ABSTRACT
The deviation from the common relationship (Figure 7.1) for methyl p-hydroxy-benzoate and L-679,336 could be due to several reasons. In the case of L-679,336, it is probable that the size of the molecule is such that the computer programme (MNDO) was unable to find the correct minimum energy conformation, and that the value obtained was a secondary minimum conformation. The anomalous behaviour of methyl p-hydroxybenzoate has been noted on 'many occasions. For example, methyl p-hydroxybenzoate has a much lower contact angle with water, and therefore a higher surface energy than 4-hydroxybenzoic acid (Sheridan et al, 1994b). Since methyl p-hydroxybenzoate has an extra methylene group, it would be expected to be more hydrophobic, and therefore have a higher contact angle with water and a lower surface energy. The surface energy of methyl-p-hydroxybenzoates is idiosyncratic in relation to the other members of the homologous series (Forster et al, 1991). Several of the mechanical properties, namely the fracture toughness, critical stress intensity factor, and critical strain energy release rate, are greater for methyl p-hydroxybenzoate than would be expected by comparison with 4-hydroxybenzoic acid (Newton et al, 1993). These variations in physical properties may be due to the significantly different surface energy value exhibited by this material (although cause and effect need to be addressed). Further discussion of this matter is given in Textbox 7.4.
