2444-111811

2444-111811 - Quiz 8 Avg. 21.7 / 25 Stnd. Dev. 4.6...

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Unformatted text preview: Quiz 8 Avg. 21.7 / 25 Stnd. Dev. 4.6 Polynuclear Aromatic Hydrocarbons 2 122 123 124 1 2 3 4 5 6 7 8 9 10 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 10 Naphthalene is a bicyclic aromatic compound with the formula C 10 H 8 and structure 122 . Naphthalene is planar, with 10 !-electrons in a !-cloud above and below the plane of the ten carbon atoms and, like benzene, it is aromatic and particularly stable. Another polycyclic aromatic compound has three rings are fused together as in 123 , and this aromatic molecule (14 !-electrons) is called anthracene (formula, C 14 H 10 ). There is an isomer of anthracene called phenanthrene , in which the point of attachment of the "third ring" on the "middle ring" is different. Phenanthrene has the same empirical formula but its structure is 124 . Naphthalene. Electrophilic Aromatic Substitution 3 Cl Cl 2 , AlCl 3 122 126 In naphthalene, it is important to recognize that there are only two different positions, C1 and C2 (see 122 ). This means that C1, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at C1, C4, C5, and C8 as labeled in 122 , only one product is formed, 1-chloronaphthalene ( 121 ), which is the actual product isolated from the chlorination Naphthalene. Electrophilic Aromatic Substitution 3 Cl Cl 2 , AlCl 3 122 126 In naphthalene, it is important to recognize that there are only two different positions, C1 and C2 (see 122 ). This means that C1, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at C1, C4, C5, and C8 as labeled in 122 , only one product is formed, 1-chloronaphthalene ( 121 ), which is the actual product isolated from the chlorination reaction. Naphthalene. Electrophilic Aromatic Substitution 4 Chlorination of naphthalene at C1 leads to the Fve resonance structures shown for arenium ion intermediate 127 . Similar attack at C2 gives arenium ion 128 , which also has Fve resonance structures shown. Note that 127A and 127B have a fully aromatic benzene rings (an "intact benzene ring"), which means that there is an additional Kekul structure for each intact ring. Therefore, 127 has a total of seven resonance structures, but 128 has only six resonance contributors. This observation indicates that 127 is more stable than 128 , and it will form faster and lead to the major product, 1-chloronaphthalene. Cl + 127A H Cl 127B H Cl 127C H Cl 127D H Cl H Cl 127E 1 2 intact benzene rings (2 Kekule structures for each) 122 Cl + Cl H 128A Cl H 128B 128C Cl H 128D Cl H Cl H intact benzene ring (2 Kekule structures) 1 2 122 128E Phenanthrene 5 Electrophilic substitution of phenanthrene is more complicated since there is less difference in the stability of the intermediate cations formed from each substitution position.the stability of the intermediate cations formed from each substitution position....
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This note was uploaded on 02/25/2012 for the course CHEM 244 taught by Professor Jardin,j during the Fall '08 term at UConn.

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2444-111811 - Quiz 8 Avg. 21.7 / 25 Stnd. Dev. 4.6...

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