Salicylaldehyde can be prepared from phenol by the Reimer–Tiemann reaction, which involves the ortho-formylation of phenol. The procedure is as follows:
- Start with phenol (C6H5OH) dissolved in a strong base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), which forms the phenoxide ion (C6H5O⁻).
- Chloroform (CHCl3) is added to the basic solution. Under these conditions, chloroform reacts with the base to generate dichlorocarbene (CCl2), a reactive intermediate.
- The phenoxide ion then attacks the dichlorocarbene electrophile at the ortho position relative to the hydroxyl group, forming an intermediate.
- This intermediate rearranges and hydrolyzes to yield salicylaldehyde (2-hydroxybenzaldehyde), which has both an aldehyde group and a hydroxyl group ortho to each other on the benzene ring.
The overall reaction can be summarized as:
C6H5OH+CHCl3+KOH→C6H4(OH)CHO+KCl+H2O\text{C}_6\text{H}_5\text{OH}+\text{CHCl}_3+\text{KOH}\rightarrow \text{C}_6\text{H}_4(\text{OH})\text{CHO}+\text{KCl}+\text{H}_2\text{O}C6H5OH+CHCl3+KOH→C6H4(OH)CHO+KCl+H2O
This method produces predominantly ortho-salicylaldehyde due to intramolecular hydrogen bonding that stabilizes the product
. In brief, the key steps are:
- Formation of phenoxide ion from phenol in base
- Generation of dichlorocarbene from chloroform and base
- Electrophilic attack of phenoxide on dichlorocarbene at ortho position
- Hydrolysis to salicylaldehyde
This reaction is known as the Reimer–Tiemann reaction and is a classical method for the synthesis of salicylaldehyde from phenol
