The incorrect method to synthesise benzaldehyde is. 

The incorrect method to synthesize benzaldehyde is the oxidation of toluene using strong oxidizing agents like KMnO4 or acidified dichromate, which generally leads to the formation of benzoic acid rather than benzaldehyde.

Explanation:

• Benzaldehyde can be synthesized by:
  • Rosenmund reduction of benzoyl chloride.
  • Hydrolysis of benzal chloride.
  • Controlled oxidation of benzyl alcohol.
  • Hydrolysis of benzylidene derivatives.
• However, direct oxidation of toluene with strong oxidizing agents is typically not a selective method for benzaldehyde; instead, it results in over-oxidation to benzoic acid.
• Therefore, this method is considered incorrect for synthesizing benzaldehyde if pure benzaldehyde is the desired product.

Summary: Avoid oxidation of toluene with strong oxidants for benzaldehyde synthesis because it yields benzoic acid. Prefer selective methods like Rosenmund reduction or benzal chloride hydrolysis.