Question

What class of organic compounds results when cyclopentanone reacts with ethylamine in the presence of trace acid?

Answer 1

This is the conversion of a ketone to an imine. An imine is basically an unsaturated amine.

THE REACTION MECHANISM

1. The amine must nucleophilically backside-attack the ketone.
2. A proton is transferred from a suitable acid.
3. Proton transfer pt2.
4. Proton transfer pt3.
5. Finally, the `pi`-compatible electrons can conjugate towards the electrophilic carbon to boot off the water molecule in a tetrahedral collapse.
6. Lastly, the product precursor is deprotonated to form the imine.

pH CONSIDERATIONS

You can tell that on steps 2-4, several sequential proton transfers are required for the mechanism to be successful. If there is too little acid, the tetrahedral intermediate cannot be sufficiently protonated.

On the other hand, if there is too much acid, the amine will be deactivated, i.e. if the amine is protonated, then the mechanism cannot even start!

Thus, it is crucial that the solution be acidic enough, but not too acidic.

WHAT DOES "TRACE ACID" IMPLY?

When we consider what "trace acid" actually implies, we can consider the pH-rate profile below for the reaction of acetone with hydroxylamine.

We see the maximum rate constant is observed near pH `4.5`. Thus, a pH of about `4.5` is indeed generally optimal.

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CHALLENGE: Can you figure out the mechanism for your compounds?