Alpha Hydrogens

For compounds containing a carbonyl group, Greek letters are used to describe the proximity of each carbon atom to the carbonyl.

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The carbonyl itself does not receive a Greek letter. In this example, there are two carbon atoms designated as alpha ($$\alpha$$) positions. Hydrogen atoms are designated with the Greek letter of the carbon to which they are attached; for example, the hydrogen atoms (protons) connected to the $$\alpha$$ carbon atoms are called $$\alpha$$ protons.

The alpha ($$\alpha$$) hydrogens of carbonyl compounds are unusually acidic. Do you know why? The pKa ranges from 16-20 for aldehydes and ketones and is a bit higher (24-ish) for esters. (Compare this to pKa=60 for an ordinary $$sp^3$$ hybridized C-H bond.)

Well, acidity is related to the stability of an acid's conjugate base. So, if we consider the conjugate base, maybe that will help.

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You may notice that the conjugate base has a lone pair next to a pi bond, which is a pattern of resonance you may recognize. The presence of resonance makes this base more stable, and therefore, the C-H bond of an $$\alpha$$ carbon more acidic.

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Next, read through section 13.1 and try the problems within. You want to be able to explain why $$\alpha$$-H's next to esters are less acidic than those next to aldehydes and ketones.

There are a couple questions to practice with in Canvas!