂ → C₆H₅NO₂ + N₂ + Cu

Replacement by a cyano group

The cyano group usually cannot be introduced by nucleophilic substitution of haloarenes, but such compounds can be easily prepared from diazonium salts. Illustrative is the preparation of benzonitrile using the reagent cuprous cyanide:

[C₆H₅N₂] + CuCN → C₆H₅CN + Cu + N₂

This reaction is a special type of Sandmeyer reaction.

Replacement by a trifluoromethyl group

Two research groups reported trifluoromethylations of diazonium salts in 2013. Goossen reported the preparation of a CuCF₃ complex from CuSCN, TMSCF₃, and Cs₂CO₃. In contrast, Fu reported the trifluoromethylation using Umemoto's reagent (S-trifluoromethyldibenzothiophenium tetrafluoroborate) and Cu powder (Gattermann-type conditions). They can be described by the following equation:

[C₆H₅N₂] + [CuCF₃] → C₆H₅CF₃ + [Cu] + N₂

The bracket indicates that other ligands on copper are likely present but are omitted.

Replacement by a thiol group

Diazonium salts can be converted to thiols in a two-step procedure. Treatment of benzenediazonium chloride with potassium ethylxanthate followed by hydrolysis of the intermediate xanthate ester gives thiophenol:

[C₆H₅N₂] + C₂H₅OCS−2 → C₆H₅SC(S)OC₂H₅ + N₂

C₆H₅SC(S)OC₂H₅ + H₂O → C₆H₅SH + HOC(S)OC₂H₅

Replacement by an aryl group

The aryl group can be coupled to another using arenediazonium salts. For example, treatment of benzenediazonium chloride with benzene (an aromatic compound) in the presence of sodium hydroxide gives diphenyl:

[C₆H₅N₂]Cl + C₆H₆ → (C₆H₅)₂ + N₂ + HCl

This reaction is known as the Gomberg–Bachmann reaction. A similar conversion is also achieved by treating benzenediazonium chloride with ethanol and copper powder.

Replacement by boronate ester group

A Bpin (pinacolatoboron) group, of use in Suzuki-Miyaura cross coupling reactions, can be installed by reaction of a diazonium salt with bis(pinacolato)diboron in the presence of benzoyl peroxide (2 mol %) as an initiator:. Alternatively similar borylation can be achieved using transition metal carbonyl complexes including dimanganese decacarbonyl.

[C₆H₅N₂]X + pinB−Bpin → C₆H₅Bpin + X−Bpin + N₂

Replacement by formyl group

A formyl group, –CHO, can be introduced by treating the aryl diazonium salt with formaldoxime (H₂C=NOH), followed by hydrolysis of the aryl aldoxime to give the aryl aldehyde. This reaction is known as the Beech reaction.

Other dediazotizations

• by organic reduction at an electrode
• by mild reducing agents such as ascorbic acid (vitamin C)
• by gamma radiation from solvated electrons generated in water
• photoinduced electron transfer
• reduction by metal cations, most commonly a cuprous salt.
• anion-induced dediazoniation: a counterion such as iodine gives electron transfer to the diazonium cation forming the aryl radical and an iodine radical
• solvent-induced dediazoniation with solvent serving as electron donor

Meerwein reaction

Benzenediazonium chloride reacts with compounds containing activated double bonds to produce phenylated products. The reaction is called the Meerwein arylation:

[C₆H₅N₂]Cl + ArCH=CH−COOH → ArCH=CH−C₆H₅ + N₂ + CO₂ + HCl

Metal complexation

In their reactions with metal complexes, diazonium cations behave similarly to NO. For example, low-valent metal complexes add with diazonium salts. Illustrative complexes are [Fe(CO)₂(PPh₃)₂(N₂Ph)] and the chiral-at-metal complex Fe(CO)(NO)(PPh₃)(N₂Ph).

Grafting reactions

In a potential application in nanotechnology, the diazonium salts 4-chlorobenzenediazonium tetrafluoroborate very efficiently functionalizes single wall nanotubes. In order to exfoliate the nanotubes, they are mixed with an ionic liquid in a mortar and pestle. The diazonium salt is added together with potassium carbonate, and after grinding the mixture at room temperature the surface of the nanotubes are covered with chlorophenyl groups with an efficiency of 1 in 44 carbon atoms. These added substituents prevent the tubes from forming intimate bundles due to large cohesive forces between them, which is a recurring problem in nanotube technology.

It is also possible to functionalize silicon wafers with diazonium salts forming an aryl monolayer. In one study, the silicon surface is washed with ammonium hydrogen fluoride leaving it covered with silicon–hydrogen bonds (hydride passivation). The reaction of the surface with a solution of diazonium salt in acetonitrile for 2 hours in the dark is a spontaneous process through a free radical mechanism:

So far grafting of diazonium salts on metals has been accomplished on iron, cobalt, nickel, platinum, palladium, zinc, copper and gold surfaces. Also grafting to diamond surfaces has been reported. One interesting question raised is the actual positioning on the aryl group on the surface. An in silico study demonstrates that in the period 4 elements from titanium to copper the binding energy decreases from left to right because the number of d-electrons increases. The metals to the left of iron are positioned tilted towards or flat on the surface favoring metal to carbon pi bond formation and those on the right of iron are positioned in an upright position, favoring metal to carbon sigma bond formation. This also explains why diazonium salt grafting thus far has been possible with those metals to right of iron in the periodic table.

Reduction to a hydrazine group

Diazonium salts can be reduced with stannous chloride (SnCl₂) to the corresponding hydrazine derivatives. This reaction is particularly useful in the Fischer indole synthesis of triptan compounds and indometacin. The use of sodium dithionite is an improvement over stannous chloride since it is a cheaper reducing agent with fewer environmental problems.

Biochemistry

Alkanediazonium ions, otherwise rarely encountered in organic chemistry, are implicated as the causative agents in the carcinogens. Specifically, nitrosamines are thought to undergo metabolic activation to produce alkanediazonium species.

Safety

Solid diazonium halides are often dangerously explosive, and fatalities and injuries have been reported.

The nature of the anions affects stability of the salt. Arenediazonium perchlorates, such as nitrobenzenediazonium perchlorate, have been used to initiate explosives.

See also

• Diazo
• Diazo printing process
• Benzenediazonium chloride
• Triazene cleavage
• Dinitrogen complex

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External links

• W. Reusch. "Reactions of Amines". VirtualText of Organic Chemistry. Michigan State University. Archived from the original on 2012-12-12.

• Organic compounds
• Carbon-heteroatom bond forming reactions
• Functional groups
• Organonitrogen compounds