Chapter 16. Chemistry of Benzene: Electrophilic Aromatic Substitution. Introduction. Electrophilic aromatic substitution is the most common reaction of aromatic compounds It replaces a proton ( H + ) on an aromatic ring with another electrophile ( E + )
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Chemistry of Benzene:
Electrophilic Aromatic Substitution
Loss of aromaticity
Retention of aromaticity
Practice Problem: Monobromination of toluene gives a mixture of three bromotoluene products. Draw and name them.
F is too reactive for monofluorination
Practice Problem: How many products might be formed on chlorination of o-xylene (o-dimethylbenzene), m-xylene, and p-xylene?
Practice Problem: When benzene reacts with D2SO4, deuterium slowly replaces all six hydrogens in the aromatic ring. Explain.
Carbocation rearrangements of Friedel-Crafts alkylation
Practice Problem: The Friedel-Crafts reaction of benzene with 2- chloro-3-methylbutane in the presence of AlCl3 occurs with carbocation rearrangement. What is the structure of the product?
Practice Problem: Which of the following alkyl halides undergo Friedel-Crafts reaction without rearrangement? Explain.
Practice Problem: What is the major monosubstitution product from Friedel-Crafts reaction of benzene with 1- chloro-2-methylpropane in the presence of AlCl3?
Practice Problem: Identify the carboxylic acid chloride that might be used in a Friedel-Crafts acylation reaction to prepare each of the following acylbenzenes
A substituent present on an aromatic ring affects:
Substituents present on the ring determine the position of the 2nd substitution: ortho, meta, and para
Substituents can be classified as:
The directing effects of the groups correlate with their reactivities:
Reactivity and orientation in electrophilic aromatic substitutions are controlled by an interplay of inductive effects and resonance effects:
Inductive effects-withdrawal or donation of electrons through a s bond due to electronegativity and polarity of bonds in functional groups
Resonance effect - withdrawal or donation of electrons through a bond due to the overlap of a porbital on the substituent with a p orbital on the aromatic ring
–Z is more electronegative than Y
– Y has a lone pair of electrons
Practice Problem: Predict the major product of the monosulfonation of toluene.
Practice Problem: Write resonance structures for nitrobenzene to show the electron-withdrawing resonance effect of the nitro group.
Practice Problem: Write resonance structures for chlorobenzene to show the electron-donating resonance effect of the chloro group.
Practice Problem: Predict the major products of the following reactions
Practice Problem: Rank the compounds in each group in order of their reactivity to electrophilic substitution
Practice Problem: Explain why Friedel-Crafts alkylations often give polysubstitution but Friedel-Crafts acylations do not
Practice Problem: Would you expect trifluoromethylbenzene to be more reactive or less reactive than toluene toward electrophilic substitution? Explain.
Practice Problem: Acetanilide is less reactive than aniline toward electrophilic substitution. Explain.
Practice Problem: Draw resonance structures for the intermediates from reaction of an electrophile at the ortho, meta, and para positions of nitrobenzene. Which intermediates are most stable?
Three rules for the additive effects of two different groups:
Practice Problem: What product would you expect from bromination of p-methylbenzoic acid?
Practice Problem: At what positions would you expect electrophilic substitution to occur in the following substances?
Practice Problem: Show the major product(s) from reaction of the following substances with (i) CH3CH2Cl, AlCl3 and (ii) HNO3, H2SO4
Practice Problem: Propose a mechanism for the reaction of 1- chloroanthraquinone with methoxide ion to give the substitution product 1-methoxyanthraquinone. Use curved arrows to show the electron flow in each step.
Practice Problem: Treatment of p-bromotoluene with NaOH at 300oC yields a mixture of two products, but treatment of m-bromotoluene with NaOH yields a mixture of three products. Explain
There are two reactions of alkylbenzene side chains:
Aromatic ring activates neighboring benzylic (C-H) position toward oxidation
Practice Problem: What aromatic products would you obtain from the KMnO4 oxidation of the following substances?
Practice Problem: Refer to Table 5.3 for a quantitative idea of the stability of a benzyl radical. How much stable (in kJ/mol) is the benzyl radical than a primary alkyl radical? How does a benzyl radical compare in stability to an allyl radical
Practice Problem: Styrene, the simplest alkenylbenzene, is prepared commercially for use in plastics manufacture by catalytic dehydrogenation of ethylbenzene. How might you prepare styrene from benzene?
There are two reduction reactions:
Practice Problem: Show how you would prepare diphenylmethane (Ph)2CH2, from benzene and an appropriate acid chloride
Practice Problem: Synthesize p-bromobenzoic acid from benzene
Br – Bromination using Br2/FeBr3
CO2H – Friedel-Crafts alkylation or acylation followed by oxidation
Practice Problem: Propose a synthesis of 4-chloro-1-nitro-2- propylbenzene from benzene
Cl – Chlorination using Cl2/FeCl3
NO2– Nitration using HNO3/H2SO4
CH2CH2CH3– Friedel-Crafts acylation followed by reduction
Practice Problem: Propose syntheses of the following substances from benzene:
Practice Problem: In planning a synthesis, it is important to know what NOT to do as to know what do. As written, the following reaction schemes have flaws in them. What is wrong with each?