Kinetic hydrogen isotope effects: An organic/physical laboratory experiment

Abstract

The kinetic hydrogen isotope effect, the difference observed in the rate constant of a reaction due to isotopic substitution of a hydrogen atom involved in the rate-determining step of a reaction, is mentioned in many beginning organic chemistry textbooks. The effect has been used as supporting evidence for E-2 elimination (la, 2, 3, 4a, Sa), nitration of benzene (lb, 4b,5b, 61, and oxidation of alcohols by chromic acid (7). However, while textbooks are using kinetic hydrogen isotope effects to arrive a t certain mechanisms, an examination of current beginning organic chemistry laboratory texts reveals a stark paucity of experiments dealing with the subject. In fact, a review of eight laboratory manuals yielded not one experiment using kinetic isotope effects. This lack may be due, in part, to the difficulty in preparing isotopically labeled reactants. The experiment presented here is designed to allay some of these difficulties. The isotopically labeled reactant, deutrochloric acid, is simple to prepare, and it and the other isotopically labeled compound, deuterium oxide, are relatively inexpensive. Deuterium oxide costs about $0.50/g and denterochloric acid (37 wt%) can be purchased for about $l.OO/g. The same concentration of deuterochloric acid can be prepared for less than $0.85/g.Vinyl ethers are hydrolyzed in aqueous solutions to either aldehydes or ketones and to alcohols. The reaction is catalyzed by acids, and the hydrolysis mechanism involves ratedetermining proton transfer to carbon (8,9) as the first step. For example, the mechanism for ethyl vinyl ether hydrolysis catalyzed by hydronium ion is H,C=CHOC,H~ + H~O + -H~C-C+HOC~H~ (1)The carbocation intermediate (CH&+H-OCHzCHJ is the same intermediate as that postulated for the acid-catalyzed hydrolysis of acetals and ketals.The experimentally observed kinetic hydrogen isotope effect, k~l k~. for a mechanism involving rate-determining proton transfer from hydronium ion, is a product of two terms (10) where the first term, h"Vho', represents the effect on the reaction rate due to isotopic substitution of a hydrogen atom involved in actual bond making or bond breakingin the ratedetermining step (in this case the proton being transferred from H30f). This is called the primary isotope effect. The second term, h # / k~I~, represents effects due to snbstitntious of anv other hvdroaen atoms (in this case the two . . remainingon the hy~lronnimion~ that also influence the rate of rmrtion. This is known as t h~ secundarv isoro~e effect.These isotope effects can be predicted the ore ti^&^ using the simplified approach of Bell (11) and Kreevoy (12). The maximum primary effect is calculated as 10.6 (II), and the minimum secondary effect as 0.48 (10). The maximum observed isotope effect, h~l h~, that can be expected, therefore, is 5.1 (eq 3). In practice values lower than this are generally observed-being around 3.In the present experiment ethyl vinyl ether, EVE, is the substrate, and the hydrolysis produets are acetaldehyde and ethanol. The rate of the hydrolysis re...

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