2 Nitration of Anisole ACTIVATED RING HNO3 H2SO4 anisole ortho para +H2SO4anisoleorthoparaReacts fasterthan benzene= “ACTIVATED”The -OCH3 group when it preexists on the ring gives onlyortho and para products, and no meta.Substituents that cause this result are called o,p directorsand they usually activate the ring.
3 Nitration of Methyl Benzoate DEACTIVATED RINGNitration of Methyl BenzoateHNO3H2SO4metamethyl benzoateReacts slowerthan benzene= “DEACTIVATED”The -COOMe group when it preexists on the ring gives onlymeta, and no ortho or para products.Substituents that cause this result are called m directorsand they usually deactivate the ring.
4 SUBSTITUENT CATEGORIES Most ring substituents fall into one of these two categories:o,p - directorsm- directorsactivate the ringdeactivate the ringWe will look at one of each kind in order tounderstand the difference…..
15 ortho, para - Directing Groups Groups that donateelectron densityto the ring.PROFILE::E+increasedreactivity+I Substituent+R Substituent..These groups also“activate” the ring, ormake it more reactive.CH3-O-CH3-....R-CH3-N-..-NH2..The +R groups activatethe ring more stronglythan +I groups.-O-H..
16 meta - Directing Groups Groups that withdrawelectron density fromthe ring.PROFILE:d+d-E+decreasedreactivity-I Substituent-R SubstituentThese groups also“deactivate” the ring,or make it less reactive.++--SO3H
17 Halides - o,p Directors / Deactivating THE EXCEPTIONHalides - o,p Directors / DeactivatingHalides represent a special case:..::They are o,p directing groupsthat are deactivatingE+They are o,p directors (+R effect )They are deactivating ( -I effect )Most other other substituents fallinto one of these four categories:+R / -I / o,p / deactivating1) +R / o,p / activating-F-Cl-Br-I2) +I / o,p / activating3) -R / m / deactivating4) -I / m / deactivating
20 GROUPS ACTING IN CONCERT stericcrowdingo,p directorm-directorverylittleformedHNO3H2SO4When groups direct to thesame positions it is easy topredict the product.majorproduct
21 GROUPS COMPETING X o,p-directing groups win over m-directing groups toocrowdedXHNO3+H2SO4
22 RESONANCE VERSUS INDUCTIVE EFFECT HNO3H2SO4majorproduct+Iresonance effects are moreimportant than inductive effects
23 SOME GENERAL RULES 1) Activating (o,p) groups (+R, +I) win over deactivating (m) groups (-R,-I).2) Resonance groups (+R) win over inductive (+I) groups.3) 1,2,3-Trisubstituted products rarely form due toexcessive steric crowding.4) With bulky directing groups, there will usually be morep-substitution than o-substitution.5) The incoming group replaces a hydrogen, it will notusually displace a substituent already in place.
26 BROMINE IN WATER .. .. .. .. .. .. - : : : : : .. .. .. .. .. .. : .. This reagent works only with highly-activated ringssuch as phenols, anisoles and anilines.............-:::::........+....:+..+bromoniumionetc
27 PHENOLS AND ANILINES REACT Br2H2OAll availablepositions arebromiated.Br2H2O
29 AROMATICITY QUESTION: Are all fully-conjugated, cyclic systems aromatic?36 kcal/mole RE???KNOWNAROMATIC??Do these other rings have thesame kind of stability as benzene?
30 .. HUCKEL 4n+2 RULE AROMATICITY Prediction: Compounds that have 4n+2 pi electrons ina cyclic array will be aromatic.4n+2 series = 2, 6, 10, 14, 18, 22, 26, 30 …….. etc.The rule was derived by observation ofPOLYCYCLIC AROMATIC COMPOUNDS61014benzenenaphthaleneanthracene1418
31 Aromatic Compounds Have Special Properties 1) Must be cyclic and fully conjugated2) Must have 4n+2 p electrons in the system3) Must have the entire system planar4) Will have no unpaired electrons inthe p system molecular orbitalsplanarsystemCharacteristic Properties:1) Special chemical stability2) Give substitution reactions instead of addition3) Show a ring currrent in the NMR(Chapter 13, Section 13.8)
32 Ring Current in Benzene Circulating p electronsDeshieldedHHA proton placed in themiddle of the ring wouldbe shielded!BoSecondary magnetic fieldgenerated by circulating pelectrons deshields aromaticprotons
33 RING CURRENTS CAN BE SEEN IN THE NMR AROMATIC - SHOWSA RING CURRENT-1.0 ppm2.0 ppm18 pinner hydrogens ppmouter hydrogens ppm-1.4 ppm
34 HUCKEL MNEMONIC CYCLIC p MOLECULAR ORBITAL ARRAYS 1) Draw a circle 2) Inscribe the ring in the circle point down3) Each point where the polygon (ring)touches the circle represents anenergy level.4) Place the correct number of electrons inthe orbitals, starting with the lowestenergy orbital first.ENRGY
35 BENZENE p M.O. levels Aromatic compounds will have all of the occupied completelyfilled with nounpairedelectrons.6p electronsAROMATICENRGYclosed*shell(*completedlevel)
36 CYCLOBUTADIENE . . ANTI-AROMATIC Does not have a completed shell and has unpaired electrons.4p electronsDoes not have 4n+2 p electrons.ENRGYopen*shell..(*incompletelevel)DIRADICAL
37 CYCLOOCTATETRAENE ANTI-AROMATIC 8p electrons Does not have a completed shell and has unpaired electrons.Does not have 4n+2 p electrons.ENRGYopen*shellnot planar(*incompletelevel)
38 SOME CYCLIC POLYENES 10 p = 4(2) + 2 16 p 12 p 18 p = 4(4) + 2 ANNULENES10 p = 4(2) + 216 pBUT CANNOT BE PLANAR(see the hydrogens)-annulene-annulene12 p18 p = 4(4) + 2AROMATIC-annulene-annulene-annulene14 p = 4(3) + 220 pAROMATIC-annulene
39 HETEROCYCLIC COMPOUNDS The unshared pairs are in thecyclic pi system (one pair in each case).............pair not inp systempyridinepyrrolefuranthiopheneAll have 6p electrons in a cyclic array.These compounds have reactions similar to benzene,rather than to alkenes. They will give substitution reactionsunder conditions similar to those for benzene.
40 X CYCLOPENTADIENYL ANION AND CATION .. - - Cl The methylene hydrogens areacidic.This compounddoes not dissolvein water.NaOEtEtOHX6p4p..AROMATIC-ANTI-AROMATIC+-ClThe cation does notform at all.The anion forms readily.
41 X CYCLOHEPTATRIENYL ANION AND CATION - .. - Cl This compound ionizes easily inwater.The methylenehydrogens arenot acidic.XNaOEtEtOH8p6p..+-ANTI-AROMATICAROMATIC-ClDoesn’t formeasily.Dissolves in water.