Dihedral angle of least stable conformer of ethane is:

Which of the following is a free radical substitution reaction?

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}={\mathrm{CH}}_2\underset{{\mathrm{H}}_2\mathrm{O},{\mathrm{H}}_2{\mathrm{O}}_2,{\mathrm{OH}}^{-}}{\overset{{\mathrm{B}}_2{\mathrm{H}}_6}{\to }}\mathrm{Z}$

What is $\mathrm{Z}?$

Among the following, the reaction that proceeds through an electrophilic substitution, is: