Organic chemistry B

Chapters 14, 15, 16Aromatic Compounds

By Prof. Dr.Adel M. Awadallah

Islamic University of Gaza

Aromatic CompoundsHydrocarbons (contain only carbon and hydrogen)

a) Saturated: (Contain only single bonds)Alkanes (CnH2N + 2 )

Cycloalkanes (CnH2N )

b) Unsaturated: contain Alkenes: double bonds (,,,CnH2N)

Alkynes: triple bonds ((CnH2N - 2)

Aromatic: benzene like compounds

Facts about double and triple bonds

HH

bond angle 109.5 120 o 180 o

bond length 154 pm 134 pm 121 pm

rotation possible restricted restricted

geometry tetrahedral triagonal planer linear

Hypridization sp3 sp2 sp

Bond Length in Benzene 139 pm (plannar, sp2 hypridized)

Structure of BenzeneBenzene is the parent of a class of especially stable compounds

called aromatic compounds. It was first isolated by Faraday in 1825 from compressed illuminating gas

Different structures were suggested for benzene (C6H6). The regular conjugated hexagon structure was suggested by Kekuli` in 1865

Orbital Model for Benzene

Nomenclature of Aromatic CompoundsCH3

CH3CH3 CH2 OH

OMeOH OCH3 COOH NH2

Br NO2

CH3 CH3

benzene toluene cumene styrene phenol

anisole benzaldehyde acetophenone benzoic acid aniline

bromobenzene nitrobenzene ethyl benzene propylbenzene

Monosubstituted benzenes are named as derivatives of benzene

Disubstituted Benzene

Assigning Priority

• Halogens < alkanes < alkenes < amines < OH < ketone < aldehyde < acid < ester

• Examples

Cl

BrCH3

NO2

CH2

CH3

Cl OH

OH

NO2

NO2

O2N

O CH3

OHOOH

O CH3

OH

OH

m-nitrotoluene m-methylstyrene

CH3

NO2

NO2

O2N

o-bromochlorobenzene (alphabetical order)

p-chlorophenol

2,4,6-trinitrophenol (picric acid)

2,4,6-trinitrotoluene (TNT)

4-hydroxybenzaldehyde 4-acetylbenaldehyde 4-acetylbenzoic acid

Aromatic Compounds in Nature and Health

O

COOH

O

CH3

OH

CHO

OMe

CH3

CH3

CH3 COOHAspirinVanilin

Ibuprofen

Aromatic Character• Aromatic compounds are those that resembles benzene:

• 1) High degree of unsaturation• 2) Do not undergo addition reactions• 3) Undergo electrophilic substitution• 4) Unusual stability• 5) Cyclic five, six, seven• 6) flat or nearly flat molecules• 7) Delocalized pi clouds must contain a total of • (4N + 2) pi electrons [Hueckel 4n+2 rule]• 8) Protons appear around 7 ppm in NMR

Resonance Energy of Benzene

Hetrocyclic Aromatic Compounds

Rearrangement of carbocations

Rearrangement of carbocations

• Reaction of benzene with n-propyl chloride produces isopropyl benzene because of rearrangement

How can you prepare the following compound? Starting from benzene

CH3

COCH3

CH3

NO2

COCH3

BrBr

NO2

start by methylation

start by nitration start by acylationstart by bromination

Reactions of the side chain of alkylbenzene

Arenes: (Alkyl-, Alkenyl- & Alkynylbenzenes)

Resonance structure of the allyl radical

Triphenylmethyl: a stable free radical

Halogenation of the side chain

Alkenylbenzenes: Preparation of styrene

Polycyclic aromatic compoundscigarettes

naphthaline anthracene Phenanthrene

Pyrene benzo[a]pyrenecarcenogenic compound resulting from burning cigarettes

• C60, An aromatic sphere • The Fullerenes