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Topic 3 Summary: "Reactions Of Alkenes & Alkynes"

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Chemistry 1027A/B
Mel Usselman

Organic Chemistry Key Concepts: Reactions Of Alkenes & Alkynes (Usselman Version) -Multiple bonds are both strong and reactive because the π bond will attract electron-seeking reagents. The characteristic reaction of all alkenes and alkynes is an addition reaction. Understanding addition reactions requires looking at the sequence of steps, the mechanism, from reagents to products. -Recognize that two structural isomers are possible as addition products. The constitutional isomers (2 products) formed by reactions are referred to as regioisomers. When one regioisomers (either isomer) is formed in greater amount, the reaction is said to be regioselective. All additions of polar AB species to π bonds proceed by a common sequence of steps known as an electrophilic addition reaction. -Species A is termed an electrophile. The electrophilic step (1 step) results in the formation of a carbon atom with a +ve charge, called a carbocation. This step is endothermic and therefore is the slower, rate- - nd determining step of the mechanism. Species B is termed a nucleophile. The nucleophilic step (2 step) results in the formation of a C-B σ bond. This step is exothermic and fast. -Notice that the E far the first step (E ) a1presents the largest energy barrier on the energy curve (rate- determining). The second step proceeds rapidly once the carbocation has been formed. The addition reaction is exothermic. The overall shape of the energy diagram for this mechanism is similar for all polar electrophilic addition reactions to alkenes. -The rate of the addition reaction will increase as the stability of the intermediate carbocation increases. Of the two carbocations that can be formed, the more stable one will form in greater amount. The regioisomer formed in greater amount will be the one in which the nucleophile ends up bonded to the C atom which is the more stable carbocation. -Groups that can donate electron density to carbocations will increase its stability. Resonance stabilization and inductive stabilization are to way to increase the stability of carbocations. In terms of inductive stabilization, the more alkyl groups (sp ) there are on a C , the more stable it is. -In this course you only need to predict the major product(s) of a reaction, not the actual % yield. It is possible to predict the relative rates of addition reactions by comparing relative stabilities of intermediate carbocations. *Electron donation by resonance greatly outweighs electron withdrawal by induction. -In the addition of hydrogen halides reaction, the non-reactive part of the molecule remains unchanged. In the addition of water reaction, a small amount of acid is required. Also, the second step of the mechanism must use H O as 2he nucleophile. After H O has for2ed a new σ bond to C, the O atom acquires a formal charge of +1 (and is then called an oxonium ion
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