9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.PunjabIn This Chapter You Will Learn1. To name simple aromatic hydrocarbons, and their derivatives.2. The Kekule and resonance approaches to explain the structure andstability of benzene.3. About the preparation of benzene.4. About the electrophilic substitution, oxidation and addition reactions ofbenzene.5. About the isomerism which arises when a second substituent enters thering.6. How does the presence of a group alters the reactivity of benzene ringtowards electrophilic substitution reactions.7. The comparison of reactivities of alkanes, alkenes and benzene.9.1 INTRODUCTIONThe term aromatic was derived from the Greek word ‘aroma’ meaning“fragrant” and was used in Organic Chemistry for a special class ofcompounds. These compounds have a low hydrogen to carbon ratioin their molecular formula and have a characteristic odour. However,it was soon realized that many aromatic compounds are odourlesswhereas many others are fragrant though they are not aromatic.Further, when aromatic compounds of higher molecular mass were subjectedto various methods of degradation, they often produced benzene or derivativesof benzene. It was observed that almost all the aromatic compounds havea six carbon unit in their molecules like benzene. Hence, benzene wasrecognized as the simplest and the parent member of this class of compounds.Animation 9.1 : Polycyclic aromatic hydrocarbnSource & Credit : QchitoolIt appears from the definition of aromatic hydrocarbons that any studyof this class of compounds must begin with the study of benzene.Benzene has characteristic structural features. It has a regular planarhexagonal structure. On the basis of the number of benzene ringsaromatic hydrocarbons can be categorized into following classes.a. Monocyclic Aromatic Hydrocarbons and their derivativesb. Polycyclic Aromatic Hydrocarbonsa. Monocyclic Aromatic Hydrocarbons and their DerivativesAromatic hydrocarbons containing one benzene ring in their molecules arecalled Monocyclic Aromatic Hydrocarbons, e.g. benzene and its derivatives.So aromatic hydrocarbons include benzene and all those compounds thatare structurally related to benzene.23

9. Aromatic HydrocarbonseLearn.Punjabb. Polycyclic Aromatic HydrocarbonsAromatichydrocarbonscontainingtwozene rings in their molecules are calledic Hydrocarbons. They may be divided into9. Aromatic HydrocarbonseLearn.Punjab9.2 NOMENCLATUREormorebenPolycyclic Aromattwo main classes.(i) Those in which benzene rings are isolated, e.g. biphenyl,diphenylmethane, etc.(Monocyclic Aromatic Hydrocarbons and their Derivatives)The nomenclature of the aromatic hydrocarbons is much more complexthan that of aliphatic hydrocarbons. The system used for naming benzeneand its derivatives generally depends on the number of substituents on thebenzene ring.Some important rules of naming are given below.(ii) Those in which the benzene rings are fused together at ortho positions so that the adjacent rings have a common carbon to carbon bonds, e.g. naphthalene,phenanthrene and anthracene1. Mono-substituted benzene derivatives are named by prefixing benzene withthe name of the substituent. The whole name is written as one word, e.g.2. There are certain monosubstituted benzene derivatives which are given thespecial names, like methylbenzene as toluene, hydroxybenzene as phenol etc.Animation 9.2 : Larger cyclic aromatic hydrocarbonsSource & Credit : sandia45

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.PunjabAll the six positions in benzene are exactly equivalent so there is only onemonosubsituted benzene.When a hydrogen atom is removed from benzene, we get a phenyl group symbolized by C6H5- or Ph-. Substituted phenyl groups are called aryl groups.3. The second substituent in benzene would give rise to three isomeric products designated as ortho (1,2), meta(1,3) and para(1,4),e.g.The order of priority of the groups (left to right):— COOH, — CN, — CHO, — COCH3, — OH, — NH, — OR, — R.5. If the two substituents are different and they are notpresent in priority order list, they are named in alphabetical order. The last named substituent will be at position 1, e.g.,4. If two or more substituents are different, then the substituent that is treatedas a high priority group, is given the number 1 position in the benzene ring. Other groups are numbered by counting from position 1 in the manner which givesthem the lowest number.6. If there is a substituent on the ring which gives a special name to themolecule, then special name is used as parent name to the molecule, e.g.Animation 9.3 : Monocyclic ring breathing mode in TolueneSource & Credit : Kemi67

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.PunjabBut benzene also gives substitution reactions with conc. HNO3 andconc. H2SO4 which indicate that benzene has a saturated structure.9.3 BENZENEii) Considering a straight chain structure for benzene andfurther assuming that each carbon carries one H-atom, itshould be capable of forming three monosubstitution products.X Benzene was discovered by Michael Faraday in 1825 in the gas produced by the destructive distillation of vegetable oil and twenty years later it was also found in coal-tar by Hoffmann.9.3.1 Structure of BenzeneMolecular Formula1. The empirical formula of benzene is determined by the elemental analysis.2. Its molecular mass determined by the vapour density method is 78.108.This is six times the empirical formula mass (CH 12 1 13). Therefore, themolecular formula of benzene is C6H6.3. The molecular formula of benzene indicates that it is highly unsaturatedcompound.9.3.2 Straight Chain Structures Ruled Out:i) Two of the possible straight chain formulas suggested for benzene are :HC C CH 2 CH 2 C CH(1,5-Hexadiyne).H 2 C CH C C CH CH 2(l,5-Hexadiene-3-yne).A compound having a structure as above should behave like analkene or alkyne, both are oxidized by alkaline KMnO4 solution. On thecontrary, benzene is stable to KMnO4 solution i.e. it does not decolorizeKMnO4 solution. Benzene gives addition reactions with hydrogenand halogens, which indicate the presence of three double bonds.8C.C.C.C.C.CXX C.C.C.C.C.C C.C.C.C.C.CBut benzene yields only one monosubstituent product.iii) The molecular formula of benzene C6H6 does not correspondto any of the open chain hydrocarbons, such as 2n 2(C6H14),It means benzene does not belong to open chain hydrocarbonand therefore possibility of a straight chain structure is ruled out.9.3.3 Kekule’s StructureThe structure of benzene continued to be a serious problem for chemistsfor about 40 years. A German chemist, Kekule at last solved the problemin 1865. Kekule proposed a cyclic regular hexagonal structure for benzene,which contains three double bonds alternating with three single bonds.He supported his theory by the following arguments.(i) Benzene gives only one monosubstituted product.9

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.Punjabiv) Benzene adds three molecules of chlorine in the presence of sunlight.(ii) Benzene gives only three disubstituted products.These two reactions confirm the presence of three double bonds alternatingwith three single bonds.These points confirm the regular hexagonal structure for benzene in whichall the carbon atoms are occupying identical positions in the molecule.Therefore, benzene forms only one toluene, one phenol and one nitrobenzene.(iii) Benzene adds three hydrogenmolecules in the presence of a catalyst.Animation 9.4 : Phenol and AlcoholSource & Credit : footdoc9.3.4 X-Ray Studies of Benzene StructureThe X-ray studies of benzene haveconfirmed the hexagonal structure forit. These studies have also revealedthat all the carbon and hydrogenatoms are in the same plane. All theangles are of 120 . All Co - C and C -Hobond lengths are 1.397A and 1.09 A,respectively.Objections to Kekule’s FormulaKekule’s formula with three double bonds demands a high degree ofunsaturation from benzene while usually it exhibits a saturated character.Thus benzene yields substitution products readily and forms addition productsreluctantly. Benzene is also a very stable compound. All these propertiesof benzene can be easily explained using the modem theories about itsstructure.1011

9. Aromatic HydrocarbonseLearn.Punjab9.3.5 Modern Concepts About the Structure of Benzene Atomic OrbitalTreatment of BenzeneThe hexagonal frame-work of benzene can beconveniently explained using hybridizationapproach. According to this, each carbonin benzene is sp2 hybridized. The three sp2hybrid orbitals on each carbon are utilizedto form three s-bonds, two. with adjacentcarbon atoms and one with hydrogen.The unhybridized 2pz orbitals remain at rightangle to these sp2 orbitals. Since all the sp2orbitals are in the same plane therefore allthe carbon and hydrogen atoms are coplanar.All the angles are of 120 which confirms theregular hexagonal structure of benzene. Theunhybridized 2pz orbitals partially overlap toform a continuous sheath of electron cloud,enveloping, above and below, the six carboncarbon sigma bonds of the ring. Since each 2pzorbital is overlapped by the 2pz orbitals of adjacentcarbon atoms, therefore, this overlappinggives, 'diffused' or 'delocalized' electroncloud.9. Aromatic HydrocarbonseLearn.PunjabCyclohexene, a six membered ring containing one double bond, can be easilyhydrogenated to give cyclohexane. When the H for this reaction is measuredit is found to be -119.5 kJ/mole, very much like that of any similarly substitutedalkene.We would expect that hydrogenationof 1,3-cyclohexadiene would liberateroughly twice as much heat and thushave H equal to about -239 kJ/mole.When this experiment is done, the resultis H -231.5 kJ/mole. This result isquite close to what we calculated, andthe difference can be explained by takinginto account the fact that compoundscontaining conjugated double bondsare usually somewhat more stablethanthosecontainingisolateddouble bonds.Animation 9.5 : Kekulé benzeneSource & Credit : Ch.imperial9.3.6 The Stability of BenzeneAs mentioned earlier benzene is an extraordinary stable molecule. Thisstability is due to the extensive delocalization of electron cloud. The extentof stability of benzene can be measured by comparing it with hypotheticalcompound, 1,3,5- cyclohexatriene. This can be done by estimating their heats rved H H2(-119.5)If we extend this kind1,3,5-cyclohexatriene, we -239 kJ/mole -231.5 kJ/moleofthinking,13andifbenzeneissimply

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.Punjabwould predict that benzene would liberate approximately -358.5 kJ/mole(3 x -119.5) when it is hydrogenated. When the experiment is actually donethe result is surprisingly different. The reaction is exothermic but onlyby -208 kJ/mole.Calculated H (3 x -119.5) -358.5 kJ/moleObserved H -208 kJ/moleDifference 150.5 kJ/moleWhen the results are represented by the following figure, it becomesclear that benzene is much more stable than we calculated it to be.Animation 9.6 : Benzene derivativeSource & Credit : Cod.eduAnimation 9.7 : Bonding in a molecule of benzene from a molecular orbital pointSource & Credit : lite.msu.eduIndeed, benzene is more stable than the hypothetical 1,3,5-cyclohexatrieneby 150.5 kJ/mole. This difference between amount of heat actuallyreleased and that calculated on the basis of the Kekule’s structure is nowcalled the ‘Resonance energy’ of the compound. It means benzene showsthe phenomenon of resonance which makes it more stable than others.In benzene electrons are delocalized making it a very stable molecule.9.3.7 The Resonance ns of atoms is calledstructuresthusarrangedareThe resonance is represented( ) e.g. the following differentvalence (the p-electrons) of carbonFig.9.1 Relative stabilities of Cyclohexene,1,3- Cyclohexadiene, 1,3,5-Cyclohexatriene(hypothetical) and benzene.1415pairingschemesofvalenceresonance” and the differentcalled“Resonancestructures”.by a double headed arrowpairing schemes of the fourthatoms are possible in benzene.

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.PunjaboIn alkanes the C-C bond length is 1.54A.oIn alkenes the C C bond length is 1.34A.o In alkynes the C C bond length is 1.20A.oIn benzene the C-C bond length is 1.397A.The C - C bond length in benzene is intermediatebetween those in alkanes and alkenes. The resonatingsingle and double bonds in benzene can better berepresented as a complete circle inside the ring.e(a), (b) were proposed by Kekule and c, d, e, were proposed by Dewar.The stability of a molecule increases with increase in the numberof its resonance structures. Thus molecule of benzene is chemicallyquite stable.In Dewar structure the carbon atoms at opposite positions 1-4,2-5 and 3-6 are at larger distances than those in the adjacentpositions 1-2,2-3,3-4,4-5,5-6 and 6 -1. Therefore the bondingsbetween C1-C4, C2-C5 and C3-C6 are not favourable ne.Infact, the structure of benzene is a resonance hybrid of all the fivestructures (a), (b), (c), (d) and (e) in which the Kekule’s structure (a)and (b) have the larger contribution. Therefore, benzene moleculecan be represented by either of the two Kekule’s structure.The three alternate single anddouble bonds in the Kekule’sstructures are called conjugatebondsorresonatingbonds.9.4 PREPARATION OF BENZENEBenzene and other aromatic hydrocarbons are readily obtained in largequantities from coal and petroleum. Benzene and some other hydrocarbonscan also be obtained from petroleum by special cracking methods. Some ofthe methods generally used for the preparation of benzene are as follows.1. Dehydrogenation of genatedwe get benzene or a substituted benzene. The reaction iscarried out by the use of a catalyst at elevated temperature.2. From Acetylene:Since the structure of benzene is a resonance hybrid, therefore all the C-Cbond lengths are equal but different from those in alkanes, alkenes andalkynes.16Benzene is formed by passing acetylene under pressure over an organonickel catalyst at 70 C.17

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.Punjab3. FromAlkanesBenzene and toluene are also prepared by passing the vapours of n-haxaneor n-heptane over a mixture of catalysts Cr2O3 AI2O3 SiO2 at 500 C.ii. By distilling phenol with zinc dust.BenzeneCH34. Preparation in the rofbybenzenesulphonicboilingwithdil.acidHCl.5. Wurtz-Fittig ReactionBenzene can be prepared in the laboratory by any one of the followingmethods.The Wurtz reaction for the synthesis of alkanes was extendedby Fittig in 1864 to the synthesis of alkyl aromatic hydrocarbons.i. By heating sodium salt of benzoic acid with soda lime.1819

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.Punjab9.5 REACTIONS OF BENZENE9.5.1 General Pattern of Reactivity of Benzene Towards ElectrophilesThe highly stable, delocalized electrons of benzene ring are not readilyavailable for the nucleophillic attack like the electrons of alkenes.Therefore, the electrons of benzene ring do not assist in the attack of weakelectrophiles. It means that more powerful electrophiles are required topenetrate and break the continuous sheath of electron cloud in benzene,e.g., substitution of halogen in benzene requires iron or correspondingferric halide as a catalyst. Infact iron too is first converted into FeX3 whichfurther reacts with halogen molecule to produce a powerful electrophile.2Fe 3X 2 2FeX 3FeX 3 X 2 FeX 4 X Tetra haloferrate H a l o g e n o n i u mion (III)ionThe halogenonium ion X thus produced attacks as a powerful electrophile onthe electrons of benzene ring.Animation 9.8 : Alkenes due to resonanceSource & Credit : Chemistry.boisestate.eduion2021

9. Aromatic HydrocarbonseLearn.PunjabThe addition product is not favourable because in its formation thecharacteristic stability of benzene is lost. The only possible product isthe substitution product in which the stability of benzene is retained.Therefore, the general pattern of the chemical reactivity of benzene towardselectrophiles can be shown as follows.9.5.2 Electrophilic Substitution Reactions1. Halogenation9. Aromatic HydrocarbonseLearn.PunjabMechanism:The actual halogenating agent is X ( i.e. CI or Br ) is formed by the followingmechanism.When alkyl benzenes are treated with chlorine or bromine inthe presence of sunlight, only the alkyl groups are substituted.The introduction of halogen group in benzene ring is called “Halogenation”Benzene reacts with halogen in the presence of a catalyst like FeBr3,AlCl3, etc. Chlorination and bromination are normal reactions butfluorination is too vigorous to control. Iodination gives poor yield.Benzal2223

9. Aromatic HydrocarbonseLearn.Punjab2. Nitration9. Aromatic HydrocarbonseLearn.Punjab3. SulphonationThe introduction of NO2 group in benzene ring is called “Nitration”.The nitration of benzene takes place when it is heated with a1:1 mixture of con. HNO3 and con.H2SO4 at 50- 55 C. Sulphuricacid reacts with nitric acid to generate nitronium ion, (N02 ).The introduction of sulphonic acid group in benzene ring iscalled Sulphonation. When benzene is heated with O4Mechanism:50-550 CHNO 2 H 2SO 4 NO 2 HSO 4 - H 2 OMechanism:When sulphuric acid alone is used, the actual electrophile in thisreaction is SO3.Animation 9.9 : Nitration of benzene mechanismSource & Credit : Sustainability.sellafieldsites2425

9. Aromatic HydrocarbonseLearn.Punjab3. Friedel-Crafts ReactionsThe alkylation and acylation of benzene are called friedel-Crafts reactions.a) Alkylation9. Aromatic HydrocarbonseLearn.Punjabb) Acylation:The introduction of an acyl group R- c in the benzene ring in the presenceof an acyl halide and a catalyst AICI3 is called Friedel Crafts Acylation orAcylation.The introduction of an alkyl group in the benzene ring in the presenceof an alkyl halide and a catalyst AICI3 is called Friedel-Crafts alkylation orAlkylation.Mechanism:R Cl AlCl3 AlCl 4 R CarbocationAnimation 9.10 : Friedel crafts reactionSource & Credit : Friedelcraft.chez.com2627

9. Aromatic HydrocarbonseLearn.Punjab9.5.3 Reactions in which Benzene Ring is Involved9. Aromatic HydrocarbonseLearn.Punjab(d) Oxidationi) Catalytic Oxidation1. Addition Reactions(a) hightemperature with hydrogen in the presence of Pt in anacidic solvent (acetic acid) or Ni at 200 C as a catalyst.Benzene is not oxidized by KMnO4 or K2Cr2O7 at ongly heated with air in the presence of V2O5 as a catalyst.(b) HalogenationBenzene reacts with chlorine and bromine in the presenceof sunlight to give addition products, hexachlorobenzene orhexabromobenzene. Fluorination is too vigorous while iodination is slow.ii) OzonolysisBenzene reacts with ozone and gives glyoxal through benzene triozonide.Side Chain Oxidation(c) CombustionWhen benzene is burnt in free supply of air, it is completely oxidized to CO2and H2O.2C6 H 6 15O 2 12CO 2 6H 2 O28Alkyl benzenes are readily oxidized by acidified KMnO4 or K2Cr2O7. In thesereactions, the alkyl groups are oxidized keeping the benzene ring intact.29

9. Aromatic HydrocarbonseLearn.Punjab9. Aromatic HydrocarbonseLearn.PunjabOn chance basis 40% ortho, 40% meta and 20% para disubstituted productsare expected.Butthisthethe actual disubstitution of benzene does not followprinciple of chance, e.g. m - Chloronitro benzene atever the length of an alkyl group may be it gives only onecarboxyl group. Moreover the colour of KMnO4 is discharged.Therefore this reaction is used as a test for alkyibenzenes.On the other hand a mixture of o- Chloronitrobenzene andp-Chloronrtrobenzene is obtained from the nitration of chlorobenzene.9.5.4 Orientation in Electrophilic Substitution reactionsWhen an electrophilic substitution reaction takes place on benzene ring,we get only one monosubstituted benzene because all the six positionsin the ring are equivalent. However, the introduction of a second groupinto the ring may give three isomeric disubstituted products, ortho, metaand para.It means that the group present in the mono-substituted benzene ringhas the directive effect and thus determines the position or orientationfor the new incoming groups. Therefore, there are two types of groups:1. ortho- and para- directing groups2. meta- directing groups1. ortho and para Directing GroupsThese groups release electrons to the benzene ring, thereby facilitatingthe availability of electrons to the electrophiles at ortho and parapositions.3031

9. Aromatic HydrocarbonseLearn.PunjabThis results in the increased chemical reactivity of benzene ringtowards electrophilies. The benzene ring can offer more thanone positions (ortho and para) to the new incoming groups.These groups are called ortho and para directing groups, e.g.29. Aromatic HydrocarbonseLearn.Punjab2. meta -Directin g GroupsThese groups withdraw the electrons of the benzene ring towardsthemselves, thereby reducing their availability to the electrophiles.This results in the decreased chemical reactivity of benzene. Moreover,due to the electron withdrawing effect of such substituents, the orthoand para positions are rendered more electron deficient than the metaposition. Thus the incoming electrophile will prefer to attack on metaposition rather than ortho and para positions. These groups are calledmeta-directing groups, e.g.The electron releasing effect of methyl group is significantand it makes the ring a good nucleophile. Due to thisincreased reacivity, more nitro groups can enter the ring.The substitution of third nitro group is not possible, other examples ofmeta directing groups are:N R 3 ,C N,COOH,CHO,COR9.6 COMPARISON OF REACTIVITIES OF ALKANES,ALKENESAND BENZENEOther examples of ortho and para directing groups are:- N(CH 3 ) 2 , NH 2 , OH, OCH 3 , Cl, Br, I32We have studied that alkanes are unreactive class of compoundsand their unreactivity is due to their non-polar nature andthe inertness of s-bond. However, theyundergo substitutionreactions relatively easily and these reactions involve free radicals.Alkenes, on the other hand, are very reactive class of compoundsand their reactivity is due to the inherent weakness of the π -bondand the availability of π electrons for the electrophilic reagents.33

9. Aromatic HydrocarbonseLearn.PunjabThey undergo electrophilic addition reactions easily. Being relativelyunstable, alkenes undergo polymerization reactions and they are also readilyoxidized.Benzene is unique in its behaviour. It is highly unsaturated compoundand at the same time it is very stable molecule. The stability of benzene,as described earlier, is due to the extensive delocalization of π -electrons.It resembles alkenes when it gives addition reactions. The substitution ofbenzene, however, does not involve free radicals. These are ophiles.Its addition reactions require more drastic conditions than those for alkenes.Benzene does not undergo polymerization and it is also resistant to oxidation.KEYPOINTS1. Aromatic hydrocarbons include benzene and all those compounds that arestructurally related to benzene.2. Aromatic hydrocarbons containing one benzene ring in their molecules arecalled monocyclic aromatic hydrocarbons.3. Aromatic hydrocarbons containing two or more benzene rings in their moleculesare called polycyclic aromatic hydrocarbons.4. Benzene was discovered by Michael Faraday in 1825.5. The electrons in benzene are loosely held and the ring acts as a source ofelectrons. Hence benzene is readily attacked by electrophiles in the presenceof a catalyst.6. Since electrophilic substitutions reaction lead to resonance stabilized benzenederivatives so substitutions are the main reactions of benzene.7. Resonance energy of benzene is 150.5 kJ/mole.8. Structure of benzene is the resonance hybrid of two Kekule’s structures andthree Dewar’s structures.9. Groups like NH2, NHR, - OR, - SH, - OCOR, - X, - OH, etc. which increase theelectron density in the nucleus and facilitate further electrophilic substitutionsare known as ortho- and para-directing groups.10. Groups like CN, - CHO, NH3, NR3, CCl3 which hinder further substitution in thebenzene nucleus are known as meta- directing groups.349. Aromatic HydrocarbonseLearn.PunjabEXERCISEQ1. Fill in the blanks.i) The term aromatic was derived from Greek word meaning.ii) Aromatic hydrocarbons include benzene and all those compounds whichare related to benzene.iii) is recognized as the simplest member of the class ofAromatic Hydrocarbons.iv) Benzene has structure.v) These removal of hydrogen atom from aromatic hydrocarbons gives aradical. The radicals are called .vi) Benzene was discovered by Michael Faraday in .vii) The unhybridized 2pz orbitals in benzene partially overlap to form aof electron cloud.viii) The introduction of halogen group in benzene ring is called .ix) The molecular formula of C6H6 indicates that it is highlycompound.x) On oxidation in the presence of V2O5 benzene gives .Q.2 Indicate True or False.i) Benzene is more reactive than alkene and less reactive than alkane.ii) Benzene has a pentagonal structure.oiii) The C-C bond length in benzene molecule is 1.397 A.iv) The state of hybridization of carbon in benzene molecule is sp3.v) There are six sigma bonds in benzene Halogenonium ion produced in electrophilic substitution reactions is apowerful electrophile.vii) In electrophilic substitution reactions, addition products are favourable.viii) Sulphonation is carried out when benzene is heated with conc. HNO3.ix) In ozonolysis benzene directly gives glyoxal.x) Benzene has five resonace contributing structures.Q.3. Multiple choice questions. Encircle the correct answer.i) The benzene molecule contains(a) Three double bonds(c) One double bond(b) Two double bonds(d) Delocalized π -electron charge35

9. Aromatic HydrocarbonseLearn.Punjab(b) alkene(d) cyclohexaneiii) Which of the following acid can be used as a catalyst in Friedel-Craftsreactions?(b) HNO3(c)BeCl2(d)NaCl(a)AlCl3iv) Benzene cannot undergo(a) substitution reactions(c) oxidation reactions(b) addition reactions(d) elimination reactionsv) Amongst the following, the compound that can be most readilysulphonated is(a) toluene(b) benzene(c) nitrobenzene(d) chlorobenzenevi) During nitration of benzene, the active nitrating agent is(a)NO3(b)NO2 (c)NO2vii) Which compound is the most reactive one:(a) benzene(b) ethene(c) ethaneviii) The electrophile in aromatic sulphonation is:(a)H2SO4(b)HSO4(c) SO3(d)HNO3(d) ethyneQ. 5. What happens when:(a) Benzene is heated with conc. H2SO4 at 250 C.(b) Chlorine is passed through benzene in sunlight.(c) A mixture of benzene vapours and air are passed over heated vanadiumpentoxide.(d) Benzene is burnt in free supply of air.Q.6. What is meant by the terms:i) Aromaticii) Oxidationiv) Nitrationv) Halogenationiii) SulphonationQ. 7. (a) Draw structural formulas for the following compounds.i) m-Chlorobenzoic acidvi) 2,4,6 Trinitrotolueneii) p-Hydroxybenzoic acidvii) m-Nitrophenoliii) o-Bromonitrobenzeneviii) p-Dibenzylbenzeneiv) o-Ethyltolueneix) 2-Amino-5-bromo-3 nitrobenzenesulphonicacidv) p-Nitroaniline(b) Give names and the possible isomeric structuresof the following.i) Xylenesii) Trimethylbenzeneiii) BromonitrotolueneQ. 8. Write IUPAC names of the following molecules.(d) SO3 ix) Aromatic compounds burn with sooty flame because:(a) They have high percentage of hydrogen.(b) They have a ring structure.(c) They have high percentage of carbon.(d) They resist reaction with air.x) The conversion of n-hexane into benzene by heating in the presence of Ptis called:(a) Isomerization(b) Aromatization(c) Dealkylation(d) Rearrangement36eLearn.PunjabQ. 4. What are aromatic hydrocarbons? How are they classified?ii) Aromatic hydrocarbons are the derivatives of(a) normal series of paraffins(c) benzene9. Aromatic HydrocarbonsQ. 9. Give the general mechanism of the electrophilic aromatic substitutionreactions.Q. 10. (a) Describe the structure of benzene on the basis of following.i) Atomic orbital treatmentii) Resonance method(b) Prove that benzene has a cyclic structure.37

9. Aromatic HydrocarbonseLearn.PunjabQ. 11. Predict the major products of bromination of the following compounds.(a) Toluene(b) Nitrobenzene(c) Bromobenzene(d) Benzoic acid(e) Benzaldehyde(f) PhenolQ. 12. How will you prepare the following compounds from benzene in twosteps.(a) m-chloronitrobenzene(b) p-chloronitrobenzeneQ. 13. Complete the following reactions. Also mention the conditionsneeded to carry out these reactions.Q. 14. Detail out three reactions in which benzene behaves as if it is asaturated hydrocarbon and three reactions in which it behaves as if it isunsaturated.Q. 15. What are Frediel-Crafts reactions. Give mechanism with example ofthe following reactions.i) Friedel-Crafts alkylation reactionsii) Friedel-Crafts acylation reactions.38

Fig.9.1 Relative stabilities of Cyclohexene,1,3- Cyclohexadiene, 1,3,5-Cyclohexatriene (hypothetical) and benzene. Indeed, benzene is more stable than the hypothetical 1,3,5-cyclohexatriene by 150.5 kJ/mole. This difference between amount of heat actually released and that calculated on the basis of the Kekule's structure is now Resonance