Question

2-Pentanone can be converted to butanoic acid by reaction with (a) Fehling's solution (b) \(\mathrm{HCN} / \mathrm{H}_{3} \mathrm{O}^{+}\) (c) \(\mathrm{H}_{2} \mathrm{NNH}_{2}, \mathrm{KOH} / \mathrm{H}_{3} \mathrm{O}^{+}\) (d) \(\mathrm{NaOH}, \mathrm{I}_{2} / \mathrm{H}_{3} \mathrm{O}^{+}\)

Short answer

Question: Identify the correct reagent for converting 2-Pentanone to butanoic acid. Answer: (d) \(\mathrm{NaOH}, \mathrm{I}_{2} / \mathrm{H}_{3} \mathrm{O}^{+}\)

Step-by-step solution

Identify the structure of 2-Pentanone

2-Pentanone is a ketone with the structure: CH3CH2COCH2CH3 It contains a carbonyl group (C=O) in the middle of the carbon chain.

Understand the function of each reagent

(a) Fehling's solution: It is a solution used to differentiate aldehydes from ketones. Fehling's solution can oxidize aldehydes, but not ketones. (b) \(\mathrm{HCN} / \mathrm{H}_{3} \mathrm{O}^{+}\): This mixture is used in a nucleophilic addition reaction, which adds a cyanide group (CN) to the carbonyl carbon, forming a cyanohydrin. (c) \(\mathrm{H}_{2} \mathrm{NNH}_{2}, \mathrm{KOH} / \mathrm{H}_{3} \mathrm{O}^{+}\): This mixture is used in the Wolff-Kishner reduction, which reduces carbonyl groups (aldehydes and ketones) to methylene groups (CH2). (d) \(\mathrm{NaOH}, \mathrm{I}_{2} / \mathrm{H}_{3} \mathrm{O}^{+}\): This mixture is used in the iodoform reaction, which cleaves methyl ketones to form carboxylic acids and iodoform (CHI3).

Determine which reagent converts 2-Pentanone to butanoic acid

To convert 2-Pentanone into butanoic acid, we need to find the reagent that will cleave the ketone and form a carboxylic acid. Based on the functions of the reagents, reagent (d) \(\mathrm{NaOH}, \mathrm{I}_{2} / \mathrm{H}_{3} \mathrm{O}^{+}\) fits our requirement. The iodoform reaction selectively cleaves methyl ketones, like 2-Pentanone, to form carboxylic acids and iodoform. Thus, the correct reagent for the conversion of 2-Pentanone to butanoic acid is (d) \(\mathrm{NaOH}, \mathrm{I}_{2} / \mathrm{H}_{3} \mathrm{O}^{+}\).

Key concepts

Ketone Reactions

Ketones are versatile compounds in organic chemistry that undergo a variety of reactions, each with its own utility. A ketone has the general structure R₁CO R₂, where R₁ and R₂ are alkyl or aryl groups, and CO represents the carbonyl group. Reactions of ketones often involve the carbonyl group due to its polar nature, which makes it electrophilic.

Nucleophilic additions are common to ketones, where a nucleophile attacks the electrophilic carbon of the carbonyl group. Reductions can convert a ketone to a secondary alcohol. Furthermore, under the right conditions, ketones can undergo oxidation to form carboxylic acids or cleavage reactions to give smaller fragments. Each reaction requires specific conditions and reagents, highlighting the significance of understanding both the reactivity pattern and the required reaction conditions for successful transformations.

Iodoform Reaction

The iodoform reaction is a specific and noteworthy reaction that involves the conversion of a methyl ketone (i.e., a ketone with a methyl group adjacent to the carbonyl) into a carboxylic acid and iodoform (CHI₃). This reaction is particularly useful for identifying methyl ketones, as well as for breaking down complicated structures into simpler molecules.

The reaction proceeds in the presence of a base like sodium hydroxide (NaOH) and iodine (I₂). Initially, the base deprotonates the methyl group, which then reacts with iodine to form triiodomethane and leaves behind a carboxylate ion. Acidification with an acid like H₃O⁺ finally releases the carboxylic acid. The iodoform reaction not only serves as a functional group transformation but also as a qualitative test for the presence of methyl ketones in a sample.

Organic Chemistry Transformations

Organic chemistry involves the transformation of one molecule into another through various reactions, each with its own mechanism and outcome. Such transformations are designed to introduce new functional groups, modify molecular structures, or adjust the oxidation state of organic compounds.

The conversion of 2-pentanone to butanoic acid is an excellent example of a transformation where a functional group (ketone) is converted to another (carboxylic acid). Transformations require careful selection of reagents and reaction conditions to achieve the desired product. They are fundamental to synthesizing complex molecules from simpler precursors. Understanding the reactivity of different functional groups and the conditions under which they react is pivotal for success in organic synthesis.

Nucleophilic Addition

Nucleophilic addition is a fundamental reaction type in organic chemistry, where a nucleophile donates a pair of electrons to an electrophilic carbon, forming a new covalent bond.

Ketones, with their polar carbonyl group, are particularly susceptible to nucleophilic addition. During the reaction, the nucleophile attacks the carbon of the carbonyl group, and the oxygen of the carbonyl becomes protonated, resulting in the addition of the nucleophile to the molecule. Protector groups like cyanide (HCN) can add to carbonyls to form cyanohydrins, which can be further transformed into other functional groups. Understanding nucleophilic addition is essential for creating a vast array of complex molecules in organic chemistry. The right choice of nucleophile and reaction conditions is crucial in controlling the outcome of the reaction.