Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

Which compound does not give cannizzaro's reaction? (a) trichloro acetaldehyde (b) benzaldehyde (c) formaldehyde (d) acetaldehyde

Step-by-Step Solution

Answer

Acetaldehyde does not give Cannizzaro's reaction.

Step 1: Understanding Cannizzaro's Reaction

Cannizzaro's reaction is a chemical reaction that involves the base-induced disproportionation of non-enolizable aldehydes to form a primary alcohol and a carboxylic acid salt. The reaction requires aldehydes without alpha hydrogens (non-enolizable).

Step 2: Analyzing Each Option

We need to assess whether the compounds given can undergo Cannizzaro's reaction by checking for the presence of alpha hydrogen atoms adjacent to the carbonyl group.

Step 3: Option (a): Trichloroacetaldehyde

Trichloroacetaldehyde has no alpha hydrogen on the carbon adjacent to the carbonyl group, because all hydrogens are replaced by chlorine atoms. It can undergo Cannizzaro's reaction.

Step 4: Option (b): Benzaldehyde

Benzaldehyde does not have any alpha hydrogen because the carbon directly attached to the carbonyl group is part of the benzene ring. Thus, benzaldehyde can participate in Cannizzaro's reaction.

Step 5: Option (c): Formaldehyde

Formaldehyde also lacks an alpha hydrogen as there is only one hydrogen attached to the carbonyl carbon, making it a good candidate for Cannizzaro's reaction.

Step 6: Option (d): Acetaldehyde

Acetaldehyde has three alpha hydrogens attached to the carbon next to the carbonyl group, making it enolizable and unsuitable for Cannizzaro's reaction. Acetaldehyde will instead undergo aldol condensation.

Step 7: Identifying the Answer

Among the given compounds, acetaldehyde is the only compound that contains alpha hydrogens. Thus, it is not suitable for Cannizzaro's reaction.

Key Concepts

Non-enolizable aldehydes

In the fascinating world of organic chemistry, an important concept is that of non-enolizable aldehydes. These are specific types of aldehydes that cannot undergo a process called enolization, due to the absence of alpha hydrogens. To understand this better, let's break it down:

• Aldehydes are organic compounds containing a carbonyl group (C=O) bonded to at least one hydrogen atom.
• The term "non-enolizable" refers to the lack of alpha hydrogens bonded to the carbon adjacent to the carbonyl group.
• In the absence of these alpha hydrogens, the aldehyde cannot form enols, which are crucial intermediates in many reactions.

Non-enolizable aldehydes are essential for Cannizzaro's reaction, where they undergo disproportionation to form a primary alcohol and a carboxylic acid. Their inability to form enols makes them unique and allows them to react differently than their enolizable counterparts.

Alpha hydrogen

The concept of an alpha hydrogen is central to understanding the chemical behavior of aldehydes and ketones. Here's what you need to know:

• Alpha hydrogen refers to any hydrogen atom that is attached to the carbon adjacent to the carbonyl group in a molecule.
• Having alpha hydrogens makes a compound enolizable, allowing it to participate in reactions like aldol condensation, where enolate ions are formed.
• In the case of Cannizzaro's reaction, the absence of alpha hydrogens is vital, as it leads to the reaction's preference for non-enolizable aldehydes.

A classic example can be seen in acetaldehyde, which contains alpha hydrogens. This makes it unsuitable for Cannizzaro's reaction, as it tends to undergo aldol condensation instead. In contrast, compounds like benzaldehyde and formaldehyde do not have alpha hydrogens, qualifying them for Cannizzaro's reaction.

Disproportionation

Disproportionation is an intriguing chemical process, particularly evident in Cannizzaro's reaction involving non-enolizable aldehydes. Here's a clearer picture of this concept:

• Disproportionation is a type of redox reaction where a single substance is simultaneously oxidized and reduced, forming two different compounds.
• In Cannizzaro's reaction, a non-enolizable aldehyde undergoes base-induced disproportionation, resulting in an alcohol and a carboxylic acid salt.
• This requires no alpha hydrogens, marking a distinction from mechanisms like aldol reaction.

An example of this can be seen with formaldehyde. In the presence of a strong base, it disproportionates to yield methanol (an alcohol) and formate (a carboxylate salt). Understanding disproportionation helps highlight the unique properties and reactivity of non-enolizable aldehydes, distinguishing their path in chemical transformations.