Q.Explain the following reactions with equations.

These four named reactions build C–C, C–O, and C–X bonds at specific positions in phenols, ethers, and carboxylic acids using characteristic reagents.

a) Kolbe's reaction:

Sodium phenoxide (formed by treating phenol with NaOH) is heated with carbon dioxide at about 400 K under a pressure of 4–7 atmospheres. The $CO_2$ electrophile attacks the ortho position of the phenoxide ring (favoured due to the chelation/ionic character of the intermediate), and the resulting product is acidified to give salicylic acid (2-hydroxybenzoic acid):

$C_6H_5ONa + CO_2 \xrightarrow{400\,K,\ 4-7\,atm} \text{sodium salicylate} \xrightarrow{H^+} \underset{\text{salicylic acid}}{o\text{-}HOC_6H_4COOH}$

b) Riemer–Tiemann reaction:

When phenol is treated with chloroform ($CHCl_3$) and concentrated sodium hydroxide (NaOH) at 340 K, dichlorocarbene ($:CCl_2$), generated from CHCl3 and NaOH, attacks the electron-rich ortho position of the phenoxide ring. Subsequent hydrolysis of the intermediate (a gem-dichloride) gives an ortho-hydroxybenzaldehyde called salicylaldehyde:

$C_6H_5OH \xrightarrow{CHCl_3,\ NaOH} \text{(dichlorocarbene attack at ortho)} \xrightarrow{H_3O^+} \underset{\text{salicylaldehyde}}{o\text{-}HOC_6H_4CHO}$

c) Williamson's ether synthesis:

An alkyl halide is treated with sodium alkoxide (prepared by reacting an alcohol with sodium metal). The alkoxide ion acts as a nucleophile and attacks the alkyl halide in an SN2 reaction, displacing the halide ion and forming an ether:

$R-O^-Na^+ + R'-X \rightarrow R-O-R' + NaX$

(For best yield, a primary alkyl halide is used, since a bulky/secondary or tertiary halide favours elimination instead of substitution.)

d) HVZ (Hell–Volhard–Zelinsky) reaction: …