4/18/ Substituent Effects in Electrophilic Substitutions. Substituent Effects in Electrophilic Substitutions

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1 9.8 Substituent effects in the electrophilic substitution of an aromatic ring Substituents affect the reactivity of the aromatic ring Some substituents activate the ring, making it more reactive than benzene Some substituents deactivate the ring, making it less reactive than benzene Relative rates of aromatic nitration Substituents affect the orientation of the reaction The three possible disubstituted products ortho, meta, and para are usually not formed in equal amounts The nature of the substituent on the ring determines the position of the second substitution Substituents can be classified into three groups ortho- and para-directing activators ortho- and para-directing deactivators meta-directing deactivators There are no meta-directing activators All activating groups are ortho- and para- directing All deactivating groups other than halogens are meta-directing The halogens are unique in that they are deactivating but ortho- and para-directing 1

2 Activating and Deactivating Effects The common characteristic of all activating groups is that they donate electrons to the ring Makes the ring more electron-rich Stabilize the carbocation intermediate Lower activation energy The common characteristic of all deactivating groups is that they withdraw electrons from the ring Makes the ring more electron-poor Destabilizes the carbocation intermediate Raising the activation energy for its formation Electrostatic potential maps of benzene, phenol (activated), chlorobenzene (weakly deactivated), and benzaldehyde (more strongly deactivated) The OH substituent makes the ring more negative (red) The Cl makes the ring less negative (orange) The CHO makes the ring still less negative (yellow) 2

3 The electron donation or electron withdrawal may occur by either an inductive effect or a resonance effect Inductive effect Due to an electronegativity difference between the ring and the attached substituent Resonance effect Due to overlap between a p orbital on the ring and an orbital on the substituent Inductive effects and resonance effects don t necessarily act in the same direction Halogen, hydroxyl, alkoxyl, and amino substituents have electron-withdrawing inductive effects due to the electronegativity of the X, O, or N atom and electrondonating resonance effects because of the lone-pair electrons on those X, O, or N atoms When the two effects act in opposite directions the stronger effect dominates Hydroxyl, alkoxyl, and amino substituents are activators because of their stronger electron-donating resonance effect Halogens are deactivators because of their stronger electron-withdrawing inductive effect 3

4 Orienting Effects: Ortho and Para Directors Alkyl groups have an electron-donating inductive effect Nitration of toluene occurs ortho and para to the alkyl group because a resonance form places the positive charge directly on the alkyl-substituted carbon where it can be stabilized by the electron-donating inductive effect of the alkyl group Orienting Effects: Ortho and Para Directors Halogen, hydroxyl, alkoxyl, and amino groups are ortho and para directors because the atom attached to the ring halogen, O, or N has a lone pair of electrons Ortho and para substitutions allow the intermediate to be stabilized by more resonance forms than the meta substitution intermediate Nitration of phenol 4

5 Orienting Effects: Meta Directors Any substituent that has a positively polarized atom (d+) directly attached to the ring increases the activation energy leading to the intermediate hybrid for ortho and para substitutions Substitution at ortho or para position gives a higher energy intermediate Meta substitution avoids higher energy intermediate and has a lower activation energy Approaching electrophile is directed to the meta positions Orienting Effects: Meta Directors Nitration of benzaldehyde A Summary of 5

6 Worked Example 9.3 Predicting the Product of an Electrophilic Aromatic Substitution Reaction Predict the major product of the sulfonation of toluene. 9.9 Nucleophilic Aromatic Substitution Aryl halides that have electron-withdrawing substituents can undergo a nucleophilic substitution reaction Nucleophilic Aromatic Substitution Reaction of proteins with 2,4-dinitrofluorobenzene (Sanger s reagent) attaches a label to the terminal NH 2 group of an amino acid by a nucleophilic aromatic substitution reaction 6

7 Nucleophilic Aromatic Substitution Mechanism of nucleophilic aromatic substitution reactions Nucleophilic Aromatic Substitution Nucleophilic aromatic substitution occurs only if the aromatic ring has an electron-withdrawing substituent in a position ortho or para to the leaving group to stabilize the anion intermediate through resonance Nucleophilic Aromatic Substitution Comparison of electrophilic and nucleophilic aromatic substitution reactions Electrophilic substitutions are favored by electrondonating substituents which stabilize the carbocation intermediate Nucleophilic substitutions are favored by electronwithdrawing substituents which stabilize a carbanion intermediate Electron-withdrawing groups that deactivate rings for electrophilic substitution (nitro, carbonyl, cyano, and so on) activate rings for nucleophilic substitution 7

8 9.10 Alkyl substituents on aromatic rings containing a benzylic hydrogen react readily with common laboratory oxidizing agents such as aqueous KMnO 4 or Na 2 Cr 2 O 7 and are converted into carboxyl groups CO 2 H Net conversion of an alkylbenzene into a benzoic acid Ar-R Ar-CO 2 H Oxidation of butylbenzene into benzoic acid The mechanism of side-chain oxidation involves reaction of a C-H bond at the position next to the aromatic ring (the benzylic position) to form an intermediate benzylic radical Benzylic radicals are stabilized by resonance and thus form more readily than typical alkyl radicals Side chain oxidations occur in various biosynthetic pathways The neurotransmitter norepinephrine is biosynthesized from dopamine by a benzylic hydroxylation reaction Radical reaction Reaction catalyzed by the copper-containing enzyme dopamine b-monooxygenase 8

9 Hydrogenation of Aromatic Rings Alkene double bonds can be reduced selectively in the presence of an aromatic ring To hydrogenate an aromatic ring it is necessary to use a platinum catalyst with hydrogen gas at several hundred atmospheres pressure or a more effective catalyst such as rhodium on carbon Reduction of Aryl Akyl Ketones Aromatic ring activates a neighboring carbonyl group toward reduction An aryl alkyl ketone prepared by Friedel-Crafts acylation of an aromatic ring can be converted into an alkylbenzene by catalytic hydrogenation over a palladium catalyst Propiophenone is reduced to propylbenzene by catalytic hydrogenation 9

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