Using the method of orientational elastic deformations in an electric field we investigated the electrooptical properties of nematics of the bis-4-cyanobiphenyloxyalkanes series differing in the number n of CH2 groups in the oxymethylene chain of the molecule. The value of optical anisotropy of nematics (Delta n=n(e)-n(o)) and threshold electric potentials V-o of deformations of their monodomain planar texture were determined at different temperatures. In both dependences of Delta n and I/V-o on n strong odd-even effect is manifested. However, the characters of alternations observed in them differ greatly. These differences are due to the fact that the value and sign of I/V-o depend not only on the intermolecular (nematic) orientational order but is determined to a much greater extent by intramolecular order which depends on the polar and polarizing architecture of the molecule.