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I've seen several procedures using thionyl chloride (SOCl2) to make an ester, where the first step is to add the thionyl chloride to the reagent/solvent on ice or subzero temperature, followed immediately by raising the temperature to reflux.

A labmate asked me a question that stumped me: what is the point of subzero cooling during addition, if you only plan to elevate the temperature immediately after for reflux?

My "real" answer is that I don't want deviate from procedure in case that is detrimental for some unspecified reason. Perhaps a better answer is that on initial addition you don't want a runaway reaction while hand is over the RBF or a strong spike in temperature. Given that the substrate/rxn mix survives reflux, I'm not sure if the latter explanation is sufficient or whether there is actually a good rationale for cooling during reagent addition prior to heating/reflux.

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    $\begingroup$ Sometimes adding thionylchloride will lead to gas development, cooling down the mixture will lead to a decreased speed in the reaction. Also, sometimes you can avoid side reactions by adding the reactants in a slow and controlled fashion. $\endgroup$
    – raptorlane
    Mar 28 at 18:33
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    $\begingroup$ It is all about control of the reaction. Thionyl chloride, as noted, can readily evolve gas with a great deal of bubbling and a rapid exotherm - very easy to lose half your reaction mixture out of the reaction vessel if it takes off. $\endgroup$
    – Waylander
    Mar 28 at 20:05
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    $\begingroup$ Thionyl chloride reacts in a 2-step mechanism to first form a chlorosulfite ester that internally reacts to give the chloride or possibly can react with a second alcohol to give a dialkyl sulfite. For some reason the thionyl chloride is usually added to the alcohol in a manner to quickly give an excess; cooling controls the reaction which is then allowed to warm. I have run several of these with yields in the 90%+ range but there was always a small amount of high melting solid that spectroscopically seemed to be a sulfite ester. Possibly the alcohol should be added to SOCl2? $\endgroup$
    – jimchmst
    Mar 28 at 21:08
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    $\begingroup$ Adding a catalytic amount of DMF generally improves the reaction and reduces sulfite ester formation. You don't reflux until the initial gas evolution is complete. $\endgroup$
    – Waylander
    Mar 29 at 6:04
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    $\begingroup$ When the side product is a problem, you could also try Oxalylchloride for the same purpose. As mentioned, DMF will form the Vilsmeier haack reagent as an intermediate. A very small amount e.g. 1-5 drops from a syringe are enough (30-150 microliter) and will simplyfy the workup. $\endgroup$
    – raptorlane
    Mar 29 at 22:50

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