Problem 93 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the final product in the following sequence of reactions. $$ \begin{aligned} &\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{MgBr} \stackrel{\mathrm{CH}_{2}-\mathrm{CH}_{2}}{\mathrm{Mg}} \rightarrow(\mathrm{a}) \stackrel{\mathrm{H}_{3} \mathrm{O}^{+}}{\longrightarrow} \text { (b) } \\ &\stackrel{\mathrm{KMnO}_{4}}{\longrightarrow}(\mathrm{c}) \end{aligned} $$ (a) \(\mathrm{CH}_{3} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{COOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\)

Step-by-Step Solution

Answer

The final product is (c) CH₃CH₂CH₂COOH.

Step 1: Understand the Reaction Sequence

The sequence starts with ethyl magnesium bromide (Grignard reagent) reacting with ethylene. This reaction typically leads to an extension of the carbon chain.

Step 2: First Reaction Product

When ethyl magnesium bromide (\(\text{CH}_3\text{CH}_2\text{MgBr}\) reacts with ethylene (\(\text{CH}_2=\text{CH}_2\)), it forms 1-butene. The reaction adds the ethyl group to the ethylene, resulting in \(\text{CH}_3\text{CH}_2\text{CH}=\text{CH}_2\) (1-butene).

Step 3: Hydrolysis Step

Next, the hydrolysis (\(\text{H}_3\text{O}^+\) treatment) of 1-butene results in the formation of butane-1-ol. This reaction converts the alkene to an alcohol and gives \(\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}\).

Step 4: Oxidation with KMnO4

Finally, oxidation of butane-1-ol with \(\text{KMnO}_4\) leads to the formation of a carboxylic acid. This step converts the primary alcohol into a carboxylic acid, forming butanoic acid (\(\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}\)).

Step 5: Determine the Final Product

The final product of the sequence is the carboxylic acid formed in the last step. Comparing it to the given options, the final product matches \(\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}\).

Key Concepts

Grignard Reagents

Grignard reagents are essential tools in organic chemistry used to form carbon-carbon bonds. They are typically organomagnesium compounds with the formula \( ext{R-MgBr}\) where \(\text{R}\) is a hydrocarbon chain. These reagents are highly reactive due to the polar nature of the carbon-magnesium bond. This polarity makes the carbon an excellent nucleophile, meaning it is eager to attach to electron-deficient centers, such as carbons in carbonyl groups or unsaturated chemical bonds like double bonds in alkenes.
When Grignard reagents, such as ethyl magnesium bromide, react with compounds like ethylene, they initiate reactions that expand carbon chains. This reaction with ethylene results in the formation of a longer alkene chain. The Grignard reagent effectively transfers its alkyl group, extending the carbon framework of the initial molecule.

Hydrolysis Reaction

Hydrolysis involves breaking down molecules using water. In organic chemistry, this reaction is crucial for converting one functional group to another. When discussing Grignard reagents, hydrolysis refers to using aqueous acid to transform Grignard-adducts into alcohols.
In the provided reaction sequence, the hydrolysis stage occurs after the Grignard reaction. The compound formed from the initial Grignard reaction is an unsaturated molecule (1-butene). Exposing this compound to acidic water (\( ext{H}_3 ext{O}^+\)) leads to hydration, turning the double bond into an alcohol group. As a result, 1-butene becomes butane-1-ol. This conversion involves breaking the pi bond of the alkenic carbon and adding hydroxyl groups, finalizing into an alcohol form.

Oxidation Reaction

Oxidation reactions involve increasing the oxidation state of a molecule. This often occurs when substances gain oxygen or lose hydrogen. In organic compounds, oxidation commonly transforms alcohols into either aldehydes, ketones, or carboxylic acids, depending on the oxidation conditions.In the reaction sequence, oxidation is achieved using potassium permanganate, \( ext{KMnO}_4\). This strong oxidizing agent converts the alcohol group of butane-1-ol into a carboxylic acid. Specifically, it targets primary alcohols, causing them to oxidize to carboxylic acids. Thus, butane-1-ol oxidizes to butanoic acid (\( ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{COOH}\)), establishing the carbon-oxygen double bond characteristic of carboxylic acid groups.

Carboxylic Acids

Carboxylic acids are one of the most versatile functional groups in organic chemistry, containing a carbonyl (\(C=O\)) and a hydroxyl (\(OH\)) group bonded to the same carbon atom. This group is depicted as \(-COOH\). Their unique structure leads to acidic properties due to the potential to donate protons, which influences their reactivity.Carboxylic acids, like butanoic acid, typically form when alcohols or aldehydes undergo oxidation, especially in the presence of strong oxidizing agents like \( ext{KMnO}_4\). In the exercise, the culmination of a Grignard reaction followed by hydrolysis and oxidation results in synthesizing a carboxylic acid. This outcome illustrates the critical role of carboxylic acids in chemical synthesis and their considerable reactivity, which makes them valuable intermediates in creating more complex organic molecules.