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

The most reactive formula for hydrolysis is - COCl.

Step 1: Understand Hydrolysis

Hydrolysis is a chemical process where a compound reacts with water to break down into two or more substances. In this context, it involves the cleavage of different acyl groups ( - COOR, - CONH_{2}, - COOH, or - COCl) with water.

Step 2: Analyze Acyl Group Stability

The reactivity of an acyl compound towards hydrolysis depends on the stability of the leaving group. More stable the leaving group, less reactive it is towards hydrolysis. Conversely, if the leaving group is less stable, the acyl group is more reactive.

Step 3: Evaluate Leaving Groups

Examine the leaving groups: -R ( - COOR), -NH_{2} ( - CONH_{2}), -OH ( - COOH), -Cl ( - COCl). Chloride (Cl^{-} ) is a weaker base compared to -OH, -NH_{2}, and -OR. Thus, chloride is a better leaving group, indicating that - COCl is more reactive.

Step 4: Determine Reactivity Based on Leaving Groups

Since chloride ion ( Cl^{-} ) is a very good leaving group, acyl chlorides ( - COCl) are highly reactive compared to esters ( - COOR), amides ( - CONH_{2}), and carboxylic acids ( - COOH).

Key Concepts

acyl group stability

To understand acyl group stability, let's consider the structure of an acyl group, which is typically a carbonyl group (\( C=O \)) attached to another atom or group. The stability of an acyl compound is largely determined by its leaving group—the part of the molecule that departs during a hydrolysis reaction.In general, the more stable the leaving group, the less reactive the acyl compound becomes towards hydrolysis. This is because a stable leaving group is less likely to depart, making the overall reaction less favorable. Conversely, a less stable leaving group readily departs, facilitating the hydrolysis process.When faced with different acyl groups, consider their leaving group stability:

• The ester group (\(-\mathrm{COOR}\)) is relatively stable, making esters less prone to hydrolysis.
• Amides (\(-\mathrm{CONH}_{2}\)) feature the least reactive group due to the particularly stable amide linkage, generally making them resistant to hydrolysis.
• Carboxylic acids (\(-\mathrm{COOH}\)) are somewhat reactive, but not as much as acyl chlorides.

Understanding these relation helps determine which acyl compounds are more or less susceptible to hydrolysis.

leaving groups in chemistry

In chemistry, leaving groups play a crucial role in many reactions, including hydrolysis. A leaving group is an atom or a group of atoms that detaches from the parent molecule and can carry away the pair of electrons that formerly bonded them to the molecule. The efficiency of a leaving group depends on its ability to stabilize the excess negative charge after detachment. More stable leaving groups generally make the parent compound more reactive. When comparing leaving groups, consider the following:

• Chloride (\(Cl^{-}\)) is a good leaving group because it can disperse the negative charge effectively, making it more stable.
• In contrast, groups like \(OH^{-}\), \(NH_2^{-}\), and \(OR^{-}\) are stronger bases and thus poorer leaving groups. They are less stable with a negative charge, leading to lower reactivity of the compounds they reside within.

Always remember, a strong acid's conjugate base (like \(Cl^{-}\)) tends to be a better leaving group due to its increased ability to hold onto the negative charge, contributing significantly to the reactivity observed in chemical processes.

acyl chlorides reactivity

Acyl chlorides have a distinct position in chemistry, prominently due to their high reactivity. This stems from the presence of the chloride ion (\(Cl^{-}\)) as an exceptional leaving group. During hydrolysis, acyl chlorides (\(-\mathrm{COCl}\)) break apart easily, which makes them more reactive than other acyl derivatives, like esters or amides.Several factors contribute to the reactivity of acyl chlorides:

• The chloride ion, after leaving, is highly stable and disperses the negative charge well, lowering the energy barrier for the reaction.
• This allows acyl chlorides to undergo a rapid reaction with water, breaking down quickly.

This reactivity isn't only limited to water; acyl chlorides react quickly with a variety of nucleophiles, making them excellent starting materials in organic synthesis.While the high reactivity of acyl chlorides is advantageous in many chemical processes, it also means that special handling and storage conditions are required to prevent unwanted reactions during storage.