Question

Which will undergo Friedal-Craft alkylation reaction? (1) Cc1ccc([N+](=O)[O-])cc1 (2) CCc1ccccc1 (3) O=C(O)c1ccccc1 (4) Oc1ccccc1 (a) 1 and 3 (b) 2 and 4 (c) 1 and 2 (d) 1,2 and 4

Short answer

Option (b) 2 and 4 will undergo Friedel-Crafts alkylation.

Step-by-step solution

Identify the Friedel-Crafts Alkylation Reaction

Friedel-Crafts alkylation is a reaction that introduces an alkyl group into an aromatic ring. This reaction typically requires a carbocation, often formed with an alkyl halide and a Lewis acid. Aromatic rings with electron-donating groups generally facilitate the reaction, while strong electron-withdrawing groups hinder it.

Analyze Compound 1

Compound 1, represented by the SMILES "Cc1ccc([N+](=O)[O-])cc1", contains a nitro group \([N^+(O^-)]=O\), which is a strong electron-withdrawing group. This deactivates the aromatic ring to electrophilic substitution reactions like Friedel-Crafts alkylation. Thus, compound 1 will not undergo the reaction.

Analyze Compound 2

Compound 2 SMILES "CCc1ccccc1" has an ethyl group attached directly to the aromatic ring. Alkyl groups are electron-donating and activate the ring towards electrophilic substitution. Therefore, compound 2 can undergo Friedel-Crafts alkylation.

Analyze Compound 3

The SMILES for compound 3 is "O=C(O)c1ccccc1". This compound has a carboxylic acid group, which is electron-withdrawing due to resonance and inductive effects. Consequently, the ring is deactivated towards Friedel-Crafts alkylation.

Analyze Compound 4

Compound 4, with the SMILES "Oc1ccccc1", has a hydroxy group \(OH\) attached to the benzene ring. The hydroxyl group is an electron-donating group, making the ring more reactive to electrophilic substitution. Hence, compound 4 will undergo Friedel-Crafts alkylation.

Evaluate the Options

Based on the analysis, compound 2 can undergo Friedel-Crafts alkylation due to the presence of an activating alkyl group, and compound 4 can react due to the activating effect of the hydroxyl group. Hence, the correct option is (b) 2 and 4.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental chemical reaction where an electrophile replaces a hydrogen atom on an aromatic ring. Aromatic compounds, like benzene, have a stable conjugated structure due to their pi-electron cloud. This stability makes them less reactive, requiring the presence of a strong electrophile to facilitate a reaction. In EAS, the aromaticity of the ring is temporarily broken when the electrophile attacks, but it is quickly restored once the hydrogen is substituted. Electrophiles are electron-poor species, which can be positively charged or have a partial positive charge. Their attraction to the electron-rich aromatic ring initiates the substitution.

• Examples of electrophilic aromatic substitution reactions include nitration, sulfonation, halogenation, and Friedel-Crafts reactions.
• The reaction usually involves a step where a Lewis acid, like AlCl3, is used to generate a strong electrophile.

Understanding EAS is crucial to grasping how different substituents on the aromatic ring can influence the rate and outcome of these reactions.

Electron-Donating Groups

Electron-donating groups (EDGs) play a significant role in electrophilic aromatic substitution reactions. These groups push electron density towards the aromatic ring through resonance or inductive effects. This increased electron density stabilizes the positively charged intermediate formed during the reaction, making the substitution process more favorable.

• Typical examples of EDGs include alkyl groups , such as methyl and ethyl, and functional groups like hydroxyl and amino groups.
• EDGs often direct the incoming electrophile to ortho and para positions on the aromatic ring, due to increased electron density at these locations.

When an aromatic ring contains EDGs, it becomes more reactive towards electrophilic aromatic substitution, allowing reactions like Friedel-Crafts alkylation to occur efficiently.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are groups that pull electron density away from the aromatic ring. This makes the ring less electron-rich and thus less reactive in electrophilic aromatic substitution reactions. EWGs stabilize the negative charge in the ring by resonance or inductive effects, which is the opposite of what is needed in EAS.

• Common examples of EWGs include nitro groups ( \([-N^+(O^-)]=O\)) , carbonyl groups , and nitrile groups.
• The presence of EWGs often leads to electrophilic substitution at the meta position, as the ortho and para positions are less electron-dense.

When an aromatic ring is substituted with EWGs, the electrophilic aromatic substitution reactions are usually hindered or require more extreme conditions to proceed. In Friedel-Crafts alkylation, for example, the reaction might not occur at all if strong EWGs are present, as they deactivate the ring.