GEMINAL SUBSTITUENT EFFECTS .13. STANDARD ENTHALPIES OF FORMATION OF ALKYLSUBSTITUTED ALPHA-AMINO CARBOXYLIC-ACID ESTERS

The title amino acid esters (1a) (1c) and (1e) were prepared in high p urity. The enthalpies of combustion Delta H-c(0)(1) of (1a-e) were mea sured using an isoperibolic calorimeter. The enthalpies of vaporizatio n Delta H-vap of(1b-e) were obtained from the temperature function of the vapour pressure measured in a flow system. The following enthalpie s of formation have been obtained in the liquid and the gas phase resp ectively: Delta H-f(0)(1) and Delta H-f(0)(g) (in kJ mol(-1)) for (1a) -468.61 +/- 0.84, -419.4 +/- 1.4; (1b) -472.7 +/- 1.0; (1c) -542.1 +/- 1.1, -488.1 +/- 1.2; (1d) -534.80 +/- 0.96 -479.9 +/- 1.1; (1e) -504. 66 +/- 0.83. The values of the synergetic interaction of geminal subst ituents are discussed in relation to the branching at the central C-at om.