Reactions of Arenes: Electrophilic and Nucleophilic Substitution

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Chapter 12. Carey & Giuliano.

Acylation-Reduction is required to synthesize a compound of which type?

ArCH2R (unable to be done using friedel crafts because rearrangement would occur)

Describe the Friedel-Crafts alkylation of benzene:

Alkyl halides react with benzene in the presence of aluminum chloride to yield alkylbenzenes

Electron-releasing groups have what effect on rate and regioselectivity in EaS?

Activate all ring carbons toward electrophilic aromatic substitution. Ortho and Para positions are most activated.

Electron-withdrawing groups have what effect on rate and regioselectivity in EaS?

Deactivates all ring carbons toward electrophilic aromatic substitution. Destabilizes carbocations leading to ortho and para substitutions. Meta substitutions are most likely to occur.

How can you make a monohalogenatedbenzene compound more reactive towards nucleophilic aromatic substitution?

Adding o or p nitro groups

How is the electrophilic carbocation for the Friedel-Crafts alkylation generated?

By alkyl halide bonding to AlX3, then leaving to create carbocation and AlX4

How would you use Friedel-Crafts alkylation to create an alkenyl substituted benzene ring from a plain benzene ring?

Treat with alkene, HCl, and Aluminum chloride to get alkane substituted benzene, then reduce with ZnO and high temp to get an alkenyl-substituted benzene ring

Name in order of increasing reactivity: Benzene, Pyridinium ion, Pyridine

Pyridinium ion, pyridine, benzene

Name the 7 substituents which are strongly deactivating towards electrophilic aromatic substitution:

formyl, acyl, carboxylic acid, ester, acyl chloride, cyano, sulfonic acid

Name the EaS activating substituents:

Alkyl, aryl, alkenyl

Name the EaS deactivating substituents:

halogen, halomethyl

Name the five main electrophilic aromatic substitution reactions described in this chapter:

Nitration, Sulfonation, Halogenation, Friedel-Crafts alkylation, Friedel-Crafts acylation

Name the strongly activating (towards Eas) substituents:

acylamino, alkoxy, acyloxy

Name the very strongly (electrophilic aromatic substitution) activating substituents:

amino, alkylamino, dialkylamino, hydroxyl

Name the very strongly (electrophilic aromatic substitution) deactivating substituents:

Nitro, trifluoromethyl

What are products when benzene is reacted with acetic anhydride and aluminum chloride?

Acetophenone and acetic acid

What are the 3 steps in the sulfur trioxide sulfonation of benzene?

1. Electrons in the pi system of benzene attack sulfur trioxide and form the cyclohexadienyl cation 2. A proton is abstracted from the sp3 hybridized carbon of the intermediate to restore the aromaticity of the ring 3. A rapid proton transfer from the oxygen of sulfuric acid to the oxygen of benzenesulfonate completes the process

What are the products when hexafluorobenzene reacts with sodium methoxide and methanol at 65 C?

2,3,4,5,6-Pentafluoroanisole

What are the reagents and conditions needed for halogenation of benzene

X2, Iron, and heat

What are the reagents and conditions needed to nitrate benzene?

Nitric Acid, Sulfuric acid, 30-40 C

What are the reagents and conditions needed to sulfonate benzene with a much faster rate of sulfonation and equilibrium pushed forward?

Sulfur trioxide and sulfuric acid

What are the reagents and conditions needed to sulfonate benzene?

Sulfuric acid and Heat. Water must be removed to drive reaction forward.

What are the steps in bromination of benzene?

1. Two of the pi electrons of benzene form a bond to the bromine-Iron(III)bromide complex to give a cyclohexadienyl cation 2. Loss of a proton from the cyclohexadienyl cation to the FeBr4 yields bromobenzene

What are the steps in nucleophilic aromatic substitution?

1. Addition: The nucleophile adds to the carbon bearing the leaving group to give a cyclohexadienyl anion intermediate 2. Elimination: loss of halide from the cyclohexadienyl intermediate restoring the aromaticity of the ring and giving the product of NaS

What are the steps in the basic mechanism of electrophilic aromatic substitution?

1. Electrophile accepts an electron pair form the pi system of benzene to form a carbocation (stabilized by electron delocalization) 2. Cation rapidly loses the proton, restoring the aromaticity of the ring

What are the steps in the Friedel-Crafts acylation of benzene?

1. A pair of pi electrons of benzene is used to form a covalent bond to the carbon of the acyl cation. 2. Aromaticity of the ring is restored when it loses a proton to the aluminum tetrachloride to give the aryl ketone

What are the three main points of the substituent effect table for electrophilic aromatic substitution reactions?

1. All activating substituents are ortho, para directors 2. Halogen substituents are slightly deactivating, but are ortho, para directing 3. Strongly deactivating substituents are meta directors

What are two methods available to reduce aldehydes and ketones?

Clemmensen Reduction(ZnHg, HCl), Wolff-Kishner Reduction(KOH, H2NNH2, heat, triethylene glycol)

What effect does a cyano group have on rate and regioselectivity towards electrophilic aromatic substitution?

Electron-withdrawing, deactivating, and meta-directing

What effect does six fluorine substituents have on benzene?

The combined electron-attracting effects stabilize the cyclohexadienyl anion intermeidate and permit nucleophilic aromatic substitution to proceed readily

What is another carbocation precursor that can be used in electrophilic aromatic substitution?

any alkene. Use it with sulfuric acid... then just bind it on there.

What is another reagent besides acyl chlorides which can be used to synthesize aryl ketones from benzene rings?

Acid anhydrides

What is the drawback to the Friedel-Crafts alkylation?

Rearrangements occur, especially when primary alkyl halides are used

What is the effect of a nitro group in electrophilic aromatic substitution? in nucleophilic aromatic substitution?

Strongly deactivating; strongly activating

What is the electrophile in halogenation of benzene?

Bromine-Iron(III) bromide complex. Br-Br-FeBr3. Positive charge on mid br, negative charge on Fe.

What is the electrophile in the Friedel-Crafts acylation of benzene?

Acyl cation (Alkane with one carbon triple bonded to oxygen which has a lone pair and positive charge on it)

What is the electrophile in the nitration of benzene?

Nitronium ion (N double bonded to two oxygens. Positive charge on the N)

What is the electrophile in the sulfonation of benzene?

Sulfur trioxide

What is the most common method for reducing an aryl ketone to an alkylbenzene?

Zn(Hg) and HCl, aka the Clemmensen Reduction

What reagents/conditions are needed to iodinate benzene?

Iodine (I2) and carboxylic acid

Where does substitution usually take place in furan?

C2

Where does substitution usually take place in napthalene?

C1

Where does substitution usually take place in pyridine?

C3

Which elements are the most reactive leaving groups and least reactive leaving groups in nucleophilic aromatic substitution?

Fluoride is most reactive, Iodide is the least reactive

Which is more reactive towards nucleophilic aromatic substitution, 2-chloropyridine or sodium methoxide?

2-chloropyridine (because nitrogen is more electronegative than carbon, the intermediate is more stable and forms faster)

Which reaction replaces a benzylic halide with a nucleophile?

Nucleophilic Aromatic Substitution

Which reaction replaces a benzylic hydrogen with an electrophile?

Electrophilic Aromatic Substitution

Why are hydroxyl, alkoxy and acyloxy substituents great activators towards electrophilic aromatic substitution?

Although they withdraw electrons, they have a larger resonance effect which releases the unshared electron pairs to circulate freely

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