Many Substituted Benzenes Are Found in Nature and a Few Have Physiological Activity

Organic 7.1-7.2

Benzene -isolated in 1825 by Micheal Faraday -many substituted benzenes are found in nature and a few have physiological activity

Ex:adrenaline, ephedrine(bronchodilator) -many physiological active substituted benzenes are synthesized EX: banned diet drug(fenphen)mixture of two synthetic substituted benzenes Amphetamine-central nervous system stimulant Methamphemina(speed),aspirin Aromatic compounds-compounds like benzene that have relatively few hydrogens in relation to the number of carbons that are typically found in oils produced by trees and other plants

Aliphatic compounds-have higher hydrogen-to-carbon ratios

Criteria for aromaticity -benzene is stable because it has resonance stabilization -extra stability if from having delocalized electrons with a cyclic cloud of electrons above and below the plane of the ring Delocalized electrons-neither belong to a single atom nor are confined to a bond between two atoms-they are shared by three or more atoms -must have an uninterrupted cyclic cloud of pi electrons above and below the plane of the molecule -molecule must be cyclic -for the pi bond to be uninterrupted, every atom in the ring must have a p orbital -each p orbital must overlap with the p orbital on either side of it(molecule must be planar) -the pi cloud must contain an odd number of pairs of pi electrons

7.2-aromatic hydrocarbons Cyclopentadiene -is NOT aromatic because it does not have an uninterrupted ring of p hybridized-one ring has an sp3 and ONLY sp2 and sp hybridized carbons have p orbitals

Cyclpentadienyl-is NOT aromatic because its pi cloud has two(even number) of pi elecrons ?Why is the cyclopentadienyl anion aromatic? Cyclopentadienyl anion-is aromatic as it has an uninterrupted ring of p orbital bearing atoms and its pi cloud contains three(odd number) pairs of delocalized pi electrons -the resonance hybrid shows that all the carbons in the cyclopentadienyl anion are equivalent-each carbon has exactly one-fifth of the negative charge associated with the anion 7.3 aromatic heterocyclic compounds

Heterocyclic compound-cyclic compound in which one or more of the ring atoms is an atom other than carbon Heteroatom-a ring atom that is not a carbon with the most common being N,O, and S

?How is pyridine aromatic? -each of the six ring atoms are sp2 hybridized-and remember that the electrons on the N are NOT pi electrons -nitrogen actually has three sp2 orbitals and a p orbital -furan and thiophen are stable as both the oxygen and the sulfur are sp2 hybridized and have one lone pair in an sp2 orbital

7.4 nomenclature of monosubstitued benzenes -some monosubstitued benzenes are named simply by stating the name of the substituent followed by the word benzene Name the following

Some monosubstitued benzenes have names that incorporate the name of the substituent: Phenyl group-a benzene ring with a substituent Benzyl group-benzene ring with a methylene group

Aryl group-general term for either a phenyl group or a substituted phenol group

7.5 how benzene reacts -electrophilic aromatic substitution reaction-an electrophile substitutes for one of the hydrogens attached to the benzene ring -first step similar to an alkene with a carbocation intermediate being formed -2nd step being the carbocation reacting with the nucleophile to form the subbed product

-the aromatic substitution product is more stable than the nonaromatic addition product benzene than undergoes an electrophilic substitution reaction to preserve aromaticity rather than an electrophilic addition reaction so we officially name this reaction an electrophilic aromatic substituted reaction

7.6 electrophilic aromatic substitution reactions

Halogenation-a bromine or a chlorine substitutes for a hydrogen

Nitration-a nitro(NO2) group substitutes for a hydrogen

Sulfonation-a sulfonic acid(SO3H)group substitutes for a hydrogen -bromination or chlorination of benzene requires a Lewis acid catalyst such as ferric bromide or ferric chloride Lewis acid-compound that accepts a share in an electron pair

-the need for a catalyst during a reaction with Br2 or Cl2 comes from the fast that benzene is less reactive than an alkene so it needs a better electrophile

-donating a lone pair to the Lewis acid weakens the Br-Br bond making Br2 a better electrophile 7.8 nitration of benzene -nitration of benzene with nitric acid requires sulfuric acid as a catalyst

-to generate the necessary electrophile, sulfuric acid protonates nitric acid and the loss of water from the protonated nitric aced forma nitronium ion which is the required electrophile

-reminder that any bass present such as water can remove the proton in the second stop of the aromatic substitution reaction

7.9 sulfonation of benzene -fuming sulfuric acid(a solution of SO3) in sulfuric acid or concentrated sulfuric acid is used to sulfonate aromatic rings

-notice that the electrophile is very similar to the electrophile in the sulfonation reaction