Molecular ordering of a 2D array of hydrocarbon chains in alkane crystals or Langmuir monolayers arises from the anisotropic pair interaction for rotations about the molecular axes. Starting from atom-atom potentials for H and C, we determine the relevant Fourier coefficients of the molecular pair interaction and, in particular, their distance dependence. In terms of a molecular-field approximation (MFA), we find two first-order transitions between a high-temperature hexagonal phase, an intermediate orthorhombic phase, and at lower T a herringbone structure with a nonhomogeneous order parameter, in agreement with the phase diagrams of various systems. It turns out that molecular ordering is closely related to an orthorhombic distortion of the hexagonal lattice. The calculated distortion agrees well, without free parameters, with measured values. In order to account for the weak variation of the critical temperatures with the chain length, we propose a simple mean-field model for conformational defects that provides a good fit for alkanes with 10-35 carbon atoms. (C) 2000 American Institute of Physics. [S0021-9606(00)70108-5].