Which of the following will not respond to Ferric chloride test ?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Glycerol
The ferric chloride test distinguishes phenols from alcohols. A compound with an –OH group attached to an sp2 carbon (phenols and enols) gives a violet, blue, green or red colour with neutral \( \ce{FeCl3} \) solution, because a coloured iron–phenoxide complex forms. Alcohols, in which –OH is attached to an sp3 carbon, do not give this colour.
Option (1) o-Cresol — Responds to the ferric chloride test. o-Cresol (2-methylphenol) is a phenol, with –OH directly on the benzene ring.
Option (2) Resorcinol — Responds to the ferric chloride test. Resorcinol (benzene-1,3-diol) is a dihydric phenol.
Option (3) Quinol — Responds to the ferric chloride test. Quinol or hydroquinone (benzene-1,4-diol) is a dihydric phenol.
Option (4) Glycerol — Does not respond to the ferric chloride test. Glycerol (propane-1,2,3-triol, \( \ce{CH2OH-CHOH-CH2OH} \)) has three –OH groups, but all of them are on sp3 carbons. It is an alcohol, not a phenol, so it gives no colour with neutral \( \ce{FeCl3} \).
Note: The question is from beyond the NCERT textbook.