Chapter 5, Part 1: Benzene and Aromaticity Based on McMurry’s Organic Chemistry, 6 th edition

2 BenzeneToluene Naphthalene Examples of Aromatic Hydrocarbons HHH HH H CH 3 H H HH H HH H HH HHH

4 Some history 1825 Michael Faraday determined C:H ratio to be 1: Eilhardt Mitscherlich isolates a new hydrocarbon and determines its empirical formula to be C n H n. Compound comes to be called benzene August W. von Hofmann isolates benzene from coal tar August Kekulé proposes structure of benzene.

6 Kekulé proposed a cyclic structure for C 6 H 6 with alternating single and double bonds. Kekulé Formulation of Benzene HHH HH H

-7 Later, Kekulé revised his proposal by suggesting a rapid equilibrium between two equivalent structures. Kekulé Formulation of Benzene HHH HH H HHH HH H

8 However, this proposal suggested isomers of the kind shown were possible. Yet, none were ever found. Kekulé Formulation of Benzene HXX HH H HXX HH H

The cyclic structure, with alternating single and slightly shorter double bonds, yields four disubstituted products—a 1,3- disubstituted product, a 1,4- disubstituted product, and two 1,2- disubstituted products—because the two substituents can be placed either on two adjacent carbons joined by a single bond or on two adjacent carbons joined by a double bond

9 Structural studies of benzene do not support the Kekulé formulation. Instead of alternating single and double bonds, all of the C—C bonds are the same length. Structure of Benzene Benzene has the shape of a regular hexagon.

pm 146 pm 134 pm All C—C bond distances = 140 pm 140 pm is the average between the C—C single bond distance and the double bond distance in 1,3-butadiene.

13 Orbital Hybridization Model of Bonding in Benzene Each carbon contributes a p orbital Six p orbitals overlap to give cyclic  system; six  electrons delocalized throughout  system

17 express the structure of benzene as a resonance hybrid of the two Lewis structures. Electrons are not localized in alternating single and double bonds, but are delocalized over all six ring carbons. Resonance Formulation of Benzene HHH HH H HHH HH H

The Stability of Benzene benzene is the best and most familiar example of a substance that possesses "special stability" or "aromaticity" aromaticity is a level of stability that is substantially greater for a molecule than would be expected on the basis of any of the Lewis structures written for it

Heats of Hydrogenation as Indicators of Stability The addition of H 2 to C=C normally gives off about 120 kJ/mol – 3 isolated double bonds would give off 360 kJ/mol –Two conjugated double bonds in cyclohexadiene add 2 H 2 to release 240 kJ/mol Benzene has 3 units of unsaturation but gives off only 206 kJ/mol on reacting with 3 H 2 molecules Therefore it has about 152 kJ more “stability” than an isolated set of three double bonds

21 heat of hydrogenation: compare experimental value with "expected" value for hypothetical "cyclohexatriene"  H°= – 208 kJ Thermochemical Measures of Stability + 3H 2 Pt

kJ/mol 231 kJ/mol 208 kJ/mol 360 kJ/mol 3 x cyclohexene

kJ/mol 360 kJ/mol 3 x cyclohexene "expected" heat of hydrogenation of benzene is 3 x heat of hydrogenation of cyclohexene

kJ/mol 360 kJ/mol 3 x cyclohexene observed heat of hydrogenation is 152 kJ/mol less than "expected" benzene is 152 kJ/mol more stable than expected 152 kJ/mol is the resonance energy of benzene

25 hydrogenation of 1,3- cyclohexadiene (2H 2 ) gives off more heat than hydrogenation of benzene (3H 2 )! 231 kJ/mol 208 kJ/mol

فصل پنجم: بنزن و آروماتیسیته پایداری بنزن :

20 Structure and Stability of Benzene  Benzene reacts slowly with Br 2 to give bromobenzene (where Br replaces H)  This is substitution rather than the rapid addition reaction common to compounds with C=C.

21 Aromatic Compounds  Aromatic was used to described some fragrant compounds in early 19 th century Later they are grouped by chemical behavior (unsaturated compounds that undergo substitution rather than addition)  Current: distinguished from aliphatic compounds by electronic configuration

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23 Sources of Aromatic Hydrocarbons  From high temperature distillation of coal tar  Heating petroleum at high temperature and pressure over a catalyst

Substituted Derivatives of Benzene and Their Nomenclature

33 1) Benzene is considered as the parent and comes last in the name. General Points

34 ExamplesExamples Bromobenzene tert-Butylbenzene Nitrobenzene NO 2 C(CH 3 ) 3 Br

35 1) Benzene is considered as the parent and comes last in the name. 2) List substituents in alphabetical order 3) Number ring in direction that gives lowest locant at first point of difference General Points

36 2-bromo-1-chloro-4-fluorobenzene ExampleExample Br Cl F

Dr. Wolf's CHM 201 & Certain monosubstituted derivatives of benzene have unique names Benzene Derivatives

Dr. Wolf's CHM 201 & Benzaldehyde Benzene Derivatives CHO

Dr. Wolf's CHM 201 & Benzoic acid Benzene Derivatives COHO

Dr. Wolf's CHM 201 & Styrene Benzene Derivatives CH 2 CH

Dr. Wolf's CHM 201 & Toluene Benzene Derivatives CH 3

Dr. Wolf's CHM 201 & Acetophenone Benzene Derivatives CCH 3 O

Dr. Wolf's CHM 201 & Phenol Benzene Derivatives OH

Dr. Wolf's CHM 201 & Anisole Benzene Derivatives OCH 3

Dr. Wolf's CHM 201 & Aniline Benzene Derivatives NH 2

Dr. Wolf's CHM 201 & Benzene derivative names can be used as parent OCH 3 NO 2 OCH 3 Anisole p-Nitroanisole or 4-Nitroanisole

39 The Phenyl Group  When a benzene ring is a substituent, the term phenyl is used (for C 6 H 5  ) You may also see “Ph” or “” in place of “C 6 H 5 ”  “Benzyl” refers to “C 6 H 5 CH 2  ”

Dr. Wolf's CHM 201 & Easily confused names phenylphenolbenzyl OH CH 2 —

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37 Ortho, Meta, and Para Disubstituted Benzenes alternative locants for disubstituted derivatives of benzene 1,2 = ortho (abbreviated o-) 1,3 = meta (abbreviated m-) 1,4 = para (abbreviated p-)

38 ExamplesExamples o-ethylnitrobenzene NO 2 CH 2 CH 3 ClCl m-dichlorobenzene (1-ethyl-2-nitrobenzene)(1,3-dichlorobenzene)

44 Disubstituted Benzenes

45 Disubstituted Benzenes  Describes reaction pattern—product orientation:

46 Naming Benzenes With More Than Two Substituents  Choose numbers to get lowest possible values  List substituents alphabetically with hyphenated numbers  Common names, such as “toluene” can serve as root name (as in TNT)

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48 Problem: IUPAC names?

آروماتیسیته و قاعده هوکل(1931): سیستمهای حلقوی مسطح با پیوندهای دوگانه که بوسیله پیوند های یگانه جداشده اند ( پیوند های مزدوج = یک در میان) در صورت داشتن تعداد الکترون های برابر با 2+ n4 ((n=1,2,3,4,…Nآروماتیک می باشند. 6=4n+2 n=1

51 Aromatic Ions  The 4n + 2 rule applies to ions as well as neutral species  Both the cyclopentadienyl anion and the cycloheptatrienyl cation are aromatic  The key feature of both is that they contain 6  electrons in a ring of continuous p orbitals

52 AROMATICITY Three Simple Rules 1. Continuous cyclic cloud of delocalized pi (  ) electrons which produce a ring current. 2. The  cloud must contain (4n + 2) pi electrons (Huckel’s rule). 3. Ring must be planar. Cycloheptatriene and Cyclopentadiene are not aromatic

53 Other “Annulenes”  Willstatter (1911) made cyclooctatraene  Not aromatic; non-planar:

54 Other “Annulenes”  Cyclobutadiene wasn ’ t prepared until 1965, by Pettit, but it dimerizes (Diels-Alder) even at - 78 o C:

ترکیبات آروماتیک چندحلقه ای:

56 Aromatic Heterocycles: Pyridine and Pyrrole  Heterocyclic compounds contain elements other than carbon in a ring, such as N,S,O,P  Aromatic compounds can have elements other than carbon in the ring  There are many heterocyclic aromatic compounds and many are very common  Cyclic compounds that contain only carbon are called carbocycles (not homocycles)  Nomenclature is specialized

هتروسیکل های آروماتیک :

58 Pyridine, Pyrrole, & Furan

59 Pyridine  A six-membered heterocycle with a nitrogen atom in its ring   electron structure resembles benzene (6 electrons)

60 Pyridine  The nitrogen lone pair electrons are not part of the aromatic system (perpendicular orbital)  Pyridine is a relatively weak base compared to normal amines but protonation does not affect aromaticity:

61 Pyrrole  A five-membered heterocycle with one nitrogen  Four sp 2 -hybridized carbons and one sp 2 - hybridized nitrogen with 4 p orbitals perpendicular to the ring and 6  electrons

62 Pyrrole  Since lone pair electrons are part of the 6 electron aromatic ring, protonation destroys aromaticity, making pyrrole a very weak base

63 Problem: Imidazole

64 Which Nitrogen is Basic?

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66 Polycyclic Aromatic Compounds: Naphthalene  Aromatic compounds can have rings that share a set of carbon atoms (fused rings)  Compounds from fused benzene or aromatic heterocycle rings are themselves aromatic

67 Naphthalene Orbitals  Three resonance forms and delocalized electrons

68 Problem: Azulene is polar (  = 1.0 D), why?

69 Problem: Azulene’s polarity

 Which of the following compounds has the greater dipole moment?