Question

Rank the following aromatic compounds in the expected order of their reactivity toward Friedel-Crafts alkylation. Which compounds are unreactive? (a) Bromobenzene (b) Toluene (c) Phenol (d) Aniline (e) Nitrobenzene (f) \(p\) -Bromotoluene

Short answer

Aniline > Phenol > Toluene > p-Bromotoluene > Bromobenzene, Nitrobenzene is unreactive.

Step-by-step solution

Understand Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a reaction where an alkyl group is introduced onto an aromatic ring. The reactivity of these compounds towards the reaction largely depends on the electron density on the benzene ring. Groups that donate electrons activate the ring, making it more reactive, while groups that withdraw electrons de-activate the ring, rendering it less reactive.

Identify Activating and Deactivating Groups

Investigate each of the given substituents: Toluene has a methyl group which is an electron-donating group; Phenol has an -OH group which is a strong activating group due to resonance; Aniline has an -NH2 group which is also a strong activating group due to resonance; Bromobenzene and p-Bromotoluene have a bromine atom which is a weak deactivating group, but it can still donate electrons via resonance; Nitrobenzene has a nitro group which is a strong electron-withdrawing group.

Rank by Reactivity

Rank the compounds by their predicted reactivity toward Friedel-Crafts alkylation based on the analysis of their substituents: 1. Aniline (strongly electron-donating) 2. Phenol (electron-donating) 3. Toluene (electron-donating) 4. p-Bromotoluene (electron-donating methyl, but weak deactivating bromine) 5. Bromobenzene (weakly electron-withdrawing) 6. Nitrobenzene (strongly electron-withdrawing)

Identify Unreactive Compounds

Compounds with strongly deactivating groups like nitrobenzene are often unreactive in Friedel-Crafts alkylation due to insufficient electron density in the benzene ring to stabilize the carbocation intermediate. Thus, nitrobenzene is expected to be unreactive.

Key concepts

Aromatic Compounds

Aromatic compounds are a unique class of organic molecules featuring one or more benzene rings. The benzene ring is characterized by its planar and cyclic structure with conjugated pi bonds. These pi bonds result in a stabilization known as aromaticity. Due to their distinct structure, aromatic compounds generally undergo substitution reactions instead of addition reactions. This is because addition reactions would disrupt the aromatic stability. The pi electrons in the benzene ring form a cloud above and below the planar ring, contributing to this stability. Understanding the behavior of aromatic compounds is essential when predicting their reactivity in chemical reactions like the Friedel-Crafts alkylation.

Electron-Donating Groups

Electron-donating groups (EDGs) increase the electron density in the benzene ring. They activate the aromatic compound towards electrophilic substitution reactions like Friedel-Crafts alkylation. Common electron-donating groups include:

• -OH (hydroxyl group)
• -NH₂ (amino group)
• -CH₃ (methyl group)

These groups can donate electrons through resonance or hyperconjugation. For example, in phenol, the -OH group donates electrons through resonance, thus activating the ring. Similarly, the methyl group in toluene donates electrons through hyperconjugation, making it reactive in alkylation reactions.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) reduce the electron density in the benzene ring. They deactivate the aromatic compound and make it less reactive in electrophilic substitution reactions. Important electron-withdrawing groups include:

• -NO₂ (nitro group)
• -CN (cyano group)
• -CF₃ (trifluoromethyl group)

These groups pull electrons away from the benzene ring, often due to electronegative atoms or multiple bonded structures, such as the nitro group in nitrobenzene. These groups make it difficult for a positive charge (the electrophile) to attack the benzene, leading to reduced reactivity in Friedel-Crafts alkylation.

Reactivity Order

The reactivity of aromatic compounds in Friedel-Crafts alkylation depends on the type of substituents present in the ring. The presence of electron-donating or electron-withdrawing groups dictates how readily the ring will undergo substitution reactions. Here's the order of reactivity for the compounds in the exercise:

• Aniline - the -NH₂ group is a strong electron-donating group
• Phenol - the -OH group provides resonance stabilization
• Toluene - the methyl group slightly activates the ring
• p-Bromotoluene - shows mixed effects due to the -CH₃ and bromine
• Bromobenzene - bromine slightly deactivates but can donate via resonance
• Nitrobenzene - the -NO₂ group strongly deactivates the ring

This order highlights the influence of substituent groups on the benzene ring and their ability to affect reactivity.

Substituent Effects

The presence of different substituents on a benzene ring leads to various effects that can influence its reactivity. These effects can be broadly categorized into inductive and resonance effects.

• Inductive effect: This effect occurs through sigma bonds, where electronegative atoms pull electron density away from the benzene ring. For instance, chlorine exhibits a -I effect, slightly deactivating the ring through the inductive effect.
• Resonance effect: Occurs when pi electrons are redistributed in the compound. Substituents like -OH and -NH₂ increase electron density through resonance, enhancing reactivity.

These effects explain how and why different substituents on aromatic rings either activate or deactivate them in reactions such as Friedel-Crafts alkylation. Understanding these effects is crucial when predicting the outcome of reactions involving aromatic compounds.