Problem 77 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following formula is more reactive for hydrolysis? (a) \(-\mathrm{COOR}\) (b) \(-\mathrm{CONH}_{2}\) (c) \(-\mathrm{COOH}\) (d) \(-\mathrm{COCl}\)

Step-by-Step Solution

Answer

Acid chlorides \\(-\mathrm{COCl}\\) are the most reactive towards hydrolysis.

Step 1: Understanding the Functional Groups

First, understand each functional group given in the options. The groups are esters \(-\mathrm{COOR}\), amides \(-\mathrm{CONH}_{2}\), carboxylic acids \(-\mathrm{COOH}\), and acid chlorides \(-\mathrm{COCl}\). Hydrolysis refers to the chemical breakdown of a compound due to reaction with water.

Step 2: Analyzing Ester Hydrolysis

Esters (\(-\mathrm{COOR}\)) undergo hydrolysis to form alcohols and acids. They require acidic or basic conditions and generally are more stable than acid chlorides, but less stable than amides. Hydrolysis can be a slow process.

Step 3: Analyzing Amide Hydrolysis

Amides (\(-\mathrm{CONH}_{2}\)) undergo hydrolysis to form acids and ammonia or amines. Amides are very stable compared to esters and acid chlorides when it comes to hydrolysis, often requiring strong acid or base conditions to proceed efficiently.

Step 4: Analyzing Carboxylic Acid Hydrolysis

Carboxylic acids (\(-\mathrm{COOH}\)) are not susceptible to hydrolysis under normal conditions, as they are the end product of hydrolysis for many other compounds such as esters and amides.

Step 5: Analyzing Acid Chloride Hydrolysis

Acid chlorides (\(-\mathrm{COCl}\)) are very reactive towards hydrolysis, readily reacting with water to form carboxylic acids and hydrochloric acid. This reaction is typically very fast and does not require a catalyst.

Step 6: Comparing Reactivity

Compare the reactivity of all the functional groups: - Acid chlorides are the most reactive towards hydrolysis due to their strong electrophilic nature.- Esters follow next, requiring additional conditions for hydrolysis.- Amides are even less reactive, needing stronger conditions.- Carboxylic acids do not undergo hydrolysis as they are the products of such reactions. Thus, acid chlorides (\(-\mathrm{COCl}\)) are the most reactive towards hydrolysis.

Key Concepts

Functional Groups

Functional groups are specific groups of atoms within molecules that determine the characteristic chemical reactions of those molecules. In the context of hydrolysis, different functional groups react differently with water. For example, esters, amides, carboxylic acids, and acid chlorides are all functional groups involved in hydrolysis processes. Understanding the distinctiveness of each group allows chemists to predict how compounds will behave in various chemical reactions and under different conditions.

Reactivity Order

Reactivity order refers to how readily a functional group participates in a chemical reaction, such as hydrolysis. In hydrolysis, acid chlorides are known to be the most reactive due to their high electrophilic nature, making them prone to reacting quickly with water. Next in line are esters, which generally require acidic or basic conditions to proceed with hydrolysis. Amides are considerably less reactive and demand more vigorous conditions. Carboxylic acids, on the other hand, usually stand as end products of hydrolysis, illustrating their low reactivity in this context. Understanding reactivity order helps in predicting the likelihood of a chemical transformation.

Acid Chlorides

Acid chlorides, characterized by the functional group \(\-\mathrm{COCl}\), are exceptionally reactive towards hydrolysis. This is because the chlorine atom is a good leaving group, facilitating the reaction with water. When acid chlorides undergo hydrolysis, they convert into carboxylic acids and hydrochloric acid. This rapid reaction often occurs without the need for a catalyst. Because of their high reactivity, acid chlorides are often used in organic synthesis to introduce acyl groups.

Esters

Esters are functional groups represented by the formula \(\-\mathrm{COOR}\). They are known to undergo hydrolysis to yield alcohols and carboxylic acids. However, ester hydrolysis tends to be slower compared to acid chlorides. It often requires acidic or basic conditions to proceed. The relatively lower reactivity of esters is due to the poor leaving ability of the alkoxy group. Despite this, esters are prevalent in nature and industry, especially in lipids and fragrances, because they can be stabilized to resist premature reactions.

Amides

Amides, with the formula \(\-\mathrm{CONH}_{2}\), show significant resistance to hydrolysis, which requires harsher conditions like strong acids or bases to occur. When subjected to hydrolysis, amides typically yield carboxylic acids and ammonia or amines. Their stability stems from the amide bond, which has resonance structures that contribute to its robustness. This stability makes amides important in biochemical compounds, such as proteins, where they form peptide bonds.