15 - Chapter 15 Making Predictions Based on Mechanistic...

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Chapter 15 – Making Predictions Based on Mechanistic Reasoning: Reactions of alkynes In this chapter we will extend what we have learned to a different class of compounds; namely, the alkynes: compounds with carbon-carbon triple bonds. Once again we will reason through the chemistry by drawing analogies to the mechanisms of alkenes. The success of applying the concepts we have learned to a new class of compounds illustrates the power of the mechanism-based approach for the study of organic chemistry. The triple bond is an electron dense region and it should therefore not be surprising that it possesses nucleophilic character. Addition of Hydrogen Halides to Alkynes The addition of HBr to a triple bond follows an analogous mechanistic pathway as the alkene addition mechanism. The alkyne is nucleophilic, just like the alkene. When the alkyne reacts with electrophilic H-Br, the result is a carbocation intermediate. The intermediate is called a vinylic cation. The mechanism is completed by the attack of the nucleophilic bromide anion on the vinylic cation. CH 3 CH 2 CC H + H–Br CH 3 CH 2 CC Br a vinylic cation intermediate CH 3 CH 2 CC Br H H H H The vinylic cation possesses a positively charged carbon atom having only two electron pair domains (EPDs); the carbon bearing the positive charge is thus sp-hybridized. The addition of the electrophilic H + takes place so as to place the positive charge on the most highly substituted carbon, just as for alkenes. As before, the more highly substituted the vinylic cation, the greater its stability. This preference determines the regioselectivity of the addition. As a consequence, the bromide is located on the more highly substituted carbon (i.e., the internal carbon). CH 3 CH 2 CC H 2 CH 3 CH 2 CH CH more stable less stable
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Relative Stability of the Vinylic Cation How does the stability of the vinyl cation compare to alkyl carbocations? It turns out that the vinyl cation is much less stable (i.e., much more reactive) than the corresponding alkyl cation. Let’s compare a primary alkyl cation to a vinylic cation. This is a valid comparison since the positively charged carbon atoms in both structures are attached to one other carbon atom. The primary alkyl cation has a three-EPD, positively charged carbon atom and thus is sp 2 hybridized, whereas the primary vinylic cation has a two-EPD, positively charged carbon atom and is, therefore, sp hybridized. RC H H RCH C–H primary alkyl cation primaryv iny l iccat ion 3-EPDs (sp 2 hybridized) 2-EPDs (sp hybridized) Calculations as well as experimental data show that the primary alkyl cation is considerably more stable than the primary vinylic cation. Chemists have come to know that
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This note was uploaded on 10/02/2008 for the course CHEM 232 taught by Professor Vanderdonk during the Fall '08 term at University of Illinois at Urbana–Champaign.

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15 - Chapter 15 Making Predictions Based on Mechanistic...

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