Salicylaldehyde can be prepared from phenol by the Reimer-Tiemann reaction. In this process, phenol is heated with chloroform (CHCl3) and a strong base such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). The base deprotonates phenol to form a phenoxide ion and also deprotonates chloroform to generate dichlorocarbene (CCl2). The phenoxide ion then attacks the dichlorocarbene, forming an intermediate which, upon rearrangement and hydrolysis, gives salicylaldehyde with the aldehyde group ortho to the hydroxyl group on the aromatic ring.
Key Steps:
- Phenol reacts with base to form phenoxide ion.
- Chloroform reacts with base to form dichlorocarbene.
- Phenoxide ion nucleophilically attacks dichlorocarbene.
- Intermediate rearranges and hydrolyzes to salicylaldehyde.
- The product is typically isolated by acidifying the reaction mixture and steam distillation.
Overall reaction:
C6H5OH+CHCl3+NaOH→C6H4(OH)CHO+NaCl+H2O\text{C}_6\text{H}_5\text{OH}+\text{CHCl}_3+\text{NaOH}\rightarrow \text{C}_6\text{H}_4(\text{OH})\text{CHO}+\text{NaCl}+\text{H}_2\text{O}C6H5OH+CHCl3+NaOH→C6H4(OH)CHO+NaCl+H2O
This reaction is usually carried out at elevated temperatures (around 50°C to reflux temperature) for a few hours to ensure maximum conversion. Salicylaldehyde forms primarily at the ortho position due to stabilizing intramolecular hydrogen bonding between the hydroxyl and aldehyde groups.
