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 \(\mathrm{C}=\mathrm{C} .\) (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: Analyzing Statement (a)

The compound \( \text{CH}_3-\text{CH}=\text{CH}-\text{CHO} \) has both an alkene (\(\text{C}=\text{C}\)) and a carbonyl (\(\text{C}=\text{O}\)) group. Nucleophilic addition of HCN typically adds across the carbonyl \(\text{C}=\text{O}\) bond, as the oxygen's lone pairs create a stronger dipole, attracting the nucleophile to the carbon atom. Thus, statement (a) is correct.

Step 2: Analyzing Statement (b)

Statement (b) proposes the addition of HCN across \(\text{C}=\text{C}\) bond. However, the carbonyl \(\text{C}=\text{O}\) bond is more reactive towards nucleophiles, making it the major site of addition. Therefore, statement (b) is incorrect.

Step 3: Analyzing Statement (c)

For the compound \( \text{PhCH}=\text{CH}-\text{COCMe}_3 \), the alkene part (\(\text{C}=\text{C}\)) is generally less reactive compared to the carbonyl part (\(\text{C}=\text{O}\)) in such nucleophilic additions. PhMgBr, being a Grignard reagent, will typically add to the carbonyl carbon. Hence, statement (c) is incorrect.

Step 4: Analyzing Statement (d)

In statement (d), \( \text{PhMgBr} \) is a Grignard reagent which attacks the electrophilic carbon in the \(\text{C}=\text{O}\) bond of \( \text{PhCH}=\text{CH}-\text{CHO} \). This reaction results in the formation of an alcohol after workup. Therefore, statement (d) is correct.

Key Concepts

Carbonyl Group

In organic chemistry, a carbonyl group is composed of a carbon atom double-bonded to an oxygen atom, denoted as \( \text{C} = \text{O} \). This group is highly significant because it plays a crucial role in many reaction mechanisms.
Why is the Carbonyl Group Reactive?
- The presence of the electronegative oxygen atom creates a polarization in the molecule. The carbon atom becomes partially positive due to the electron-withdrawing nature of the oxygen.
- This polarity makes the carbon atom a good target for nucleophilic attack, as nucleophiles are species that donate an electron pair to positively charged or electron-deficient atoms.
- Such reactivity at the carbonyl carbon is crucial in many organic reactions, including nucleophilic addition.

Nucleophilic Addition at the Carbonyl Group
- Nucleophiles will preferentially attack the electrophilic carbon in the \( \text{C} = \text{O} \) bond, which often leads to the opening of the \( \text{C} = \text{O} \) bond and forming a tetrahedral intermediate.
- The final product commonly involves the transformation of a carbonyl into an alcohol or other functional groups, depending on the nucleophile used.

Grignard Reagents

Grignard reagents are a fascinating class of compounds used in organic synthesis, and they are usually organomagnesium halides, denoted as \( \text{R-MgX} \) where \( \text{R} \) is an organic group and \( \text{X} \) is a halogen.
Properties and Reactivity
- These reagents are highly reactive and excellent for forming carbon-carbon bonds.
- Grignard reagents behave as nucleophiles because the \( \text{Mg} \) atom donates electrons to the carbon, making the carbon part of \( \text{R-MgX} \) highly nucleophilic.
- They are sensitive to moisture, as they can react with water to give hydrocarbons, rendering them ineffective for their intended nucleophilic reactions.

Grignard Reaction with Carbonyl Compounds
- When a Grignard reagent attacks a carbonyl group, it typically targets the electrophilic carbon.
- This leads to the formation of an alcohol after the addition of acid in a process called acidic workup.
- An important aspect of Grignard reactions is their utility in creating complex molecules by building carbon skeletons.

Alkenes

Alkenes are hydrocarbons with a carbon-carbon double bond (\( \text{C} = \text{C} \)). These unsaturated compounds are key intermediates in diverse organic reactions.
Alkene Reactivity
- Alkenes are more reactive than alkanes because the double bond involves a \( \pi \)-bond, which is weaker than the \( \sigma \)-bonds in alkanes.
- The presence of the \( \pi \)-bond provides a region of electron density that can participate in reactions like electrophilic additions.

Comparison with Carbonyls
- While alkenes are reactive, they are less so than carbonyl groups in terms of nucleophilic addition.
- This is because the \( \text{C} = \text{O} \) bond in carbonyls is more polarized compared to the \( \text{C} = \text{C} \) bond in alkenes, making the carbon of carbonyl more electrophilic.
- Therefore, in reactions where both alkenes and carbonyls are present, the nucleophile will more likely target the carbonyl's electrophilic carbon.