Chemistry Reference
In-Depth Information
6
Evaluation of Phosgenation Reactions
6.1
Criteria for Selecting a Reagent
Whenever a phosgenation reaction has to be performed, one first has to consider
whether a safe phosgene equivalent or substitute can be used, or whether phos-
gene itself is required. What are the criteria for making this decision?
It should first be ascertained whether one is trying a new synthesis (research) or
developing a procedure for a recurrent preparation (R&D) or a production. In the
first case, attention should be focussed (under the prevailing conditions) on gain-
ing access to the desired product as simply and as rapidly as possible, and in su
cient
amount. The phosgenation reagent should be selected accordingly - often it could
be phosgene due to reasons of chemical character (see below). In other cases,
choice of the reagent depends on other assessment criteria as listed below.
6.1.1
Preparative Criteria
A second class of criteria for choosing an appropriate reagent is based on chemical
properties such as reactivity, selectivity, and yield, as well as reaction time, handling,
work-up, purity, and, last but not least, costs (see also Chapter 3). These preparative
criteria are more reaction-dependent and can be optimized either by modifying the
reaction or by selecting an appropriate reagent. They are less dependent on equip-
ment or infrastructure.
Criterion: yield
This is the primary criterion in that it gives an indication of the degree of success
of the chosen chemistry. Yield is a complex criterion, comprising a lot of param-
eters, such as reactivity, selectivity, reaction time, and product stability, as well as
environmental impact (possibly caused by unwanted or unknown by-products), and
is very sensitive to any alteration of these.
Criterion: reactivity
As shown in Table 3.1 in Chapter 3, phosgene is 19 times more reactive than di-
phosgene and 170 times more reactive than triphosgene [1], the main phosgene
