Question

The reagent that can distinguish between benzaldehyde and propionaldehyde is (A) Tollen's reagent (B) 2,4 -dinitrophenylhydrazine (C) Sodium hydrogen sulphite (D) Fehling's solution

Short answer

The reagent that can distinguish between benzaldehyde and propionaldehyde is (A) Tollen's reagent and (D) Fehling's solution. Both these reagents react with propionaldehyde but not with benzaldehyde, allowing them to differentiate between the two aldehydes.

Step-by-step solution

Understanding the structure of the two aldehydes

Benzaldehyde and propionaldehyde are both aldehydes, having a carbonyl group (C=O) and one hydrogen atom attached to the carbonyl carbon. However, their structures are different: 1. Benzaldehyde - It has a phenyl group (C₆H₅-) attached to the carbonyl carbon. Its chemical formula is \(C_6H_5CHO\). 2. Propionaldehyde - It has an alkyl group (CH₃CH₂-) attached to the carbonyl carbon. Its chemical formula is \(CH_3CH_2CHO\).

Identifying the specific reactions

Now that we know the structure of the two aldehydes, we will go through each reagent and analyze its reaction with the two aldehydes: (A) Tollen's reagent - Tollen's reagent is an ammoniacal silver nitrate solution (\(Ag(NH_3)_2OH\)) that oxidizes aliphatic aldehydes like propionaldehyde to carboxylic acids, and simultaneously reduces silver in the form of silver mirror. Aromatic aldehydes like benzaldehyde do not react with Tollen's reagent. (B) 2,4-dinitrophenylhydrazine - This reagent reacts with both aliphatic and aromatic aldehydes to form yellow or orange-red precipitates called 2,4-dinitrophenylhydrazones. Thus, this reagent cannot differentiate between benzaldehyde and propionaldehyde. (C) Sodium hydrogen sulphite - This reagent reacts with both aliphatic and aromatic aldehydes to form stable crystalline bisulphite addition compounds. Hence, it cannot differentiate between benzaldehyde and propionaldehyde. (D) Fehling's solution - It is a deep blue solution, which contains copper(II) ions complexed with tartrate ions in an alkaline solution. Fehling's solution oxidizes aliphatic aldehydes, like propionaldehyde, to carboxylic acids and reduces copper(II) ions to reddish-brown cuprous oxide. Aromatic aldehydes, like benzaldehyde, do not react with Fehling's solution.

Choosing the correct reagent

Our task was to choose the reagent that can distinguish between benzaldehyde and propionaldehyde. Based on the reactions discussed in Step 2, we can see that: - Tollen's reagent and Fehling's solution react with propionaldehyde but not with benzaldehyde. - 2,4-dinitrophenylhydrazine and sodium hydrogen sulphite react with both benzaldehyde and propionaldehyde - they cannot differentiate between the two aldehydes. Thus, the correct answer is (A) Tollen's reagent and (D) Fehling's solution, as both of these reagents can differentiate between benzaldehyde and propionaldehyde.

Key concepts

Tollen's Reagent

Tollen's reagent is a classic tool in organic chemistry used to distinguish between different types of aldehydes and ketones. It is a specific reagent, particularly useful for identifying aliphatic aldehydes. Tollen's reagent consists of an ammoniacal silver nitrate solution. The chemical formula for Tollen's reagent is represented as \( Ag(NH_3)_2OH \).

When Tollen's reagent reacts with an aliphatic aldehyde, such as propionaldehyde, it results in the oxidation of the aldehyde to a carboxylic acid. At the same time, the silver ions present in the solution are reduced, forming a thin layer of silver metal, often referred to as the 'silver mirror'.

• This distinctive reaction does not occur with aromatic aldehydes, such as benzaldehyde. Therefore, Tollen's reagent can successfully distinguish between aliphatic and aromatic aldehydes.
• The 'silver mirror' test is not only visually attractive but also serves as clear evidence of an oxidation-reduction reaction.

For students studying organic chemistry, understanding how Tollen's reagent interacts with different aldehydes helps in grasping the broader topic of oxidation-reduction reactions in organic molecules.

Fehling's Solution

Fehling's solution is another reagent used in chemical tests designed to differentiate between types of aldehydes. It is composed of copper(II) ions complexed with tartrate ions in an alkaline medium. The deep blue color of Fehling's solution arises from the presence of the copper(II)-tartrate complex.

When Fehling's solution interacts with an aliphatic aldehyde, such as propionaldehyde, the aldehyde is oxidized to a carboxylic acid, which in turn reduces the copper(II) ions to form a reddish-brown precipitate of cuprous oxide \([Cu_2O]\).

• This reaction indicates a positive test for aldehydes with Fehling's solution, distinguished by the change in color and the formation of a precipitate.
• Aromatic aldehydes, like benzaldehyde, do not react with Fehling's solution, allowing for the elegant separation between aliphatic and aromatic aldehydes.

Therefore, Fehling's solution is an effective way to distinguish propionaldehyde from benzaldehyde in the lab due to its selective reactivity.

Chemical Reactions of Aldehydes

Aldehydes are a class of organic compounds characterized by the presence of a carbonyl group \((C=O)\) bonded to at least one hydrogen atom. The reactivity of aldehydes largely stems from this carbonyl group, which makes them versatile in numerous chemical reactions, specifically oxidation reactions.

Aldehydes can undergo oxidation to form carboxylic acids. This is a key reaction that forms the basis of tests like Tollen's reagent and Fehling's solution. These tests exploit the aldehyde's susceptibility to oxidation, either through the formation of a carboxylic acid or a simple reduction of metal ions present in the reagent.

• The reactivity can vary significantly between different aldehydes. For instance, aliphatic aldehydes like propionaldehyde readily oxidize, while aromatic aldehydes like benzaldehyde do not.
• Understanding these differences is crucial for students to predict and explain results from common lab tests and apply this knowledge to synthesize expected products in chemical reactions.

Exploring the chemical reactions of aldehydes equips students with fundamental insights into broader organic reaction mechanisms and is essential for mastering organic chemistry.