Aromatic Compounds

Lecture Outline Aromatic vs aliphatic compounds Aromatic vs aliphatic compounds Benzene Benzene Structure/ stability Structure/ stability Electronic configuration Electronic configuration The concept of aromaticity The concept of aromaticity Huckel’s rule Huckel’s rule Aromatic compounds other than benzene Aromatic compounds other than benzene Nomenclature of benzene derivatives Nomenclature of benzene derivatives

Aromaticity hydrocarbons aliphatic aromatic alkanes alkenes alkynes

unsaturated cyclic hydrocarbons organic compounds had an aroma or odour contain single rings or groups of rings simplest aromatic compound is benzene aromatic compounds include benzene or are benzene based and have benzene- like structures and properties. Aromatic Compounds

♫♪ Sweet Dreams are Made of Benzene ♫♪ Kekul é The idea of the benzene structure came to Kekul é in a dream

Benzene, C 6 H 6 6 carbon ring with a hydrogen bonded to each carbon one electron from each carbon is free to participate in a double bond

Resonance Theory 1.Resonance forms are imaginary benzene has a single hybrid structure which combines the characteristics of both resonance forms Resonance forms Hybrid structure

Structure of Benzene Structure of Benzene Benzene - Kekul é The first structure for benzene was proposed by August Kekulé in 1872 this structure, however, did not account for the unusual chemical reactivity of benzene

Molecular Orbital *  electron cloud delocalized all over the ring * the resonance picture this helps to explain lack of reactivity of benzene * great stability (substitution not addition )

Benzene - Resonance Energy Resonance energy: Resonance energy: the difference in energy between a resonance hybrid and its most stable hypothetical contributing structures in which electrons are localized on particular atoms and in particular bonds one way to estimate the resonance energy of benzene is to compare the heats of hydrogenation of benzene and cyclohexene

Benzene - Resonance Energy heats of hydrogenation for both cyclohexene and benzene are negative (heat is liberated) these results are shown graphically on the next slide

The Concept of Aromaticity Requirements for aromaticity Molecule must be cyclic Molecule must be cyclic Molecule must be planar Molecule must be planar Each atom of the ring (s) must have a p-orbital perpendicular to the plane of the ring Each atom of the ring (s) must have a p-orbital perpendicular to the plane of the ring Huckel’s rule must be obeyed Huckel’s rule must be obeyed Planar compounds must have (4n + 2) pi ( electrons, where n is an integer Planar compounds must have (4n + 2) pi (  electrons, where n is an integer n= integer (1,2,3,…) Ring with 2, 6, 10 or 14 pi electrons may be aromatic Ring with 2, 6, 10 or 14 pi electrons may be aromatic Ring with 8, 12 or 16 pi electrons will not be aromatic Ring with 8, 12 or 16 pi electrons will not be aromatic

For aromaticity, all pi ( electrons must be paired and all bonding orbitals filled For aromaticity, all pi (  electrons must be paired and all bonding orbitals filled Maximum and complete overlap is required for stabilization Maximum and complete overlap is required for stabilization With unpaired pi ( electrons, overlap is not maximized With unpaired pi (  electrons, overlap is not maximized The pi ( electrons in an aromatic compound are delocalized over the entire ring leading to stabilization The pi (  electrons in an aromatic compound are delocalized over the entire ring leading to stabilization

02/19/13 Farshid Zand 15 So what makes a molecule aromatic? It must be cyclic It must be cyclic It must be conjugated It must be conjugated It must be flat so that the p orbital overlap can occur It must be flat so that the p orbital overlap can occur It must also have It must also have 4n + 2 pi electrons…

The Cyclopentadienyl anion The Cyclopentadienyl anion The Cyclopentadienyl cation The Cyclopentadienyl cation

Structure Resonance theory of benzene All bonds are equivalent!  electrons are delocalised around the ring

Aromaticity Example 1: Benzene cyclic planar conjugated 6  electrons

Aromaticity Other Examples? cyclic planar conjugated X 6  electrons X cyclic planar conjugated X 6  electrons X

© Prentice Hall 2001 Chapter Aromaticity cyclooctatetraene is nonaromatic cyclooctatetraene is nonaromatic It is not planar It is not planar

Physical Properties of Benzene MP benzene = 5.5°C, BP benzene =80.1°C MP benzene = 5.5°C, BP benzene =80.1°C non-polar Molecule non-polar Molecule insoluble in water insoluble in water less dense than water less dense than water

Nomenclature of aromatic compounds A] Mono substituted benzene: B] Disubstituted benzene. *because all C equal no numbers *Substituent and then benzene

Monosubstituted Benzenes Monosubstituted aromatics are named using -benzene as the parent name preceded by the substituent name (as a prefix; all one word): fluorobenzene nitrobenzene ethylbenzene Chlorobenzene

24 II. Nomenclature A. Single group methoxybenzene (anisole) benzenecarbaldehyde (benzaldehyde) benzenecarboxylic acid (benzoic acid) benzenamine (aniline) benzenesulfonic acid

Common Names of Subs. Benzenes

B] Disubstituted benzene :

When two substituents are present on the benzene ring, the prefixes ortho, meta and para are used. When two substituents are present on the benzene ring, the prefixes ortho, meta and para are used.

28 II. Nomenclature B. Two groups ortho (or 1,2) meta (or 1,3) para (or 1,4)

02/19/13 Farshid Zand 29 Disubstituted Benzenes Disubstituted benzenes use prefixes ortho-(o), meta(m), or para-(p). These prefixes are useful when discussing reactions. Disubstituted benzenes use prefixes ortho-(o), meta(m), or para-(p). These prefixes are useful when discussing reactions.

Disubstituted Benzenes ortho-= o-= 1,2-disubstituted (two groups on adjacent carbons on the ring)

Br 4-Aminophenol p- 2-Chlorophenol O- 4-Nitrotoluen p- 3-Bromobenzoic acid m-

A.Preparation of Benzene 1. Decarboxylation 2.Reduction of phenol

C.Chemical Properties 1. Electrophilic substitution reaction a. Nitration (Forming nitro-compound) b. Sulphonation (Forming sulphonic acid) c. Halogenation (Forming halobenzene) d. Friedal-Craft Alkylation (Forming alkylbenzene) e. Friedal-Craft Acylation (Forming carbonyl compound)

Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon

Reactions of Benzene Friedel –crafts Alkylation Friedel –crafts Acylation

C.Chemical Properties 1. Addition Reaction (NOT electrophilic) a.Catalytic hydrogenation

Summary of

Reactions of Toluene I. At side chain 1. Oxidation of alkyl benzene With MnO 4 - /H +

I. At side chain 2.Substitution at side chain

E.S. Reactions of Toluene Nitration of toluene (O.S.) Trinitrotoluene (TNT) is explosive substance.

Toluene faster than the same reactions with benzene

Nitrobenzene slower than the same reactions with benzene