Tilden's Reagent is nitrosyl chloride ($\text{NOCl}$), a yellow gas commonly prepared by the reaction of aqua regia (mixture of conc. $\text{HNO}_3$ and conc. $\text{HCl}$):

• Historical Significance: Discovered by British chemist Sir William A. Tilden in 1874 for the characterization and structural elucidation of terpenes (such as pinene and limonene).
• Reaction with Alkenes: Adds across carbon-carbon double bonds ($\gt \text{C}=\text{C}\lt $) to form crystalline nitroso chlorides or chloro-oximes with well-defined melting points.
• Reaction with Primary Amines: Reacts with primary aliphatic amines to convert them into alkyl chlorides with evolution of nitrogen gas:
  $$\text{R-NH}_2 + \text{NOCl} \rightarrow \text{R-Cl} + \text{N}_2 + \text{H}_2\text{O}$$
• Identity: Nitrosyl chloride (Option 1).