The diffusion of ethane and n-pentane on metallic surfaces is studied by molecular dynamics simulations. On atomically smooth, faced centered cubic (111) crystal surfaces, n-pentane has several conformational states, each of which diffuses at a different rate. Diffusion barriers are distinct from intramolecular conformational isomerization barriers. Weak friction is observed. Hopping diffusion occurs near the diffusion threshold. As the temperature is raised, correlated barrier crossing and recrossing increases. n-Pentane slides, transverse to its axis, without changing its orientation with respect to the surface. Conformational changes occur on a longer time scale than diffusional barrier crossing and are restricted by the surface. Far above the diffusion threshold temperature, partial molecular desorption occurs and conformational isomerization is less restricted. The molecules slide, roll and tumble continuously across the surface. A similar pattern of temperature-dependent change in diffusion mechanisms occurs in ethane but without conformational changes.