Ketones on Acid

by Kiley Lynch

Acids are like an aphrodisiac for carbonyl compounds: it makes them more likely to react with nucleophiles.

Let me explain.
I said the last two days that carbonyl carbons are important electrophiles: they bear a partial positive charge. 

Now, I’m going to show how you can make them even more electrophilic – more reactive. This means that reactions that normally wouldn’t happen, will now happen.

First, a question: How do we make electrophiles more electrophilic?

Simple: we take electrons away from them!

How can we take electrons away? With carbonyls, the answer might be a bit counterintuitive. We’re going to add acid to the oxygen, and this will make the carbon more electron-poor.

Sounds weird, but it actually makes sense when you think about it.

Think about the resonance forms of the carbonyl:

  • its most stable resonance form has a carbon-oxygen double bond (neutral) and the less stable resonance form has a positive charge on carbon and a negative charge on oxygen.
  • So the “resonance hybrid” has a small partial positive charge on carbon, because of that resonance form on the right.

Now let’s add acid – say, H+ .

  • The oxygen will go from “owning” a pair of electrons, to “sharing” it with the hydrogen.
  • So it formally “loses” an electron to give a positively charged oxygen. (Watch out though: remember that “formal charge” doesn’t tell us about electron densities: electronegativity does. So even though there’s a “formal charge” of +1 on the oxygen, it’s still electron-rich compared to hydrogen and carbon)

With me so far? If this isn’t clear, write me! If everything is OK, let’s keep going.

Think about what this does to the resonance forms.

  • Now, both resonance forms have a charge of +1 . This means that the right-hand resonance form (with a positive charge on carbon), although still less stable than the resonance form on the left (where everything has a full octet)  will be more significant than before.
  • And this means it will make a greater contribution to the resonance hybrid, which means that the carbon will have a greater partial positive charge. 

That means that the carbon will be more electrophilic – and therefore, react faster with nucleophiles!

So why is this important? It’s important because it allows certain reactions to happen that would never proceed otherwise. 

When will it be important? Really soon! It’s important for the formation of acetals, which is coming right up. It will also be KEY for the next few chapters involving carbonyl chemistry.

Tomorrow: let’s talk about the SECOND-most important mechanism step for carbonyls.
Thanks for reading! James