Aldehydes And Ketones – Addition

by James

The next part of the course is going to cover aldehydes and ketones.

Here’s the biggest tip I have about these molecules.

The most important mechanism for these molecules is “addition”. Sometimes I call it “1,2-addition”, although the “1,2” part isn’t so important.

Learn this, and you’re well on your way to understanding aldehyde and ketone chemistry.

Let’s walk through it.

Aldehydes and ketones contain the “carbonyl” group. That’s a carbon-oxygen double bond.

Where are the electrons in an aldehyde or ketone? Oxygen is more electronegative than carbon. So the carbon bears a partial positive charge, and oxygen bears a partial negative charge. Put another way, carbon is electrophilic.

If carbon is electrophilic, that means it’s going to react with nucleophiles.

Let’s look at the Grignard reaction for instance. Where are the electrons on a Grignard? Carbon is more electronegative than magnesium, so there’s a partial negative charge on the carbon! (Carbon is nucleophilic in a Grignard)

Trying to figure out what’s going on, we line up the negative charge on the Grignard carbon (nucleophilic) with the positive charge on the carbonyl carbon (electrophilic). Now we’re going to form and break our bonds through a reaction called “addition”.

Here’s how the addition works.
We form a bond between the nucleophile and the electrophile. This is going to exceed the legal bonds on carbon, so we break the C-O Pi bond, and move the pair of electrons to the oxygen.

carbonyls.1

That’s it! We’ve done the addition. Two arrows.  Now, the next step in the Grignard is to add acid. That’s a common next step because we’re often much more interested in a neutral product in the end (an alcohol) than a charged product.

Learn this mechanism well: carbonyl chemistry (aldehydes, ketones, esters, etc) is dominated by this one mechanism!

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