chapter ten
Working up the reaction
10.1 Introduction
It is important to give some thought to the work-up of the reaction before
you attempt it. There are several aspects that need to be considered, the
most important of which are as follows: First of all, make sure that the
reaction has actually nished (by careful analysis using your chosen
monitoring system). When using TLC analysis, it is sometimes difcult to
judge by spotting the reaction mixture directly on to the TLC plate. This is
because other components in the reaction mixture can sometimes obscure
the spots of interest. High-boiling solvents such as DMF or pyridine can
also obscure TLC spots of interest. If you suspect this is a problem, it is
often possible to get a more accurate TLC analysis by withdrawing a small
aliquot of reaction mixture by syringe and performing a “mini work-up
in a small vial. To do this, add your aliquot to a vial containing 0.5 cm
each of diethyl ether and an appropriate aqueous solution. Shake the vial
vigorously, then TLC the diethyl ether layer. This technique can also be
used to screen alternative work-up conditions, for example, adding to
water or an aqueous base rather than aqueous acid, or using other organic
solvents in place of diethyl ether.
Having satised yourself that the reaction has run to completion or
that it is time to end the experiment, the appropriate “quench” is added to
the reaction mixture. Choice of this reagent can be crucial and can greatly
affect the isolated yield of desired product. It is also vital to use a quench
reagent or procedure that is safe. If the product is expected to be reason-
ably stable, which usually is the case, then the choice of procedure for
quenching the reaction is determined by the reagent(s) used in the reac-
tion. Clearly, we cannot cover all possibilities in this chapter, but general
procedures that should cover most of the situations that are likely to be
encountered are provided in Section 10.2. The classication is made on the
basis of the nature of the reaction mixture that is to be worked up.
10.2 Quenching the reaction
If the reaction has been carried out under an inert atmosphere, then it is
advisable to add the quench before exposing the reaction mixture to the air.
It is best added as you would add a reagent (typically dropwise by syringe).
If the reaction was run at low temperature, then add the quench at this

Get Advanced Practical Organic Chemistry, Third Edition, 3rd Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.