aromatic hydrocarbons I: structures and reactions

aromatic compounds

benzene is a resonance hybrid of these resonance structures. Individual resonance forms are imaginary, not real. The real structure is a composite, or resonance hybrid, of the different forms

orbital representation

each carbon of benzene has a 2p_z orbital with its one electron sideways overlaps the 2p_z orbitals of its two neighbors to give an extended pie cloud containing 6 pie electrons

all carbon-carbon bonds are equal length

discovery of benzene

1852 Michael faraday

kekule

proposed structure of benzene

kekule’s structure of benzene

the correct explanation is that it is not two structures in equilibrium but a single structure which is a resonance between the two above

Huckel’s rule for aromaticity

to be aromatic, a compound should have a planar ring(s), each atom in the ring has a p_z orbital and there must be (4n + 2)pie electrons (n = integer). i.e. for benzene n = 1

benzene has an aromatic sextet or an aromatic pie cloud containing 6 pie electrons

benzenoid aromatic compounds

• benzene

• toluene

• 1,2-dimethylbenzene

• aniline

• phenol (aromatic alcohol)

• hexamethylbenzene

naming Di-substituted benzenes

the prefixes ortho-, meta, and para are commonly used for 1,20, 1,3- and 1,4- positions respectively

polycyclic and heterocyclic aromatic compounds

Compounds that contain multiple fused aromatic rings or have one or more heteroatoms in the ring structure. These include naphthalene (10 pie electrons (n=2)) and anthracene (14 pie electrons (n=3))

electrophilic substitution

typical reactions are substitution reactions since these preserve the aromatic sextet (6 pie electrons)

typically electrophilic substitution since in these reactions electrophiles react with electron rich aromatic ring

F₂ reacts violently without catalyst

Br₂ reacts in the presence of catalyst AlBr₃

no light is needed to initiate the reaction (not free radical mechanism)

mechanism of chlorination of benzene

i) generation of electrophile

ii) reaction between ring and electrophile (one bond formed and one bond broken)

ii) loss H⁺ from Benz onium ion (carbocation) (one bond formed and one bond broken)

the driving force for this step is the regeneration of the aromaticity which accounts for the mechanism pathway

iv) regeneration of catalyst

AlCl₃ as a Lewis acid

AlCl₃ acts as a Lewis acid (electron pair receptor) abstracting Cl^- and Cl₂ thereby generating the key electrophile Cl⁺

Cl^- acts as a Lewis base (electron pair donor)

BF₃ and all group III trihalides (e.g. AlCl₃) are Lewis acids since there have only six electrons around central atom

Friedel Craft’s alkylation of benzene

important carbon-carbon forming reaction

reaction does not work with aryl halides such as chlorobenzene

Friedel crafts alkylation of benzene mechanism

follows the same general reaction pathways as chlorination

nitration of benzene

i) formation of the electrophile

the rest follows the same general reaction pathways as chlorination and alkylation