Alpha-Carbon Acidity
The C-H bonds on a carbon directly bonded to a carbonyl (the alpha-carbons) are anomalously acidic. Ordinary alkane C-H bonds have pKa around 50. But the alpha-C-H of a ketone has pKa around 20, and a 1,3-dicarbonyl alpha-H can have pKa around 10. That is 30-40 pKa units lower than a plain alkane - a factor of 10³⁰ in acidity.
Why? The conjugate base after deprotonation (the enolate) has its negative charge delocalized from carbon onto oxygen via resonance. The oxygen lone-pair contributor stabilizes the anion enormously, because charge on O is much lower-energy than charge on C.
pKa Values You Should Memorize
| Substrate | Alpha-H pKa | Notes |
|---|---|---|
| Alkane (R-CH₂-R’) | ~50 | Essentially not acidic |
| Nitrile (R-CH₂-CN) | 25 | One alpha-acidifying group |
| Ester (R-CH₂-COOR’) | 25 | One alpha-acidifying group |
| Ketone / aldehyde (R-CH₂-COR’) | 20 | One alpha-acidifying group, slightly more acidifying than ester |
| Nitro (R-CH₂-NO₂) | 10 | Very strong activator |
| 1,3-dicarbonyl (R-CH₂ between two C=O) | 10-13 | Two alpha-acidifying groups |
| Malonate diester | 13 | Two ester groups |
| Acetoacetate (ester + ketone) | 11 | One ester, one ketone |
| Cyanoacetate | 9 | CN + ester |
| Nitroalkane + carbonyl | 5-6 | Very acidifying |
These values match the ARIO framework from Chapter 4. Resonance stabilization onto carbonyl oxygens dominates - more carbonyls next door means more resonance delocalization means more acidic alpha-H.
Why Ketones Are More Acidic Than Esters
Ketone alpha-H pKa ≈ 20; ester alpha-H pKa ≈ 25. Why the difference?
The ester has an OR group attached to the carbonyl. This OR donates a lone pair by resonance, stabilizing the starting ester carbonyl and partially “using up” the carbonyl’s electrophilic character. When the alpha-H is removed, the resulting enolate has the negative charge delocalized onto the carbonyl oxygen - but the OR’s resonance donation has made this oxygen less eager to accept charge. The enolate is therefore less stable in an ester than in a ketone.
Net: ketone alpha-H is about 5 pKa units (100,000×) more acidic than ester alpha-H.
Choosing a Base for Deprotonation
The base must have a conjugate acid with pKa HIGHER than the target alpha-H pKa (Chapter 4.2 rule). Common base choices:
| Target pKa | Sufficient bases | Sufficient? |
|---|---|---|
| 10-13 (1,3-dicarbonyl) | NaOH (water pKa 15.7), NaOEt (ethanol pKa 16), K₂CO₃ (carbonic acid pKa 6.4, so not enough for pKa 10 - but its sodium salt is basic enough for some 1,3-dicarbonyls) | Yes for 10-13 |
| 20 (simple ketone) | NaH (H₂ pKa 36), LDA (HN(iPr)₂ pKa 36), n-BuLi (butane pKa 50) | Yes |
| 25 (ester) | NaH, LDA, n-BuLi | Yes |
Hydroxide and alkoxide are NOT strong enough to fully deprotonate a simple ketone (pKa 20). They would only generate a small equilibrium fraction of enolate (~,000). For clean enolate chemistry on simple ketones, you need NaH, LDA, or organolithium.
For 1,3-dicarbonyls (pKa 10-13), hydroxide and ethoxide work fine because the target pKa is well above the conjugate acid’s pKa.
Stereochemistry Alert
Deprotonation of an alpha-carbon can create a new stereocenter (the alpha-carbon becomes sp² in the enolate, then sp³ again after protonation or alkylation). Reprotonation from either face gives a racemic mixture. This is how racemization of alpha-chiral carbonyls can happen in base - the molecule interconverts enolate and ketone forms repeatedly, equilibrating the stereochemistry.
Alpha-Acidity Is Substrate-Dependent Within One Molecule
If a ketone has two different alpha-carbons (one with more substituted C, one less), both are acidic but to slightly different degrees. The more hindered one usually gives the thermodynamic enolate; the less hindered one gives the kinetic enolate (covered in Section 7.4).