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).

Pinacol is a vicinal diol (glycol) whose IUPAC name is 2,3-dimethylbutane-2,3-diol, with the chemical formula $(\text{CH}_3)_2\text{C(OH)-C(OH)}(\text{CH}_3)_2$.

• Structure: Consists of a 4-carbon butane chain with two methyl groups and two tertiary hydroxyl groups positioned symmetrically at carbon-2 and carbon-3.
• Synthesis: Produced by the reductive dimerization (pinacol coupling) of acetone using magnesium amalgam ($\text{Mg-Hg}$) in the presence of water:
  $$2\text{CH}_3\text{COCH}_3 + 2[\text{H}] \xrightarrow{\text{Mg-Hg} / \text{H}_2\text{O}} (\text{CH}_3)_2\text{C(OH)-C(OH)}(\text{CH}_3)_2$$
• Pinacol-Pinacolone Rearrangement: When heated with dilute mineral acid ($\text{H}_2\text{SO}_4$), pinacol undergoes dehydration and a 1,2-methyl shift to form pinacolone (3,3-dimethylbutan-2-one):
  $$(\text{CH}_3)_2\text{C(OH)-C(OH)}(\text{CH}_3)_2 \xrightarrow{\text{H}^+, \Delta} (\text{CH}_3)_3\text{C-COCH}_3 + \text{H}_2\text{O}$$
• Identity: Option 2 correctly depicts 2,3-dimethylbutane-2,3-diol.