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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?

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  • $\begingroup$ Diazonium salts are not stable. If the mixture is warmed or just let stand over time, decomposition will occur. If the decomposition is not controlled, it will be explosive. $\endgroup$ – Zhe Jun 19 '17 at 18:08
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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.

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).

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    $\begingroup$ +1 "run between x and y" usually means "it failed outside, but we were not keen on finding out why exactly". Or even "that's what we tried and it worked". It's a thankless job, pinning down the limitation of a reaction. $\endgroup$ – Karl Jun 19 '17 at 21:23
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    $\begingroup$ 0 °C and 5 °C are both "magic" in the same sense of -78 °C; 0 °C is obviously the temperature of an ice water bath, while 5 °C is about the temperature used in most refrigerators, where diazonium salts are often stored. So basically, the range means "diazonium salts can be stored in fridges for a while without major decomposition, and reactions need to be done with mild cooling". $\endgroup$ – Nicolau Saker Neto Jun 19 '17 at 21:58
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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.

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

                   Creator: Souna23, Wikipedia

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

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  • $\begingroup$ "The first step of this reaction is probably the reason why aryl diazonium salts are unstable at higher temperatures." I fail to see how the first step in the picture above is explaining why the diazonium salt is unstable. Please could you explain? Thanks! $\endgroup$ – Gaurang Tandon Mar 19 '18 at 2:34
  • $\begingroup$ The first step shows the decay path of diazonium salts. The driving force is the stability of the formed nitrogen. The above procedure is sometimes called phenolverkochung (german for to cook something till decay) which suggests that it is favored by high temperatures. $\endgroup$ – aventurin Mar 19 '18 at 16:45
  • $\begingroup$ I understand, but: your answer correctly identifies that the diazonium salt decomposes at higher temperature, but does not identify why it was stable at low temperatures as well. If formation of dinitrogen leads to stability, why couldn't it have formed at $\pu{0-5^\circ C}$ as well? (Sorry if the point is already explicit, but I am not able to see it :( ) $\endgroup$ – Gaurang Tandon Mar 20 '18 at 12:05
  • $\begingroup$ If you heat an organic substance, it eventually will decompose. Some substances sooner, some later. As it happens, diazonium salts are somewhat stable at around 0°C. I'm quite sure that they decompose even at 0°C, but the reaction rate is low enough to let handle them as intermediate products e.g. in dye synthesis. $\endgroup$ – aventurin Mar 20 '18 at 17:13
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Aryldiazonium salts are unstable at high temperature and decompose to produce hydrogen chloride, nitrogen gas and chlorobenzene.

From nature journal:

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(to produce phenol) and alcohol(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. 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. 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.

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