Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement(s) is/are correct? (a) The nucleophilic addition of \(\mathrm{HCN}\) to \(\mathrm{CH}_{3}-\mathrm{CH}=\) \(\mathrm{CH}-\mathrm{CHO}\) results in major addition to \(\mathrm{C}=\mathrm{O}\). (b) The nucleophilic addition of \(\mathrm{HCN}\) to \(\mathrm{CH}_{3}-\mathrm{CH}=\) \(\mathrm{CH}-\mathrm{CHO}\) results in major addition to \(\overrightarrow{\mathrm{C}}=\mathrm{C}\). (c) The nucleophilic addition of \(\mathrm{PhMgBr}\) to \(\mathrm{PhCH}=\) \(\mathrm{CH}-\mathrm{COCMe}_{3}\) results in addition to \(\mathrm{C}=\mathrm{C}\). (d) The nucleophilic addition of \(\mathrm{PhMgBr}\) to \(\mathrm{PhCH}\) \(\mathrm{CH}-\mathrm{CHO}\) results in addition to \(\mathrm{C}=\mathrm{O}\).

Step-by-Step Solution

Answer

Statements (a) and (d) are correct.

Step 1: Understand the Reaction Mechanisms

In nucleophilic addition reactions, the nucleophile attacks the carbon with the highest positive charge, usually found in carbonyl groups \( (C=O) \) or at the double bonds \( (C=C) \) when sufficiently activated. We need to consider the reactivity and the structural aspects of the compounds in question.

Step 2: Analyze Statement (a)

Statement (a) mentions the addition of \( \text{HCN} \) to \( \text{CH}_3-\text{CH}=\text{CH}-\text{CHO} \). This compound contains both \( C=O \) and \( C=C \) groups. Generally, \( \text{HCN} \) as a nucleophile will preferentially add to the carbonyl group because the \( C=O \) group is more polar and thus more electrophilic. Therefore, major addition to \( C=O \) is expected.

Step 3: Analyze Statement (b)

Statement (b) suggests major addition of \( \text{HCN} \) to the \( C=C \) bond. However, due to the preference of \( \text{HCN} \) to add to more electrophilic centers such as carbonyl \( C=O \) over less polar \( C=C \), this statement doesn't align with typical nucleophilic addition behavior. Thus, this is incorrect.

Step 4: Analyze Statement (c)

Statement (c) considers \( \text{PhMgBr} \) (a Grignard reagent) adding to \( \text{PhCH}=\text{CH}-\text{COCMe}_3 \). Grignard reagents strongly favor reaction with carbonyl groups \( C=O \), not \( C=C \) unless the circumstance is unique, such as activation of \( C=C \) by adjacent electron-withdrawing groups. Here, primary activity would be at the \( C=O \), so addition to \( C=C \) is unlikely to be the major pathway.

Step 5: Analyze Statement (d)

Statement (d) states that \( \text{PhMgBr} \) adds to the \( C=O \) bond in \( \text{PhCH}=\text{CH}-\text{CHO} \). Grignard reagents are well-known for attacking carbonyl groups \( C=O \) efficiently, making this statement correct.

Key Concepts

Grignard Reaction

The Grignard reaction is a crucial transformation in organic chemistry that involves the use of Grignard reagents. These are organomagnesium compounds, which are typically represented as R-Mg-X, where R denotes an organic group (often an alkyl or aryl group), and X is a halogen.

• Grignard reagents are highly reactive and serve as powerful nucleophiles.
• Their main function is to attack electrophilic carbon atoms, particularly in carbonyl groups (C=O), to form new carbon-carbon bonds.

During a Grignard reaction, the carbon in the carbonyl group, which bears a partial positive charge due to the polar nature of the C=O bond, becomes the target. This reactivity makes Grignard reagents invaluable in synthesizing alcohols. For example, reacting a Grignard reagent with an aldehyde or ketone typically results in the formation of a secondary or tertiary alcohol, respectively.
Although Grignard reagents are also capable of reacting with other electrophilic sites, their selectivity towards carbonyl groups makes them a preferred tool for organic chemists looking to construct complex molecules.

Carbonyl Group Reactivity

Carbonyl groups, characterized by a carbon double-bonded to oxygen (C=O), are notable for their high reactivity in organic chemistry. This prominence is attributed to the polar nature of the C=O bond.

• The oxygen atom is more electronegative, attracting electrons and creating a partial negative charge.
• The carbon atom, conversely, holds a partial positive charge, making it an electrophilic center.

Carbonyl compounds, such as aldehydes and ketones, readily participate in nucleophilic addition reactions where nucleophiles target the partially positive carbon atom. The polarity of the carbonyl bond not only dictates the site of attack but also the favorable conditions under which these reactions occur.
Consequently, carbonyl groups are more reactive than non-polar C=C double bonds in compounds that contain both. In competitive scenarios, as in the presence of both C=O and C=C, nucleophiles often prefer engaging with carbonyl sites, barring any exceptional structural influences.

Electrophilic Centers

Electrophilic centers in a molecule are regions that are electron-deficient and thus seek electrons to achieve a stable electronic configuration. Such centers often arise from polar bonds where one atom is significantly more electronegative than the other.

• In a carbonyl group, the electrophilic center is typically the carbon atom due to the partial positive charge it bears.
• These centers are hotspots for nucleophilic attacks where electron-rich species donate electrons to these electron-deficient atoms.

In analyzing organic reactions, identifying electrophilic centers helps predict the course of nucleophilic attacks. For instance, in the reaction involving HCN, the nucleophile C^- attacks the carbon within the carbonyl group due to its electrophilicity. Understanding the concept of electrophilicity is critical in forecasting how a molecule will react with nucleophiles, which aids in strategic synthesis planning and chemical research.