Chemistry 3B Lecture 9 - Chem 3B Lecture 9 Thursday...

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Chem 3B Lecture 9 Thursday February 19 th , 2009 We are really going to be talking about the transition states. In a situation like this, you are looking at transition’s state but you are actually looking at intermediates; you need to compare various resonance structures. We start with ortho: we have 3 possible resonance structures, as we do for meta and para. They are all allylic and have the same associated stability in terms of being allylic so lets look for more subtle differences. In ortho, we have a secondary carbocation, and we also have a tertiary. Then we look at meta to notice there are only secondary carbocations. In para, there is one tertiary carbocation. The difference is that tertiary resonance structures are unique relative to meta. Tertiary carbocations are more stable due to induction and hyperconjugation. More electron density is being put onto that site which will stabilize the delta positive of that center so the overall transition state will be stabilized. In meta, you never get a delta positive on the carbon that bears the methyl group. Ortho and para are favored relative to meta. Alkyl groups on benzene rings on an EAS reactions are said to be ortho/para directing; they will direct the electrophile to the ortho and para positions. The ratio of ortho and para might not be exactly what we would think. Suppose we add Br2 and we get 100% para, we would still call the alkyl groups ortho and para directing. If you don’t get any ortho products, then it may be because of steric hinderance. As you increase the size of the alkyl group, you will get virtually no reaction occurring at the ortho position and almost exclusive reaction at the para position even though we would call the increased alkyl group ortho/para directing. Alkyl groups are also said to be activating relative to benzene itself. If you react benzene with Br2 in FeBr3, you get bromo benzene; if you react toluene with Br2 and FeBr3, you get 2 possible products. The rate of the benzene reaction is 1, and the rate of the
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This note was uploaded on 02/04/2010 for the course CHEM 3BL taught by Professor Chunmei during the Spring '08 term at University of California, Berkeley.

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Chemistry 3B Lecture 9 - Chem 3B Lecture 9 Thursday...

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