Chemistry 125: Lecture 65 April 4, 2011 Addition to C=O Mechanism & Equilibrium Protecting Groups Imines This For copyright notice see final page of this file

Basic Carbonyl Reactivity R R C O H 1) Addition to C=O (initial nucleophile attack) 2) Addition to C=O (acid catalysis) 5) Nucleophilic substitution of acid derivatives 3) Allylic rearrangement to enol (acid-catalyzed or via enolate) 4)  -Substitution A Nu R2CR2C L A H

RCOOH Reactions (sec. 20.7, pp ) OH RC O H substitution at  -C substitution at C R substitution at O R addition A Nu

RC O OH Fischer Esterification (sec. 20.8, p. 965) H + H + + RO H + Tetrahedral Intermediate (A/D, not the pentavalent transition state of S N 2) substitution at C OR starts with addition to C=O H + + H OH Why protonate this oxygen instead of this one? and is stabilized still more after protonation on top because this pair was stabilized by  * (acid catalyzed) analogous to nucleophilic aromatic substitution RNH 2 F NO 2 +

Victor Meyer 9/8/48 - 8/8/97 “Geliebte Frau! Geliebte Kinder! Lebt wohl! Meine Nerven sind zerstört; ich kann nicht mehr.” Victor Meyer ( ) introduced the idea of Steric Hindrance (1894) “…the source of this behavior is stereochemical…the space filling of the neighboring groups” van’t Hoff (1874) Sachse (1891) Configuration Conformation Baeyer (1885) Strain

Tetrahedral Intermediate Fischer Esterification Didn’t Work with 2,6-Dimethylbenzoic Acid STRAIN! Carboxylic Acid

Tetrahedral Intermediate (A/D) R Linear Acylium Intermediate (D/A, like S N 1) OH R C O substitution at C Fischer Esterification (sec. 20.8, p. 965) H + H + O + OR H + 100% H 2 SO 4 Pour into ROH Melvin Newman’s Method (1941) (attaching second H + inhibits reversal) H+H+ + O H H H+H+ + sometimes

Hydration (acid or base) (e.g. J&F Sec ) imine2amino acid

Acid-Catalyzed Hydration of Formaldehyde + H HO CH 2 + H HOH H-O-CH 2 + O=CH 2 HOH HO CH 2 (Hydrate, gem-diol) + H Base-Catalyzed Hydration of Formaldehyde O=CH 2 HO - HO -O-O CH 2 + HO - HO CH 2 HOH H-O-HH-O-H +

2 × × × bond strength sp 2 -H < sp 2 -C (destabilize CH 2 O) 1 H2OH2O + electron donation sp 3 > sp 2 (stabilize hydrate) 37 2 = 1369 conjugation stabilizes aldehyde smallness stabilizes hydrate Hydration Equilibrium Constant K

Acetal Formation (acid only) (e.g. J&F Sec. 16.9) cyclic

Acid-Catalyzed Hemiketal from Formaldehyde + H HO RO CH 2 + H ROH H-O-CH 2 + O=CH 2 HOR RO HO CH 2 (Hemiketal) + H Base-Catalyzed Hemiketal from Formaldehyde O=CH 2 RO - RO -O-O CH 2 + RO - RO HO CH 2 R-O-HR-O-H + can attack either O reverse or continue + H R-O-CH 2 + Ether inert to base, can only reverse! RO CH 2 ROH - H 2 O - H + no way to get to ketal or to attack ketal (Ketal)

Protecting Groups (& deprotection) (e.g. J&F Sec ) protect C=O as ketal

Protecting Groups (& deprotection) (e.g. J&F Sec ) protect C=O as ketal protect ROH as THP or TBDMS derivative

Imines (e.g. J&F Sec )

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