Lec 10-12

Lec 10-12 - Addition Reactions of Alkenes" Chapter 4...

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Addition Reactions of Alkenes Chapter 4 (pp. 147–172) Chapter 5 Chapter 7 (pp. 298–314)
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General Reactivity of C=C Electrons in π bond are loosely held. Electrophiles are attracted to the π electrons. Carbocation intermediate may form. Nucleophile then adds to the carbocation. Net result is addition to the double bond. C C R R Nu = + E ! + X ! C C R R X E R R R R
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Regioselectivity 1-bromo-2-methyl propane (not observed) (CH 3 ) 2 C-CH 2 Br H 2-bromo-2-methyl- propane (CH 3 ) 2 C-CH 2 H Br Reactions in competition: compete for the same starting materials carbocation formed in preference (exclusively) determines the regioselectivity of HBr addition
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Carbocations Carbocation: a species in which a carbon atom has only six electrons in its valence shell and bears positive charge Carbocations are classifed as 1°, 2°, or 3° (or methyl) depending on the number oF carbons bonded to the carbon bearing the positive charge Electrophiles (Lewis acids); “electron-loving” H 3 C C CH 3 CH 3 H C CH 3 CH 3 H C CH 3 H H C H H Methyl
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Carbocation Stability relative stability methyl and primary carbocations are so unstable that they are never observed in solution carbocations are stabilized by alkyl substituents 2 ways: (1) inductive effect : donation of electron density along the sigma bonds. (2) hyperconjugation ( no-bond resonance ): overlap of sigma bonding orbitals with empty p orbital. Methyl cation (methyl) Ethyl cation (1°) Isopropyl cation (2°) tert -Butyl cation (3°) Increasing carbocation stability + + + + C H H CH 3 C CH 3 CH 3 H C CH 3 CH 3 CH 3 C H H H
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The Inductive Effect the positively charged carbon polarizes electrons of adjacent sigma bonds toward it the positive charge on the cation is thus localized over nearby atoms the larger the volume over which the positive charge is delocalized, the greater the stability of the cation H 3 C C + CH 3 CH 3 C C C C ! + ! + ! + ! + H H H H H H H H H ! + ! + ! + ! + ! + ! + ! + ! + ! + H C + H H H C H H ! + ! + ! + ! +
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Hyperconjugation involves partial overlap of the σ -bonding orbital of an adjacent C-H or C-C bond with the vacant 2 p orbital of the cationic carbon the result is delocalization of the positive charge R R H H H R R H + H H
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Reaction Free-Energy Diagram Δ G ˚ : minimum energy required to reach the transition state Reaction free-energy diagram: a graph showing the changes in energy that occur during a chemical reaction Reaction coordinate: a measure in the change in positions of atoms during a reaction
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Activation Energy Standard free energy of activation, Δ G ˚ : minimum kinetic energy the molecules must have to overcome the repulsions between their electron clouds when they collide if Δ G ˚ is large ( E a > 20 kcal/mol), few collisions occur with sufFcient energy to lead to a product; reaction is slow if Δ G ˚ is small ( E a 20 kcal/mol), many collisions occur with sufFcient energy to lead to a product; reaction is fast
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Energy Barrier Negative exponential relationship between rate and Δ G ˚
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Lec 10-12 - Addition Reactions of Alkenes" Chapter 4...

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