(a) Decreasing order of acidic strength:
Step 1 (Governing Principle): [0.5 Mark]
The acidic strength of phenols is determined by the stability of the phenoxide ion formed after the loss of a proton ($H^+$). Electron-withdrawing groups ($-NO_2$) stabilize the phenoxide ion through $-I$ and $-M$ effects, thereby increasing acidity.
Step 2 (Step-by-Step Analysis): [0.5 Mark]
The $-NO_2$ group is a strong electron-withdrawing group. As the number of $-NO_2$ groups increases, the electron density on the oxygen atom of the phenoxide ion decreases, leading to greater stabilization:
- 2,4,6-Trinitrophenol (Picric acid): Three $-NO_2$ groups (strongest stabilization).
- 4-Nitrophenol: One $-NO_2$ group.
- Phenol: No electron-withdrawing group.
Step 3 (Final Concluded Answer): [0.5 Mark]
The decreasing order of acidic strength is:
$$2,4,6\text{-Trinitrophenol} > 4\text{-Nitrophenol} > \text{Phenol}$$
(b) Solubility of alcohols vs. hydrocarbons:
Step 1 (Governing Principle): [0.5 Mark]
Solubility in water depends on the ability of a molecule to form hydrogen bonds with water molecules ($H_2O$).
Step 2 (Reasoning): [0.5 Mark]
Alcohols contain a polar $-OH$ group, which allows them to form intermolecular hydrogen bonds with water. In contrast, hydrocarbons are non-polar and lack the ability to form hydrogen bonds with water; they only interact via weak London dispersion forces.
💡 EL10 Concept Insight: Think of water molecules like a group of friends holding hands (hydrogen bonds); alcohols can easily join this group because they have an $-OH$ "hand" to hold onto, while hydrocarbons are like outsiders who can't join in, so they stay separated.
(c) Reagent for oxidation of primary unsaturated alcohol to an aldehyde:
Step 1 (Governing Principle): [0.5 Mark]
To oxidize a primary alcohol to an aldehyde without further oxidizing it to a carboxylic acid, a mild oxidizing agent is required that does not affect the carbon-carbon double bond ($C=C$).
Step 2 (Final Concluded Answer): [0.5 Mark]
The reagent used is Pyridinium chlorochromate (PCC) in dichloromethane ($CH_2Cl_2$).