Effet de la Longueur de la Chaîne Aliphatique Sur L'Association Des Alcools

The curves of the dipole moments μ of the primary aliphatic alcohols in function of the concentration in CCl4 present a minimum that increases with the length of the hydrocarbon chain, as showed by R. Mecke. We have determined these μ values for several primary alcohols from methanol to octanol at 25, 35, 45, and 55°C. Assuming that the minimum is due to the presence of cyclic hydroxylic chains. it is possible to calculate from the curves the ratio r of alcohols molecules implicated in cyclic structures. The variation of r with the concentration C seems more similar to these of C1/2/C, where C1 is the monomer concentration, than to these of C3/1/C, C4/1/C, etc. One concludes to a predominance of trimeric cycles in the cyclic species, although it seems that the importance of higher cycles increases with the molecular weight in the alcohol. The theory of Mecke and Kempter, with the correction for the dimerisation constant, introduced by Coggeshall and Saier, is only applicable on the open species. The length of the aliphatic chain of the alcohol seems to have a rather moderate effect on the dimerisation and polymerisation constants of the open species. This factor has a greater influence on the closing of the open hydroxylic polymers, the cyclisation being more important when the molecular weight of the alcohol increases.