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Lesson24 - Electrophilic Addition Reactions Adding XY...

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Electrophilic Addition Reactions - Adding XY Across Unsaturated Carbon-Carbon Bonds 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 This page summarizes the main classes of chemical transformations. Having studied the process of elimination, we are now set to examine the reverse reaction - addition of the elements “XY” across carbon-carbon multiple bonds.
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Examples of Addition Reactions to Carbon-Carbon Double Bonds CHCH 3 H 2 C + H 2 O H H H 2 C H C CH 3 OH Br Br CH 2 Cl 2 + Br Br a non-nucleophilic solvent (i.e., inert) cyclopentene H H a vicinal dibromide (or vic -dibromide for short) Cl Cl H 2 O + OH Cl a nucleophilic solvent participates in the reaction cyclopentene H H + HCl a chlorohydrin CH 3 + H–Br CH 3 H H Br H 2 1 2 1
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Addition Mechanisms Require a New Elementary Step: Association of an Electrophile with a π -bond [A E ] Empty a Filled π [A E ] involves π -bond breaking and σ -bond making 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 Curved Arrows Imply the Frontier Orbitals Curved arrows indicate the HOMO-LUMO pair (Frontier Orbitals) involved in an elementary step. For [A E ] the tail of the arrow implies the filled orbital (HOMO) is an electron pair in a pi-bond. The head of the arrow points between one carbon atom of the π - bond and the E + suggesting that an atom-centered empty orbital (LUMO, a ) accepts the electron pair with σ -type orbital interaction (note : the HOMO for [A E ] will always be π but the LUMO will vary according to the specific E + involved). [A E ] This symbol means an electrophile association with a π -bond [A E ] involves a π a σ -type interaction E + is the electrophile
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The Electrophilic Pathways for C=C Addition (two variations of [Ad E 2]) When E + has no lone pair (e.g., H + ) When :E + has a lone pair (e.g., :Br + ) Two-step mechanism: [Ad E 2] = [A E ] then [A N ] Two step mechanism: [Ad E 2] = [A E ] then [S N 2]
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