Introduction to Reactions of Aryl Diazonium Salts
Aryl diazonium salts are significant intermediates. They are prepared in cold (0 º to 10 ºC) aqueous solution, and usually react with nucleophiles with loss of nitrogen. Some of the more usually used substitution reactions are displayed in the diagram below. These reactions are energetically favored, Because the leaving group (N2) is thermodynamically very stable. Those substitution reactions that are catalyzed by cuprous salts are termed as Sandmeyer reactions. Fluoride substitution takes place on treatment with BF4(-), a reaction termed as the Schiemann reaction. The Stable diazonium tetrafluoroborate salts might be isolated, and on heating these lose nitrogen to provide an arylfluoride product. The top reaction with the hypophosphorus acid, H3PO2, is notable since it achieves the reductive removal of an amino (or nitro) group. Different the nucleophilic substitution reactions, this reduction possibly proceeds by a radical mechanism.
These aryl diazonium substitution reactions considerably expand the tactics available for the synthesis of polysubstituted benzene derivatives. Refer the following options:
(i) The typical precursor to an aryl amine is the equivalent nitro compound. A nitro substituent deactivates the aromatic ring and directs electrophilic substitution to meta locations. (ii) Reduction of a nitro group to an amine might be achieved in various ways. The resultant amine substituent strongly activates an aromatic ring and directs electrophilic substitution to para & ortho locations. (iii) The activating character of an amine substituent might be attenuated by formation of an amide derivative (reversible), or even changed to deactivating and meta-directing by formation of a quaternary-ammonium salt (irreversible). (iv) Conversion of the aryl amine to a diazonium ion intermediate permits it to be exchanged by a range of distinct groups (including hydrogen), which may in turn be employed in subsequent reactions.
A resonance explaination of diazonium ions depicts that the positive charge is delocalized over the two nitrogen atoms. It is not feasible for nucleophiles to bond to the inner nitrogen but bonding or coupling of negative nucleophiles to the terminal nitrogen gives neutral azo compounds. As displayed in the equation below, this coupling to the terminal nitrogen should be comparatively reversible and fast. The azo products may present as E / Z stereoisomers. In practice it is establish that the E-isomer predominates at equilibrium.
Except these azo products are trapped or stabilized in a number of manner, reversal to the diazonium ion and slow nucleophilic substitution at carbon (with irreversible nitrogen loss) will be the ultimate course of reaction, as explained in the preceding section. For an instance, if phenyldiazonium bisufate is added quickly to a cold solution of sodium hydroxide a comparatively stable solution of sodium phenyldiazoate (the conjugate base of the initially formed diazoic acid) is obtained. Lowering pH of this solution regenerates phenyldiazoic acid (pKa ca. 7), which disassociates back to the diazonium ion and eventually goes through substitution, generating phenol.
C6H5N2(+) HSO4(-) + NaOH (cold solution) ↔ C6H5N2-OH + NaOH (cold) ↔ C6H5N2-O(-) Na(+)
phenyldiazonium bisulfate phenyldiazoic acid sodium phenyldiazoate
Aryl diazonium salts might be reduced to the equivalent hydrazines by mild reducing agents like sodium bisulfite, zinc dust or stannous chloride. The bisulfite reduction may continue by an initial sulfur-nitrogen coupling, as displayed in the following equation.
The most significant application of diazo coupling reactions is electrophilic aromatic substitution of activated benzene derivatives by diazonium electrophiles. The results of such type of reactions are highly colored aromatic azo compounds that find use as synthetic dyestuffs, generally considered as azo dyes. Azobenzene (Y=Z=H) is light orange in the diagram; Though, the color of other azo compounds may range from red to deep blue depending on the nature of the aromatic rings and the substituents they carry. Azo compounds may present like cis/trans isomer pairs but mainly the well-characterized and stable compounds are trans.
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