Q.The reagent which does not react with both acetone and benzaldehyde :

Only compounds with a free −CHO group reduce Fehling's solution; a carboxylic acid without −CHO does not.

Fehling's solution is reduced by aldehydes (which get oxidised to carboxylate while Cu$^{2+}$ is reduced to Cu$_2$O, a brick-red precipitate). $CH_3CHO$ and $HCHO$ are aldehydes, so they reduce it. $HCOOH$ (formic acid) also reduces Fehling's solution because it still contains an aldehydic $-CHO$-type hydrogen (it can be regarded …

Fehling's solution reacts with neither acetone (a ketone) nor benzaldehyde (an aromatic aldehyde), so it is the odd reagent — option (c).

Concept. We need a reagent that reacts with neither acetone (CH$_3$COCH$_3$, a ketone) nor benzaldehyde (C$_6$H$_5$CHO, an aromatic aldehyde).

Checking each reagent:

• (a) Sodium hydrogen sulphite (NaHSO$_3$) forms crystalline bisulphite addition products with aldehydes and with reactive methyl ketones like acetone → reacts with both.
• (b) Phenylhydrazine condenses with the >C=O of both aldehydes and ketones to give phenylhydrazones → reacts with both. …

Fehling's reagent is made of two separate solutions, A and B, mixed just before use.

Fehling's reagent is used as a test to distinguish aldehydes (especially aliphatic aldehydes) from ketones. It consists of:

• Fehling solution A: an aqueous solution of copper sulphate (CuSO4).
• Fehling solution B: an alkaline solution of sodium potassium tartrate (Rochelle salt) and sodium hydroxide. …

Fehling's test is positive for compounds containing a free -CHO group (or one that can tautomerise to expose one); acetic acid (CH3COOH) has no such group and fails the test, unlike formic acid, acetaldehyde, and formaldehyde.

Fehling's solution (an alkaline Cu2+ complex) is reduced by aldehydes to a brick-red precipitate of Cu2O, with the aldehyde being oxidised to a carboxylic acid.

• Acetaldehyde (CH3CHO): has a -CHO group — reduces Fehling's solution readily.
• Formaldehyde (HCHO): has a -CHO group — reduces Fehling's solution readily (and is oxidised all the way to CO2/formic acid). …

Fehling's solution is a test specific to aliphatic aldehydes; it gives a negative result with ketones (like acetone) AND with aromatic aldehydes (like benzaldehyde) — so it is the one reagent that reacts with neither.

Checking each reagent against BOTH acetone and benzaldehyde:

• $NaHSO_3$ (sodium bisulfite): forms a bisulfite addition product with both methyl ketones (acetone) and aldehydes (benzaldehyde) — reacts with both.
• Phenylhydrazine: forms a phenylhydrazone with any aldehyde or ketone via nucleophilic addition-elimination at the carbonyl — reacts with both. …

Fehling's solution (an alkaline Cu2+ complex) is reduced by aldehydes like acetaldehyde to a brick-red Cu2O precipitate — a classic aldehyde test.

Fehling's solution consists of Fehling A (CuSO4 solution) and Fehling B (alkaline sodium potassium tartrate). When an aldehyde such as acetaldehyde (CH3CHO) is warmed with Fehling's solution, the Cu2+ ion (as the deep-blue tartrate complex) oxidises the aldehyde to the corresponding carboxylate (acetate) ion, while itself getting reduced from Cu2+ to Cu+, which precipitates as a reddish-brown/brick-red solid of Cu2O.

…

Formic acid reduces Fehling's solution because it contains an aldehydic (-CHO) group.

Formic acid, HCOOH, structurally contains an aldehyde-like -CHO group attached to -OH, so it behaves like an aldehyde and gives a positive Fehling's test (reducing $Cu^{2+}$ of Fehling's solution to a red precipitate of $Cu_2O$). Acetic …

Fehling's reagent is made of two separately-stored solutions; solution A is aqueous copper sulphate.

Fehling's solution is prepared fresh by mixing two solutions just before use: Fehling's solution A = aqueous solution of copper sulphate (CuSO4), and Fehling's solution B = alkaline solution of sodium potassium tartrate (Roche …

Fehling's reagent is prepared by mixing two solutions, A and B, just before use; solution B is alkaline sodium potassium tartrate.

Fehling's solution is used as a test for reducing sugars (aldehydes, especially aliphatic aldehydes, and reducing carbohydrates). It is made of two separate solutions that are mixed in equal volumes immediately before the test, because the mixture does not keep well:

• Fehling's solution A: an aqueous solution of copper sulphate (CuSO4).
• Fehling's solution B: an alkaline solution of sodium potassium tartrate (Rochelle salt, NaKC4H4O6) dissolved in sodium hydroxide (NaOH). …