Aromatic NucleophilicSubs. Reac." -(Ar SN Reac")The benzene SN reac." due are to not the very repulsion common wlively changely IT e density aboveb/w charge Nu also andring andbelow aromatic ring. Samadly which theSeaving group will H totherefore these reac". are uncommonhighly unstable difficulty form,like in aliphatic SN reac" they may beSN' as well as SN2Ar. SN' Reaction -The um only compound example is decomposition of diazoniw Nufast 8Hs: Vist N slow 8H5(BDC)- -u- ---NENNu(fast)(SNI)MCGH5Nu(C Nu = OH, OR,CN) The substitution of -N2cl group is exampleof SN' Reaction which is stepwise Theelemination of N2 2 from phenyldiazo cation4the rate determining step . so, it Ar issn' reachz2Ar SN ReactionThese are common reactions but verydifficult in benzene ring due to repulsionbetween K electrons and attacking Nu .The electron attracting groups if alreadypresent in ring facilitate SN Reaction.The common example is substitution ofhaloging from halobenzene st is possibledue to -I effect of halogen atom Thegeneral mechanism takes place as:-S-NuNuxslowSt +Nu+H(SN2)Resonance in phenonium Carbania111NuNufast-x-However, the mechanism of above reacn not(so easy and required high temperature200 - 300°) and high pressure (150-2000)/ MOREDATEdifficult The substitution due to of the halogen additional from halobergane becomeresults with conjugation sp 2 hybridised of lone pair carbon of electron of of which of halogenfactorpartial double bond character ring inCbond , the bond length become shorter,increase internuclear forces increase bond energySo, it will be difficult forlike in vinylchloride case.substitution halogen atom just somethingceIIIclconjugation in c-cl bondgroups stabilised -ve charge of phenoniumst means the presence of electron attractingsupport SN Reaction while electron releasingcarbanion attracting towards itself andgroups destabilised -ve charge by incleaningreaction. electron density and make difficult SNxNuNuGstabilizeGchargedetstabilizecharge The strong electron attracting at groups ortho make like and NOL halogenation Br, decreasing to I2 order of of reactivityZero) so, powerful O.S and oxidised Alkane. fromSince 0 (O.S) in to the - reacn halogen reducehalogenationary attracting and faster will, y the oxidising nature (electionThe oxidising nature be decrease asso, rate of halogenation decrease fromfrom F2 to I2 and electronegative F2 decrease to I2The order can also be explained on the basismolecule stronger and stable the HXof formation of stable and strong is H-Xbond more in the aach - Thestability of H-X as4F HCl HBrHIBond energy decreasestability decreaseTherefore formation of H-X decrease fromHF to HI and also rate of halogenationdecrease in same order.For AlkanesIn higher alkanes weaker the bond energyof carbon-hydrogen bond more easily breakand substitute The decreasing bond energy ofvarious C-H become-1 C-H 2°C-H 3° C-HBond energy decreaseRate of subs increase PAGE Nothe stability of various degree iThe reactivity can also be explained carbon onfree radical The stability of becomei i2° i3° cStability increaseRate of subs increaseMore stable is the free radical ,moreeasily form during substitution so,highly branched alkanes hydrogen atombecome more reactive and easily substitute10CH-CH - 1 i clf-45-413 3 20 2 1° cry 10 CH 4H3 H3 101°3°Reactivity 10 increaseQcth1°1° 2thuCHCHH-CH3[42+hoX33CH3CH3CH5CCH2-HH32CH3CHG CAceCece Allylic halogenationStereachemistry of free radical subsIn free radical reaction / the intermediatecarbon 2 free radical having planar structureandare in same plane at 120°. so, theof sp hyberdisation . The three bond pairsnew entering free radical species can equallyattack at both side of planargeometry and two opposite tetrahedral -geometry equally form for examplehalogunation halogenation of in-butane and formationmolecule Both d and l isomers formof 2 -halobutane which is assymmetricin equal amount called Racimic mixtureBoth the isomers cancel out the rotationof light have no net rotation . so, in freeradical subs n the Recimisation takes place.H4CH3-CH3CH3 + BrBretzCH2-tH.Br44C*+*BrBr41 cthCH2City (+) (I) Racemic crb mixture (-) Allylio halogenationdC.CC = Callylin 'E'Halogenation of alkenes involving allyliccarbon atom by N- Bromosuccinimide (NBS)Oh N-chloro susciminide (NCS) But NBSdue most commonly used halagenating agentits physical stateadequate (optimum) reactivity 4The Carbon atom just adjacent to the( L carbon)Olefinic bond is called allylic carbon 'sand NBS substitute is very H-atom regioselectivereagent onlyof allylia carbon. so, Olefin mustcontain at least one H- - atom atallylic coubon otherwise there will beno reaction. The reach also known aswohe -Zeigler Reac "BLNBSCC =Chv,C C c=c/HBrBRCHNBS3C- C =No Reachv,alyRNo. allylistiation DATEiCH2CCH5 CH= th +N-BrenCH2cTO12CH=CH,2Br(NBS)+6NHSuccinimideIn Benzenoid compound such a -carbon k/a abenzylic carbon and undergo substitution wNBS The reach also takes place at Lcarbon to Acetylimic bond and Carbonylgroup also (x=0).&-CE GHNBSCtzzctf-c CHhucclyBr,H3ftb-BrNBShu,cctuBenzyl bromideboth In a molecule containing definic and AcetylineL-then the bromination takes place tothecarbonwithrespecttoIIIbondacetylinica&CH CH= CH cH NBS0iv,uly Br MechanismAllylic in bromination cely solvent, is the catalysed other by non-polar light - orsolvents heat can also be used In allylic brominalthe low or adequate concentration of Br2required white at high can the addition NBS atlittle soluble in cly and provide con nmultiple bond takes place since low isof Br2 constantly If direct Br2 is used, thenaddition tapes placeNBSinhu CH2-CH=CH2 (Allylic brominationccly Brby subs )low conn of BromineBr,inhy CH2-CH-CH2 (Bromination by add'by Br high con of Br2)NBS is hoasclyBrBr2 inhuBrCH3COOHso, it must noted that low and const conof subs Br, is reac" required and in the allylic bromination canbe given as below:- mechanism MechanismSince the reac catalysed in light and non-polar solvent, / the mechanism takeshomolysis and formation of free place byintermediate.Initiation0hoByN° + BrPropagationBr +CHCH = CH2CH- CH= CH2]+ HBs-g?HAuyi free radicalBs + HBEN-1H+ Br2dissolvedincclyCHCH- 4/2 + Br. taBeBe-CH -CH= CH2 + BeTerminationBr + BeBr2 PAGE NODATEThe Bly molecule attack on allylic carbonandin propagation step generated in cily so is situ conn of reachwill not slightly be soluble strong but low and const. so Br2it can be said NBS is highly regioselectivereagent for allylic brominationthe belomination takes place with higherIf different allylic carbon atoms present,degace allylic carbon atom because it wellproduce higher degree more stable carbonfree radical. for ex- -ctf-44-CH=CH-CH-CH, ,H3 NASinhv/uly cth (H3CH3ByOuis Why NBS doesn't give addition reacnunsaturated bond ?Ans Since NBS produce low conn of Br, whichis adeguate only for subs but not enoughion for addn reac In addition reacn highconn of Bs required because is addition , theonly high con'n can forward the reac? sointermediate carbocation may reverse anddirect Brg in high conn required forreach at unsaturated carbon atomaddition Therefore . NBS doesn't give additionCth-CH=Cth S+ 5-2 + Brt CH2-CHICH IIBridgearbocationintermediate Perkin ReactionIt is an example of carbon- hetero multiple bond(c=0) addition followed by eleminationThe auomatic aldihyde when condensed withacid anhydride containing at least twoor potassum salt of respective acid formx - hydrogen aloms presence of sodiumL, B unsaturated acidBCH=OLisCH=CH COOHCH,COONACH3-Ot+ CH3 COOHLCHc3toLIB unsaturated(AGO)and( cinnamic Acid JB CH3CH t- COOHH=OaCH the coCH 3 CH COONa0++CH CH-COCH CH COOH3 2propionicanhydrideIt is not able that reacn takes place wbond.L. carbon atom and form L1 B unsaturatedof there are no two L-H atom, thereac n can't take place. theus45CH GOMy CH COONANoReow.+ofMechanismThe nbase reac' is catalysed by carboxylate anionfrom anhydride molecule. The generatedwhich generate carbanion NutNu- react nucleophile addition at aldehydhegroup as follow:-CH-Go-NaCHCOOT + Na+CH5-To+HOFOFCHit:fitcth+CHCCOOH7carbanion (N4)8H5 C Kst +State:CH O- 4NG addnH/CHHi -BiiCHt-ItCH2-from CHECOOHo-C-CH3(th Coo 6th5C. H CH. to OF CH 150 A 4H5CH = CH no ii c O- a withiOHH4+110Cg HfCH=44-COOH + CH3COOHcinnamic Audplace the carboxylate moleculeHowever, According to fitting , the reach takesnot by anhydride molecule. It is justreverse to the Perkin mechanism as -GHFCH=0 + 4Hs CH2 - COONa + (cycoloDdCHHCCH= &- COOHB8H5X- phenyl cinnamic Acid -In above meac Cinnamic acid shouldform according to perkin conceptfout product is just reversedfrenkin justified formation of a methylCinnamic Vacid rather than cinnamicacid , the reacn takes place byexchange reach tacitic -betweenarhydride and Sodium Johnnyl autate PAGE HyDATE2C6H5 CHg CooNa ++ 2CH Coonapheryl Actic anhydride(2d 'H' atom)Here again Sheryl acetic anhydrids is theactate as al base to generate Nutmain reading molecule Sediumfrom pheryl actic arhydrideas he used another base pyridingThe Perkin concept Supported by Kalmirquinoline , anine ) from Sodium acetateand form expected product as peukinproductpruidineCCM5-CH=0 + CCH3(0220CH5-CH=CH-GNFitting Concept it discauded by followingreach means thespeciel place. Otherwise reac" was takesCarboxylate ion is net main reachingC6H5CH=0 + CH3COONapyridineXFactors ffiring Kate of Since Reaction it ! -is PAGENCDATEnucleophilic e addition to-CHO group Sopara reac CN polition speed groups increase up foreat specially +ve Charge Ortho at & andattracing groups (-Nog, CN phalogeRnitro than berzaldehlde brenzaldehydl react On the more other falter D handpauaprevence rate of e- donating group decreaseat -CHO group . for O- methylof reach by decreasing tve chargebenzaldehydevery poorvery slowly reacts withyieldExtension of Reaction :- perkin reachof unlaturated compounds forea:used varities of SynthesisSalicyaldehydeCH=0AcONaCHS0OH+ Ac20 (Perkin)0CHHOHO- hydrycimanic acidtheCH(Countric acid)0CHC=OCoumarin+ (CH3C0)20CH3COONaCH=O(Peukin)acrylic LacidB- - brough fungl