Marcus Theory Applied To Acetal Cleavage And Aldol Reactions

The acid catalyzed and uncatalyzed hydrolysis of the acetal 2,2-dimethoxypropane (DMOP) and the ortho ester trimethylorthoacetate (TMOA) have been studied in aqueous solution at 25{dollar}\sp\circ{dollar}C. Theoretical and experimental evidence have been found for a base catalyzed E2 mechanism of DMOP hydrolysis proceeding through an enol ether. This mechanism is neither predicted nor observed for TMOA hydrolysis. The specific acid catalyzed and uncatalyzed rate constants for hydrolysis are for DMOP k{dollar}\rm\sb{lcub}H+{rcub}=(2.5\pm0.3)\times10\sp3\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub},\ k\sb w=(3.6\pm0.2)\times10\sp{lcub}-8{rcub}\ s\sp{lcub}-1{rcub},\ k\sb{lcub}OH-,E2{rcub}=(2.8\pm0.4)\times10\sp{lcub}-8{rcub}\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub},\ and\ for\ TMOA\ k\sb{lcub}H+{rcub}=(2.05\pm0.12)\times10\sp4\ M\sp{lcub}-1{rcub}s\sp{lcub}-1{rcub},\ k\sb w=(3.7\pm0.3)\times10\sp{lcub}-6{rcub}\ s\sp{lcub}-1{rcub}.{dollar} General acid catalysis was observed for TMOA with four general acids. From the Bronsted plot, {dollar}\alpha=0.76\pm0.05.{dollar} Evidence of general acid catalysis in the hydrolysis of DMOP with phenol as general acid was suggestive but inconclusive.;The rate and equilibrium constants for formation, dehydration and, when appropriate, isomerization, have been determined for four hydroxide catalyzed aldol condensations at 25.0{dollar}\sp\circ{dollar}C in aqueous solution. The four reactions studied were acetone, acting as carbon acid, with {dollar}\alpha,\alpha,\alpha{dollar}-trifluoroacetophenone and with p-nitroacetophenone, and p-nitroacetophenone, acting as carbon acid, with p-nitroacetophenone and with acetone. The experimentally determined rate constants agree well with rate constants predicted by Marcus Theory, supporting the utility of Marcus Theory in the prediction of rates of unstudied aldol reactions