Question

What are X and Y respectively in the following reaction? $$ CH_3CH_2CHO + 2Cu^{2+} + 5OH^- \rightarrow X + Y + 3H_2O $$

• A. \( CH_3CH_2COOH \), \( Cu(OH)_2 \)
• B. \( CH_3CH_2COO^- \), \( Cu \)
• C. \( CH_3CH_2COOH \), \( Cu_2O \)
• D. \( CH_3CH_2COO^- \), \( Cu_2O \)

Hint:

Recognize that the reaction described is analogous to Fehling's test for aldehydes. In Fehling's test, aldehydes are oxidized to carboxylate ions in a basic medium, and \( Cu^{2+} \) ions are reduced to reddish-brown \( Cu_2O \) precipitate.

Answer 1

The Correct Option is D

The given reaction involves the oxidation of an aldehyde (\( CH_3CH_2CHO \), propanal) using copper(II) ions in a basic medium.
This is characteristic of Fehling's test, which is used to detect the presence of aldehydes (and some ketones with \( \alpha \)-hydrogens).
In Fehling's test, copper(II) ions in Fehling's solution (complexed with tartrate ions to keep them in solution in basic conditions) oxidize aldehydes to carboxylate ions.
The copper(II) ions are reduced to copper(I) oxide (\( Cu_2O \)), which is a reddish-brown precipitate.
The balanced ionic equation for the reaction of an aldehyde with copper(II) ions in a basic solution can be generalized as: $$ RCHO + 2Cu^{2+} + 5OH^- \rightarrow RCOO^- + Cu_2O(s) + 3H_2O $$ In our case, \( R \) is \( CH_3CH_2 \).
So, propanal (\( CH_3CH_2CHO \)) is oxidized to propanoate ion (\( CH_3CH_2COO^- \)), and copper(II) ions are reduced to copper(I) oxide (\( Cu_2O \)).
Therefore, X is the propanoate ion (\( CH_3CH_2COO^- \)), and Y is copper(I) oxide (\( Cu_2O \)).
This corresponds to option (D).
Note that option (B) suggests the formation of metallic copper (\( Cu \)), which would involve a further reduction of copper(I) to copper(0).
While this can happen under more forcing conditions or with different reducing agents, Fehling's test typically results in the formation of \( Cu_2O \).
Option (A) suggests the formation of propanoic acid (\( CH_3CH_2COOH \)), which would require acidic conditions, contrary to the basic medium indicated by \( OH^- \).
Option (C) also suggests propanoic acid.