Problem 162 - A Complete Resource Book in Chemistry for JEE Main

Question

The correct order of reactivity of PhMgBr with \(\mathrm{Ph}-\mathrm{C}-\mathrm{Ph} \mathrm{CH}_{3}-\mathrm{C}-\mathrm{H} \quad \mathrm{CH}_{3}-\mathrm{C}-\mathrm{CH}_{3}\) is (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (b) III > II > I (c) \(\mathrm{II}>\mathbb{I I I}>\mathrm{I}\) (d) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\)

Step-by-Step Solution

Answer

(d) I > III > II

Step 1: Understand the Reactivity Nature

PhMgBr is a Grignard reagent, which is a highly reactive organometallic compound. It is nucleophilic and reacts primarily with carbonyl compounds. The reactivity often depends on the presence of carbonyl or similar groups in the molecule to facilitate a nucleophilic attack.

Step 2: Identify the Structures for Attack

The structures provided are: 1. Phenyl group (Ph)-C-Ph 2. Methyl group (CH3)-C-H 3. Dimethyl group CH3-C-CH3 Each structure needs to be evaluated for its potential to undergo a nucleophilic attack by PhMgBr. Typically, carbonyl groups or more unsaturated (e.g., aryl groups) centers will be more reactive.

Step 3: Evaluate Structure I: Phenyl-C-Phenyl

In structure I, the presence of two phenyl groups surrounding the central carbon can stabilize the transition state of an attack by PhMgBr due to resonance effects. However, without a polar carbonyl group, the activation is lower compared to a typical carbonyl compound.

Step 4: Evaluate Structure II: Methyl-C-H

In structure II, there is a methyl group attached to a seemingly hydrogen-protected central carbon. This structure is least likely to undergo a nucleophilic attack as it lacks a leaving group or polarizable bond that typically reacts with Grignard reagents.

Step 5: Evaluate Structure III: Dimethyl-C

In structure III, a central carbon is flanked by two methyl groups. Although it's similar to structure II in lacking a carbonyl or polar bond, the exposure of the central carbon is higher due to less steric protection, potentially making it slightly more reactive than structure II.

Step 6: Determine the Order of Reactivity

Based on the structures' evaluation, structure III (dimethyl group) may have some subtle heightened reactivity due to less steric hindrance compared to methyl-C-H (structure II), but both have minimal reactivity due to lacking a polar carbonyl bond. Structure I may be slightly more reactive due to resonance stabilization, resulting I > III > II being the most fitting reactivity order.

Key Concepts

Nucleophilic Attack

A nucleophilic attack is a fundamental concept in organic chemistry. It occurs when a molecule with an extra pair of electrons, called a nucleophile, seeks out a positively charged or electron-deficient site on another molecule. In the case of Grignard reagents, the nucleophile is typically the carbon atom bonded to magnesium. This nucleophilic carbon is attracted to electron-poor sites, which are often found in carbonyl groups. These interactions are crucial for forming new carbon-carbon bonds.

Grignard reagents are excellent nucleophiles, meaning they aggressively search for these electron-deficient sites to bond with. This makes them incredibly useful in synthetic chemistry, especially for forming complex organic compounds. However, the efficiency of these attacks depends heavily on the molecular structures involved.

Organometallic Compounds

Organometallic compounds are fascinating molecules characterized by direct bonds between carbon atoms and metals. These compounds are incredibly versatile and useful in chemistry because they can behave as both nucleophiles and catalysts for various reactions. The metal in these compounds, like magnesium in Grignard reagents, donates electron density to the carbon atom.

This electron-rich carbon can then perform a nucleophilic attack on other molecules. These properties make organometallic compounds a staple in creating new chemical bonds, turning them into essential tools for organic synthesis. Their reactivity, however, also means they need to be handled carefully, often requiring an inert atmosphere to prevent premature reactions with moisture or oxygen.

Reactivity Order

Reactivity order refers to the sequence in which different compounds or functional groups undergo chemical reactions. When looking at compounds in terms of reactivity, we focus on how readily they interact with other substances. For the Grignard reagent PhMgBr, the reactivity is influenced by the structure and specific functional groups of the interacting molecules.

In the original exercise, the Grignard reagent reacts differently with three compounds. Structure I, with its phenyl groups, allows for some stabilization but lacks a carbonyl group to enhance reactivity. Structures II and III, with methyl groups, are less reactive, but a subtle difference makes structure III slightly more accessible due to less steric hindrance. Understanding these subtle structural differences helps predict how molecules behave in chemical reactions.

Phenyl Group

A phenyl group is a ring-shaped array of six carbon atoms, making it a type of aryl group. This group significantly influences chemical properties due to its stable aromatic structure. Phenyl groups are non-polar and generally don't participate in nucleophilic attacks. However, their presence can affect reactivity indirectly.

For example, in structure I from the original exercise, the phenyl groups provide resonance stabilization. This stabilization doesn't make them reactive sites for Grignard reagents, but it does affect the surrounding chemical environment. Resonance allows phenyl groups to disperse charge through their ring structure, consequently impacting the nucleophilic attack by Grignard reagents on adjacent carbon atoms.

Carbonyl Group

Carbonyl groups are a central focus in many organic reactions, especially with Grignard reagents. These groups, characterized by a carbon double-bonded to oxygen, are polar due to the difference in electronegativity between carbon and oxygen. This polarity makes the carbon atom within the carbonyl group an excellent target for nucleophilic attacks.

In the presence of Grignard reagents, the carbonyl carbon becomes electron-deficient, attracting the nucleophilic carbon from the Grignard reagent. This reaction results in the formation of a new carbon-carbon bond, a crucial step in building more complex molecules. The absence of a carbonyl group in the original exercise's structures explains the low reactivity of the compounds with PhMgBr, highlighting the carbonyl group's importance in increasing reactivity.