Benzene Diazonium Salts and its application

The ppts is a brief introduction to Benzene Diazonium Chloride Salts and its applications in diazonium dyes and organic synthesis.

1.                               UNIT II Aromatic amines, Preparation of Aromatic amines (ArSN2 reaction), Preparation, Properties & Applications of BDC in Azo dyes synthesis, Applications

2. UNIT-II Topic-III Aryl diazonium salts (BDC) The German organic chemist Johann Peter Griess  (1829-88

3. Aryl diazonium salts: Introduction Aryl diazonium salts: The aromatic salt of the formula ArN 2 X are called as aryl diazonium salts. The aryl diazonium salts are stable salts as compare to alkane diazonium salts due to delocalization of positive charge on N-atom with the ring. Preparation: Diazotization The preparation of aryl diazonium salts from primary aromatic amines is known as diazotization. or The reaction of aromatic primary amines with sodium nitrite in strong acid medium at 0-5 0C is called as diazotization .

4. If electron withdrawing groups are attached to aromatic nucleus then the aromatic amines are difficult to diazotize because the nucleophilicity of the amino-nitrogen is reduced by the partial withdrawal of the unshared electron pair. Aryl diazonium salts: diazotization Mechanism:

5. Aryl diazonium salts: Properties of BDC Properties of BDC: It is unstable organic salts , soluble in water, decomposed above 5 0C, therefore, reactions are carried out below 5 0C . 2 . It is weak electrophile reacts only with activated aromatic compounds (o/p-directing & activating compounds (known as secondary component). e.g. -------------------------------------------------- 3 . It is highly sensitive to pH of reaction . 4 . It is bulkie r electrophile, shows ArSE2 reactions at para positions only. This ArSE2 reaction of BDC is known as C-coupling reaction, gives azo compounds / dyes. 5. It also shows equally important reaction, replacement reaction.

6. It is unstable organic salts , decomposed above 5 0C, therefore, reactions are carried out below 5 0C This is required condition for any reactions of BDC, t is decomposed above 10 0C because of high negative free energy of loss of N 2 gas from the BDC. Aryl diazonium salts: Properties of BDC 2. It is weak electrophile reacts only with activated aromatic compounds (o/p-directing & activating compounds (known as secondary component). e.g. Phenols and anilines. Therefore, the ArSE2 reactions of BDC with activated aromatic compounds such as phenols or anilines are called as C-coupling reactions (bond formation with C-atom) this gives formation of azo compounds, azo dyes are important class of dyes. e.g. Consider a C-coupling reaction

7. Aryl diazonium salts: Properties of BDC C-coupling reactions: Primary component of reaction: --------------------------------------------------------------------------------------------- Secondary component of reaction: ---------------------------------------------------------------------------------------- Coupling takes place at position: ------------------------------------------------------------------------------------------- e.g. Note: Condition of reaction-- Temp., pH Substrate & reagents--- Both are activating C-coupling at ----- Para position Product ----- Azo compound

8. Aryl diazonium salts: Coupling reactions Identify, primary & secondary component of the C-coupling reaction. Comp. No. Primary component (P) Secondary component (S) 1 P S 2 S 3 S 4 P S 5 P S 6 P S

9. Aryl diazonium salts: Reactions of BDC Coupling reactions Secondary component Product Example C-coupling - ArSE2 Phenols, Anilines Azo dyes N-coupling - SN Primary aromatic amines Diazo amino dyes Types of reaction Replacement reactions Reactants Catalysts Product Sandmeyer’s reaction Cu 2 Cl 2 or CuCl --- Chloro aryl CuBr --- Bromo aryl deamination reaction H 3 PO 2 --- Aryl Synthesis of phenol dil. acid --- Phenol

10. The ArSE2 reactions of BDC with activated aromatic compounds such as phenols or anilines are called as C-coupling reactions (bond formation with C-atom) this gives formation of azo compounds, azo dyes are important class of dyes. e.g. Consider C-coupling reaction of N,N- dimethyl aniline with sulphanilic acid diazonium salt, gives Methyl orange dye Aryl diazonium salts: C-coupling reaction

11. e.g. Consider C-coupling reaction of N,N- dimethyl aniline with BDC, gives Butter yellow dye Aryl diazonium salts: C-coupling reaction

12. Aryl diazonium salts: N-coupling reaction The SN reactions of BDC with primary or secondary aromatic amines are called as N-coupling reactions (bond formation with N-atom) this gives formation of diazo amino compounds or dyes. e.g. Consider N-coupling reaction of BDC with aniline, gives diazo amino dye. Tertiary amines does not give the reaction. (reason ?). Give the reaction taking secondary component as N-methyl aniline

13. Aryl diazonium salts: pH correlation BDC is highly sensitive to pH in its C-coupling reactions , affects both reactivity and yields of the diazo dyes. We know, the secondary component of C-coupling reactions are activated compounds either anilines or phenols, anilines are basic while phenols are acidic in nature . Therefore, Which is the best pH for BDC in its reaction with two different classes of compounds? Case I: Consider the case of anilines as secondary component. We know, anilines are weakly basic in nature, therefore reacts with acids (at low pH– below 4) giving salts. e.g., Thus, the available pH range for the reaction is from 4 to 8. and the optimized pH is 4-5.

14. Aryl diazonium salts: pH correlation BDC is highly sensitive to pH in its C-coupling reactions , affects both reactivity and yields of the diazo dyes. We know, the secondary component of C-coupling reactions are activated compounds either anilines or phenols, anilines are basic while phenols are acidic in nature . Therefore, Which is the best pH for BDC in its reaction with two different classes of compounds? Case II: Consider the case of phenols as secondary component. We know, phenols are weakly acidic in nature, therefore reacts with bases (at high pH–above 7) giving salts. e.g., When pH is 3. Explain its effects ? Thus, the available pH range for the reaction is from 6 to 9. and the optimized pH is 8-9 .

15. Aryl diazonium salts: Replacement reactions Replacement reactions: The replacement of N 2 group from BDC by halo atoms (Cl, Br), cyano (CN), OH, or by H atom is known as replacement reactions. Sandmeyer’s reactions (Aniline– BDC—Halo aromatics) 2. deamination reaction (Aniline—BDC—Aromatics) Phenol synthesis (Aniline—DBC– Phenol)

16. Aryl diazonium salts: Sandmeyer’s reactions Sandmeyer reaction: The replacement of N 2 group by chlorine / bromine atom by CuCl or CuBr is known as Sandmeyer reaction. e.g. Consider general reaction Mechanism: The mechanism is free radical catalysed takes place in following steps- Oxidation reaction

17. Aryl diazonium salts: Sandmeyer’s reactions Sequence of reaction is-- Loss of N 2 gas, Free radical generation

18. Aryl diazonium salts: deamination reactions deamination reaction/ Reduction/ Removal of –NH 2 group: The conversion of aryl amines to aryl by diazotization and subsequently reduction by H 3 PO 2 (hypo phosphorus acid) and CuCl is known as deamination reaction. e.g. Consider general reaction--- Mechanism: The mechanism is free radical catalysed takes place in following steps- Oxidation reaction

19. Aryl diazonium salts: deamination reactions Loss of N 2 gas, Free radical generation Radical abstraction, Oxidation of H 3 PO 2 to H 3 PO 3

20. Ullmann’s reaction Ullmann’s reaction: The biaryl synthesis involving aryl halide in presence of Cu(s) is called as Ullmann reaction. Thus, it is arylation of aryl (not possible by F. C. Alkylation). e.g., Consider general reaction- Mechanism: Oxidation reaction Cleavage of Aryl halide & formation of biaryl

21. Applications of ArSN2, BDC & Ullmann reactions We see, one circle of synthesis is completed by using ArSN2, BDC and Ullmann reactions. Q. How will one synthesize 5 from 1?

22. Benzidine rearrangement Introduction: The acid catalysed intra molecular rearrangement reaction of diaryl hydrazine to diamino biphenyl is known as benzidine rearrangement. Benzidine is important precursor for azo dyes. e.g. Consider general reaction-- Benzidine (p-benzidine) is major product while other o-benzidine & sumidine are other products in minor amount.

23. Mechanism: The reaction takes in three steps--- [As intramolecular rearrangement, indicates breaking of N-N bond and ArSE2 reaction]. Benzidine rearrangement Step I: Protonation of substrate, diaryl hydrazine

24. Benzidine rearrangement Step II: Breaking of N-N bond and ArSE2 reaction Step III: deprotonation

25. Azo dyes Dyes: The brilliant colour molecules having following properties are known as dyes. 1. It should be fast to action of acids, alkali & sunlight 2. The colour should be fasten during washing 3. It should dissolves in water during dyeing process. e.g., Consider few examples of azo dyes, benzidine and m- phenylene diamine based azo dyes. Synthesis of Congo red tetra azo dye Continue….

26. Azo dyes It is used as indicator, to colour biological specimens. pH range is 3.0 (blue) ------ 5.2 (red)

27. Azo dyes Synthesis of Bismark brown tetra azo dye- It is the mixture of diazo and tetra azo dyes. How many molecules of m- phenylene diamine were used for synthesis of 1 mole mixture of Bismark brown? Used in varnishes, boot polishes etc.

28. Methods of synthesis of anilines Anilines or aromatic primary amines can be synthes ize by following methods--- ArSN2 reaction Reduction of nitro aromatics Reduction of nitro aromatics: How will your prepare m- phenylene diamine ? e.g.,