Simple Landau free energy function is presented to describe the Liquid-R II -R I phase sequence of alkanes and transitions between them. The order parameters necessary to describe these rotator phase transitions are identified. We present a mean-field description of the Liquid-R II and Liquid-R I transitions. General arguments are presented for the topology of the phase diagram in the vicinity of the Liquid-R II -R I triple point. Within this model the Liquid-R II and Liquid-R I transitions are found to be always strongly first order. Calculations based on this model agree qualitatively with experiments.