Chemistry 211
Fall 2005
- Equilibrium Controlled Reactions:
Class Group Activity #15: Carbonyl Reactions
-
2
- Original Reactions:
Thus far in our analysis of the Reactions
of
Carbonyl Compounds, we have found three types of reactions that
appeared
to occur at the carbonyl carbon &/or oxygen. In all reaction types,
the Carbonyl seemed to act as an electrophile.
- Addition Reactions.
e.g. Rxns a., b. & d.
- Substitution for the carbonyl oxygen.
e.g. Rxns. c. & j.
- Substitution for an a-atom
group.
e.g. f., h., & i.
f.
- Further Examples:
- Addition Reactions.
- Substitution for Carbonyl Oxygens.
- Substitution for the alpha-atom group.
- Exploration:
- Consider the 15 examples above.
- Recall our earlier discussions of the
structure
of the carbonyl
group, how it seems to behave in these reactions and how each of the
individual
bonds might be made or broken. Note also, that we found that the
alpha-atom, when
it is involved, acts as either a leaving group or loses a proton and
attaches
to a carbonyl carbon atom. With this in mind, try to write
reaction
mechanisms (sequences of simple reaction steps) for the three addition
reactions reproduced below. These reactions are equilibrium
reaction
(fast in both the forward and reverse directions). So, it is
important
that energies
of the highest energy electrons in ALL intermediate products
in your mechanisms be kept a low as
possible. One high energy intermediate
will essentially stop the progress of the reaction along that path.
- Identify the highest energy e-'s
at the
beginning of the reaction. Don't forget to consider the solvent
and catalyst. (structures over and under the reaction arrows)
- Consider the bonds that need to be made and
broken to convert
the reactants to the products.
- Start from the highest energy e-'s and
consider how
they can be used to make or break one or more of the bonds that move
the reactants toward the products. Draw the arrows that indicate
the bond changes and then draw the intermediate structures that result
from your arrows. If there seem to be more than one
possibility, draw arrows and structures for all possibilities. [NOTE: A reaction step may include no more
than two different structures. The probability of properly
aligning more than two separate structures is so small that such
"ter-reactant" processes are unlikely.]
- Identify the highest energy e-'s in
your intermediate
structures. If you are evaluating more than one
possibility, identify the highest energy e-'s in each
set of intermediate structures. Compare the energies of each
group
of highest energy e-'s to that of the highest energy e-'s
at the beginning of the reaction. Any set of intermediates is
acceptable if the highest energy e-'s are no more than 5 pK
units higher in energy than those at the beginning of the
reaction. If
there is more than one acceptable intermediate, choose the one with the
lowest energy highest energy e-'s.
- Start from the highest energy e-'s in
the chosen
intermediate and consider how they can
be used to make or break one or more of the bonds that move the
intermediate toward the products. Draw the arrows that indicate
the bond
changes and then draw the intermediate structures that result from your
arrows. If there seem to be more than one possibility, draw
arrows
and structures for all possibilities.
- Continue steps iv. & v. until you reach the
product
structures.
Use the analysis sequence provided for
reaction d. to devise a
mechanism for reactions k. and a. See Activity
14 for the analysis of bonds made and broken in reaction a.
- Are there any similarities in your three
mechanisms?
Class Group Activity # 15 - Out
of Class Applications:
Class Group Activity # 15 - Summary
of Class Discussion:
