Question

An organic compound \(\mathrm{B}\) is formed by the reaction of ethyl magnesium iodide \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{MgI}\right)\) with a substance \(\mathrm{A}\), followed by treatment with a dilute aqueous acid. Compound B doesn't react with PCC or PDC in dichloromethane. Which of the following is the most suitable as A? (A) \(\mathrm{CH}_{3} \mathrm{CHO}\) (B) \(\mathrm{HCHO}\) (C) \(\mathrm{H}_{2} \mathrm{C}-\mathrm{CH}_{2}\) (D) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CCH}_{3}\)

Short answer

The most suitable substance A is (A) \(\mathrm{CH}_{3}\mathrm{CHO}\), as it forms compound B when reacted with ethyl magnesium iodide followed by treatment with dilute aqueous acid and fulfills the additional conditions mentioned.

Step-by-step solution

Grignard reagent (R-MgX, where R is an organic group and X is a halogen) can react with various functional groups like aldehydes, ketones, esters, etc. The treatment with dilute aqueous acid is to hydrolyze the product, separating the ions and releasing the original R group from ethyl magnesium iodide. #Step 2: Analyze reaction of given options with Grignard reagent and subsequent reactions #

We need to examine each option and analyze the reaction with ethyl magnesium iodide and the subsequent treatment with dilute aqueous acid. Then, check if compound B formed in each case doesn't react with PCC or PDC in dichloromethane. #Step 3: Analyze option A #

Option A is an aldehyde \(\mathrm{CH}_{3}\mathrm{CHO}\). The reaction of ethyl magnesium iodide with aldehydes followed by treatment with dilute aqueous acid forms a secondary alcohol. In this case, the secondary alcohol formed is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH}\) (butanol), which doesn't react with PCC or PDC in dichloromethane. This fulfills the given conditions; hence option A is a possible choice. #Step 4: Analyze option B #

Option B is HCHO, a formaldehyde. The reaction of ethyl magnesium iodide with formaldehyde followed by treatment with dilute aqueous acid forms a primary alcohol. In this case, the primary alcohol formed is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CH}_{2}\mathrm{OH}\) (propanol), which doesn't react with PCC or PDC in dichloromethane. Though this fulfills the given conditions, option A is a better choice since it forms a higher order of alcohol (secondary). #Step 5: Analyze option C #

Option C is an alkene (\(\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2}\)). Grignard reagents don't react with simple alkenes. Hence, there will be no reaction in this case, and option C can be ruled out. #Step 6: Analyze option D #

Option D is a tertiary alkyl halide (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CCH}_{3}\)). Grignard reagents don't react with tertiary alkyl halides. Hence, there will be no reaction in this case, and option D can be ruled out. Considering the analysis, option A (\(\mathrm{CH}_{3}\mathrm{CHO}\)) is the most suitable substance A, as it forms compound B when reacted with ethyl magnesium iodide followed by treatment with dilute aqueous acid and fulfills the additional conditions mentioned.

Key concepts

Organic Chemistry

Organic chemistry focuses on the study and understanding of carbon-based compounds. It is fascinating because carbon's ability to form four bonds allows for the creation of diverse, complex structures. In the context of this exercise, we delve into the intriguing Grignard reaction—a staple in organic synthesis.
The Grignard reaction involves a Grignard reagent, such as ethyl magnesium iodide in our exercise. This reagent displays a unique chemical nature. It has a carbon atom with a partial negative charge, which makes it highly reactive towards electrophiles, substances that accept electrons.
In our case, the reagent reacts primarily with carbonyl-containing compounds like aldehydes and ketones. This interaction ultimately alters molecular structures to yield alcohols. Understanding these reactivity patterns is central to grasping the transformations in organic synthesis.

Secondary Alcohols

Secondary alcohols are hydroxyl compounds ( -OH group) bonded to a carbon atom that is attached to two other carbon atoms. These alcohols arise from distinctive reactions involving ketones or aldehydes with organometallic reagents.
In our exercise, the interaction of ethyl magnesium iodide with acetaldehyde ( CH₃CHO) exemplifies this process. The alkyl group from the Grignard reagent attaches itself to the carbonyl carbon of the aldehyde, forming an alkoxide intermediate. The subsequent treatment with water or dilute acid displaces the magnesium group, resulting in a secondary alcohol, specifically butanol ( CH₃CH₂CH₂CH₂OH).
Such reactions illuminate how secondary alcohols can be synthesized systematically, underlining their importance in organic chemistry and industrial applications. They serve as intermediates in synthesizing pharmaceuticals, dyes, and perfumes.

Chemical Reactivity Analysis

Chemical reactivity analysis examines how different compounds interact, predicting the outcome based on their electronic and structural properties. This analysis aids in understanding why certain reactions proceed and others do not.
Evaluating our exercise, compound B is tested for non-reactivity with PCC and PDC, common oxidizing agents. These agents traditionally oxidize alcohols; however, in this case, the secondary alcohol, butanol, remains unchanged. This suggests stability against oxidation, congruent with our analysis of option A ( CH₃CHO) as the right aldehyde.
Moreover, when analyzing alkenes and tertiary alkyl halides, these compounds show no reactivity with Grignard reagents due to their inherent chemical structures. Alkenes lack polar functional groups, and tertiary alkyl halides are sterically hindered, preventing nucleophilic attack. This knowledge reinforces identifying the best suitable reactions and the importance of compatibility in organic synthesis strategies.