Conversions (Roadmaps through Diazonium Salts)

CHEDiazonium Salts & Reactions

Benzenediazonium chloride is the universal hub for converting aniline into aryl halides, phenols, nitriles and arenes, because $\text{-}N_2^+$ is an excellent leaving group replaceable by many nucleophiles. ISC roadmap questions reward knowing which reagent installs which group and how to route around aniline's over-reactivity.

Diazotisation (the first step of every roadmap)

C_6H_5NH_2 \xrightarrow{NaNO_2/HCl,\ 273\text{-}278\,K} C_6H_5N_2^+Cl^-

HNO_2

is generated in situ from

NaNO_2 + HCl

Halide and nitrile replacements

C_6H_5N_2^+Cl^- \xrightarrow{CuCl} C_6H_5Cl \qquad C_6H_5N_2^+Cl^- \xrightarrow{CuCN} C_6H_5CN

Sandmeyer-type replacement;

N_2

is expelled in each case

Phenol and deamination

C_6H_5N_2^+Cl^- \xrightarrow{H_2O,\ \Delta} C_6H_5OH \qquad C_6H_5N_2^+Cl^- \xrightarrow{H_3PO_2} C_6H_6

warm water gives phenol; hypophosphorous acid replaces

\text{-}N_2^+

\text{-}H

Always make the diazonium salt first: $C_6H_5NH_2 \xrightarrow{NaNO_2/HCl,\,273\text{-}278\,K} C_6H_5N_2^+Cl^-$ (diazotisation), then replace $\text{-}N_2^+$ .
Aryl halides: $Cl$ / $Br$ via Sandmeyer ( $CuCl$ / $CuBr$ in HX) or Gattermann (Cu powder + HX); $I$ by simply warming with $KI$ ; $F$ by Balz-Schiemann ( $HBF_4$ then heat).
Phenol: warm the diazonium salt with water (steam/dilute acid) $\rightarrow C_6H_5OH + N_2 + HCl$ .
Benzonitrile: treat with $CuCN/KCN$ (Sandmeyer-type) $\rightarrow C_6H_5CN$ ; this adds the new $\text{-CN}$ carbon onto the ring and can be hydrolysed onward to benzoic acid.
Deamination (aniline $\rightarrow$ benzene): diazotise, then reduce $\text{-}N_2^+$ with hypophosphorous acid $H_3PO_2$ (or ethanol), replacing $\text{-}N_2^+$ by $\text{-}H$ .
Benzene $\rightarrow$ aniline (multi-step): nitrate ( $conc.\,HNO_3/H_2SO_4$ ) to nitrobenzene, then reduce ( $Sn/HCl$ or $Fe/HCl$ , then $NaOH$ ).
$p$ -bromoaniline: direct bromine water gives the $2,4,6$ -tribromo product, so first acetylate the $\text{-}NH_2$ (acetic anhydride), brominate to the $para$ position, then hydrolyse off the acetyl group.
Azo dye route: diazotise aniline at $273\text{-}278\,K$ and couple with phenol in mild alkali to give the orange $p$ -hydroxyazobenzene.
Diazonium routes give a single, correctly positioned product where direct electrophilic substitution would give isomer mixtures — that is their main synthetic value.
The $\text{-}NH_2$ (and hence $\text{-}N_2^+$ ) acts as a removable 'positioning handle': install it, use it to direct/place groups, then remove it by deamination if not wanted in the product.
Roadmap logic: decide the final substituent, pick the diazonium reagent that installs it ( $CuX$ , $KI$ , $HBF_4$ , $H_2O$ , $CuCN$ , $H_3PO_2$ ), and add any acetylation/protection steps needed upstream.

Where the marks go

Forgetting to diazotise first — you cannot replace $\text{-}NH_2$ directly; it must be converted to $\text{-}N_2^+$ before any nucleophile can displace it.
Using bromine water directly for $p$ -bromoaniline — it gives $2,4,6$ -tribromoaniline; the acetyl-protect / brominate / deprotect sequence is required.
Quoting the wrong reagent for iodo or fluoro: $I$ needs only $KI$ (no copper), and $F$ needs $HBF_4$ + heat (Balz-Schiemann), not a cuprous salt.
Writing deamination with $Sn/HCl$ or $LiAlH_4$ — the correct reagent to replace $\text{-}N_2^+$ by $\text{-}H$ is $H_3PO_2$ (hypophosphorous acid) or ethanol.
Running diazotisation or coupling warm — above $278\,K$ the salt hydrolyses to phenol, ruining the intended conversion.

How the board asks it

Conversionthe halide, phenol and nitrile replacements of the diazonium group
How will you convert aniline into (i) chlorobenzene, (ii) phenol and (iii) benzonitrile? Give the reagents and conditions for each step.
Identify / classifyroadmap logic of installing then replacing the $\text{-}N_2^+$ group
Compound $A$ ( $C_6H_5NH_2$ ) is treated with $NaNO_2/HCl$ at $273\text{-}278\,K$ to give $B$ , which on warming with $KI$ gives $C$ . Identify $A$ , $B$ and $C$ and write the equations involved.
Predict the productdiazotisation followed by coupling at $273\text{-}278\,K$
Write the structure of the product formed when benzenediazonium chloride is coupled with phenol in a mildly alkaline medium at $273\text{-}278\,K$ , and name the type of reaction.
Give reasonsdiazonium routes giving single, correctly-positioned products
Account for the fact that aniline cannot be directly brominated to give $p$ -bromoaniline, and state how this difficulty is overcome.
Distinguishthe correct reagent for chloro, iodo and fluoro replacement
How will you obtain (i) iodobenzene and (ii) fluorobenzene from benzenediazonium chloride, and how do the reagents used differ from that used to obtain chlorobenzene?

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.