Why should the temperature be maintained at 0–5 °C in a diazotisation?

I know that in a diazotisation reaction, the temperature should be carefully maintained within the range of 0–5 °C by use of an ice bath. Why is this so? How does the temperature affect the stability of the aryl diazonium salt?




The temperature at which many reactions are carried out is often arbitrary, with certain values being favoured due to the ease at which cooling baths can be prepared. As an example -78°C is highly prevalent, and a question about this '[magic temperature' has been asked here previously](https://chemistry.stackexchange.com/questions/46479/why-is-%E2%88%9278-c-a-magic-temperature-for-organic-reactions?rq=1).


In the case of reactions forming diazonium salts, there has been a common rule of thumb that they *must* be kept somewhere between 0 - 5°C, which is even quoted on Wikipedia:



> 
> In aqueous solution diazonium salts are unstable at temperatures above 5 °C (**Ref**: Wikipedia)
> 
> 
> 


At temperatures above 5 °C, diazonium salts (in aqueous solution) have been known to decompose, often explosively, to liberate nitrogen gas, temperatures too low however can cause crystallisation, which causes issues later on (diazonium salts should **never** be dried upon isolation - they explode). 


Although people often keep to this rule of thumb, theres no real body of evidence for its generality (i.e. different diazonium species will have vastly different decomposition temperatures). 


Often the only time people carry out in-depth studies of a *particular* reaction is when the reaction needs to be scaled up, such as in the the pharmaceutical industry. Process chemists go to extraordinary lengths to study their reactions to avoid any possible safety issues (or even unintended side reactions). In *Organic Process Research & Development* **2004**, *8*, 1059−1064, a team of process chemists at Lundbeck carried out extensive thermal studies on a diasotisation reaction they were doing as part of a Sandmeyer reaction, and they found no issues well above 5 °C (though as a precaution they still kept the reaction below 15 °C). 


 8

Warming aryl diazonium salts in the presence of water is a well-known method for synthesizing phenols (with moderate yields).


                   [![Creator: Souna23, Wikipedia](https://i.stack.imgur.com/iyyJl.png)](https://i.stack.imgur.com/iyyJl.png)


The first step of this reaction is probably the reason why aryl diazonium salts are unstable at higher temperatures.


 4

Obviously temperature affects decomposition rates, but maintaining generation and stability of the nitrous acid is key to actually achieving aryl diazonium salt production. Different counterions impart lesser or greater stability of the aryldiazonium salt, to the point where certain counterions, such as tetrafluoroborate or acetate are stable at room temperature, so would be of lesser concern in the reaction.


 3

Aryldiazonium salts are unstable at high temperature and decompose to produce hydrogen chloride, nitrogen gas and chlorobenzene.


[From nature journal:](http://www.nature.com/nature/journal/v140/n3541/abs/140466c0.html) 



> 
> The decomposition of aromatic diazo compounds is non-ionic in
>  mechanism. When suspended in an organic liquid like benzene,
>  **diazonium chloride appears to melt at about 50° C and then immediately a violent decomposition sets in.** There is great heat
>  evolution and except on the small scale, **the reaction tends to
>  become explosively violent.** **Often hydrogen chloride is evolved,**
>  and whenever its formation has been observed, **chlorobenzene has been
>  found amongst the reaction products.**
> 
> 
> 


Decomposition of aryldiazonium salts has also been studied in other solvents like [water](http://pubs.rsc.org/-/content/articlepdf/1940/tf/tf9403600948)(to produce phenol) and [alcohol](http://pubs.acs.org/doi/pdf/10.1021/ja01855a074)(to produce aldehyde and alcohol).


Hence, the salts are never dried and always kept at low temperature. It has also been studied that among all diazonium salts, [benzenediazonium fluoroborate is by far safest to use.](http://sites.chem.colostate.edu/diverdi/C475/miscellanea/sample_presentation_475.pdf) It is not known to explode unless heated at high temperature. It is stable and can be stored for [more than a month at room temperature or for a few years at -20 °C under $\ce{N2}$ in the dark.](http://reag.paperplane.io/00000122.htm) Like other diazonium salt, it decomposes but slowly and should not be prepared well in advanced and should be stored in refrigerator.


Why benzenediazonium fluoroborate is stable is discussed in [another question](https://chemistry.stackexchange.com/questions/58461/benzenediazonium-fluoroborate).


 1