310_20 - Organic Organic Lecture Series CH 310 N LECTURE 20...

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Unformatted text preview: Organic Organic Lecture Series CH 310 N LECTURE 20 Textbook Assignment: Chapter 22 Reactions of Benzene Homework (for credit): Problem Set – 7 Posted Today’s Topics: Di-substitution Rxns; Directing Effects Notice & Announcements: EXAM 2: Pick up @ office hrs 1 Organic Lecture Series Reactions of Benzene & Its Derivatives Chapter 22 2 Organic Organic Lecture Series Di- and Polysubstitution Only a trace 3 Organic Lecture Series Di- and Polysubstitution Orientation on nitration of monosubstituted benzenes: 4 Organic Organic Lecture Series Di- and Polysubstitution • Orientation: –certain substituents direct preferentially to ortho & para positions; others to meta positions –substituents are classified as either ortho-para directing or meta directing toward further substitution 5 Di- and Polysubstitution Organic Lecture Series • Rate –certain substituents cause the rate of a second substitution to be greater than that for benzene itself; others cause the rate to be lower –substituents are classified as activating or deactivating toward further substitution 6 Organic Organic Lecture Series 7 Di- and Polysubstitution Organic Lecture Series – -OCH3 is ortho-para directing: – -CO2H is meta directing 8 Di- and Polysubstitution Organic Organic Lecture Series 9 Di- and Polysubstitution Organic Lecture Series • Generalizations: – alkyl, phenyl, and all other substituents in which the atom bonded to the ring has an unshared pair of electrons are orthopara directing; all other substituents are meta directing – all ortho-para directing groups except the halogens are activating toward further substitution; – the halogens are weakly deactivating 10 Di- and Polysubstitution Organic Organic Lecture Series the order of steps is important: 11 Organic Lecture Series Theory of Directing Effects • The rate of EAS is limited by the slowest step in the reaction • For almost every EAS, the ratedetermining step is attack of E+ on the aromatic ring to give a resonancestabilized cation intermediate • The more stable this cation intermediate, the faster the ratedetermining step and the faster the overall reaction 12 Organic Organic Lecture Series Theory of Directing Effects • For ortho-para directors, ortho-para attack forms a more stable cation than meta attack – ortho-para products are formed faster than meta products • For meta directors, meta attack forms a more stable cation than ortho-para attack – meta products are formed faster than ortho-para products 13 Theory of Directing Effects Organic Lecture Series Nitration of anisole -OCH3; examine the meta attack: 14 Organic Organic Lecture Series Nitration of anisole -OCH3: examine the ortho-para attack: This resonance structure accounts for the selectivity 15 Organic Lecture Series Theory of Directing Effects Nitration of benzoic acid -NO2; examine the meta attack: 16 Nitration of benzoic acid Organic Organic Lecture Series -NO2: assume ortho-para attack: This resonance structure accounts for the selectivity Activating-Deactivating 17 Organic Lecture Series • Any resonance effect, such as that of NH2, -OH, and -OR, that delocalizes the positive charge on the cation intermediate lowers the activation energy for its formation, and has an activating effect toward further EAS • Any resonance effect, such as that of NO2, -CN, -CO, and -SO3H, that decreases electron density on the ring deactivates the ring toward further EAS 18 Activating-Deactivating Organic Organic Lecture Series • Any inductive effect, such as that of CH3 or other alkyl group, that releases electron density toward the ring activates the ring toward further EAS • Any inductive effect, such as that of halogen, -NR3+, -CCl3, or -CF3, that decreases electron density on the ring deactivates the ring toward further EAS 19 Di- and Polysubstitution Organic Lecture Series • Generalizations: – alkyl, phenyl, and all other substituents in which the atom bonded to the ring has an unshared pair of electrons are orthopara directing; all other substituents are meta directing – all ortho-para directing groups except the halogens are activating toward further substitution; – the halogens are weakly deactivating 20 Organic Organic Lecture Series Activating-Deactivating for the halogens, the inductive and resonance effects run counter to each other, but the former is somewhat stronger the net effect is that halogens are deactivating but ortho-para directing 21 Di- and Polysubstitution Organic Lecture Series 22 Organic Organic Lecture Series Medicinal Chemistry Benzodiazepins 1) Sedative-hypnotic 2) Anticonvulsant 3) Muscle relaxant ® Valium 4) Anxiolytic 23 Retrosynthetic Analysis Organic Lecture Series Medicinal Chemistry 24 Organic Organic Lecture Series Medicinal Chemistry 25 Organic Lecture Series Medicinal Chemistry 26 Organic Organic Lecture Series Medicinal Chemistry 27 Organic Lecture Series Medicinal Chemistry 28 ...
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This note was uploaded on 11/21/2011 for the course CH 51455 taught by Professor Colapret during the Spring '11 term at University of Texas at Austin.

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