tailieunhanh - Lecture Organic chemistry: Chapter 22 - L. G. Wade, Jr.

Chapter 22 - Condensations and alpha substitutions of carbonyl compounds. In this chapter, we consider two more types of reactions: substitution at the carbon atom next to the carbonyl group (called alpha substitution) and carbonyl condensations. Carbonyl condensations are among the most common biological methods for building up and breaking down large molecules. | Chapter 22 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Condensations and Alpha Substitutions of Carbonyl Compounds Chapter 22 Alpha Substitution Alpha substitution is the substitution of one of the hydrogens attached to the alpha-carbon for an electrophile. The reaction occurs through an enolate ion intermediate. Chapter 22 Condensation with an Aldehyde or Ketone The enolate ion attacks the carbonyl group to form an alkoxide. Protonation of the alkoxide gives the addition product: a b-hydroxy carbonyl compound. Chapter 22 Condensation with Esters The enolate adds to the ester to form a tetrahedral intermediate. Elimination of the leaving group (alkoxide) gives the substitution product (a b-carbonyl compound). Chapter 22 Keto–Enol Tautomers Tautomerization is an interconversion of isomers that occur through the migration of a proton and the movement of a double bond. Tautomers are not resonance form. . | Chapter 22 Copyright © 2010 Pearson Education, Inc. Organic Chemistry, 7th Edition L. G. Wade, Jr. Condensations and Alpha Substitutions of Carbonyl Compounds Chapter 22 Alpha Substitution Alpha substitution is the substitution of one of the hydrogens attached to the alpha-carbon for an electrophile. The reaction occurs through an enolate ion intermediate. Chapter 22 Condensation with an Aldehyde or Ketone The enolate ion attacks the carbonyl group to form an alkoxide. Protonation of the alkoxide gives the addition product: a b-hydroxy carbonyl compound. Chapter 22 Condensation with Esters The enolate adds to the ester to form a tetrahedral intermediate. Elimination of the leaving group (alkoxide) gives the substitution product (a b-carbonyl compound). Chapter 22 Keto–Enol Tautomers Tautomerization is an interconversion of isomers that occur through the migration of a proton and the movement of a double bond. Tautomers are not resonance form. Chapter 22 Base–Catalyzed Tautomerism In the presence of strong bases, ketones and aldehydes act as weak proton acids. A proton on the a carbon is abstracted to form a resonance-stabilized enolate ion with the negative charge spread over a carbon atom and an oxygen atom. The equilibrium favors the keto form over the enolate ion. Chapter 22 Acid-Catalyzed Tautomerism In acid, a proton is moved from the a-carbon to oxygen by first protonating oxygen and then removing a proton from the carbon. Chapter 22 Racemization For aldehydes and ketones, the keto form is greatly favored at equilibrium. If a chiral carbon has an enolizable hydrogen atom, a trace of acid or base allows that carbon to invert its configuration, with the enol serving as the intermediate. This is called racemization. Chapter 22 Acidity of Hydrogens pKa for H of aldehyde or ketone ~20. Much more acidic than alkane or alkene (pKa > 40) or alkyne (pKa = 25). Less acidic than water (pKa = ) or .

• Hoá học
• Organic chemistry
• Lecture Organic chemistry
• Hóa học hữu cơ
• Electronic Structure
• Carbonyl compounds
• Alpha substitutions