The Pople and Karasz theory of melting of molecular crystals, which is based on the theory of melting of inert gas crystals by Lennard-Jones and Devonshire, is extended using a third energy parameter, W ''. The extension is done as follows: the previous repulsive energy parameter W is divided into two parts. The first part is the interaction between molecules which are on different sites with the same orientations, called again W. The second is the interaction between molecules on different sites and also different orientations, namely W ''. W '' is combined with the previous W' and the new W energy parameters by the arithmetic mean including an adjustable parameter. The thermodynamic properties are evaluated by the Bragg-Williams approximation. The theory is applied to plastic crystals and compared with the Pople and Karasz theory. By introducing a physical realistic coupling between orientational and positional order, v, the theory gives a solid state rotational transition and melting transition. For values of v less than or equal to 0.331 the two transitions are separate, while for values of v greater than or equal to 0.331 they coalesce. The quantitative predictions of the theory is compared with experimental results for plastic crystals by plotting entropy and volume changes versus melting/boiling temperature ratios. The agreement between our theory and the experimental data is better than the calculations of the Pople and Karasz theory, as result of the introduction of the parameter W '' in the present theory.