Methanyklsys Essay

1699 Words Oct 2nd, 2014 7 Pages
David O. Johnston David Lipscomb College

I
II

The Methclnolysir of Acetal
A chemical kinetics a n d g a r chromatography experiment

Nashville,

Tennessee

The methanolysis of acetal and gas chromatography complement perfectly to provide a rare and interesting experiment for the physical chemistry student. The reaction of acetal with methanol is a consecutive reaction in which diethyl acetal (aretal) replaces its two ethoxy groups with methoxy groups forming methy1et)hylacetal first and dimethyl acetal in the second step. Investigating the reaction kinetics for this type of reaction mas almost impossible before the advent of gas chromatography (I). There are many advantages to using the methanolysis of acetal in this study:
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inlet pressure. The recorder was a Sargent Model SRL with a 5-mv range plug. To begin a run 25 ml of the methanol-HC1 solution is pipetted into the reaction flask followed by 10 ml of acetal. These quantities give initial concentrations of 1.14X l W 4M HC1,1.96 M acetal,and 17.6 M methanol. The timer is started when half of the acetal has passed into the flask. The stoppered flask is agitated for a few seconds in the bath. Two-ml samples are removed about 20 sec prior to quenching at approximately 1, 2.5,4.2, 6,7.5,20, 30, 35, 40, 45, 50, 60,70, a n d 8 0 min. To quench the samples, the tip of a 2-mm glass rod is moistened with concentrated ammonium hydroxide and swished through the sample. The time of quenching is recorded. This quenching technique is very effective as shown by aliquots of the same sample reproducing after 18 hr within the experimental error of injecting samples. Thirty-five microliter aliquots of the samples are injected into the gas chromatograph between taking samples in order to conserve time. The elution time per sample is about 10 min for the instrument and flow conditions stated above. As shown by the typical chromatogram in Figure 1, these conditions do not give complete separation of the alcohols and the dimethyl acetal. However, the diethyl acetal and methylethyl acetal peaks are completely separated, and these are
Table of Consecutive First Order Rote Constants at 2 . ' C. 50 for the

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