Which of the following is Baeyer's reagent?

Identify $"\mathrm{Z}"$ in the following series of reaction

$\mathrm{Butan}-2-\mathrm{ol}\stackrel{{\mathrm{PCl}}_3}{⟶}\mathrm{X}\stackrel{ \mathrm{alco}.\mathrm{KOH} }{⟶}\mathrm{Y}\underset{\mathrm{ii})\mathrm{H}-\mathrm{OH}/\mathrm{heat} }{\overset{ \mathrm{i}) {\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}\mathrm{Z}$