Problem 232 - A Complete Resource Book in Chemistry for JEE Main

Question

Trihloroacetaldehyde was subjected to cannizzaro's reaction by using \(\mathrm{NaOH}\). The mixture of the products contains sodium trichloroacetate and another co mpound. The other compound is: (a) Trichloromethanol (b) \(2,2,2\)-Trichloropropanol (c) Chloroform (d) \(2,2,2\)-Trichloroethanol

Step-by-Step Solution

Answer

The other compound is 2,2,2-Trichloroethanol (d).

Step 1: Understanding Cannizzaro Reaction

The Cannizzaro reaction is a redox reaction where an aldehyde lacking an alpha hydrogen reacts with a strong base to form an alcohol and a carboxylate salt. It's important to note that this reaction occurs with aldehydes that do not have a hydrogen atom attached to the alpha carbon atom, making trihaloacetaldehydes suitable candidates.

Step 2: Analyzing Trichloroacetaldehyde

Trichloroacetaldehyde ( ext{CCl}_3 ext{CHO}) is an aldehyde with three chlorine atoms attached to the alpha carbon, which means it lacks alpha hydrogen atoms. Thus, it is suitable to undergo a Cannizzaro reaction.

Step 3: Applying the Cannizzaro Reaction

In the Cannizzaro reaction of trichloroacetaldehyde, one molecule of the aldehyde gets oxidized to form the carboxylate salt, sodium trichloroacetate ( ext{CCl}_3 ext{COONa}), and another molecule gets reduced to form an alcohol. The alcohol in consideration here is ext{2,2,2-Trichloroethanol}.

Step 4: Identifying the Other Compound

According to the Cannizzaro reaction mechanism, the aldehyde (trichloroacetaldehyde) will split into sodium trichloroacetate and a corresponding alcohol. The alcohol formed from this reaction is ext{2,2,2-Trichloroethanol}.

Key Concepts

Trihaloacetaldehydes

Trihaloacetaldehydes are a special class of aldehydes, characterized by having three halogen atoms attached to the carbon that is near the aldehydic group. The halogens are usually chlorine atoms in these compounds, making them highly reactive in certain chemical reactions.
These compounds are ideal candidates for the Cannizzaro reaction due to their structural attributes. Because they lack hydrogen atoms on the alpha carbon (the carbon adjacent to the carbonyl group), they do not readily undergo other common reactions like aldol condensation.
Understanding the role of alpha hydrogens is key when studying chemical reactivity in aldehydes, and trihaloacetaldehydes serve as an excellent example. Their lack of alpha hydrogens is the reason they participate in the Cannizzaro reaction, resulting in the formation of a carboxylate and an alcohol.

Aldehydes without alpha hydrogen

Aldehydes without alpha hydrogen are unique in their chemical behavior. Typically, aldehydes can participate in reactions such as the aldol condensation if they have an alpha hydrogen. This hydrogen is necessary for the formation of enol intermediates.
However, when the alpha carbon is fully substituted with groups like halogens, as in trihaloacetaldehydes, the aldehyde loses this capability. Instead, these aldehydes undergo the Cannizzaro reaction, where one molecule reduces and another oxidizes simultaneously.
During the Cannizzaro reaction, the absence of alpha hydrogen enables the set of redox processes that transform the aldehyde into a carboxylate salt and an alcohol. This distinct pathway showcases the chemistry of aldehydes in situations where the usual reactivity routes are blocked by the absence of alpha hydrogens.

Redox reaction in aldehydes

A redox reaction involves the transfer of electrons between two chemical species, and in the Cannizzaro reaction, this occurs within aldehyde molecules. One molecule acts as a reducing agent and gets oxidized, while another acts as an oxidizing agent and gets reduced.

• The aldehyde that oxidizes loses electrons to form a carboxylate salt.
• The aldehyde that reduces gains electrons to become an alcohol.

This internal transfer and transformation highlight the dual nature of the aldehyde under basic conditions such as those provided by \( ext{NaOH}\). The mechanism demonstrates the fascinating versatility of chemical compounds. It's crucial to understand that the absence of alpha hydrogens is what makes this dual transformation possible in aldehydes like trichloroacetaldehyde.

Trichloroacetaldehyde

Trichloroacetaldehyde, chemically represented as \( ext{CCl}_3 ext{CHO}\), is a classic example of an aldehyde suitable for the Cannizzaro reaction. With three chlorine atoms attached to the carbon adjacent to the carbonyl group, it effectively lacks any alpha hydrogen atoms.
This specific configuration makes trichloroacetaldehyde highly reactive under alkaline conditions, facilitating its transformation into two distinct products through the Cannizzaro reaction.
When subjected to \( ext{NaOH}\), trichloroacetaldehyde undergoes a split: one part forms sodium trichloroacetate, a carboxylate, while the other becomes \( ext{2,2,2-Trichloroethanol}\), an alcohol. This demonstrates a unique application of redox chemistry in organic reactions, highlighting how specific structures can dictate the path and products of a chemical reaction.