Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will react with water? (a) \(\mathrm{CHCl}_{3}\) (b) \(\mathrm{Cl}_{3} \mathrm{CCHO}\) (c) \(\mathrm{CCl}_{4}\) (d) \(\mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\)

Step-by-Step Solution

Answer

\( \mathrm{Cl}_{3} \mathrm{CCHO} \) (trichloroacetaldehyde or chloral) will react with water.

Step 1: Understanding Reactivity with Water

To determine which compounds will react with water, we must analyze the structures and the presence of functional groups that typically react with water. In organic chemistry, possible reactions with water often involve hydrolysis or other transformations where water acts as a nucleophile.

Step 2: Examining \( \mathrm{CHCl}_{3} \)

This compound is chloroform, a stable molecule with a central carbon atom bonded to one hydrogen and three chlorine atoms. It lacks functional groups like an epoxide or a carbonyl with adjacent electron-withdrawing groups that could make the carbon susceptible to nucleophilic attack by water. Therefore, \( \mathrm{CHCl}_{3} \) is unlikely to react with water.

Step 3: Examining \( \mathrm{Cl}_{3} \mathrm{CCHO} \)

This compound is trichloroacetaldehyde, also known as chloral. It has a carbonyl group (\( \text{C=O} \)) adjacent to a trichloromethyl group (\( \text{CCl}_{3} \)). The carbonyl carbon is highly electrophilic due to the electron-withdrawing effect of the \( \text{CCl}_{3} \) group, making it susceptible to nucleophilic attack by water. Thus, \( \mathrm{Cl}_{3} \mathrm{CCHO} \) will react with water to form chloral hydrate.

Step 4: Examining \( \mathrm{CCl}_{4} \)

Carbon tetrachloride is a stable molecule similar to chloroform but with no hydrogen atoms. The carbon has a full complement of chlorine atoms, which makes it very stable and resistant to reaction with water. Therefore, \( \mathrm{CCl}_{4} \) will not react with water.

Step 5: Examining \( \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl} \)

This compound is 1,2-dichloroethane, a simple alkyl dihalide. There is no reactive group present such as a carbonyl or an epoxide, and the carbon-chlorine bonds are not readily hydrolyzed by water under normal conditions. Consequently, \( \mathrm{ClCH}_{2} \mathrm{CH}_{2} \mathrm{Cl} \) will not react with water.

Key Concepts

Understanding Hydrolysis

Hydrolysis is a chemical reaction involving water, often used to break down compounds. In organic chemistry, it frequently involves the breaking of a bond in a molecule with the addition of the elements of water. In many cases, this can lead to the transformation of a functional group in a molecule.
For example, esters can undergo hydrolysis to form an acid and an alcohol. Amides can also hydrolyze to produce an acid and an amine. The effectiveness of hydrolysis often depends on the stability and nature of the bonds within a molecule.
Two types of hydrolysis reactions are typical:

• Acid-catalyzed hydrolysis: Utilizing an acidic environment to speed up the reaction.
• Base-catalyzed hydrolysis (saponification): Utilizing a basic environment to facilitate the reaction.

Understanding the specific conditions under which hydrolysis occurs can help predict which substances may react with water, as seen in our exercise.

The Role of Nucleophilic Attack

Nucleophilic attack is an essential concept in organic reactions, where a nucleophile, an electron-rich species, donates an electron pair to an electron-deficient atom or group, known as an electrophile. This process typically results in the formation or breaking of bonds.
A common scenario for nucleophilic attack is when water, a weak nucleophile, attacks a carbon in a polar bond such as a carbonyl group. In such reactions, water can add to a carbonyl group, forming a hydrate, or initiate other transformation reactions.
The nucleophilicity of water and its ability to attack specific functional groups was central to determining why only trichloroacetaldehyde reacts with water in the exercise. This understanding is crucial for predicting and controlling chemical reactions in organic synthesis.

Identifying Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. They determine how molecules behave in chemical reactions and interact with other substances.
Some common functional groups include:

• Alcohols \(( ext{-OH})\)
• Carbonyl groups \(( ext{C=O})\), found in aldehydes and ketones
• Carboxylic acids \(( ext{-COOH})\)
• Amines \(( ext{R-NH}_2)\)
• Halides \(( ext{-Cl, -Br})\)

These groups dictate the chemical behavior of the compounds they're in. In this scenario, identifying a carbonyl group in trichloroacetaldehyde highlighted its reactivity with water due to nucleophilic attack, showcasing the powerful influence of these groups.

The Importance of the Carbonyl Group

A carbonyl group is a functional group consisting of a carbon atom double-bonded to an oxygen atom \(\text{(C=O)}\). It's a prominent feature in many organic compounds, like aldehydes, ketones, carboxylic acids, and their derivatives.
The carbon atom in a carbonyl group is electrophilic, meaning it's electron-deficient and can attract nucleophiles. This makes it a key site for chemical reactions such as nucleophilic additions.

• The presence of a carbonyl group often indicates potential reactivity with nucleophiles.
• In the case of trichloroacetaldehyde, the carbonyl carbon becomes especially susceptible to attack by water due to its adjacent electron-withdrawing groups.

Understanding the carbonyl group's role and behavior helps predict how compounds will interact in chemical reactions.

Electron-Withdrawing Effect Consequences

The electron-withdrawing effect refers to the ability of a substituent to pull electron density away from the rest of the molecule. This makes certain parts of molecules more electrophilic and reactive towards nucleophiles.
Trichloroacetaldehyde is a perfect example, where the trichloromethyl group \(\text{(CCl}_3)\) pulls electrons away from the carbonyl carbon, enhancing its electrophilicity.
Key Points:

• Increases reactivity of adjacent carbon atoms.
• Stabilizes negative charges due to increase in electron deficiency.
• Influences the acidity of a hydrogen atom due to increased positive character.

The electron-withdrawing nature of groups surrounding a functional center can largely determine the reactivity and outcome of organic reactions. In our exercise, this effect facilitated the reaction with water, differentiating trichloroacetaldehyde from other compounds.