carey_6 - Chapter 6 Addition reactions to alkenes Well for...

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Chapter 6 Addition reactions to alkenes Main points of the chapter Well, for this chapter I won’t write up a list because I would just duplicate the chapter sections. It is all mechanism, all the time and it is all important. But we can make one useful categorization. Sections 6.4-6.17 are all electrophilic addition. Section 6.1-6.3 is hydrogenation. Sections 6.18-6.19 are oxidations. I separate these because all of the electrophilic additions behave in the same basic manner. The differences are in the nature of the electrophile. The commonalities are in the formation of stable carbocations which governs Markovnikoff’s rule which determines regiochemistry. Don’t be fooled by the different product outcomes, they are determined by the same cation stability rule. 1. Hydrogenation This is a minor part of this chapter but a major part of the course. By that I mean that all pi bonds are subject to hydrogenation so many functional groups can be reduced (a common term for hydrogenation). You should view these as of a single type with the same characteristics over all the functionalities, making it easier to remember and see the commonality. We will see different reagents for hydrogenation and the hydride reagents are a type whose mechanism is more explicable in the usual way we describe these processes. So, form a “box”, a category where you can add reduction reactions as we encounter them in succeeding chapters. The mechanism is poorly understood as it occurs at the surface and involves formation of metal-H and metal-C bonds, see Fig 6.1 but we will not do the mechanism. The reaction R 2 C=CR 2 + H 2 + Metal catalyst R 2 HC-CHR 2 R is usually an abbreviation for alkyl groups but here these may be hydrogens as well. Basically no limitation on what can be bonded to the carbons of the double bond. There are a variety of metal catalysts, Pt, Ni, Pd, Rh and later we will see modifications of these. The main point resulting from the mechanism is that the hydrogens are added from the same side of the double bond (syn addition). So the hydrogens are cis. In this example since there is free rotation about the single bond formed that is not observable but in a ring it is evident. There is also a discussion of heats of hydrogenation but it yields the same conclusions as heats of combustion did. More substituted alkenes are more stable.
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H 3 C Elec-Nuc Step 1 H H H H 3 C H H H Elec This is a secondary cation. Addition to C2 would yield a primary cation Step 4 Nuc H 3 C H H Elec H Nuc 2. Electrophilic Addition reactions. All of the principal reactions of the chapter are fundamentally the same . 1. An electrophile adds to the nucleophilic pi bond forming a carbon-electrophile bond. 2. This leaves the other carbon of the double bond electron deficient, a carbocation.
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carey_6 - Chapter 6 Addition reactions to alkenes Well for...

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