(a) Arrange the following compounds in the decreasing order of their acidic strength :
Phenol, 4-Nitrophenol, 2,4,6-Trinitrophenol
(b) Why are alcohols more soluble in water than hydrocarbons of comparable molecular masses ?
(c) Name the reagent used in the oxidation of a primary unsaturated alcohol to an aldehyde.
(a) Arrange the following compounds in the decreasing order of their acidic strength :
Phenol, 4-Nitrophenol, 2,4,6-Trinitrophenol
(b) Why are alcohols more soluble in water than hydrocarbons of comparable molecular masses ?
(c) Name the reagent used in the oxidation of a primary unsaturated alcohol to an aldehyde.
Show Hint
Solution and Explanation
A phenol loses \( \mathrm{H^+} \) to form a phenoxide ion.
Electron-withdrawing groups such as \( \mathrm{-NO_2} \) stabilise this ion and raise acidity.
Step 2: Part (a), compare the three compounds:
Phenol has no extra group, so it is the weakest.
4-Nitrophenol has one nitro group at the para position. It pulls electron density by the -I and -M effects.
2,4,6-Trinitrophenol has three nitro groups at the ortho and para positions. It stabilises the ion the most, so it is the strongest (picric acid).
\[ \text{2,4,6-Trinitrophenol} > \text{4-Nitrophenol} > \text{Phenol} \]
Step 3: Part (b), solubility in water:
Alcohols have an -OH group. It forms hydrogen bonds with water molecules.
Hydrocarbons are non-polar and cannot form hydrogen bonds with water.
So alcohols dissolve more easily.
Step 4: Part (c), choose the oxidant:
The reagent must stop at the aldehyde and leave the C=C bond alone.
Pyridinium chlorochromate (PCC) does this.
Collins reagent (\( \mathrm{CrO_3} \) in pyridine) also works.
Final Answer:
(a) 2,4,6-Trinitrophenol > 4-Nitrophenol > Phenol.
(b) Alcohols form hydrogen bonds with water; hydrocarbons cannot.
(c) Pyridinium chlorochromate (PCC).
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