In the following skew conformation of ethane, H'–C–C–H'' dihedral angle is :

The major product of the following addition reaction is :
H₃C–CH=CH₂ $$\buildrel {C{l_2}/{H_2}O} \over \longrightarrow $$

But-2-ene on reaction with alkaline KMnO₄ at elevated temperature followed by acidification will give :

Increasing order of reactivity of the following compounds for S_N1 substitution is: