Acetone is polar molecule due to presence of polar carbonyl group. Hence dipole exists in acetone molecule. Therefore, as a result, dipole-dipole attraction force exists between acetone molecules along with inherent london-dipole force.
2-methylpropane is a non-polar saturated alkane. Hence only london dispersion force exists between 2-methylpropane molecules.
Therefore intermolecular force is stronger in acetone than 2-methylpropane. Hence higher temperature is required to remove this intermolecular force and boil liquid acetone molecule as compared to 2-methylpropane.
