Diazonium Salts: Preparation & Stability

CHEDiazonium Salts & Reactions

Aromatic diazonium salts, made by diazotisation of aniline at low temperature, are the gateway to a huge range of substituted benzenes. ISC focuses on the reagents and the strict $273\text{-}278\,K$ window, and on why aryl salts are stable enough to use while alkyl ones are not.

Diazotisation of aniline

C_6H_5NH_2 + NaNO_2 + 2\,HCl \xrightarrow{273\text{-}278\,K} C_6H_5N_2^+Cl^- + NaCl + 2\,H_2O

HNO_2

is generated in situ from

NaNO_2 + HCl

Thermal decomposition (why kept cold)

C_6H_5N_2^+Cl^- + H_2O \xrightarrow{\Delta} C_6H_5OH + N_2 + HCl

above

278\,K

the salt hydrolyses to phenol with loss of

N_2

Preparation (diazotisation): $C_6H_5NH_2 + NaNO_2 + 2HCl \rightarrow C_6H_5N_2^+Cl^- + NaCl + 2H_2O$ , with $HNO_2$ generated in situ.
The reaction is run at $273\text{-}278\,K$ ( $0\text{-}5^\circ\text{C}$ ) because the diazonium salt is unstable and decomposes above this range to phenol and $N_2$ .
The product (benzenediazonium chloride) is used immediately in solution; it is rarely isolated as the dry solid, which can be explosive.
Aryl diazonium salts are more stable than alkyl ones because the $\text{-}N_2^+$ group is stabilised by resonance/delocalisation with the benzene ring.
The positive charge of $\text{-}N_2^+$ is dispersed onto the ring carbons through resonance, which is what stabilises the aryl cation.
Alkyl diazonium salts have no such ring stabilisation, so they decompose instantly, releasing $N_2$ even at low temperature.
Because alkyl diazonium ions decompose at once, aliphatic $1^\circ$ amines with $HNO_2$ give an alcohol plus brisk $N_2$ , not a usable salt.
Benzenediazonium chloride is colourless/crystalline and water-soluble, and reacts further by either replacement (loss of $N_2$ ) or retention (coupling) of the $\text{-}N_2^+$ group.
The driving force for all diazonium replacement reactions is loss of the very stable $N_2$ gas, making $\text{-}N_2^+$ an excellent leaving group.

Where the marks go

Running diazotisation at room temperature — above $278\,K$ the salt decomposes to phenol and $N_2$ before it can be used.
Claiming alkyl diazonium salts can be prepared and stored like aryl ones — they decompose instantly because they lack ring resonance stabilisation.
Writing $HNO_2$ as a bottled reagent rather than generating it in situ from $NaNO_2 + HCl$ .
Balancing the diazotisation with only one $HCl$ — two equivalents are needed (one makes $HNO_2$ , one provides the chloride counter-ion).
Confusing the two fates of $\text{-}N_2^+$ : replacement reactions LOSE $N_2$ , but coupling reactions RETAIN the $\text{-}N=N\text{-}$ unit.

How the board asks it

Give reasonsthe $273\text{-}278\,K$ window and thermal instability
Account for the following: The diazotisation of aniline is carried out at a low temperature of $273\text{-}278\,K$ .
Give reasonsaryl vs alkyl resonance stabilisation
Give reasons: Aromatic diazonium salts are more stable than aliphatic (alkyl) diazonium salts.
Conversiondiazotisation as the gateway step
How will you convert aniline into benzenediazonium chloride? Give the reagents and conditions.
Predict the productdiazotisation vs reaction of aliphatic amines with $HNO_2$
Complete the reaction: $C_6H_5NH_2 + NaNO_2 + HCl \xrightarrow{273\text{-}278\,K} ?$ , and state what is obtained instead when a primary aliphatic amine is treated with $HNO_2$ .
Structure / namingdiazotisation and in-situ $HNO_2$
Define diazotisation and name the reagents used to convert aniline into benzenediazonium chloride.
Assertion–Reasonthermal instability of the dry salt
Assertion: Benzenediazonium chloride is used immediately in solution and is rarely isolated as the dry solid. Reason: The dry salt decomposes on warming to give phenol and $N_2$ and can be explosive. Select the correct option.

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.