Diazonium compound

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Benzenediazonium cation. Benzenediazonium cation.png
Benzenediazonium cation.

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.

Contents

Structure and general properties

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, [1] which is almost identical to that for dinitrogen molecule (N≡N).

The linear free energy constants σm and σp indicate that the diazonium group is strongly electron-withdrawing. Thus, the diazonio-substituted phenols and benzoic acids have greatly reduced pKa values compared to their unsubstituted counterparts. The pKa of phenolic proton of 4-hydroxybenzenediazonium is 3.4, [2] versus 9.9 for phenol itself. In other words, the diazonium group lowers the pKa (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. [3]

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.[ citation needed ]


Alkanediazonium salts are synthetically unimportant due to their extreme and uncontrolled reactivity toward SN2/SN1/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. [4] [5]

Methylation with diazomethane. Diazomethanemethylation.png
Methylation with diazomethane.

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. [6]

The aqueous pKa of methyldiazonium ([CH3N2]+) is estimated to be <10. [7]

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
Sample of benzenediazonium tetrafluoroborate. PhN2BF4.jpg
Sample of benzenediazonium tetrafluoroborate.

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, [8] which are stable solids at room temperature. [9] 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. [10]

Due to these hazards, diazonium compounds are often not isolated. Instead they are used in situ. This approach is illustrated in the preparation of an arenesulfonyl compound: [11]

Preparation of m-trifluoromethylbenzenesulfonyl chloride.svg

Reactions

Diazo coupling reactions

The first and still main use of diazonium salts isazo coupling, which is exploited in the production of azo dyes. [12] [13] 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. [14] 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.. [14]

Azo-coupling-B-2D-skeletal.svg

Another commercially important class of coupling partners are acetoacetic amides, as illustrated by the preparation of Pigment Yellow 12, a diarylide pigment. [15]

PigmentYellow12corrected.png

Displacement of the N2 group

Arenediazonium cations undergo several reactions in which the N2 group is replaced by another group or ion. [16] [17]

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. [18]

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: [19]

[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. [20]

[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]. [21]

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). [22] In a related reaction, the same diazonium salt undergoes loss of N2 and CO2 to give benzyne. [23]

Replacement by hydrogen

Arenediazonium cations reduced by hypophosphorous acid, [24] ethanol, [25] sodium stannite [26] or alkaline sodium thiosulphate [27] 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. [28]

Replacement by a hydroxyl group

Phenols are produced by heating aqueous solutions of arenediazonium salts: [29] [30] [31] [32]

[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. [33] A Sandmeyer-type hydroxylation is also possible using Cu2O and Cu2+ 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]+ + C2H5OCS2 → 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:. [34] Alternatively similar borylation can be achieved using transition metal carbonyl complexes including dimanganese decacarbonyl. [35]

[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. [36] This reaction is known as the Beech reaction. [37]

Other dediazotizations

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). [39]

Grafting reactions

In a potential application in nanotechnology, the diazonium salts 4-chlorobenzenediazonium tetrafluoroborate very efficiently functionalizes single wall nanotubes. [40] 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). [41] 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: [42]

Diazonium salt application silicon wafer DiazoniumSaltApplicationSiliconWafer.png
Diazonium salt application silicon wafer

So far grafting of diazonium salts on metals has been accomplished on iron, cobalt, nickel, platinum, palladium, zinc, copper and gold surfaces. [43] Also grafting to diamond surfaces has been reported. [44] One interesting question raised is the actual positioning on the aryl group on the surface. An in silico study [45] 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.

Metabolic activation of the nitrosamine NDMA, involving its conversion to an alkylating agent. Ndma activ.svg
Metabolic activation of the nitrosamine NDMA, involving its conversion to an alkylating agent.

Safety

Solid diazonium halides are often dangerously explosive, and fatalities and injuries have been reported. [10]

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

See also

Related Research Articles

<span class="mw-page-title-main">Amine</span> Chemical compounds and groups containing nitrogen with a lone pair (:N)

In chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Formally, amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines, trimethylamine, and aniline. Inorganic derivatives of ammonia are also called amines, such as monochloramine.

<span class="mw-page-title-main">Aniline</span> Organic compound (C₆H₅NH₂); simplest aromatic amine

Aniline is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile starting material for fine chemical synthesis. Its main use is in the manufacture of precursors to polyurethane, dyes, and other industrial chemicals. Like most volatile amines, it has the odor of rotten fish. It ignites readily, burning with a smoky flame characteristic of aromatic compounds. It is toxic to humans.

<span class="mw-page-title-main">Nitrous acid</span> Chemical compound

Nitrous acid is a weak and monoprotic acid known only in solution, in the gas phase, and in the form of nitrite salts. It was discovered by Carl Wilhelm Scheele, who called it "phlogisticated acid of niter". Nitrous acid is used to make diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to give azo dyes.

In organic chemistry, an aryl halide 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.

<span class="mw-page-title-main">Azo compound</span> Organic compounds with a diazenyl group (–N=N–)

Azo compounds are organic compounds bearing the functional group diazenyl.

In organic chemistry, the diazo group is an organic moiety consisting of two linked nitrogen atoms at the terminal position. Overall charge-neutral organic compounds containing the diazo group bound to a carbon atom are called diazo compounds or diazoalkanes and are described by the general structural formula R2C=N+=N. The simplest example of a diazo compound is diazomethane, CH2N2. Diazo compounds should not be confused with azo compounds or with diazonium compounds.

<span class="mw-page-title-main">Phosphonium</span> Family of polyatomic cations containing phosphorus

In chemistry, the term phosphonium describes polyatomic cations with the chemical formula PR+
4
. These cations have tetrahedral structures. The salts are generally colorless or take the color of the anions.

The Sandmeyer reaction is a chemical reaction used to synthesize aryl halides from aryl diazonium salts using copper salts as reagents or catalysts. It is an example of a radical-nucleophilic aromatic substitution. The Sandmeyer reaction provides a method through which one can perform unique transformations on benzene, such as halogenation, cyanation, trifluoromethylation, and hydroxylation.

The Ullmann condensation or Ullmann-type reaction is the copper-promoted conversion of aryl halides to aryl ethers, aryl thioethers, aryl nitriles, and aryl amines. These reactions are examples of cross-coupling reactions.

In organic chemistry, an azo coupling is an reaction between a diazonium compound and another aromatic compound that produces an azo compound. In this electrophilic aromatic substitution reaction, the aryldiazonium cation is the electrophile, and the activated carbon, serves as a nucleophile. Classical coupling agents are phenols and naphthols. Usually the diazonium reagent attacks at the para position of the coupling agent. When the para position is occupied, coupling occurs at a ortho position, albeit at a slower rate.

<span class="mw-page-title-main">Thiophenol</span> Chemical compound

Thiophenol is an organosulfur compound with the formula C6H5SH, sometimes abbreviated as PhSH. This foul-smelling colorless liquid is the simplest aromatic thiol. The chemical structures of thiophenol and its derivatives are analogous to phenols. An exception is the oxygen atom in the hydroxyl group (-OH) bonded to the aromatic ring is replaced by a sulfur atom. The prefix thio- implies a sulfur-containing compound and when used before a root word name for a compound which would normally contain an oxygen atom, in the case of 'thiol' that the alcohol oxygen atom is replaced by a sulfur atom.

The Balz–Schiemann reaction is a chemical reaction in which a primary aromatic amine is transformed to an aryl fluoride via a diazonium tetrafluoroborate intermediate. This reaction is a traditional route to fluorobenzene and some related derivatives, including 4-fluorobenzoic acid.

The Gomberg–Bachmann reaction, named for the Russian-American chemist Moses Gomberg and the American chemist Werner Emmanuel Bachmann, is an aryl-aryl coupling reaction via a diazonium salt.

<span class="mw-page-title-main">Meerwein arylation</span> Organic reaction

The Meerwein arylation is an organic reaction involving the addition of an aryl diazonium salt (ArN2X) to an electron-poor alkene usually supported by a metal salt. The reaction product is an alkylated arene compound. The reaction is named after Hans Meerwein, one of its inventors who first published it in 1939.

The Hoesch reaction or Houben–Hoesch reaction is an organic reaction in which a nitrile reacts with an arene compound to form an aryl ketone. The reaction is a type of Friedel-Crafts acylation with hydrogen chloride and a Lewis acid catalyst.

<span class="mw-page-title-main">Sodium tetraphenylborate</span> Chemical compound

Sodium tetraphenylborate is the organic compound with the formula NaB(C6H5)4. It is a salt, wherein the anion consists of four phenyl rings bonded to boron. This white crystalline solid is used to prepare other tetraphenylborate salts, which are often highly soluble in organic solvents. The compound is used in inorganic and organometallic chemistry as a precipitating agent for potassium, ammonium, rubidium, and caesium ions, and some organic nitrogen compounds.

<span class="mw-page-title-main">Benzenediazonium tetrafluoroborate</span> Organic compound containing an –N≡N+ function

Benzenediazonium tetrafluoroborate is an organic compound with the formula [C6H5N2]BF4. It is a salt of a diazonium cation and tetrafluoroborate. It exists as a colourless solid that is soluble in polar solvents. It is the parent member of the aryldiazonium compounds, which are widely used in organic chemistry.

In organic synthesis, cyanation is the attachment or substitution of a cyanide group on various substrates. Such transformations are high-value because they generate C-C bonds. Furthermore nitriles are versatile functional groups.

IMes is an abbreviation for an organic compound that is a common ligand in organometallic chemistry. It is an N-heterocyclic carbene (NHC). The compound, a white solid, is often not isolated but instead is generated upon attachment to the metal centre.

<span class="mw-page-title-main">Organic thiocyanates</span>

Organic thiocyanates are organic compounds containing the functional group RSCN. the organic group is attached to sulfur: R−S−C≡N has a S–C single bond and a C≡N triple bond.

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