Lecture #3

Lecture #3 - Electrophilic Attack on Conjugated Dienes:...

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Electrophilic Attack on Conjugated Dienes: 1,2- and 1,4-Addition Conjugated dienes display a more complicated mode of electrophilic addition than simple alkenes. Typically, more than one product is formed. CH 2 =CH-CH=CH 2 1,3-butadiene HCl 25 o C CH 3 CH-CH=CH 2 + CH 3 -CH=CH-CH 2 Cl Cl 3-chloro-1-butene 1-chloro-2-butene (78%) (22%) The formation of the above two products can be understood within the context of the usual mechanism for electrophilic addition to alkenes, by recognizing the more stable carbocation intermediate . CH 2 =CH-CH=CH 2 1 2 H + C 1 C 2 CH 2 -CH-CH=CH 2 H + H -CH 2 -CH-CH-CH 2 + X less stable primary carbocation more stable allylic-type cation 1
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Products from 1,2- and 1,4-Additions The products are explained by attack of the nucleophilic Cl - at the two carbons of the allylic-type cation that carry positive charge. H -CH 2 -CH-CH-CH 2 + Cl - CH 3 CH-CH=CH 2 Cl CH 3 -CH=CH-CH 2 Cl The Overall Two-Step Mechanism (1) CH 2 =CH-CH=CH 2 H + slow step electrophilic addition CH 3 CH-CH=CH 2 + CH 3 CH=CH-CH 2 + CH 3 -CH-CH-CH 2 ! + ! + allylic cation (2) CH 3 -CH-CH-CH 2 ! + ! + + :Cl: : - (a) (b) competing nucleophilic additions (a) (b) CH 3 CH-CH=CH 2 Cl CH 3 -CH=CH-CH 2 Cl 1,2-addition product 1,4-addition product 2
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Other Examples CH 2 =CH-CH=CH 2 Br 2 -15 o C BrCH 2 -CH-CH-CH 2 + Br - BrCH 2 CHCH=CH 2 Br (54%) BrCH 2 CH=CHCH 2 Br (46%) 1,2 1,4 CH 2 =CH-CH=CH 2 HBr 40 o C CH 3 -CH-CH-CH 2 + Br - CH 3 CH-CH=CH 2 (20 %) + CH 3 CH=CH-CH 2 Br (80%) Br 1,2 1,4 What factors determine the mixture of 1,2- and 1,4-addition products? 3
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Kinetic Control versus Thermodynamic Control The product mixture obtained from the reaction of butadiene and HBr depends on the temperature: CH 2 =CH-CH=CH 2 + HBr CH 3 CH-CH=CH 2 + CH 3 CH=CH-CH 2 Br Br (20 %) (80%) at 40 o C at -80 o C (20 %) (80%) There is a dramatic reversal of the product mixture when the temperature is changed from 40 o C to -80 o C . Another Key Observation While the two products are stable at -80 o C , at 40 o C the less stable 3-bromo-1-butene isomerizes to the more stable 1-bromo-2-butene. CH 3 CH-CH=CH 2 CH 3 CH=CH-CH 2 Br Br 3-bromo-1-butene 1-bromo-2-butene (less stable) (more stable) 40 o C 4
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Thermodynamic Control of Product Mixture At the higher temperature, the products are not stable . They revert back to the carbocation intermediate providing a pathway for interconversion of the two products. Under this condition, the two products equilibrate , and the product mixture reflects the relative stabilities of the two products. CH
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This note was uploaded on 01/23/2011 for the course CHEM 322BL taught by Professor Singer during the Spring '07 term at USC.

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Lecture #3 - Electrophilic Attack on Conjugated Dienes:...

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