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Chap 22 Chemistry of Benzene Substituents

Chap 22 Chemistry of Benzene Substituents - Chemistry 140C...

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Chemistry 140C Winter 2010 (K. Albizati) 1 Chapter 22 – Chemistry of Benzene Substituents The benzene ring makes the C-H bonds in the benzylic positions much more reactive in a variety of transformations: For example: Bromination under free radical conditions gives rise to benzylic bromination and not ring bromination. Recall the propagation steps in radical halogenation reactions: Recall also that the more stable radicals form faster than less stabilized radicals. The benzylic radical is resonance stabilized:
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Chemistry 140C Winter 2010 (K. Albizati) 2 And the hybrid can be drawn like: or in orbital form, like: The free electron can be delocalized over 7 carbon atoms, thus endowing it with substantial stabilization. So, radical halogenation takes place on the benzylic carbon. A similar stabilization occurs with both carbonium ions and carbanions at the benzylic position: ……….A highly resonance stabilized carbonium ion. …..and a highly resonance stabilized carbanion. So………..
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Chemistry 140C Winter 2010 (K. Albizati) 3 Will this be a favorable S N 1 reaction? MECHANISM ANIMATION What do you think about the acidity of the benzylic hydrogens vs. the ring hydrogens in toluene? Which one of the compounds below has more acidic benzylic hydrogens? Why?
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Chemistry 140C Winter 2010 (K. Albizati) 4 Benzylic Oxidation An industrially important reaction that has a counterpart in biochemistry is the oxidation of arene alkyl side chains: A few features of this reaction….. - Any alkyl group is oxidized all the way down to the ring to give a carboxylic acid - This can be done with alkaline KMnO 4 or Na 2 Cr 2 O 7 or even with transition metal catalysts and oxygen (industrial process) - Proceeds first through the benzylic alcohol, then the ketone and then C-C bond cleavage occurs Human cells possess the ability to similarly oxidize alkyl benzenes to benzoic acids via an enzyme-driven biochemical process. These benzoic acids can then be excreted. This is an important detoxification mechanism our bodies have developed over the course of evolution. This is important because many aromatic compounds can interfere with biological processes involving DNA. For many years benzene was used as a common laboratory and industrial solvent. About 30 years ago, OSHA designated benzene as a carcinogen and its widespread use as a solvent was discontinued. In its place we use toluene (which our bodies can deal with) which has similar solvating properties.
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Chemistry 140C Winter 2010 (K. Albizati) 5 Mn +4 in the form of MnO 2 can be used for a very selective oxidation of benzylic alcohols: This oxidation is highly selective for benzylic alcohols over almost any other kind of alcohol, which makes it very useful, since protecting groups are not needed in cases where a substrate molecule has more than one hydroxyl.
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