Lecture_III_Day

Lecture_III_Day - ORGANIC CHEMISTRY 308 LECTURE III CHAPTER...

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ORGANIC CHEMISTRY 308 LECTURE III CHAPTER 15 1 ORGANIC CHEMISTRY 308 LECTURE III CHAPTER 15 I. Benzene Our starting point. Where we see further consequences of conjugation A. Historical Perspective. In the mid 19 th century Benzene was a well known compound. Natural sources of benzene include volcanoes and forest fires. Benzene is also found in crude oil, gasoline, and cigarette smoke. It was recognized even then that there is something very unusual about benzene. 1. Its formula was known to be C 6 H 6 indicating a high degree of unsaturation. But it did not behave like an alkene or an alkyne at all. For one thing it was observed to be much less reactive. 2. The details of its reactions were even more puzzling. For example when hexene reacts with bromine, the bromine readily adds. C 6 H 12 + Br 2 C 6 H 12 Br 2 But when benzene reacts with Br 2 , it requires a Lewis acid catalyst such as FeCl 3 for the reaction to occur. Only one product is obtained, but the product is a substitution product: C 6 H 6 + Br 2 C 6 H 5 Br + HBr The product is called bromobenzene, which has many of the same properties as benzene itself. 3. Equally puzzling was the behavior of bromobenzene. When treated with Br 2 and FeCl 3 it also undergoes a substitution reaction: C 6 H 5 Br + Br 2 C 6 H 4 Br 2 + HBr Just three isomeric products are obtained from this reaction.
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ORGANIC CHEMISTRY 308 LECTURE III CHAPTER 15 2 4. The term aromatic was used to characterize benzene and its derivatives because they often had characteristic smells and to emphasize their differences from aliphatic compounds such as alkanes, alkenes etc. 5. First approach to a correct structure for benzene was made by Kekulé in 1865 as the result of an opium dream: a snake with its tail in its mouth etc. It was further refined in 1872 by the addition of rapidly moving double bonds. 6. What Kekulé and other 19 th century scientists were trying to explain was the equivalence of all six carbon atoms (and as we now know all 6 carbon-carbon bonds. A modern description using sp 2 hybrid orbitals and unhybridized p-orbitals very simply explains the equivalence. Notice that 6 atomic orbitals are overlapping. Therefore there are six molecular orbitals spread over the six carbon atoms. Three of these MO’s are bonding and three are antibonding. Since there are six π electrons to be accommodated all six reside in bonding molecular orbitals. Similarly, a valence bond description, where benzene is described as a blend of two equivalent contributing structures also explains the equivalence of the bonds and atoms. These two contributing structures are often written as one structure with a circle instead of three double bonds.
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ORGANIC CHEMISTRY 308 LECTURE III CHAPTER 15 3 B. Nomenclature 1. Monosubstituted benzenes are often encountered. They can be named by adding a substituent prefix to the name benzene and the names are easy to remember. Examples are chlorobenzene (or fluoro, bromo, or iodo), nitrobenzene (-NO 2 ), and ethylbenzene etc. But many compounds of
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This note was uploaded on 04/04/2008 for the course CHEM 308 taught by Professor Boikess during the Spring '08 term at Rutgers.

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Lecture_III_Day - ORGANIC CHEMISTRY 308 LECTURE III CHAPTER...

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