Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Consider the following statements for Friedel Craft's reaction. Which of the following statement is incorrect? (a) The entering alkyl group is an activating group; hence polyalkylation is possible. (b) Rearrangement in alkylating group is possible when the Lewis acid used is \(\mathrm{AlCl}_{3}\). (c) Aryl halides are more effect than alkyl halide as halide component. (d) Aromatic compounds containing \(\mathrm{m}\)-directing group cannot give this reaction

Step-by-Step Solution

Answer

Statement (c) is incorrect.

Step 1: Identify the Nature of Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a reaction that introduces an alkyl group into an aromatic compound using an alkyl halide and a Lewis acid catalyst like AlCl3. The alkyl group is an activating group, which means it increases the reactivity of the benzene ring, allowing for further substitution reactions, also known as polyalkylation. Therefore, statement (a) is correct.

Step 2: Analyze Rearrangement Possibility

During Friedel-Crafts alkylation, especially when using AlCl3, carbocations can rearrange to form more stable carbocation intermediates. Since rearrangement of the alkylating group is a known characteristic, statement (b) is also correct.

Step 3: Compare Aryl Halides with Alkyl Halides

Aryl halides do not typically form carbocations as readily as alkyl halides, which are necessary intermediates for Friedel-Crafts alkylations. Therefore, aryl halides are less effective than alkyl halides in this reaction. Hence, statement (c) is incorrect.

Step 4: Examine the Influence of Meta-Directing Groups

Meta-directing groups on an aromatic ring tend to deactivate the ring towards electrophilic substitution reactions like Friedel-Crafts alkylation. As a result, compounds with m-directing groups generally do not undergo this reaction. So, statement (d) is correct.

Key Concepts

Understanding Alkylation in Friedel-Crafts Reaction

In the Friedel-Crafts alkylation process, an alkyl group is added to an aromatic ring, such as benzene. This transformation involves an alkyl halide, like methyl chloride, and a Lewis acid catalyst, commonly aluminum chloride (\(\text{AlCl}_3\)). The alkyl group becomes attached to the aromatic compound, enhancing its reactivity for further procedures. This enhanced reactivity can lead to polyalkylation, where multiple alkyl groups are successively added to the ring. Polyalkylation is possible because the original alkylation activates the aromatic ring, paving the way for additional substitutions.

• Alkyl group: Increases reactivity of the aromatic ring.
• Polyalkylation: Repeated introduction of alkyl groups due to increased reactivity.

While the process is highly useful for introducing alkyl groups, controlling the extent of alkylation to prevent excessive substitution can be challenging.

Role of Lewis Acid Catalyst in Alkylation

Lewis acid catalysts play a crucial role in the Friedel-Crafts alkylation by facilitating the formation of the reactive species essential for the reaction. A Lewis acid, like \(\text{AlCl}_3\), accepts an electron pair, enabling the alkyl halide to form a powerful electrophile known as a carbocation. This carbocation is highly reactive and can easily interact with the aromatic ring's \(\pi\)-cloud of electrons, leading to the substitution.

• \(\text{AlCl}_3\) is a common choice due to its strong electron pair-accepting properties.
• Carbocation formation is crucial for driving the reaction forward.

Without such a catalyst, the reaction would either not occur or proceed very slowly, highlighting the importance of Lewis acids in this mechanism.

Carbocation Rearrangement Explained

Carbocation rearrangement during Friedel-Crafts alkylation can occur due to the formation of a carbocation intermediate. When an alkyl halide is treated with \(\text{AlCl}_3\), it generates a carbocation which may rearrange to a more stable configuration. Stability of carbocations generally increases with greater substitution - tertiary carbocations being more stable than secondary, which are more stable than primary.

• Rearrangement results in a more stable carbocation, leading to different products.
• The phenomenon is a unique trait of Friedel-Crafts alkylation, among other reactions involving carbocations.

This rearrangement tendency can lead to unanticipated products, underlining the importance of understanding potential reaction pathways in synthesis.

Electrophilic Substitution in Aromatic Compounds

In the Friedel-Crafts alkylation, the key transformation is an electrophilic substitution, where the aromatic compound's hydrogen atom is replaced by an alkyl group. Electrophiles, like carbocations, are electron-deficient and seek electrons from electron-rich aromatic rings. This interaction substitutes a hydrogen on the aromatic ring with an alkyl group, enhancing its electron density.

• This increases the compound's reactivity towards further reactions.
• In presence of meta-directing groups: This activity can be hindered as these groups reduce the electron density circumjacent to the reactive site.

Electrophilic substitution is fundamental in the broad range of reactions involving aromatic compounds, underscoring its relevance across organic chemistry.