Lesson19 - Elimination Across sp3 Carbons Next we examine a...

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1) Acid-Base - hydrogen swap Elimination Across sp 3 Carbons Next we examine a class of elimination reactions to produce carbon-carbon multiple bonds. We recently studied substitution reactions at sp 3 carbon atoms. Elimination and substitution are competing reactions; some mechanisms of elimination and substitution have common intermediates . 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
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Elimination Across Carbon-Carbon Bonds: Three Examples CH 3 C Br CH 3 CH 3 + Br C C H H CH 3 CH 3 H O H H + H 2 O, Δ C C H H H H Br CH 2 CH 3 HOH + + C C H H H CH 2 CH 3 KOH KBr O OH O KOH HOH +
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Elimination Mechanisms Require New Elementary Steps: Electrophile Dissociation From an Adjacent Carbocation Empty a Filled σ [D E ] involves σ -bond breaking, π -bond making Curved Arrows Imply the Frontier Orbitals Curved arrows indicate the HOMO-LUMO pair (Frontier Orbitals) involved in the elementary step. The tail of the arrow implies the filled orbital (HOMO) is an electron pair in a σ -bond. The head of the arrow points between two carbon atoms and suggests a new π -bond. An empty orbital on the carbocation is available to accommodate this electron pair implying that the LUMO is an atom-centered, empty orbital ( a ). The HOMO ( σ -orbital) is beside (and nearly parallel to) the LUMO ( a ) allowing for a π -type interaction. [D E ] This symbol means dissociation of an electrophile [D E ] is a σ a π -type interaction -E is the electrophile (-E is often an -H, hydrogen) C C E E + C C σ n π σ ± a π ± σ→σ ± σ→ a σ→π ± filled empty n →σ ± n a n →π ± π→σ ± π→ a π→π ±
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Elimination Mechanisms Require New Elementary Steps: Beta Elimination [E β ] 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.
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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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Lesson19 - Elimination Across sp3 Carbons Next we examine a...

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