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Reactions of Alkenes and Alkynes- Orgo.docx

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Chemistry 1027A/B
Mel Usselman

Reactions of Alkenes and AlkynesMultiple bonds are strong and reactive o Because pi bond will attract electronseeking reagents o Has electrons looking to do something elseAlkenes are stable but have electrons that are relatively unexposedAll alkenes are very stable molecules and very reactiveCharacteristic reaction if all alkenes and alkynes is an addition reaction in which AB species reacts to break the pi bond and from two new sigma bondsElectrophilic Addition Reactions o In an exothermic reaction there can be two possible productsTwo structural isomers are possible as addition products o When an alkene reaction generates 2 constitutional isomers in the same reaction we say the reagent refers to both in certain regionsThe constitutional isomers formed by reactions are referred to as regioisomers When one regioisomer formed in larger amountreaction called regioselective o Mechanism of electrophilic reactionStep 1 an electrophilic stepTwo electrons of the pi bond attract the plus to itUse arrows to drawThe pi bond takes the positive A breaking bond between AB and leaving an extra electron on B making it BFirst step is transformation of a stable molecule is an electrophilic attack of the positive moleculeCarbon atom with a positive charge called carbocationStep 2 a nucleophilic stepVery fast reactionB species termed nucleophileExothermic and fastEnergy Diagram for Electrophilic Addition o An energy diagram for a general two step reaction o Reaction goes from left to right o When you heat stuff up you give them more energy and a greater chance for molecules to bump into each other o Carbocation is still much less stable than the neutral material o Making an unstable thing more stable o Intermediate is where the unstable carbocation is o When going over the second transition state remember you are not starting from the bottomTherefore there is easily enough energy to climb that small hill o The reaction is potentially reversible but C and D have to produce and greater amount of energy
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