Lesson29 - Substitution at the Carbonyl Carbon (Acyl...

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Substitution at the Carbonyl Carbon (Acyl Substitution) 1) Acid-Base - hydrogen swap 2 & 3) Oxidation & Reduction 4) Substitution - replace C’s substituent (-X) with another (-Y), neither being -H X C Y C Y + X + neither X nor Y being hydrogen Y replaces X 5) Elimination - loss of XY elements with concomitant pi bond formation 6) Addition - gain of XY elements with concomitant loss of pi bond reduction X C H C oxidation H replaces X X replaces H change in the number of C-H bonds in relation to the number of C-X bonds X C Z Z Y C + X Y elimination addition form π bond -XY break π bond +XY base H X H Y Y + X + base acid acid 7) Rearrangement - isomerization process (no atoms lost or gained); results in new bonding connectivity (one of many examples shown as there is no generic representation). R CH 3 R R stands for a generic "residue" Z = C, N, O We previously studied substitution reactions at sp 3 carbon and concluded that they proceed either by the [S N 1] or the [S N 2] pathway. Here we examine substitution at the sp 2 carbonyl carbon ( nucleophilic acyl substitution reactions ). We’ll see that substitution at sp 2 atoms generally proceed by pathways much different than those at sp 3 atoms.
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Nucleophilic Acyl Substitution: Three Examples CH 3 O CH 3 C O Cl + CH 3 C O OCH 3 + Cl OH + + OH C O CH 3 OCH 3 C O CH 3 esterification CH 3 OH H H CH 3 NH CH 3 O CH 3 O O O CH 3 O + CH 3 NHCH 3 O CH 3 NH 2 CH 3 NH 2 H + H
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Acyl Substitution Requires a Variation of the [E β ] Step: Beta Elimination Across the C-Z: Bond Empty σ * Filled n [E β ] involves π -bond making σ -bond breaking Curved Arrows Imply the Frontier Orbitals Curved arrows indicate the HOMO-LUMO pair (Frontier Orbitals) involved in this elementary step. The tail of the first arrow implies that the filled orbital (HOMO) is a nonbonded electron pair, n . The head of the first arrow points between C-Z: suggesting a new π - bond. An empty, low-lying σ *-orbital from the adjacent C-L bond is available to accommodate this electron pair suggesting the
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This note was uploaded on 01/25/2012 for the course CHEM 232 taught by Professor Miller during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lesson29 - Substitution at the Carbonyl Carbon (Acyl...

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