Problem 197 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following $$ \begin{array}{ll} \hline \text { Column-I } & \text { Column-II } \\ \hline \text { (a) } \mathrm{CH}_{3} \mathrm{COCH}_{3} & \text { (p) } \mathrm{H}_{2} \mathrm{SO}_{4} \text { (conc.) } \\ \text { (b) } \mathrm{CH}_{3} \mathrm{CHO} & \text { (q) } \mathrm{I}_{2}-\mathrm{OH}^{-} \\ \text {(c) } \mathrm{CH}_{3} \mathrm{CHOH}-\mathrm{CH}_{3} & \text { (r) } \mathrm{CH}_{3} \mathrm{MgI} \text { in ether } \\ \text { (d) } \mathrm{CCl}_{3} \mathrm{CHO} & \text { (s) } 2,4 \text { - dinitrophenyl } \\ \text { hydrazine } \\ & \text { (t) } \mathrm{NaOH}(\mathrm{aq}) \end{array} $$

Step-by-Step Solution

Answer

(a) -> (q), (b) -> (s), (c) -> (p), (d) -> (t)

Step 1: Identify Reagents and Reactions

First, identify the chemical reagents and the reactions they are commonly involved in. For example, \( \mathrm{H}_2\mathrm{SO}_4 \) (conc.) is used for dehydration, \( \mathrm{I}_2 - \mathrm{OH}^- \) is used in iodoform test, Grignard reagent (\( \mathrm{CH}_3 \mathrm{MgI} \)) is used in nucleophilic additions, and \( \mathrm{NaOH(aq)} \) can be used for aldol reactions.

Step 2: Match CH3COCH3 with I2-OH-

The compound \( \mathrm{CH}_3\mathrm{COCH}_3 \) is a ketone, which gives a positive iodoform test. Therefore, it matches with \( \mathrm{I}_2 - \mathrm{OH}^- \) (option q).

Step 3: Match CH3CHO with 2,4 - dinitrophenyl hydrazine

The compound \( \mathrm{CH}_3\mathrm{CHO} \) is an aldehyde, which forms a hydrazone on reaction with 2,4-dinitrophenylhydrazine, indicating option (s) is correct.

Step 4: Match CH3CHOH-CH3 with H2SO4 (conc.)

The compound \( \mathrm{CH}_3\mathrm{CHOH}-\mathrm{CH}_3 \) is an alcohol and can be dehydrated to form alkenes using \( \mathrm{H}_2\mathrm{SO}_4 \text{ (conc.)} \), so it matches part (p).

Step 5: Match CCl3CHO with NaOH(aq)

The compound \( \mathrm{CCl}_3\mathrm{CHO} \) can participate in a nucleophilic addition reaction with \( \mathrm{NaOH(aq)} \), suggesting option (t) is appropriate. Normally it gives the reaction for reactions like the Haloform reaction.

Key Concepts

Iodoform Test

The Iodoform Test is a distinctive test for identifying methyl ketones and some alcohols. It's a simple and effective method used in organic chemistry to pinpoint specific compounds. This test involves the reaction of a compound with iodine (I_2) and a base such as often represented by OH^- . When a compound containing a methyl ketone group is present, you will observe a characteristic yellow precipitate known as iodoform (CH ext{I}_3).

• Commonly Used Reagents: Iodine and a strong base like NaOH.
• Positive Result: The appearance of a pale yellow solid precipitate.

For instance, acetone (CH_3COCH_3) is a compound that produces a positive iodoform test, making it a critical tool for identifying such substances via laboratory experiments.

Aldehydes and Ketones

Aldehydes and ketones delve into a category of organic compounds featuring the carbonyl group (C=O). The major difference is the location of this carbonyl group in the molecule. Aldehydes have the structural formula R-CHO where the carbonyl carbon is at the end of the molecule, while ketones have the general structure RCOR, with the carbonyl carbon in the middle, bonded to two hydrocarbon groups.

• Aldehydes are generally more reactive than ketones due to less steric hindrance around the carbonyl carbon.
• Ketones do not oxidize as readily as aldehydes.
• Both can participate in nucleophilic addition reactions.

Aldehydes like acetaldehyde (CH_3CHO) will react with 2,4-dinitrophenylhydrazine to form stable hydrazones, highlighting their presence.

Grignard Reagent

The Grignard Reagent is a brilliant discovery in organic synthesis that involves the use of organomagnesium compounds. An example is methylmagnesium iodide (CH_3MgI), which can be used in the formation of carbon-carbon bonds.
This reagent is introduced to various organic compounds, typically resulting in the addition of the Grignard carbon to a carbonyl group. This mechanism is a straightforward and reliable method to extend carbon skeletons in organic chemistry.

• Applications: Synthesis of alcohols by reacting Grignard reagents with aldehydes and ketones.
• Precaution: Must be handled under anhydrous conditions as any water will decompose the reagent.

These characteristics highlight the importance of Grignard reagents in constructing complex molecules.

Dehydration Reactions

Dehydration reactions are a critical part of organic chemistry, focusing on the removal of a water molecule from compounds, often in the conversion of alcohols to alkenes. This process typically requires a strong dehydrating agent such as concentrated sulfuric acid (H_2SO_4).
During this transformation, an alcohol is converted into an alkene, where a hydroxyl group and a hydrogen atom are eliminated, forming a double bond.

• Common Reagents: Concentrated acids like H_2SO_4 or phosphoric acid.
• Significance: Good method for preparing alkenes from alcohols.

For example, the dehydration of isopropanol will yield propene, showcasing the powerful bond-manipulating tools that chemists utilize.