NMR spectra of alkyl vinyl ether and its structures

Abstract

The NMR spectra of some vinyl ethers at 16.2 and 40.0 MHz have been obtained by Feeney at room temperature (1) and correlated with the degree of resonance contributions in vinyl ethers (2). In this work we have measured the NMR spectra of alkyl vinyl ethers at various temperatures using 100 MHz spectrometer and studied on the resonance and the molecular conformation in alkyl vinyl ether. The spectral parameters of vinyl ethers are given in Table I, in which the vinyl ethers are arranged in increasing order of their reactivities in cationic polymerization (3) from top to the bottom. This order is generally in accordance with the increasing order of bulkiness of alkyl group in alkyl vinyl ether. From top to the bottom in this Table the geminal coupling constant of vinyl ether, J AB, generally increases and the chemical shifts of HA and HB, 6A and 6B, are to lower field and the difference, 6 -6 A B' increases. It has been suggested that alkyl vinyl ethers may take a following resonance form (1,4). ?H2-CH=&R Pople (5) has shown that the coupling constant between geminal hydrogen atoms increases as the s-character of the carbon atom between them increases. So the JAB value of alkyl vinyl ether can be used as an indication of the degree of resonance. Thus the contributions of the resonance form are greatest in methyl vinyl ether and least in t-butyl vinyl ether. On the other hand, the resonance in alkyl vinyl ether must lead to a reduction in the olefinic character of the vinyl group and consequently the terminal methylene protons will become more equivalent. The values of AA, 6B and hA-6B shows that the resonance contribution in alkyl vinyl ether decreases from top to the bottom in Table 1, that is, in the increasing order of the bulkiness of its