Diazonium salt synthesis
The most important method for the preparation of diazonium salts is treatment of aromatic amines such as aniline with sodium nitrite in the presence of a mineral acid. In aqueous solution these salts are unstable at temperatures higher than +5 °C; the N+triple bond N group tends to be lost as N2, i.e. nitrogen gas. One can isolate diazonium compounds as the tetrafluoroborate salt, which is stable at room temperature, but typically diazonium compounds are not isolated and once prepared, used immediately in further reactions. The process of forming diazonium compound is called diazotation (diazoniation) or 'diazotisation GB or diazotization US. The reaction was discovered by Peter Griess in 1858, who subsequentely discovered several reactions of the new compound.
Diazonium salt reactions
- The most important aromatic diazonium salt reactions are azo coupling with anilines and phenols to azo compounds (azo dyes) in electrophilic aromatic substitution.
- Nitrogen replacement reactions by halogens take place in nucleophilic aromatic substitution such as the Sandmeyer Reaction, the Gomberg-Bachmann reaction and the Schiemann reaction. In the so-called Craig method, 2-aminopyridine reacts with sodium nitrite, hydrobromic acid and excess bromine to 2-bromopyridine .
- In Meerwein arylation the salt also decomposes and the aryl residue reacts with an electron deficient alkene in an addition reaction
- In the Bamberger triazine synthesis and the Widman-Stoermer synthesis a diazonium salt reacts as an electrophile through its terminal nitrogen atom with an activated double bond.
- Hydrolysis of diazonium salts yields alcohols
- Reduction with hypophosphorous acid replaces the nitrogen by hydrogen, which allows amino and nitro groups to be removed easily from rings
Applications
The first use of diazonium salts was to produce water-fast dyed fabrics by immersing the fabric in an aqueous solution of the diazonium compound, then a solution of the coupler.Diazonium salts are light sensitive and break down under near UV or violet light. This property has led to their use in document reproduction. In this process, paper or film is coated with a diazonium salt. After contact exposure under light, the residual diazo is converted to a stable azo dye with an aqueous solution of coupler. A more common process uses a paper coated with diazo, coupler and an acid to inhibit coupling; after exposure the image is developed by a vapor mixture of ammonia and water which forces coupling.
In nanotechnology
In a nanotechnology application of diazonium salts, 4-chlorobenzenediazonium tetrafluoroborate is very efficient in functionalizing 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 15 minutes of grinding at room temperature the surface of the nanotubes are covered with chlorophenyl groups with an efficiency of 1 in 44 carbon atoms. These added subsituents prevent the tubes from forming intimate bundles due to large cohesive forces between them which is a reoccurring 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 :
No details on aryl density are given.
Thus far grafting of diazonium salts on metals has been accomplished on iron, cobalt, nickel, platinum, palladium, zinc, copper and gold surfaces. 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.
See also
References
External links
- W. Reusch Reactions of Amines. VirtualText of Organic CHemistry. Michigan State Uniersity. .
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Last updated on Tuesday July 08, 2008 at 20:42:46 PDT (GMT -0700)
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Diazo compounds are used as precursors to carbenes, which are generated by thermolysis or photolysis, for example in the Wolff rearrangement. They are also used in transition metal-catalyzed cyclopropanation reactions for example in a synthesis of tranylcypromine in scheme 1 in which the sodium salt of benzaldehyde tosyl hydrazone is converted to a rhodium metal carbene through the diazo intermediate.
An unusual biomolecule sporting a diazo group was synthesized in 2006 with in its final stage the reaction of a carbonyl group with the hydrazine 1,2-bis(tert-butyldimethylsilyl)hydrazine to the hydrazone followed by reaction with the periodinane difluoroiodobenzene to the diazo compound
Another commercially relevant diazocompound is Ethyl diazoacetate N2CHCOOEt.
A group of isomeric compounds with similar properties are the diazirines, where the carbon and two nitrogens are linked as a ring.
See also
References
This article is licensed under the GNU Free Documentation License.
Last updated on Tuesday July 08, 2008 at 23:15:32 PDT (GMT -0700)
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