JEE Main 2022 · 30 June · morning · Single correct · +4 / −1
The reagent neutral ferric chloride is used to detect the presence of ______________
- (a) sulphide ion and alcoholic -OH group.
- (b) acetate ion and phenolic -OH group.
- (c) sulphide ion and phenolic -OH group.
- (d) acetate ion and alcoholic -OH group.
Answer and solution
Answer (b) acetate ion and phenolic -OH group.
Solution
Chemistry ma’am · 3 steps
Neutral FeCl₃ shows a colour with just two of these: acetate turns red, phenol turns violet.
- What the reagent does. Fe³⁺ forms coloured complexes with oxygen donors that bind it strongly. It must be neutral, because free acid would break those complexes and no colour would appear.
Species With neutral FeCl₃ Usual test for it Acetate ion Deep red colour of [Fe₃(OH)₂(CH₃COO)₆]⁺; reddish-brown precipitate on boiling neutral FeCl₃ Phenolic -OH Violet colour of the iron phenoxide complex neutral FeCl₃ Sulphide ion Not used as a test purple colour with sodium nitroprusside, black PbS with lead acetate Alcoholic -OH No colour, because alcohols are far too weakly acidic to bind Fe³⁺ this way red colour with ceric ammonium nitrate - Phenol versus alcohol. The phenoxide oxygen is stabilised by the benzene ring, so phenol gives up its proton and binds Fe³⁺. An alcohol cannot do this, so it gives no colour.
- So. Neutral ferric chloride detects the acetate ion and the phenolic -OH group, which is option (b).
The idea to keep. Neutral FeCl₃ is the test for acetate (red) and phenols (violet). Alcohols stay colourless, and sulphide has its own test with sodium nitroprusside.
Answer from the NTA final answer key. Solution by PrepWiser.