Consider the following chemical reaction and identify the intermediate involved in the reaction.
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Phenol going to salicylaldehyde is the Reimer-Tiemann reaction (CHCl\(_3\) with aqueous NaOH). Find the species formed after the dichlorocarbene attacks the phenoxide ring.
Step 1: Identify the reaction.
Phenol is converted into salicylaldehyde (2-hydroxybenzaldehyde). A CHO group enters the ortho position of the ring. This change is the Reimer-Tiemann reaction.
Its reagents are chloroform (CHCl\(_3\)) and aqueous sodium hydroxide, followed by acid work up.
Step 2: Recall the mechanism.
First, NaOH removes the acidic H of phenol and gives sodium phenoxide. The negative charge of the phenoxide is spread into the ring, mostly at the ortho and para positions.
Second, NaOH removes HCl from CHCl\(_3\) and gives dichlorocarbene, \(:\mathrm{CCl_2}\). This is a neutral and electron deficient species, so it acts as the electrophile.
Third, the ortho carbon of the phenoxide attacks the dichlorocarbene. After a proton shift, the ring carries a CHCl\(_2\) group and keeps the O-Na\(^+\) group. This is the intermediate.
Fourth, aqueous alkali hydrolyses the CHCl\(_2\) group to CHO, and acid work up gives salicylaldehyde.
Step 3: Check option (1).
This structure has O-Na\(^+\) and CHO together. It is the sodium salt of salicylaldehyde, which is the result of the hydrolysis step, not the intermediate. So option (1) is wrong.
Step 4: Check option (2).
This structure has O-Na\(^+\) on one carbon and CHCl\(_2\) on the ortho carbon. It is exactly what forms when the phenoxide ion reacts with dichlorocarbene. So option (2) is correct.
Step 5: Check option (3).
This structure has a free OH group with CHCl\(_2\). The reaction runs in strong aqueous NaOH, so the phenol is present as the phenoxide and not as free OH. So option (3) is wrong.
Step 6: Check option (4).
This structure has O-Na\(^+\) with a CH\(_2\)Cl group. Dichlorocarbene has two chlorine atoms, so the group formed is CHCl\(_2\), not CH\(_2\)Cl. So option (4) is wrong.
Final Answer:
The intermediate is the ortho dichloromethyl sodium phenoxide, which is option (2).
\[ \boxed{\text{Option (2)}} \]
