Question

Arrange the following in decreasing order of nucleophilic substitution towards hydrolysis: Benzoyl chloride \(\quad\) Benzyl chloride \(\quad\) Chlorobenzene (I) (II) (III) (A) I > II > III (B) II > I > III \((\mathrm{C}) \mathrm{II}>\mathrm{III}>\mathrm{I}\) (D) \(\mathrm{I}>\mathrm{III}>\mathrm{II}\)

Short answer

The correct order of nucleophilic substitution towards hydrolysis for the given compounds is: \( \boxed{ (B) \ II > I > III }\) (Benzyl chloride > Benzoyl chloride > Chlorobenzene)

Step-by-step solution

Understand Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the exchange of a leaving group (in our case, a chloride ion) with a nucleophile. The rate of nucleophilic substitution reactions depends on factors such as the stability of the carbocation intermediate, the ability of the leaving group to leave, and the stability of the product.

Examine the Structures of the Given Compounds

Let's examine the structures of the three given compounds: 1. Benzoyl chloride: The carbonyl group in benzoyl chloride has a polarized bond, which makes the carbon atom more susceptible to nucleophilic attack. Also, the resonance between the carbonyl group and the phenyl ring increases the stability of the intermediate. 2. Benzyl chloride: The positive charge on the carbon atom in the carbocation intermediate formed during the nucleophilic substitution reaction can be delocalized by resonance with the phenyl ring. This increases the stability of the intermediate and makes it more likely to undergo hydrolysis. 3. Chlorobenzene: The carbon-chlorine bond in chlorobenzene is less polarized due to the low electronegativity of the phenyl ring. Therefore, the carbon atom is less susceptible to nucleophilic attack, and the reaction rate of hydrolysis will be slower compared to that of benzoyl and benzyl chloride.

Compare the Reactivity of the Compounds in Nucleophilic Substitution

Based on the structural properties and stability of intermediates, it can be deduced that the reactivity of nucleophilic substitution towards hydrolysis will be as follows: Benzyl chloride > Benzoyl chloride > Chlorobenzene

Match the Correct Option

Now, let's match this order to the given options: (I) Benzoyl chloride (II) Benzyl chloride (III) Chlorobenzene Our order is II > I > III, which matches option (B). #Answer# The correct order of nucleophilic substitution towards hydrolysis for the given compounds is: \( \boxed{ (B) \ II > I > III }\) (Benzyl chloride > Benzoyl chloride > Chlorobenzene)

Key concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental reaction in organic chemistry. It involves a nucleophile—a molecule or ion with a pair of electrons—attacking an electrophilic center, often a carbon atom bonded to a leaving group. The leaving group, which is usually a halide ion in many organic reactions, is displaced as the nucleophile takes its place. A key part of understanding nucleophilic substitution is knowing how to determine the nucleophilicity—the tendency of a molecule to donate a pair of electrons. Factors such as the basicity of the nucleophile, the polarizability of the group, and the steric effects play significant roles. For example, in the context of benzoyl chloride, benzyl chloride, and chlorobenzene, the nucleophilicity order is largely influenced by the electronic structure and resonance capabilities of these molecules.

Hydrolysis

Hydrolysis is a chemical process where a compound reacts with water, causing the compound to split and the water molecule to be incorporated into the products. In organic chemistry, nucleophilic hydrolysis often involves water acting as a nucleophile. The breakdown of esters, amides, and other carboxylic acid derivatives through hydrolysis is a common example. In the problem we're considering, the hydrolysis of benzoyl chloride, benzyl chloride, and chlorobenzene under nucleophilic substitution conditions shows varying speeds. This variance depends on the susceptibility of the compound to nucleophilic attack, which is primarily governed by the polarity of the carbon-leaving group bond and the stability of the resulting structure or tetrahedral intermediate.

Carbocation Stability

Carbocations are positively charged carbon atoms, common intermediates in many chemical reactions, including nucleophilic substitutions. The stability of these carbocations is a major determinant of the reaction rate. More stable carbocations are formed faster and make the reaction sequence proceed more efficiently. The benzyl carbocation, for example, benefits from resonance stabilization due to its ability to delocalize the positive charge over the phenyl ring. This resonance makes it significantly more stable than a carbocation without such delocalization potential. In contrast, the lack of carbocation stabilization in chlorobenzene is reflective in its slower reaction rate. Understanding such differences is key to predicting the order of reactivity in nucleophilic substitution reactions.

Resonance Effect

The resonance effect is a significant aspect of organic compounds with conjugated systems, influencing their stability and reactivity. Through resonance, electrons are delocalized over several atoms, increasing the molecule's stability. This delocalization can impact both the starting materials and reaction intermediates. In nucleophilic substitution reactions, the resonance can stabilize intermediates like carbocations or transition states, which can affect the rate and outcome of the reaction. Benzyl chloride showcases resonance stabilization in its intermediate, making it more reactive in nucleophilic substitutions compared to others like chlorobenzene, where resonance within the aromatic ring prevents such stabilization of the reacting carbon. In the cases of benzoyl chloride and benzyl chloride, resonance improves the efficiency of the reaction by stabilizing potential intermediates through electron sharing, a crucial factor for achieving fast and favorable reactions.