Methanyklsys

1695 words 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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The gas chromatograph used to obtain the data in the table was a Perkin and Elmer Model 811 equipped with a hot wire detector and homemade in. X 11 ft copper tubing columns packed with Anakrom ABS 70/80 mesh (Analabs, Inc., Hamden,

Acid Concn. X 10' Mole/liter 7.1 8.6

-Rate

kr X

min-I
4.25 4.95

lo2

Constants------. kri X 10' min-' 0.38 0.66

' The reaction proceeded negligibly over a. period of 105 min when 0.16 ml of water without acid was injected into the meth* no1 and acetal reaction mixture. Moles of methanal/acetal/ water were 6/0.7/0.002, respectively. To obtain the data in the tahle, Acculute Standard Volumetric Solution (Anachemia Chemicals, Ltd., Champlain, N. Y.) and distilled water were used to prepare the 1.000 M HCI. The scetal was Matheson smerior made f h 101-103°C) and the met,hanol ~ was Baker ~ n % & e ds&ctrop&tometric ~ b s o l u t ewhich were used without fnrther pnrification.
Volume 44, Number

I lanuory 1967 / 33 ,

MeOH

the rate equation, assuming steady state, becomes d(MeEt)/dl = 0 = klEt

+ k-%Me- k_,MeEt - kzMeEt,

(5)

where the rate constants are pseudo first-order. Substituting first for Me = E ~ . o MeEt - Et, then for k-1MeEt = k-I ( E b -

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