BENZENE AND AROMATIC COMPOUNDS

BENZENE AND AROMATIC COMPOUNDS The discovery of benzene: 1825 - Michael Faraday, empirical formula of C 1834 - Eilhard Mitscherlich synthesized benzin from gum benzoin, empirical formula C Aromatic The compounds with pleasant aromas or sweet-smelling odor. OC 3 C=C-COO attached to O C 1 2 Coal tar : as odorless -O also attached to - The true structure of benzene can be explained by the concept of resonance and electron delocalization due to orbital overlap. Aromatic compounds : benzene and derivatives of benzene 1866 - Friedrich Kekule proposed benzene as a cyclic structure with three double bonds (localized double bond) in equilibrium Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. 3 4

The structure of benzene It contains six-membered ring and three additional degree of unsaturation It is planar All C-C C bond lengths are equal The reaction is substitution not an addition 2 CCl 4 (an addition) Resonance energy is the extra stabilization provided by the delocalization of the electrons in the aromatic compound. Aromatic compounds tend to undergo substitution rather than addition reaction, to retain their resonance energy. This behavior is characteristic of aromatic compounds. 2 Fe 3 + (substitution) Can we predict which compound is aromatic? 5 6 Aromatic Character : The (4n+2)π Rule The criteria for aromatic character (aromaticity) 1. A molecule must be cyclic (each p orbital overlap with p orbital on adjacent atoms). 2. A molecule must be planar. 3. A molecule must be conjugated (p orbital on every atom) 4. A molecule must satisfy uckel s rule : (4n+2)π n = 0, 1,2, 3 and so on. All compounds can be classified in one of three ways : 1. Aromatic: a cyclic, planar, completely conjugated compound with 4n+2 π electrons 2. Antiaromatic: a cyclic, planar, completely conjugated compound with 4n π electrons 3. Not aromatic (or nonaromatic): a compound that lacks one (or more) of the following requirements for aromaticity ; being cyclic, planar and completely conjugated. 7 8

Example of aromatic compounds (4n+2) π electrons n 4n+2 Structure and Name 1 6 2 10 benzene N.. pyridine.. N Ọ. pyrrol furan Organic ions Tropylium ion (6π electrons) 4n+2 = 6 n = 1 Cyclopentadienyl anion 6π electrons contain 4n+2 π electrons.. + aromatic aromatic 3 14 naphthalene N quinoline Cyclopentadienyl cation 4π electrons contain 4n π electrons Cyclopentadienyl radical 5π electrons does not contain 4n or 4n+2 π electrons +. antiaromatic non aromatic anthracene phenanthrene 9 10 Common Names of Benzene Derivatives Nomenclature of Benzene Derivatives Monosubstituted benzenes: O C 3 N 2 OC 3 C 2 C 3 C 3 C C 3 C 3 Phenol Toluene Aniline Anisole ethylbenzene tert-butylbenzene O O C 3 O O 2 C Cl Benzaldehyde Acetophenone Benzoic acid Styrene 11 chlorobenzene 12

Disubstituted benzenes : Common Name o-dibromobenzene IUPAC Name 1,2-dibromobenzene If the two groups are different, alphabetize the the name of the substituents: Cl Common name : o-bromochlorobenzene IUPAC name : 1-bromo-2-chlorobenzene m-dibromobenzene 1,3-dibromobenzene Common name : m-fluoronitrobenzene IUPAC name : 1-fluoro-3-nitrobenzene NO 2 p-dibromobenzene 1,4-dibromobenzene F 13 14 If one of the substituents is part of a common root, use a common root name: Polysubstituted benzenes: 3 substituent C 3 Common name : p-bromotoluene IUPAC name : 4-bromotoluene Common name : o-nitrophenol IUPAC name : 2-nitrophenol O NO 2 1. Number to give the lowest possible numbers 2. Alphabetize the substituent names. 3. When substituents are part of common roots, the common root is located at C1. Cl C 2 C 3 N 2 Cl C 2 C 2 C 3 4-chloro-1-ethyl-2-propylbenzene Cl 2,5-dichloroaniline 15 16

Aromatic rings as substituent: Phenyl group ; C 6 5 - ; Ph- Benzyl group C ; C 6 5 C 2 - ; PhC 2 O as PhO C 3 CC 2 C 3 C 2 Cl O C 2 CC 3 benzyl acetate benzyl chloride 2-phenylbutane 17 18 Reactions of Aromatic Compounds eneral Mechanism Electrophilic Aromatic Substitution Electrophile substitutes for a hydrogen on the benzene ring 19 20

21 22 1) omination of Benzene Requires a stronger electrophile than 2. Use a strong Lewis acid catalyst, Fe 3. Chlorination and Iodination Chlorination is similar to bromination. Use AlCl 3 or FeCl 3 as the Lewis acid catalyst. Iodination requires an acidic oxidizing agent, like nitric acid, which oxidizes the iodine to an iodonium ion. 23 24

2) Nitration of Benzene Use sulfuric acid with nitric acid to form the nitronium ion electrophile. 3) Sulfonation Sulfur trioxide, SO 3, in fuming sulfuric acid is the electrophile. 25 26 4) Friedel-Crafts Alkylation Synthesis of alkyl benzenes from alkyl halides and a Lewis acid, usually AlCl 3. Reactions of alkyl halide with Lewis acid produces a carbocation which is the electrophile. Other sources of carbocations: a) alkenes + F Mechanism of Friedel-Crafts Alkylation b) alcohols + BF 3. 27 28

Other sources of carbocations: a) alkenes + F 29 30 b) alcohols + BF 3 5) Friedel-Crafts Acylation Acyl y chloride is used in place of alkyl chloride. The acylium ion intermediate is resonance stabilized and does not rearrange like a carbocation. The product is a phenyl ketone that is less reactive than benzene. 31 32

Mechanism of Friedel-Crafts Acylation 33 34 Disubstitution : Reactivity W > > What makes a substituent on a benzene ring electron donating or electron withdrawing. The answer is inductive effects or resonance effects. Then consider the net balance of both the inductive and the resonance effects. Decreasing order of reactivity = donates electron to the ring thus activating it W = withdraws electron from the thus deactivating it R R donates electrons by an inductive effects R has no resonance effects So R as an electron-donating group 35 36

opposing effects Z Ż. inductively withdraw electron density donate electron density by resonance Z = N, O, X If Z= N, O ; the resonance effect dominates and the net effect is electron donation 37 38 Summary of Activators (ortho, para directors) 39 40

Summary of Deactivators (meta directors) 41 42 Disubstituted Benzenes : Orientation or ortho product further substitution ( = second substitution) or meta product para product The proportions of each isomer on statistical basis would be: ortho = 2/5 of the total or 40% meta = 2/5 of the total or 40% para = 1/5 of the total or 20% In fact, such distribution is never observed. The actual distribution depends on the nature of the first substituent,. 43 44

Two categories: 1) ortho, para director + To understand why a substituent direct an incoming electrophile to a particular position, we must look at the stability of the carbocation intermediate that is formed in the rate determining step. 2) meta director Three different carbocation intermediates can be formed: ortho-substituted carbocation; meta-substituted carbocation; para-substituted carbocation. 45 The more stable the carbocation the more likely it is that it will formed. 46 If Z= halogen ; the inductive effect dominates and the net effect is electron withdrawal. 47 48

Substituent such as C=O, CN, and NO 2 withdraw electrons by resonance. These substituent also withdraw electrons inductively because the atom is more electronegative than a hydrogen. Thus, all substituents that withdraw electrons are meta directors. Or again we can look at the stability of the carbocation intermediate in the ortho-, meta-, para-substituted carbocations. 49 50 Effects of Multiple Substituents on Electrophilic Aromatic Substitution Classification of Substituents When there is a conflict between an activating group and a deactivating group, the activating group usually directs the substitution. Strong ortho/para directors> moderate ortho/para directors> all meta directors 51 52

53 54 Side-chain Reactions of Aromatic Compounds Clemmensen Reduction 1. alogenation of an alkyl side chain C 2 C 3 Cl 2 CC 3 uv light Cl + major product 2. Oxidation of an alkyl side chain minor product C 2 C 2 Cl Acylbenzenes can be converted to alkylbenzenes by treatment with aqueous Cl and amalgamated zinc. O [O] R C O hot O C 2 C 3 KMnO 4 C O 55 56

Synthesis of Benzenes Derivatives When designing a synthesis of substituted benzenes, the order in which the substituent are introduced is crucial. Example: Synthesize ortho-, meta-, and paranitrobenzoic from toluene. 57 58 Synthesize m-bromonitrobenzene from benzene. NO 2 NO 2 NO 3 2 2 SO 4 Fe 3 m-bromonitrobenzene If 59