AromaticityAromaticity is a property of the sp2 hybridized planarrings in which the p-orbital, one on each atom, allowcyclic delocalization of π electrons.CRITERIA:RULE 1: An aromatic compound is cyclic and planarRULE 2: Each atom in an aromatic ring has a p-orbital whichis parallel so that a continuous overlap is possible around theringRULE 3: The cyclic pi-molecular orbital formed by overlap of p-orbital must contain ( 4n+2) pi electrons.
Cycloheptatriene• Cyclic & Planar• Has a 3 double bonds & 6 pi-electrons• 1 saturated Carbon without a p-orbitalCyclooctatetriene• Cyclic & Planar• Has a p-orbital on each atom of the ring• Huckel Rule not satisfiedBenzene• Cyclic & Planar• Has a p-orbital on each atom of the ring• Obeys Huckel Rule
PHYSICAL PROPERTIES1. Benzene is a colorless liquid of boiling point80.10C, melting point 5.50C2. It is immiscible with water but miscible withalcohol, ether and chloroform3. Benzene itself is a good solvent for many organicand inorganic substance4. Its vapour is highly toxic; inhalation over aperiod of time leads to anemia.5. It burns with a luminous, sooty flame6. Benzene and its derivatives shows characteristics IRspectrum
Method of Preparation of BENZENE:Large scale preparation (From Petroleum):
Small scale preparation:• By passing Acetylene through red hot tube at 5000C• By heating Phenol with Zinc dust
Principle types of reactions of BENZENE:• Electrophilic Substitution reaction• Addition reactions• Oxidation reactions
Comparison between Alkene & Benzene:Alkene BenzeneThe Carbonium ion produced from theAlkene usually combines with aNucleophile to give the overall additionproduct.Whereas in Benzene the Nucleophileremoves a proton from the Carbonium ionintermediate. The loss of proton allows theelectrons from the C-H bond to go backinto the ring & re-generate the aromatic pi-system.Both are sucseptible to Electrophilic attack because of their exposed pi-electrons.
Chemical Reactions:+Benzenesulfonic acidSulfonation:H S O 3 HS O 3H 2 S O 4++An alkylbenzeneAlkylation:RR XA l X 3H X++Acylation:An acylbenzeneH R C XA l X 3 H XOC ROH
Friedel-Crafts AlkylationStep 1: formation of an alkyl cation as an ion pair.Step 2: attack of the alkyl cation on the ring.Step 3: proton transfer regenerates aromaticity.R Cl ClAlClClR ClClClAl Cl R+AlCl4-An ion pair containinga carbocation+-+A molecularcomplex+ R+RHRHRHA resonance-stabilized cation+++HRCl AlCl3 R AlCl3 HCl+ ++
Addition Reactions:Addition of Hydrogen:Addition of Halogens:The above two reactions involve a Free Radical Mechanism
Uses:• Benzene is an important industrial solvent and precursor in theproduction of drugs, plastics, synthetic rubber, and dyes.• It is a natural constituent of crude oil, and may be synthesized fromother compounds present in petroleum.•Benzene is widely used in the United States; it ranks in the top 20chemicals for production volume.•Some industries use benzene to make other chemicals which are used tomake plastics, resins, and nylon and synthetic fibers.•Benzene is also used to make some types of rubbers, lubricants, dyes,detergents, drugs, and pesticides.•Natural sources of benzene include volcanoes and forest fires.•Benzene is also a natural part of crude oil, gasoline, and cigarette smoke.