AROMATIC SUBSTITUTION REACTIONS . 36 kcal/mol resonance. “Double” bonds in a benzene ring do not react like those in an alkene. easier . E +. Instead of addition reactions, benzene rings give substitution reactions (resonance preserved). More powerful electrophiles
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AROMATIC SUBSTITUTION REACTIONS 36 kcal/mol resonance “Double” bonds in a benzene ring do not react like those in an alkene. easier E+ Instead of addition reactions, benzene rings give substitution reactions (resonance preserved). More powerful electrophiles are required. A few of the common aromatic substitution reactions are on the next slide. E+ more difficult
Some Substitution Reactions of Benzene Halogenation + Friedel-Crafts Alkylation + Friedel-Crafts Acylation + - + + Nitration + - Sulfonation +
MECHANISMS All of the reactions follow the same pattern of mechanism. The reagents combine to form a strong electrophile E+ ,and its partner (:X ), which react as follows: ELECTROPHILIC AROMATIC SUBSTITUTION (+) + + HX (+) + slow :X intermediate restores ring resonance benzenium ion* resonance structures are shown by the (+) symbols * also called a benzenonium ion
ENERGY PROFILE FOR AROMATIC SUBSTITUTION (+) benzenium intermediate Transition state 1 + (+) Transition state 2 intermediate Ea activation energy + + H slow fast STEP 1 STEP 2
Cl Al Cl Cl HALOGENATION - 1 Formation of the Chloronium Ion Complex .. .. : : : : d+ .. .. .. .. .. .. d- : : : : : .. .. .. .. .. .. : : : : .. .. sp2 .. : : .. .. .. + - : : : .. .. .. : : .. chloronium ion complex
HALOGENATION - 2 Chlorination of Benzene - Cl2 + AlCl3 - + + [ ] benzenium ion chloronium ion complex + HAlCl4 HCl + AlCl3
FRIEDEL-CRAFTS ALKYLATION - 1 Formation of a Carbocation Complex .. .. : : : : d+ .. .. .. .. d- : : : .. .. .. .. : : : : .. .. Other aliphatic R-Cl may be used .. : : .. .. - + : : .. .. : : .. carbocation
FRIEDEL-CRAFTS ALKYLATION - 2 Friedel-Crafts Alkylation - CH3Cl + AlCl3 - + + [ ] + HAlCl4 HCl + AlCl3
FRIEDEL-CRAFTS ALKYLATION - 3 REARRANGEMENTS ARE COMMON IN FRIEDEL-CRAFTS ALKYLATION AlCl3 - + - + carbocation rearrangement AlCl3 +
FRIEDEL-CRAFTS ACYLATION - 1 Formation of an Acylonium Complex .. .. : : : : .. .. .. .. d+ d- : : : .. .. .. .. : : : : .. .. .. Other acid chlorides (RCOCl) may be used : : .. .. - : : .. .. + : : .. acylium ion (acylonium ion) Rearrangements DO NOT occur
FRIEDEL-CRAFTS ACYLATION - 2 Friedel-Crafts Acylation - + AlCl3 - + + [ ] + HAlCl4 HCl + AlCl3
.. .. .. : : : .. .. .. + + + H2SO4 : + .. : : : : : .. .. - - .. NITRATION - 1 Formation of Nitronium Ion nitronium ion .. : Powerful Electrophile + Reacts with benzene. : ..
NITRATION - 2 Nitration of Benzene : .. .. .. HNO3 + : : + : : + H2SO4 .. - : .. .. : .. : + .. -
SULFONATION - 1 Fuming Sulfuric Acid . H2SO4 SO3 .. .. : : .. .. : .. .. : : .. .. sulfur trioxide
SULFONATION - 2 Sulfonation of Benzene - . H2SO4 SO3 + H3O+ D can be reversed in boiling water or steam (acidic) + H2SO4
DIRECTIVITY AND RING ACTIVATION / DEACTIVATION
ortho, meta and para Positions m-nitrotoluene 3-nitrotoluene ipso 1 o- ortho 6 2 1-methyl-3-nitrobenzene meta m- 5 3 4 para p-dichlorobenzene p- 1,4-dichlorobenzene
Reacts faster than benzene Nitration of Anisole HNO3 + H2SO4 anisole ortho para “ACTIVATED” The -OCH3 group when placed on the ring gives only ortho and para products, and no meta: Substituents that cause this result are called o,p directors and they usually activate the ring.
Nitration of Methyl Benzoate HNO3 H2SO4 meta methyl benzoate Reacts slower than benzene “DEACTIVATED” The -COOMe group when placed on the ring gives only meta, and no ortho or para products: Substituents that cause this result are called m directors and they usually deactivate the ring.
SUBSTITUENT CATEGORIES Most ring substituents fall into one of these two categories: O,P = ACTIVATE M = DEACTIVATE o,p - directors m- directors activate the ring deactivate the ring We will look at one of each kind in order to understand the difference…..
Nitration of Anisole ortho meta para Activated ring actual products para ortho +
: ortho EXTRA! energy lowered meta : para EXTRA!
RECALL: HAMMOND POSTULATE Ea Energy Profiles NITRATION OF ANISOLE benzenium intermediate meta benzenium intermediates have more resonance ortho para ortho-para director
Nitration of Methyl Benzoate ortho meta para Deactivated ring actual product meta
d- d+ BAD! ortho energy raised meta d- d+ para BAD! energy raised
Energy Profiles NITRATION OF METHYL BENZOATE some benzenium resonance structures have a bad situation ortho para meta meta director
ortho, para - Directing Groups Groups that donate electron density to the ring. PROFILE: : E+ increased reactivity +I Substituent +R Substituent .. These groups also “activate” the ring, or make it more reactive. CH3-O- .. CH3- .. R- CH3-N- .. -NH2 .. The +R groups activate the ring more strongly than +I groups. -O-H ..
meta - Directing Groups Groups that withdraw electron density from the ring. PROFILE: d+ d- E+ decreased reactivity -I Substituent -R Substituent These groups also “deactivate” the ring, or make it less reactive. + + - -SO3H
EXCEPTION Halides - o,p Directors / Deactivating Halides represent a special case: .. : : They are o,p directing groups that are deactivating E+ They are o,p directors (+R effect ) They are deactivating ( -I effect ) Most other other substituents fall into one of these four categories: +R / -I / o,p / deactivating 1) +R / o,p / activating -F -Cl -Br -I 2) +I / o,p / activating 3) -R / m / deactivating 4) -I / m / deactivating
GROUPS ACTING IN CONCERT steric crowding o,p director m-director very little formed HNO3 H2SO4 When groups direct to the same positions it is easy to predict the product. major product
GROUPS COMPETING o,p-directing groups win over m-directing groups too crowded X HNO3 + H2SO4
RESONANCE VERSUS INDUCTIVE EFFECT +R HNO3 H2SO4 major product +I resonance effects are more important than inductive effects
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 to excessive steric crowding. 4) With bulky directing groups, there will usually be more p-substitution than o-substitution. 5) The incoming group replaces a hydrogen, it will not usually displace a substituent already in place.