Catalytic hydrogenation or reduction of an aldehyde ($\text{R-CHO}$) using transition metal catalysts like Platinum ($\text{Pt}$) or Palladium ($\text{Pd}$) yields a primary alcohol ($\text{R-CH}_2\text{-OH}$):

• Reaction Mechanism: Molecular hydrogen adds across the polar carbon-oxygen double bond ($\gt \text{C}=\text{O}$); one hydrogen atom bonds to the carbonyl carbon and one to the oxygen atom.
• Ketone Reduction: In contrast, catalytic reduction of ketones ($\text{R-CO-R}$) gives secondary alcohols ($\text{R-CH(OH)-R}$).
• Other Reducing Agents: Sodium borohydride ($\text{NaBH}_4$) or Lithium aluminium hydride ($\text{LiAlH}_4$) also carry out this reduction cleanly.
• Product: Hence, the main product is the primary alcohol $\text{R-CH}_2\text{-OH}$ (Option 1).

The reaction of phenol with chloroform ($\text{CHCl}_3$) in the presence of aqueous alkali ($\text{KOH}$ or $\text{NaOH}$) to introduce a formyl ($-\text{CHO}$) group ortho to the phenolic hydroxyl group is the classic Reimer-Tiemann Reaction.

• Intermediate: The electrophilic species generated in situ is dichlorocarbene ($:\text{CCl}_2$), an electron-deficient reactive neutral intermediate.
• Major Product: Salicylaldehyde (ortho-hydroxybenzaldehyde) is the predominant product due to intramolecular hydrogen bonding stabilization.
• Kolbe's Reaction: Phenol reacts with $\text{CO}_2$ and $\text{NaOH}$ under pressure to form salicylic acid (o-hydroxybenzoic acid).
• Williamson Synthesis: Reaction of sodium phenoxide or alkoxide with an alkyl halide to synthesize ethers.

Heating carboxylic acids with a strong dehydrating agent like phosphorus pentoxide ($\text{P}_2\text{O}_5$) or concentrated $\text{H}_2\text{SO}_4$ causes intermolecular dehydration, yielding an acid anhydride:

• Reaction Mechanism: An $-\text{OH}$ from one carboxylic acid molecule and an $-\text{H}$ from another carboxylic acid molecule are removed as a water molecule ($\text{H}_2\text{O}$), joining the two acyl groups via an oxygen bridge.
• Example: Acetic acid on heating with $\text{P}_2\text{O}_5$ yields acetic anhydride:
  $$2\text{CH}_3\text{COOH} \xrightarrow{\text{P}_2\text{O}_5, \Delta} (\text{CH}_3\text{CO})_2\text{O} + \text{H}_2\text{O}$$
• Strong Dehydrating Property: Phosphorus pentoxide ($\text{P}_4\text{O}_{10}$) has immense affinity for water, driving the dehydration equilibrium forward to completion.
• Product: Thus, the main product is an acid anhydride (Option 1).