Dipole-­dipole interactions occur among the polar molecules. Polar molecules have permanent dipoles. The positive pole of one molecule is thus attracted by the negative pole of the other molecule. The magnitude of dipole­dipole forces in different polar molecules is predicted on the basis of the polarity of the molecules, which in turn depends upon the electronegativities of the atoms present in the molecule and the geometry of the molecule

Acetonitrile $\left(C H_{3} C N\right)$ and acetone $\left(C H_{3}\right)_{2} C O$ both are polar.