What can we predict about product distribution and
rates of formation?
Reaction control in electrophilic addition.
As we have seen in the previous tutorials, product distribution is
influenced
by temperature and time. There are a few things to keep in mind
when predicting product distribution for electrophilic addition:
-
Electrophilic addition to conjugated dienes produces a mixture of
1,2-addition and 1,4-addition products.
-
Generally, when HBr and Br2 are added to conjugated dienes at lower
temperatures, there will be more of the 1,2-product produced. Here, there
is no equilibrium established between the 1,2- and 1,4-addition products.
The 1,2-addition product predominates under these conditions because it is
produced at a greater rate than the 1,4-addition product. The
establishment of equilibrium is prevented because the time of the reaction is too
short.
-
Conversely, the addition of HBr and Br2 at higher temperatures will
produce more of the 1,4-addition product. Here the reaction is reversible because
there is an equilibrium established between the two products. The amount
of each product present at equilibrium is in direct relation to the
relative thermodynamic stability of that product. This stability is based on
the degree of substitution of the carbon bearing the positive charge. In
this case, the 1,4-addition product has a higher degree of substitution,
therefore making it the more stable product. However, if we
were to increase the time of the reaction, it would be possible to establish an equilibrium
between the two products.
Electrophilic addition energy diagram
In the energy diagram above, there is an example of electrophilic
addition. A resonance-stabilized allylic carbocation intermediate which reacts
with HCl forming two products. After the reaction proceeds, you can
see that the less stable product requires less energy for formation and is
formed much faster. The more stable product requires more energy and is
slower in formation.
Reactions that involve kinetic control are influenced by the rates of
formation, where thermodynamic control is influenced by the stability of
each product. Electrophilic addition reactions can be controlled by the
amount of time given for the reaction to occur. Under longer time
periods, the products that are formed slower can actually predominate in
the reaction. An equilibrium will eventually be reached where the more
stable product is present in larger amounts.
Keep in Mind:
Not all reactions are controlled by temperature and time, as we have seen in the previous
modules involving electrophillic addition with conjugated dienes.
Equilibrium can also be controlled by reaction conditions other than
temperature in time. The reaction mechanism used is also another factor to
consider in the establishment of equilibrium. In the next module, we will
take a look at these factors influencing the kinectic and thermodynamic control in the formation of
enolate anions.
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Last modified 6-18-98
This project was generously supported by the Natural Science Foundation.
This page was developed by Mary A. Dewan and Dr. Abby Parrill
Department of Chemistry
Michigan State University
These pages may be downloaded and linked freely for academic purposes.
Questions, problems, and errors should be sent to
parrill@cem.msu.edu.