Posted by: Mark Foreman | February 1, 2013

Carbonyl chemistry

Dear Reader,

The good news is that organic chemistry has only a few reactions in it, each reaction tends to appear again and again in different clothing. I estimate that a student can understand the first year course at Chalmers with only about seven organic reactions.

These reactions are

  1. Nucleophilic attack on a carbonyl
  2. SN2
  3. SN1
  4. E1
  5. E2
  6. Aromatic substitution
  7. Carbocation rearrangement

We will start with the carbonyl group, it is important to understand that carbonyl chemistry has a central role in our world. The following topics relate to it.

  1. Protein chemistry
  2. Soap production
  3. Biodiesel (FAME)
  4. Plastics (Polyester, polylactic acid and others)

Now the first thing we need to get under control is the synthesis of esters from carboxylic acids and alcohols, this occurs by the Fisher ester synthesis mechanism, by the same mechanism esters can be turned into carboxylic acids (or carboxylic acid salts). We will consider this chemistry using a good friend of mine (Isosaccharinic acid, ISA).

ISA can exist in an open chain form and in the form of a cyclic ester (lactone), now we should start by thinking how does it change from one form to the other.

ISA1

Now the first stage in the ester synthesis is to protonate the carboxylic acid group, today a student asked why should it be one of the oxygens or the other ?

If we protonate the oxygen bearing the H atom in the carboxylic acid, then the positive charge will be localised on one oxygen only. While the OH group has now been converted into a good leaving group, the attack on the carbonyl group has not been strongly favoured.

ISA2

If on the other hand we protonate the other oxygen in the carboxylic acid, then we can delocalise the charge by resonance. In general the more atoms a charge can be spread over by resonance the lower the energy of an ion. So for electronic effects we will have the following cationic species form.

Now what happens next is that a lone pair of electrons on the oxygen of one of the alcohol groups will attack the carbonyl carbon.

ISA4

Next the proton on the oxygen which just did this attack is transferred via the solvent to one of the new OH groups at the top of the molecule.

ISA5

Then to finish it off the electron pair in the O-H sigma bond is used to form the pi bond in the carbonyl between the oxygen and the carbon. We also get back our proton at this stage.

ISA6

I sincerely hope that you can see a beauty in this chemistry, while things in popular culture are well known for being appealing to the senses in a way that is often easy for many people to access. I would say that once you can start to see the patterns in chemistry that many pretty things will start to appear before your eyes.

If we consider another reaction in organic chemistry we will see soon how it is in many ways the same reaction dressed up differently. Next we will consider the reduction of a carbonyl using a hydride reducing agent such as sodium borohydride. I will consider both the right arrows and the wrong ones.

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