Chapter 22 – Benzene
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Chapter 22 – Benzene Rings Topics Topics G5 and and G10 from from the IB HL Chemistr Chemistr C!rric!"!m C!rric!"!m
G5
Arenes #2$5 Ho!rs%
G$5$ 1
&ssessment 'tatement -escri)e and e.p"ain the str!ct!re of )enzene !sing phsica" and chemica" e/idence$
()* Teacher Teacher+s +s ,otes TOK: 'ee 1$$1$ or phsica" reference3 inc"!de a comparison of car)on4 car)on )ond "engths in a"anes3 a"enes3 and )enzene3 )enzene3 and the n!m)er of str!ct!ra" isomers 6ith the form!"a C7H82$ or chemica" e/idence3 inc"!de a comparison of the entha"pies of hdrogenation of )enzene3 cc"ohe.ene3 134 cc"ohe.adiene3 and 13354 cc"ohe.atriene3 and the
-escri)e and e.p"ain the re"ati/e rates of hdro"sis of )enzene compo!nds ha"ogenated in the ring and in the side4chain$
G$5$ 2
9
(n" the react actions 6ith the (H ion 6i"" )e assessed$
(rganic compo!nds that are deri/ati/es of the hdrocar)on )enzene3 C 7H73 are no6n as arenes$ The form form a specia" specia" )ranch of organic chemistr #no6n as aromatic chemistr% and ha/e properties that are distinct from a"" other organic compo!nds #6hich are no6n as a"iphatics%$ The e to !nderstanding these !ni:!e properties of arenes comes from an e.p"oration of the str!ct!re of the parent arene mo"ec!"e – )enzene itse"f$
Benzen Ben zene e is a high highly ly unsat un satur urat ated ed mo mole lecu cule le Benzene 6as ;rst iso"ated )
amma)"e "i:!id at room temperat!re 6ith a s6eet sme"" and a )oi"ing point of =0?C$ It is immisci)"e in 6ater3 forming the !pper of t6o "aers3 and is itse"f a !sef!" so"/ent for organic compo!nds$ compo!nds$ It can )e o)tained from the fractiona" disti""ation of cr!de oi" and from cata"tic reforming of gaso"ine$ The 1@1 ratio of car)on to hdrogen in )enzene indicates a high degree of !nsat!ration3 greater than that of a"enes 6ith their car)on4car)on do!)"e )ond or a"nes a"nes 6ith their car)on4car)on trip"e )ond$ This high !nsat!ration of )enzene is demonstrated ) the fact that it and a"" other arenes )!rn 6ith a /er smo >ame3 the res!"t of the presence of "arge amo!nts of !n)!rned car)on $
Benzen Ben zene e does does not no t eha eha!e !e li"e li "e other ot her unsa unsatu tura rate ted d mo mole lecul cules es
Chapter 22 – Benzene
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•
Instead3 a cc"ic arrangement of the car)on atoms 6as s!ggested and e!"e proposed the ;rst reasona)" accepta)"e str!ct!re in 1=75$
The Ke"ule Ke"ul e struct str ucture ure #or enzene en zene:: $%&%5' $%& %5'cyc cycloh lohe(a e(atri triene ene e!"eAs str!ct!re for )enzene proposed a si.4mem)ered ring of car)on atoms 6ith a"ternating sing"e and do!)"e )onds$ Dsing IDP&C nomenc"at!re3 this str!ct!re is 13354cc"ohe.atriene@
The smmetr smmetr of this mode" e.p"ains e.p"ains man man of the no6n no6n properties properties of of )enzene3 )enzene3 inc"!ding inc"!ding the fact that )enzene e.ists in on" one form3 that is3 6ith no isomers$ It a"so e.p"ains 6h 6h there are no isomers of mono4s!)stit!ted deri/ati/es s!ch as C7H5Br$ Br$ B!t 6hen the e!"e mode" is tested against other no6n properties of )enzene3 there are 6as in 6hich it does not oer a f!"" e.p"anation$ These are s!mmarized in the fo""o6ing ta)"e$
C"ear"3 the e!"e str!ct!re m!st )e de/e"oped to inc"!de a /a"id interpretation of all a/ai"a)"e data$ 84ra diraction st!dies 6hich pro/ide e"ectron densit maps ha/e he"ped pro/ide f!rther insights into the str!ct!re$
The special staility o# the enzene ring is the result o# delocalized electrons The c!rrent mode" for the str!ct!re of )enzene3 "ie the e!"e mode"3 is a cc"ic str!ct!re in 6hich a frame6or of sing"e )onds attaches each car)on to one on either side and to a 2
hdrogen atom$ Each of the si. car)on atoms is sp h)ridized3 and forms three sigma F )onds 6ith ang"es of 120?3 maing a p"anar shape$ This "ea/es one !nh)ridized p e"ectron on each car)on atom 6ith its d!m))e"" shape perpendic!"ar to the p"ane of the ring3 so its e"ectron densit is in t6o regions3 one a)o/e and one )e"o6 the ring$ B!t instead of pairing !p to form discrete a"ternating )onds3 the p or)ita"s eecti/e" o/er"ap in )oth directions3 spreading themse"/es o!t e/en" to )e shared ) a"" si. car)on atoms$ This forms a de"oca"ized e"ectron c"o!d in 6hich e"ectron densit is concentrated in t6o do!ghn!t4shaped rings a)o/e and )e"o6 the p"ane of the ring$
This is a smmetrica" arrangement 6here a"" the p e"ectrons are eecti/e" shared e:!a"" ) a"" of the )onded car)on atoms$ e no6 from simi"ar e.amp"es of de"oca"ization co/ered in o!r !nit on )onding that it is associated 6ith an increase in the sta)i"it of a mo"ec!"e as it is associated 6ith a "o6ering of the interna" energ$ Benzene can )e represented as the t6o resonance str!ct!res@
Ho6e/er3 the !s!a" con/ention for depicting that
)enzene is a resonance h)rid is@
e can no6 !se this mode" of the str!ct!re of )enzene to interpret the o)ser/ations made ear"ier that 6ere not consistent 6ith the e!"e str!ct!re$ 1$ Only one isomer of, for example, 1,2-dibromobenzene exists. &s )enzene is a smmetrica" mo"ec!"e 6ith no a"ternating sing"e and do!)"e )onds3 a"" ad*acent positions in the ring are e:!a"$ or e.amp"e3 1324di)romo)enzene3 C 7HBr2
2$ All carbon-carbon bond lengths in benzene are equal and intermediate in length between single and double bonds. This is )eca!se each )ond contains a share of three e"ectrons )et6een the )onded atoms$ -ata from 84ra ana"sis is sho6n in the ta)"e )e"o6$
$ ata for enthalpy of hydrogenation of benzene suggests an unusually stable compound. The specia" sta)i"it of )enzene is the res!"t of the spreading of the e"ectrons ) de"oca"ization )eca!se this minimizes the rep!"sion )et6een them$ e can ca"c!"ate the "o6ering of interna" energ res!"ting from this ) comparing the entha"p of hdrogenation of )enzene 6ith that of other !nsat!rated cc"ic 74car)on mo"ec!"es$
e can see that the entha"p change for 134cc"ohe.adiene 6hen forming cc"ohe.ane is a"most t6ice that for cc"ohe.ene – e.act" 6hat 6o!"d )e e.pected gi/en that it has t6o do!)"e )onds$ If )enzene had three do!)"e )onds3 then its /a"!e sho!"d )e three times this ;g!re3 the /a"!e sho6n a)o/e for the hpothetica" 13354 cc"ohe.atriene$ The fact that its entha"p of hdrogenation is 152 Jmo" lower than this e.pected /a"!e is consistent 6ith the fact that it does not ha/e three discrete do!)"e )onds$ This energ "o6ering as a res!"t of the de"oca"ized ring of e"ectrons is ca""ed the resonance energ or the sta)i"ization energ of )enzene$ $ !enzene is reluctant to undergo addition reactions and is more li"ely to undergo substitution reactions. -espite its !nsat!ration3 addition reactions to )enzene are energetica"" not fa/ored as the 6o!"d in/o"/e disr!pting the entire c"o!d of de"oca"ized e"ectrons$ In other 6ords3 the
resonance energ 6o!"d ha/e to )e s!pp"ied and the prod!ct3 "acing the de"oca"ized ring of e"ectrons3 6o!"d )e less sta)"e$ Instead )enzene is more
"ie" to !ndergo substitution reactions that preser/e the arene ring$ or e.amp"e3 )enzene reacts 6ith Br 2 in the presence of a s!ita)"e cata"st to form the s!)stit!ted prod!ct )romo)enzene3 C7H5Br$
e 6i"" disc!ss these s!)stit!tion reactions of )enzene "ater in this chapter$ B!t ;rst3 6e 6i"" tae a "oo at the chemistr of some of the s!)stit!ted deri/ati/es of )enzene in/o"/ing ha"ogens$
)alogenated deri!ati!es o# enzene: reacti!ity depends on the position o# the halogen e "earned in Chapter 10 that ha"ogenoa"anes !ndergo n!c"eophi"ic s!)stit!tion reactions of the tpe@
This is no6n as a hdro"sis reaction and prod!ces an a"coho"$ It occ!rs as a res!"t of the po"ar nat!re of the car)on4 ha"ogen )ond ca!sing the car)on atom to )e e"ectron de;cient and so 9 s!scepti)"e to attac ) n!c"eophi"es s!ch as (H @
Ko! ma ;nd it !sef!" to re/ie6 that section )efore reading on3 as 6e are going to )e in/estigating to 6hat e.tent a simi"ar reaction 6i"" happen 6ith ha"ogenated arenes$ &renes can )e s!)stit!ted 6ith ha"ogens in t6o :!ite distinct 6as and as 6e 6i"" see3 this has a signi;cant eect on their reacti/it$
e 6i"" compare the tendenc of these t6o tpes of ha"ogenated arenes to !ndergo hdro"sis reactions$
Ring sustituents Compo!nds s!ch as ch"oro)enzene are re"ati/e" inert and on" !ndergo n!c"eophi"ic s!)stit!tion of the ha"ogen 6ith e.treme dic!"t$ This is "arge" d!e to the fact that the e"ectronegati/e ha"ogen dra6s the de"oca"ized e"ectron charge from the )enzene ring onto the car)on of the car)on4ha"ogen )ond$ This red!ces the magnit!de of the MN on the car)on and hence decreases its s!scepti)i"it to n!c"eophi"ic attac$
Interaction )et6een non4)onding e"ectrons in the ha"ogen and the de"oca"ized ring e"ectrons a"so strengthens the car)on4ha"ogen )ond3 f!rther contri)!ting to the "ac of reacti/it$ Conse:!ent"3 ch"oro)enzene 6i"" on" !ndergo hdro"sis 6ith ,a(H !nder conditions of high press!re and temperat!re$
*ide'chain sustituents Compo!nds s!ch as ch"orometh")enzene !ndergo hdro"sis reactions 6ith n!c"eophi"es in m!ch the same 6a as ha"ogenoa"anes do$ 'o the react re"ati/e" easi" 6hen heated 6ith a:!eo!s ,a(H3 forming the a"coho"@
This is )eca!se the en/ironment of the car)on4ha"ogen )ond is simi"ar here to that in ha"ogenoa"anes and the e"ectron deficient #MN % car)on is s!scepti)"e to n!c"eophi"ic attac in a simi"ar 6a$
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Chapter 22 – Benzene
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+(ercises: Benzene is a hdrocar)on$ rite the e:!ation for its comp"ete com)!stion and sa 6hat tpe of >ame o! 6o!"d e.pect to see 6hen it )!rns$
-escri)e the )onding in a )enzene mo"ec!"e and !se it to e.p"ain )enzeneAs energetic sta)i"it$
Ethene contains a car)on4car)on do!)"e )ond and can !ndergo an addition reaction 6ith hdrogen$ o!"d o! e.pect )enzene to react 6ith hdrogen more or "ess readi" than etheneQ E.p"ain o!r ans6er$
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Chapter 22 – Benzene
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G$, +lectrophilic sustitution reactions # Ho!rs% &ssessment 'tatement -escri)e3 !sing e:!ations3 the nitration3 ch"orination3 a""ation3 and ac"ation of )enzene$
()* Teacher+s ,otes 2 Inc"!de the !se of for the )enzene ring as 6e"" as form!"as s!ch as C7H5,(2$ The introd!ction of more than one gro!p into the )enzene ring 6i"" not )e assessed here$
-escri)e and e.p"ain the mechanisms for the nitration3 ch"orination3 a""ation3 and ac"ation of )enzene$
G$1 0$
-escri)e3 !sing e:!ations3 the nitration3 ch"orination3 a""ation3 and ac"ation of meth")enzene$
2
G$1 0$
-escri)e and e.p"ain the directing eects and re"ati/e rates of reaction of dierent s!)stit!ents on a )enzene ring$
G$1 0$1
G$1 0$2
Inc"!de the formation of ,( concentrated 2
N
from the reaction )et6een N
nitric acid and s!"f!ric acids3 and the formation of C" 3 N
N
R 3 and RC( from reactions in/o"/ing a"!min!m
Inc"!de the s!)stit!ents –CH 3 –(H3 and –,(2$ Inc"!de the terms acti/ating and deacti/ating$ (n" the introd!ction of one f!rther gro!p 6i"" )e assessed3 e.cept for the formation of 23374 trich"oropheno"$ The directing aects can )e e.p"ained in terms of the charge distri)!tion of the intermediates$ The s"ight" increased reacti/it d!e to the presence of –CH can )e e.p"ained in terms of its e"ectron4re"easing nat!re$ The great" increased reacti/it d!e to the presence of –(H can )e e.p"ained in terms of its partia" donation of a non4 )onded e"ectron pair$ The decreased reacti/it d!e to the presence of –,(2 can )e e.p"ained in terms of its
+lectrophilic sustitution o# enzene e ha/e seen that the de"oca"ized e"ectrons in )enzene gi/e it a specia" sta)i"it$ This means that addition reactions3 6hich 6o!"d "ead to "oss of the sta)"e arene ring3 are genera"" not fa/ored as the prod!cts 6o!"d )e of higher energ than the reactant$ Instead3 s!)stit!tion reactions3 in 6hich one #or more% of the hdrogen atoms is rep"aced ) an incoming gro!p3 occ!r more readi" as these "ead to prod!cts in 6hich the arene ring is conser/ed$ &s the de"oca"ized ring of e"ectrons represents an area of e"ectron densit3 )enzene is s!scepti)"e to attac ) e"ectrophi"es$ Therefore3 most tpica""3 the arenes !ndergo electrophilic sustitution reactions$
These e"ectrophi"ic s!)stit!tion reactions of )enzene ha/e high acti/ation energies and so proceed rather s"o6"$ This is )eca!se the ;rst step in the mechanism3 in 6hich an e"ectron pair from )enzene is attracted to the e"ectrophi"e3 "eads to
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Chapter 22 – Benzene
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a disr!ption of the smmetr of the de"oca"ized sstem$ The !nsta)"e car)ocation intermediate that forms has )oth the entering atom or gro!p and the "ea/ing hdrogen temporari" )onded to the ring$
The incomp"ete circ"e inside the ring sho6s its "oss of smmetr3 6ith the positi/e charge N
distri)!ted o/er the )!" of the mo"ec!"e$ Loss of a hdrogen ion3 H 3 from this intermediate "eads to the e"ectrica"" ne!tra" s!)stit!tion prod!ct as t6o e"ectrons from the C9H )ond mo/e to regenerate the aromatic ring$ This prod!ct is more sta)"e3 as sho6n )e"o6$
& /ariet of s!)stit!ents can tae part in these reactions3 so the can )e !sed to introd!ce dierent f!nctiona" gro!ps into the ring$ In the fo!r descriptions that fo""o6 #nitration3 ch"orination3 a""ation and ac"ation%3 m!ch of the disc!ssion of the )est conditions for a reaction is )ased on generating an e"ectrophi"e 6hich is a)"e to attract e"ectrons from )enzene s!cient" strong" to disr!pt the ring for a s!)stit!tion prod!ct to form$
-itration o# enzene The nitration of )enzene is the s!)stit!tion of 4H ) 4,( 2 to form nitro)enzene3 C 7H5,(2@
N
The e"ectrophi"e for the reaction2 is ,( , the nitroni!m ion$ This is generated ) !sing a nitrating mi(ture: a mi.t!re of concentrated nitric and concentrated s!"f!ric acids at 50?C$ &s the stronger of the t6o acids3 s!"f!ric acid protonates the nitric acid3 6hich then "oses a 2 mo"ec!"e of 6ater to prod!ce ,(
N
@
,( is a strong e"ectrophi"e and reacts 6ith the e"ectrons of the )enzene ring to form the N 2 car)ocation intermediate$ Loss of a proton from this "eads to re4formation of the arene ring in the prod!ct nitro)enzene3 6hich appears as a e""o6 9
oi"$ The hdrogen ion re"eased reacts 6ith the to re4form s!"f!ric acid3 H 2'(@ )ase H'(
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Chapter 22 – Benzene
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Chlorination o# enzene The ch"orination of )enzene in/o"/es the s!)stit!tion of 4H ) 4C" to form ch"oro)enzene3 C 7H5CI@
N
The e"ectrophi"e for the reaction is CI $ This is prod!ced ) !se of a halogen carrier cata"st3 s!ch as &"C" $ Beca!se this is an e"ectron4de;cient species3 it acts as a Le6is acid3 accepting a "one pair of e"ectrons from the ha"ogen and so ind!cing po"arit in the mo"ec!"e@
The positi/e" charged end of the ha"ogen mo"ec!"e is no6 e"ectrophi"ic and attacs the )enzene N ring$ Hetero"tic ;ssion of the ch"orine mo"ec!"e forms C" 6hich )onds to a car)on atom in the ring3 forming the car)ocation$ This then deprotonates to form ch"oro)enzene$ The hdrogen ion 9 re"eased reacts 6ith C" to form hdrogen ch"oride and regenerate the &"C" cata"st@
The reaction is carried o!t in anhdro!s conditions !sing dr ether3 as a"!mini!m ch"oride reacts /io"ent" 6ith 6ater$ '!)stit!tion 6ith ha"ogens ) this method 6ors 6e"" for )oth )romine and ch"orine$ The reaction goes 6ith dic!"t 6ith iodine and too /io"ent" 6ith >!orine to )e of an !se$ Ha"ogen carrier cata"sts a"so inc"!de eBr or e3 6hich reacts 6ith the ha"ogen to form the iron #III% ha"ide d!ring the reaction@ 2 e N Br2 eBr
→
2
The contrast )et6een the reacti/it of )enzene and an a"ene s!ch as ethene is c"ear" seen in their reactions 6ith ha"ogens$ hereas ethene reacts readi" 6ith the ha"ogen in the dar to prod!ce the addition compo!nd3 there is no reaction )et6een )enzene and ch"orine on their o6n in the dar$
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Chapter 22 – Benzene
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Al"ylation o# &""ation of )enzene in/o"/es the s!)stit!tion of 4H ) an a"" gro!p R #for e.amp"e3 4CH %3 to form the a"" )enzene compo!nd3 in this case meth")enzene3 C 7H5CH@
N
The e"ectrophi"e is R 3 6hich is generated !sing the ha"ogenoa"ane and a cata"st of &"C" in dr ether #anhdro!s conditions%$ &s 6ith the s!)stit!tion of ha"ogens3 the cata"st acts to accept an e"ectron pair3 so he"ping the ha"ogenoa"ane to sp"it hetero"tica"" and generate a positi/e ion3 a car)ocation@
This car)ocation then acts as the e"ectrophi"e3 attacing )enzene and proceeding ) a simi"ar mechanism to those sho6n a)o/e 6here the de"oca"ized ring is temporari" disr!pted and then reformed as a proton is "ost@
This tpe of reaction3 6here an arene reacts 6ith reagents that can gi/e rise to a positi/e" charged car)on atom3 is an e.amp"e of a .riedel'Cra#ts reaction% named for the t6o chemists 6ho de/e"oped the process in 1=$ &"tho!gh these reactions are )road" !sef!" for the snthesis of a range of a"")enzenes3 riede"4Crafts a""ations ha/e some "imitations$ (ne of the pro)"ems is that it is often dic!"t to stop the reaction after a sing"e s!)stit!tion3 )eca!se )onding of the ;rst a"" gro!p acti/ates the ring to6ards f!rther s!)stit!tion$ This 6i"" )e e.p"ained "ater in this section$
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Chapter 22 – Benzene
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Acylation o# enzene &c"ation means s!)stit!tion of an RC(4 ac" gro!p into the )enzene ring$ or e.amp"e3 s!)stit!tion ) the ethano" gro!p CH C(4 6i"" ie"d the etone3 phen"ethanone@
This is another e.amp"e of a riede"4Crafts reaction 6here the e"ectrophi"e is a car)ocation$ In N this case3 it is an ac" cation3 RC( $ This is generated from an ac" ch"oride !sing the cata"st &"C" to ind!ce po"arit3 so that the ch"oride !ndergoes hetero"tic ;ssion to prod!ce N the ac" cation RC( @
The ac" cation then attacs the )enzene ring3 proceeding 6ith a simi"ar mechanism to the a""ation reaction@
The ac"ation reaction3 !n"ie the a""ation reaction3 !s!a"" stops after the ;rst s!)stit!tion$ This is )eca!se the ac"ated )enzene prod!ct has a deacti/ated ring that maes f!rther s!)stit!tion more dic!"t$ e 6i"" e.p"ain this dierence )e"o6$ The riede"4Crafts reactions of a""ation and ac"ation pro/ide a means of attaching car)on atoms to the )enzene ring and so )!i"ding !p side4chains$ The are therefore /er important reactions in snthesis path6as$
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Chapter 22 – Benzene
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& s!mmar of the prod!cts res!"ting from the e"ectrophi"ic s!)stit!tion reactions of )enzene is sho6n on the fo""o6ing page$
.urther sustitution reactions o# arenes Beca!se )enzene is a smmetrica" mo"ec!"e3 there is on" one possi)"e monos!)stit!ted prod!ct in each of the e.amp"es a)o/e$ B!t 6hat happens 6hen an arene that a"read has a f!nctiona" gro!p attached is the reactant for these e"ectrophi"ic s!)stit!tion reactionsQ In these cases3 the ;rst attached gro!p3 not the attacing gro!p3 determines the f!rther reacti/it$ There are t6o factors to consider here$ 1$ The re"ati/e reacti/it of the ring – 6hether it is more or "ess "ie" than )enzene to !ndergo s!)stit!tion$ 'ince s!)stit!tion is e"ectrophi"ic3 it is in>!enced ) 6hether e"ectrons are dra6n a6a from or p!shed to6ards the )enzene ring ) the e.isting gro!p$ 2$ The directing eect – 6hich positions in the ring3 re"ati/e to the ;rst gro!p3 6i"" )e more "ie" to !ndergo s!)stit!tion$ Ring positions are n!m)ered 1473 6ith 1 assigned to the origina" s!)stit!ent3 and ) IDP&C r!"es3 the sma""est possi)"e n!m)ers !sed for s!)se:!ent s!)stit!ents$ ,ote that positions 2 and 73 and and 5 are identica" d!e to the smmetr of the ring$
&s 6e 6i"" see thro!gh a st!d of dierent tpes of s!)stit!ent #ring acti/ators and ring deacti/ators%3 these t6o factors are !s!a"" inter4re"ated$
Ring acti!ators '!)stit!ent gro!ps that ca!se the arene ring to )e more reacti/e to6ards e"ectrophi"ic s!)stit!tion than )enzene are no6n as ring acti/ators$ These are gro!ps that increase the e"ectron densit aro!nd the ring3 maing it more reacti/e to e"ectrophi"es$ e 6i"" st!d t6o e.amp"es here3 4CH and 4(H$ &s 6e ha/e seen in disc!ssions a)o!t the sta)i"it of car)ocations3 the meth" gro!p 4CH is an e"ectron4donating gro!p 6ith a positi/e ind!cti/e eect@
Pheno"3 C7H5(H3 is fo!nd to )e signi;cant" more reacti/e than )enzene to6ards e"ectrophi"ic s!)stit!tion$ These ring4 acti/ating gro!ps direct the incoming gro!p main" into the 23 or 7 positions$
or e.amp"e3 in reactions 6ith ch"orine@
'o the mono4ch"oro deri/ati/e of meth")enzene 6i"" )e a mi.t!re of 24ch"orometh")enzene and 4 ch"orometh")enzene$ In pheno"3 the 4(H gro!p acti/ates the ring so strong" that s!)stit!tion3 for e.amp"e ) ch"orine3 occ!rs at a"" three of the 23 and 7 positions$ The reaction proceeds 6itho!t the !se of a ha"ogen carrier cata"st$
The prod!ct is 23374trich"oropheno"$ This appears as a 6hite precipitate immediate" on contact )et6een the ch"orine and pheno"$ To e.p"ain the directing eect of a s!)stit!ted gro!p3 6e can !se a simi"ar approach to the one !sed to e.p"ain
The most sta)"e of the intermediates 6o!"d the one 6here the positi/e charge is concentrated on C1 of the ring – that is the tertiar car)on3 6hich is attached to the meth" gro!p$ This is )eca!se the positi/e ind!cti/e eect of the meth" gro!p he"ps to decrease this positi/e charge and so sta)i"ize the ion$ e can see that in the 2 and positions3 there is a resonance form corresponding to this more sta)"e intermediate$ B!t in the 4intermediate3 none of the resonance forms p!t the charge on the tertiar car)on atom and so the cannot )e sta)i"ized in this 6a$ Therefore3 the o/era"" energ of this intermediate is higher3 and s!)stit!tion into the position is not fa/ored$ In the reaction 6ith pheno"3 the 23 and 7 positions are a"so the sites for f!rther s!)stit!tion d!e to sta)i"ization of the intermediates$ B!t the reason for this is dierent$ Taing s!)stit!tion 6ith ch"orine as an e.amp"e3 6e can again consider the cationic intermediates that 6o!"d form ) possi)"e s!)stit!tion into the 23 and positions$ Here a"so the intermediates are sta)i"ized ) resonance3 as sho6n in the ;g!re on the fo""o6ing page for each case$
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Chapter 22 – Benzene
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e can see that s!)stit!tion into the 2 and positions )oth form intermediates that are sta)i"ized ) four resonance forms$ This is )eca!se s!)stit!tion in these positions "eads to the de"oca"ization )eing e.tended o!t onto the o.gen atom 6here the positi/e charge is sta)i"ized ) donation of an e"ectron pair from o.gen$ B contrast3 s!)stit!tion into position ie"ds an intermediate that is sta)i"ized ) on" three resonance forms3 and none of these can e.tend the positi/e charge to the o.gen$ Therefore in this case too the intermediate from 4s!)stit!tion is of higher energ and so its formation is not fa/ored$ In conc"!sion3 the meth" gro!p 4CH and hdro." gro!p 4(H )oth act as ring acti/ators and direct s!)se:!ent s!)stit!ents to the 23 and 7 positions$ The directiona" eect is a res!"t of charge distri)!tions in the reaction intermediates3 6here the ro!te proceeding /ia the most sta)"e intermediate is fa/ored$
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Chapter 22 – Benzene
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Ring deacti!ators '!)stit!ent gro!ps that ca!se the arene ring to )e "ess reacti/e to6ards e"ectrophi"ic s!)stit!tion than )enzene are no6n as ring deacti!ators/ These are gro!ps that decrease the e"ectron densit aro!nd the ring3 and so desta)i"ize the car)ocation intermediate and ca!se it to form more s"o6"$ e 6i"" disc!ss the nitro gro!p3 4 ,( 2 here$ The nitro gro!p 6ithdra6s e"ectrons from the ring d!e to the e"ectronegati/it of the nitrogen and o.gen atoms$ &"so3 the e"ectrons in its do!)"e )ond con*!gate 6ith the e"ectrons in the ring ca!sing the e"ectron densit in the ring to )e red!ced$ There are no "one pairs on the nitrogen atom #e:!i/a"ent to the o.gen atom in pheno" as the atom )onded direct" to the ring% to oset this eect ) donating e"ectrons to the ring@
,itro)enzene3 and other deri/ati/es possessing /er e"ectronegati/e atoms andJor do!)"e )onds3 therefore react m!ch "ess readi" then )enzene in e"ectrophi"ic s!)stit!tion reactions$ 'o3 for e.amp"e3 the nitration of nitro)enzene re:!ires more se/ere reaction conditions than the nitration of )enzene$ Ring4deacti/ating gro!ps s!ch as ,( 2 !s!a"" direct the incoming gro!p into the #and e:!i/a"ent 5% position$ or e.amp"e3 in the nitration reaction !sing the nitrating mi.t!re descri)ed ear"ier3 the o/era"" reaction is@
The prod!ct 6i"" )e 134dinitro)enzene 6ith a sma"" amo!nt of 13354trinitro)enzene@
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Chapter 22 – Benzene
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Chapter 22 – Benzene
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The directiona" eect can again )e e.p"ained ) a consideration of the sta)i"ities of the possi)"e reaction intermediates$
Here the cationic intermediate is desta)i"ized ) the e"ectron46ithdra6ing 4 ,( 2 gro!p$ The intermediate formed ) reaction at the position is the most sta)"e )eca!se it is the "east desta)i"ized ) ha/ing t6o ad*acent positi/e charges as sho6n in the ;g!re a)o/e$
*ummary o# the e0ects o# sustituent groups on electrophilic aromatic sustitution In genera"3 sat!rated gro!ps are ring acti/ators and direct to positions 2 and 6hi"e !nsat!rated gro!ps are ring deacti/ators and direct to position $ #Ha"ogens are an e.ception to this
%$ #hempocalypse
Chapter 22 – Benzene
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+lectrophilic sustitution o# methyl enzene
&s 6o!"d )e e.pected3 meth")enzene !ndergoes t6o distincti/e tpes of reaction$ 1$ 'ide4chain s!)stit!tion@ this proceeds /ia a free4radica" mechanism3 simi"ar to the reaction 6ith a"anes descri)ed in Chapter 0$ It is descri)ed )e"o6 for the reaction 6ith ch"orine$ 2$ E"ectrophi"ic s!)stit!tion of the ring@ this occ!rs more readi" than for )enzene d!e to the ring4acti/ating propert of the 4CH gro!p$ '!)stit!ents are directed to the 2 and positions$
-itration o# methyl enzene Dsing the nitrating mi.t!re of concentrated nitric and s!"f!ric acids3 meth")enzene !ndergoes s!)stit!tion ) one3 t6o or three nitro gro!ps3 depending on the conditions$ The reaction can )e sho6n th!s@
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Chapter 22 – Benzene
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The prod!ct 23374trinitrometh")enzene is a"so no6n as T,T #trinitroto"!ene%3 a 6e""4no6n e.p"osi/e$ hen s!pp"ied 6ith the /er high energ of acti/ation to achie/e detonation3 it dissociates into car)on mono.ide3 6ater and nitrogen as 6e"" as !n)!rned car)on$ The "arge /o"!me change on this reaction is 6hat gi/es rise to the e.p"osi/e eect$
Chlorination o# methylenzene Side-chain substitution hen ch"orine is )!))"ed thro!gh )oi"ing meth")enzene in the presence of DS "ight3 s!)stit!tion occ!rs in the side chain to ie"d a mi.t!re of prod!cts@
The reaction in/o"/es the homo"tic ;ssion of ch"orine to prod!ce free radica"s 6hich then s!)stit!te in the 4CH gro!p in the same 6a as descri)ed for a"anes in Chapter 0$
Ring substitution Dsing a ha"ogen carrier cata"st s!ch as &"C" 3 meth")enzene !ndergoes s!)stit!tion ) ch"orine3 ie"ding a mi.t!re of the 24ch"oro and 4ch"oro s!)stit!ted prod!cts@
The reaction occ!rs more readi" than 6ith )enzene$
Al"ylation o# methylenzene Dsing the ha"ogenomethane and &"C" as the ha"ogen carrier cata"st3 meth")enzene !ndergoes a""ation ie"ding a mi.t!re of 1324 and 134dimeth")enzene@
&gain3 the reaction occ!rs more readi" than 6ith )enzene$ B !sing dierent ha"ogenoa"anes3 dierent a"" side chains can )e introd!ced into the prod!ct$
Acylation o# methylenzene Dsing the ac" ch"oride and 6arming the mi.t!re 6ith &"C" as the ha"ogen carrier cata"st3 meth")enzene can )e ac"ated at the 2 and positions$ The prod!ct from the reaction 6ith ethano" ch"oride is a mi.t!re of the etones 24meth"phen"ethanone and 4 meth"phen"ethanone@
