Reactions of Aromatic Compounds - Nucleophilic Aromatic Substitution Ar-SN
Decomposition of Arenediazonium Salts
Certain substituents, such as the hydroxy group, the cyano group, and fluorine, cannot be directly introduced into an aromatic compound through an electrophilic aromatic substitution. Due to its extremely high reactivity, fluorine, for instance, would result in uncontrollable side reactions.
In such cases, therefore, an alternative synthetic pathway must be employed. When arenediazonium salts or aromatic diazonium ions, respectively, are heated, nitrogen () is split off, yielding aryl cations. Aryl cations are extremely good electrophiles, which may easily be attacked by a nucleophile, even though it is merely a poor nucleophile.
In fact, phenols can, in this way, simply be produced by heating an aqueous solution of the corresponding arenediazonium salt. In this reaction, water is the attacking nucleophile.
When tetrafluoroborate is used as a counter ion, even fluorine can be introduced into the aromatic compound, when no other nucleophile is present. In this reaction, called the Schiemann reaction, the isolated arenediazonium tetrafluoroborate is thermically decomposed without the need of a solvent.
Furthermore, the introduction of a methoxy group in methanolic solutions, as well as the introduction of bromine and chlorine in strongly acidic, aqueous solutions is made possible through the application of arenediazonium salts. Other substituents, such as the cyano group, cannot simply be introduced in that aromatic diazonium cations are heated in the presence of the corresponding nucleophile.
If no desired results are obtained when aromatic diazonium cations are heated in the presence of a nucleophile (Nu) that ought to be introduced as a substituent into the aromatic compound, the application of the corresponding copper(I) salt (Cu(I)Nu) is often successful instead. This is the Sandmeyer reaction. The Sandmeyer reaction is appropiate for the introduction of chlorine and bromine into aromatic compounds of high-yield. The cyano group can also be introduced through the Sandmeyer reaction by converting the aromatic diazonium cation with the aid of copper(I) cyanide (Cu(I)CN). This reaction is usually conducted in neutral solution in order to prevent the liberation of hydrogen cyanide (HCN). The Sandmeyer reaction is not a nucleophilic substitution. Rather, it follows a radical mechanism.
The mechanistic details of the Sandmeyer reaction's final step, that is, the formal transfer of a nucleophile radical from Cu(II)NuX to the aryl radical, are not completely known.
Aside from chloride, bromide, and cyanide (Nu = , , ), nitrite (Nu = ; ) may be applied as a nucleophile in the Sandmeyer reaction. Furthermore, the diazonium ion's reduction with hypophosphorous acid () results in the dediazonization reaction. That is, the positively charged diazonium group is exchanged for hydrogen. The dediazonization reaction is a very important method, which is utilized in the synthesis of multisubstituted aromatic compounds. First of all, an amino group is introduced into the aromatic compound. This amino group, which has a strong +M effect, is then used in order to control the regioselectivity of further electrophilic aromatic substitutions when introducing additional substituents. Last but not least, the amino group is removed through diazotization and subsequent dediazonization.
A reaction that resembles the Sandmeyer reaction occurs when iodine is introduced into an aromatic compound through treatment of the arenediazonium salt with potassium iodide (KI). In this reaction, catalysis by copper(I) is not mandatory, as iodide itself is easily and reversibly oxidizable and, thus, claims the function which copper(I) adheres to in the Sandmeyer reaction.