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Date: 30-9-2020
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Date: 30-9-2019
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Date: 11-10-2020
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Aryl iodides can be prepared simply by treating the diazonium salt (chloride or sulfate) with iodide ion, usually as a solution of sodium or potassium iodide. A copper(I) catalyst is not essential, though it is sometimes used.
For preparation of aryl bromides and chlorides, the Sandmeyer reaction is used, since the reaction is sluggish without copper(I) catalysis. The corresponding copper halide is used – CuBr for Br, and CuCl for Cl, for example:
The Balz–Schiemann reaction uses tetrafluoroborate ion and delivers the halide-substituted product, fluorobenzene, which is not obtained by the use of copper fluorides.
Another use of the Sandmeyer reaction is for cyanation which allows for the formation of benzonitriles, an important class of organic compounds.
The amino functional group can in effect be removed by diazotization followed by reduction. The most common reagent is hypophosphorous acid (H3PO2), but NaBH4 also works. Ethanol can also effect the reduction, and this is commoner in the older literature.
Removal of the NH2 in this way can be useful in synthesis, because the amino group is a powerful activator and ortho-para director. For example, the NH2 can be used to introduce three bromines rapidly ortho/para to itself, then the NH2 can be removed to leave the bromines intact. A compound such as 2,4,6-tribromobenzoic acid can be easily prepared in this way.
The NH2 can be replaced by OH simply by warming the acidic solution of the benzenediazonium salt, and water acts as the incoming nucleophile. Since chloride ion can act as a competing nucleophile at higher temperatures, we avoid a halogenation side reaction by using a non-nucleophilic anion such as hydrogen sulfate, HSO4¯. The diazonium hydrogen sulfate salt is prepared simply using NaNO2 in cold dilute sulfuric acid, and then warmed to produce the phenol. In some cases, the entire reaction is done in warm aqueous H2SO4.
Since there is no easy way to introduce an OH group directly onto an aromatic ring via electrophilic aromatic substitution, this reaction is the most commonly used way to prepare phenols in synthesis. Given the importance of the hydroxyl group in organic chemistry – many phenols are found in nature, for example – means that this reaction is widely used.
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تفوقت في الاختبار على الجميع.. فاكهة "خارقة" في عالم التغذية
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أمين عام أوبك: النفط الخام والغاز الطبيعي "هبة من الله"
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قسم شؤون المعارف ينظم دورة عن آليات عمل الفهارس الفنية للموسوعات والكتب لملاكاته
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