Problem 193 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following: List I (Reaction) 1\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{NaOH} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}\) \(+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\) 2\. \(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{CH}_{3} \mathrm{Cl} \stackrel{\text { Anhyd. } \mathrm{AlCl}_{3}}{\stackrel{\mathrm{Pd} / \mathrm{BaSO}_{4}}{\longrightarrow}} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}\) 3\. \(\mathrm{RCOCl}+\mathrm{H}_{2} \stackrel{\mathrm{P}}{\longrightarrow} \mathrm{RCHO}\) 4\. \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}+\mathrm{NaOH}+\mathrm{CO}_{2} \stackrel{\text { 1. heat, } 2 . \mathrm{H}^{*}}{\longrightarrow}\) List II (Name of reactions) (i) Rosenmund reduction (ii) Kolbe's reaction (iii) Claisen reaction (iv) Cannizzaro reaction (v) Friedal-Craft's reaction The correct matching is:

Step-by-Step Solution

Answer

1 -> (iv), 2 -> (v), 3 -> (i), 4 -> (ii)

Step 1: Identifying Reaction Types from List I

The first reaction in List I (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}+\mathrm{NaOH} \longrightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}+\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{OH}\)) is the Cannizzaro reaction, where an aldehyde (without \(-H\) at the alpha carbon) undergoes self-oxidation and reduction with a base, resulting in the formation of an alcohol and a carboxylic acid.

Step 2: Analyzing Reaction 2 from List I

The second reaction in List I (\(\mathrm{C}_{6} \mathrm{H}_{6}+\mathrm{CH}_{3} \mathrm{Cl} \stackrel{\text {Anhyd.}\ \mathrm{AlCl}_{3}}{\stackrel{\mathrm{Pd}\/\mathrm{BaSO}_{4}}{\longrightarrow}} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}\)) is a Friedel-Craft's reaction, where benzene reacts with an alkyl halide in the presence of AlCl₃, a Lewis acid catalyst, to form toluene.

Step 3: Identifying the Third Reaction in List I

The third reaction (\(\mathrm{RCOCl}+\mathrm{H}_{2} \stackrel{\mathrm{P}}{\longrightarrow} \mathrm{RCHO}\)) is the Rosenmund reduction. This involves the reduction of an acid chloride to an aldehyde using hydrogen gas and a palladium catalyst.

Step 4: Understanding Reaction 4 in List I

The fourth reaction in List I (\(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}+\mathrm{NaOH}+\mathrm{CO}_{2} \stackrel{\text { 1. heat, } 2. \mathrm{H}^{*}}{\longrightarrow}\)) is Kolbe's reaction. This carboxylation of phenol involves treating it with sodium hydroxide and carbon dioxide to produce salicylic acid under acidic conditions.

Step 5: Matching Reactions from List II

Now, we match the reactions from List I to the names in List II: 1. Cannizzaro reaction (iv) 2. Friedal-Craft's reaction (v) 3. Rosenmund reduction (i) 4. Kolbe's reaction (ii)

Key Concepts

Cannizzaro Reaction

The Cannizzaro reaction is a unique chemical process in organic chemistry where an aldehyde, lacking an alpha hydrogen atom, undergoes simultaneous oxidation and reduction. This reaction typically requires a strong base, like NaOH. In the process, one molecule of the aldehyde is reduced to form an alcohol, while another is oxidized to form a carboxylic acid. A classic example is the reaction of benzaldehyde \((\text{C}_{6}\text{H}_{5}\text{CHO})\) with a base to form benzyl alcohol \((\text{C}_{6}\text{H}_{5}\text{CH}_{2}\text{OH})\) and benzoic acid \((\text{C}_{6}\text{H}_{5}\text{COOH})\). *Key Features:* - Involves aldehyde molecules. - No alpha hydrogen in the aldehyde. - Self redox process (one part reduces, another oxidizes). - Commonly used with aromatic aldehydes. This reaction is particularly important for its ability to transform aldehydes into more functional products, providing a pathway for synthesizing alcohols and acids from simple aldehydes.

Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a classic type of electrophilic aromatic substitution reaction. It involves the introduction of an alkyl group into an aromatic ring using an alkyl halide and a catalyst. The most commonly used catalyst is anhydrous aluminium chloride \(\text{AlCl}_{3}\), a strong Lewis acid.This reaction is widely used for synthesizing complex organic molecules as it allows the introduction of substituents to aromatic rings (like benzene). In the classic example provided, benzene reacts with methyl chloride in the presence of \(\text{AlCl}_{3}\) to produce toluene \((\text{C}_{6}\text{H}_{5}\text{CH}_{3})\).*Essential Aspects:* - Utilizes a strong Lewis acid catalyst (\text{AlCl}_{3}). - Alkyl halides are common reactants. - Increases the electrophilicity of the alkyl group, facilitating its addition to benzene. - Conditions must be anhydrous, as water can deactivate the catalyst. Despite its utility, the reaction must be controlled as over-alkylation can occur, leading to complex mixtures.

Rosenmund Reduction

The Rosenmund reduction is a valuable method for converting acyl chlorides to aldehydes. This reaction utilizes hydrogen gas in the presence of a palladium catalyst poisoned with barium sulfate. The catalyst is often deactivated or 'poisoned' to selectively stop the reduction at the aldehyde stage without further reducing it to an alcohol. It is particularly useful in industrial applications for the selective synthesis of aldehydes and ensures high specificity in the reaction process. *Key Characteristics:* - Works with acid chlorides (acyl chlorides) as reactants. - Uses a poisoned catalyst to stop at the aldehyde level. - No excessive reduction, ensuring the formation of pure aldehydes. - Palladium/barium sulfate is the standard catalyst system. This reduction technique offers a direct pathway to aldehydes from acid chlorides, an important transformation in organic synthesis.

Kolbe's Reaction

Kolbe's reaction, also known as Kolbe's electrolysis or Kolbe-Schmitt reaction, involves the carboxylation of a phenolic compound. In the process, phenol is treated with sodium hydroxide followed by heating in carbon dioxide, ultimately forming a carboxylate salt. Upon acidification, the carboxylate salt yields a carboxylic acid.One historical application is the synthesis of salicylic acid, a precursor to aspirin, by reacting sodium phenoxide (phenol plus NaOH) with \(\text{CO}_{2}\). *Key Points to Remember:* - Involves the formation of carboxylate salts. - Requires alkaline conditions initially (NaOH). - Converts carboxylate to carboxylic acid upon acidification. - Important in pharmaceutical syntheses.This reaction is not only a fundamental transformation in organic chemistry but also demonstrates the power of chemical synthesis in producing life-enhancing drugs.