The Chemistry of Alkenes

Abstract

A detailed representative kinetic run is given in Table II.24 First-order rate plots of log [CCN]t/[CCN]o against t yielded straight lines in all cases studied (Figure 5).24 The second-order rate constants were determined by dividing the pseudo-first-order rate constants by base concentration. It may be argued that the trans olefin does indeed isomerize to the cis olefin, but that the very minor accumulation of the cis isomer is due to its faster consumption in the addition reaction , as compared to that of the trans olefin. This, however, is not the case as is evident from the fact that the second-order rate constants for the nucleophilic attack step of the ethoxide anion on the cis-and on the trans-cinnamonitrile are about the same: ~ 3.50 X 10~41. mol-1 s_1 and/?i = (3.72 ± 0.51) X 10-4 1. mol-1 s_1. Experimental Section The infrared spectra were obtained with a Perkin-Elmer Model 257 spectrophotometer. NMR spectra were recorded on a JEOL 60 MHz spectrometer. VPC measurements were done on a Varían Aerograph Model 1800 gas chromatograph. Mass spectra were recorded on a Hitachi Perkin-Elmer Model RMV-6 (70 eV) mass spectrometer. Materials. A mixture of cis-and trans-cinnamonitrile was syn-thesized22 and separated by distillation on a spinning band column. The cis and trans isomers were each obtained at 98% purity (VPC). Absolute dry ethyl [2H] alcohol (Miles-Yeda), 99.9% isotopically pure, was used. Ethanolic sodium ethoxide solutions were prepared by adding sodium metal to ethyl [2H] alcohol under reflux and nitrogen. The base concentration was determined by titration with hydrochloric acid. Liquid materials and solutions were kept under pure nitrogen in flasks fitted with self-sealing rubber caps. Aliquot portions were removed from these flasks with syringes by applying nitrogen pressure. Kinetic Runs. The reactions of trans-cinnamonitrile with ethyl [2H] alcohol catalyzed by sodium ethoxide were carried out in a 150-ml flask connected to high vacuum and nitrogen lines. The flask was fitted with a self-sealing rubber cap through which liquids were introduced by syringes. The system was dried, evacuated, and flushed with dry nitrogen prior to the introduction of solvent and reactants. The required amounts of cinnamonitrile and ethyl [2H] alcohol were introduced into the flask which was then immersed in a constant-temperature bath at (39 ± 0.5) °C. A solution of sodium ethoxide in ethyl [2H]alcohol was then introduced and time recorded. Portions of the homogeneous mixture were withdrawn at measured intervals through a capillary stopcock, by applying a nitrogen pressure. The samples withdrawn were, quenched with excess acetic acid and subjected to quantitative VPC and ir measurements. Some representative results of kinetic rate measurements of the addition reaction are presented in Table II.24 The kinetic rate measurements of the addition of ethyl [2H]alcohol to cis-cinnamonitrile and its simultaneous isomerization to the trans isomer were followed by VPC. Quantitative Ir Analysis. The baseline density method23 was used to determine the percentage of deuterium (% D) in the samples withdrawn from the reaction mixture. The ir spectrum of each of several mixtures composed of weighted amounts of-deuterated and nondeuterated trans-cinnamonitrile was recorded using no solvent in sodium chloride cells (0.025 cm thickness). The transmittance T at 975 cm-1 (due to the C=CH bond) and the transmittance V at 955 cm-1 (due to the C=CD bond) were determined. The % D of each of the same samples was determined from their mass spectra. A standard curve for the quantitative ir analysis was prepared by plotting log T/T' against % D. Solvents (ethyl [2H] alcohol and acetic acid) were evaporated from each of the quenched samples withdrawn from the reaction mixture and the residue was distilled under vacuum into a cooled trap. This distillate consisted of a mixture of the addition product of ethyl [2H]alcohol to trans-cinnamonitrile and the a-deu-terated and nondeuterated olefins. The ir spectrum of the liquid was recorded, log T/T' determined, and the % D then directly read from the standard curve. Registry No.-Ethyl [2H] alcohol, 925-93-9; trans-cinnamonitrile, 1885-38-7; sodium ethoxide, 141-52-6; «s-cinnamonitrile, 24840-05-9. Supplementary Material Available. Figures 3,4, and 5 and Table II describing the addition of ethyl [2H]alcohol to trans-and cis-cin-namonitrile (4 pages). Ordering information is given on any current masthead page. The usual orientation of nucleophilic addition at the 2 position of a 3-alkylpyridine can be changed by increasing the steric requirements of the nitrogen substituent. Thus the addition of phenylmagnesium bromide to the alkyl chloroformate ester salts of 3,4-lutidine is regioselective giving up to 90% addition of the aryl group at the 6 position. The large steric requirements of the 1-phenoxycarbonyl group or an ortho-substituted phenyl Grignard re-agent gave 95% or greater regioselectivity of reaction at the less hindered a position of the 3,4-lutidine salt.