6.8 Exceptions to the Rule

6.8 Exceptions to the Rule

• The golden rule that we saw in the beginning of this chapter helped us understand most of the chemistry that we explored.
• If you can, re-form the carbonyl, but never expel H- or C-.

• There are a few rare exceptions to this rule.
• We will look at two exceptions in this section.

• The Cannizzaro reaction has very little synthetic utility.
• It is not likely that you will use this reaction more than once.
• Look in your textbook and lecture notes for that reaction.
• You can ignore it if you don't need to know about it.
• If you are responsible for knowing that reaction, you should look at the mechanism in your textbook.
• H- is not really expelled by itself if you focus on the step where it gets expelled.
• It is transferred from one place to another.
• We can understand it a bit better.
• H- is too unstable to ever leave as a group.
• That's why we don't expel H- into solution.
• In the Cannizzaro reaction, it never leaves as a group.

• One other exception to the golden rule will be explored.
• It seems like we are re-forming a carbonyl group to expel C-.
• This reaction is very useful.
• You might use it many times to solve synthesis problems if you know how to use it correctly.
• We will cover that reaction now.

• RCO H is used in the Baeyer-Villiger reaction.

• When you see the letters MCPBA, you should know that we are talking about a peroxy acid.

• This time, the result is to put an oxygen atom next to the carbonyl group.
• Let's see the accepted mechanism for this process.

• The first three steps of acetal formation are very similar to these three steps.

• The conjugate base of H-A is deprotonated to give an un charged, tetrahedral intermediate.

• This probably happens, but it doesn't lead to a new product.
• The regenerating materials are the starting materials.
• We apply our golden rule to see if we can expel any other leaving group.
• There are no other groups that can expel H- or C-.
• This appears to be an exception to the golden rule.
• Don't worry about trying to apply this next step in any other mechanism, because something unique happens and you won't see it in any other mechanism.

• The fate of the alkyl group is highlighted above.
• The carbonyl group is expelling the R group, which migrates to the nearby oxygen atom.
• It looks like we are expelling C-.
• C- is too unstable.
• We can have an exception here because it never becomes C- for any period of time.

• This is the first time you will see a rearrangement that doesn't involve a carbocation.
• You don't need to apply this mechanism to other situations.
• Don't focus too much on this mechanism for now.
• If you can use this reaction to solve synthesis problems, it will be a very useful reaction for you to have in your back pocket.

• You will need to know how to predict where the oxygen atom is installed in order to use this reaction properly.

• We have to decide where the oxygen atom will be installed during the Baeyer-villiger reaction because this ketone is unsymmetrical.

• We need to know which alkyl group is more likely to migrate.
• The R group is connected to the oxygen atom in the product by the mechanism above.
• We have to decide which group can migrate faster.

• The phenyl groups have the same migratory tendencies as the secondary alkyl groups.
• H migrates the fastest.

• It doesn't matter what the R group is because H migrates faster than any other group.

• If you are starting with a ketone instead of an aldehyde, you should look for the most substituted alkyl group.

• The oxygen atom is on the other side.

• In order to use this reaction in synthesis, you need to know where the oxygen atom will be inserted.

• A peroxy acid is being used to treat a ketone.
• We can see that the starting ketone is notsymmetrical.
• Predicting where the oxygen atom will be inserted is important.
• We can see that the left side is being used more.
• The oxygen atom will be inserted where the R group migrates faster.