Answer is: because isomer 2 has weaker intermolecular forces.
Intermolecular forces are the forces
between molecules. The stronger are intermolecular forces, the higher is
boiling point of compound, because more energy is needed to break interaction
between molecules.
There are several types of intermolecular
forces: hydrogen bonding, ion-induced dipole forces, ion-dipole
forces andvan der Waals forces.
