Question

The final product \(\mathrm{S}\) in the following reaction sequence is \(\frac{\mathrm{CH}_{3} \mathrm{Cl}}{\text { Anhy } \cdot \mathrm{AlCl}_{3}} \longrightarrow \mathrm{P} \stackrel{\mathrm{NBS}}{\longrightarrow} \mathrm{Q} \stackrel{\mathrm{KCN}}{\longrightarrow} \mathrm{R} \stackrel{\mathrm{H}_{3} \mathrm{O}^{\oplus}}{\longrightarrow} \mathrm{S}\)

Short answer

In the given reaction sequence, the final product S is acetic acid (CH3COOH). The reaction steps involve Friedel-Crafts alkylation, reaction with NBS, nucleophilic substitution with KCN, and acid-catalyzed hydrolysis.

Step-by-step solution

Reaction of CH3Cl with anhydrous AlCl3

In the Friedel-Crafts alkylation, an alkyl chloride (CH3Cl in this case) reacts with anhydrous aluminum chloride (AlCl3) as a Lewis acid catalyst. The chloride ion of CH3Cl coordinates with aluminum in AlCl3, resulting in a carbocation (CH3+) and an AlCl4- complex. In our reaction, there is no mention of any other reactant that the carbocation would combine with. Therefore, the product at this step would just be the carbocation (CH3+). The intermediate product P is CH3+.

Reaction of P with N-bromosuccinimide (NBS)

The CH3+ carbocation intermediate is very reactive and in the presence of NBS, the carbocation reacts with NBS to form a primary bromomethyl cation. The product of this step, the intermediate Q, is CH3Br.

Reaction of Q with KCN

CH3Br (Q) reacts with the nucleophile KCN in an SN2 reaction. In this reaction, the cyanide ion (CN-) from KCN attacks the methyl carbon in CH3Br, displacing the bromide ion (Br-) in the process. The result of this step is the formation of CH3CN (methyl cyanide), our intermediate product R.

Reaction of R with H3O+

In the final step, CH3CN (R) reacts with H3O+ to yield an acid-catalyzed hydrolysis reaction, wherein the CN group is replaced by a carboxylic acid (COOH) group. The final product S will be CH3COOH (acetic acid).

Key concepts

Carbocation

A carbocation is a positively charged ion where the carbon atom carries a positive charge. In the context of Friedel-Crafts alkylation, carbocations act as key intermediates. During the reaction of an alkyl chloride like \(\mathrm{CH}_{3}\mathrm{Cl}\) with a Lewis acid catalyst such as anhydrous aluminum chloride (\(\mathrm{AlCl}_3\)), the chloride ion detaches, forming a carbocation.

• The carbocation in this scenario is \(\mathrm{CH}_{3}^+\), a simple methyl carbocation.
• Carbocations are electrophiles due to their positive charge, making them highly reactive.

Since carbocations seek electrons, they combine with other electron-rich species to form new bonds, vital in many organic reactions. Understanding carbocations lets chemists predict reaction pathways, as their formation often leads to product formation through further interactions.

SN2 Reaction

The SN2 reaction, or bimolecular nucleophilic substitution, is a hallmark mechanism in organic chemistry where one nucleophile displaces another on a substrate.In this mechanism, as seen in the reaction of \(\mathrm{CH}_{3}\mathrm{Br}\) with \(\mathrm{KCN}\), the cyanide ion (\(\mathrm{CN}^-\)) serves as the nucleophile:

• This nucleophile attacks the carbon atom bearing the leaving group (in this case, the bromide ion, \(\mathrm{Br}^-\)).
• The reaction occurs in a single concerted step, where the nucleophile bolsters its attack and the leaving group is expelled simultaneously.
• As a result, the configuration of the carbon atom is inverted, a hallmark of the SN2 mechanism.

This makes SN2 reactions well-suited for converting alkyl halides to nitriles, as exemplified by the transformation from \(\mathrm{CH}_{3}\mathrm{Br}\) to \(\mathrm{CH}_{3}\mathrm{CN}\) in the reaction sequence.

Acid-Catalyzed Hydrolysis

Acid-catalyzed hydrolysis is a fundamental process in organic chemistry, where an acid catalyst facilitates the conversion of a nitrile or other functional group into a carboxylic acid.In our reaction sequence, \(\mathrm{CH}_{3}\mathrm{CN}\) undergoes this transformation in the presence of \(\mathrm{H}_{3}\mathrm{O}^+\):

• The \(\mathrm{CN}\) group is replaced by a \(\mathrm{COOH}\) group, yielding acetic acid \(\mathrm{CH}_{3}\mathrm{COOH}\).
• The hydronium ion (\(\mathrm{H}_{3}\mathrm{O}^+\)) acts as a catalyst by donating protons in the reaction mechanism.
• This reaction type is crucial for converting nitriles to carboxylic acids, which are important in both biological systems and industrial applications.

Understanding acid-catalyzed hydrolysis offers insights into modifying organic molecules to attain desired functionalities and properties.