Advanced Topic 866 of 1102 Organic Chemistry
Alpha Hydrogen Acidity
Hydrogens next to carbonyls are acidic because the enolate is resonance stabilized.
In-depth explanation of Alpha Hydrogen Acidity
Hydrogens on the carbon next to a carbonyl group, called alpha hydrogens, are weakly acidic because removing one forms an enolate in which the negative charge is delocalized onto the oxygen. For simple aldehydes and ketones the pKa is about 17 to 25. A 1,3-dicarbonyl compound has a much lower pKa of about 9 to 13, because its enolate is stabilized by two carbonyl groups. This acidity underlies the aldol, Claisen and alkylation reactions of carbonyl compounds, which all begin with enolate formation.
Going deeper: Alpha Hydrogen Acidity
Key ideas: Hydrogens on the carbon next to a C=O are acidic because the enolate is resonance stabilised: aldehyde and ketone pKa about 17 to 20, ester about 25, amide about 30, but between two carbonyls (1,3-dicarbonyls) only about 9 to 13. Deprotonation for full enolate needs LDA (conjugate acid pKa 36), reversible conditions need ethoxide. Enolates react as nucleophiles at carbon with alkyl halides and carbonyls.
Common mistakes and connections: Students confuse alpha hydrogens (next to carbonyl) with the carbonyl hydrogen of an aldehyde. Another error is using hydroxide for full enolate formation, which allows self-condensation. Links to aldol, Claisen, iodoform, acetoacetic ester synthesis and kinetic versus thermodynamic enolates.
Visualization of Alpha Hydrogen Acidity
Carbonyl groups acidify alpha C-H bonds because the enolate is resonance-stabilized; two carbonyls make hydrogen much more acidic. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Acetone has an alpha hydrogen pKa of about 19, so it can be deprotonated by strong bases to form an enolate.
Frequently asked questions about Alpha Hydrogen Acidity
What is Alpha Hydrogen Acidity?
Hydrogens next to carbonyls are acidic because the enolate is resonance stabilized.
Is Alpha Hydrogen Acidity a beginner, intermediate or advanced topic?
Alpha Hydrogen Acidity is a advanced topic in Organic Chemistry. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Alpha Hydrogen Acidity?
Study these earlier topics in Organic Chemistry first: Degree of Unsaturation, Acidity of Terminal Alkynes, Keto-Enol Tautomerism.
What comes after Alpha Hydrogen Acidity?
The next topics in Organic Chemistry are Baeyer's Reagent and Williamson vs Elimination.
Can I practise Alpha Hydrogen Acidity with quizzes?
Yes. The interactive lesson includes a self-check question on Alpha Hydrogen Acidity, and the Organic Chemistry quiz draws on every topic in this area.
Where Alpha Hydrogen Acidity fits in Organic Chemistry
Alpha Hydrogen Acidity is topic 138 of 148 in Organic Chemistry.