Rotational isomerism about single bonds in olefinic compounds

Abstract

Analysis of the 60 Mc/sec high resolution proton n.m.r. spectra of samples of allyl chloride, allyl bromide, allyl iodide and allyl benzene both neat and in 10 % (viv) solution in CC14 yield sets of coupling constants and chemical shifts from which conclusions may be drawn concerning the relative stabilities of the rotational conformers. In allyl chloride and ally1 benzene that conformer with the substituent gauche to the proton on trigonai carbon is slightly preferred. The preference for this conformer becomes increasingly more pronounced in allyl bromide and allyl iodide.Numerous studies 1-9 have been directed at establishing the nature of rotational isomerism about the single bond joining trigonal carbon to tetrahedral carbon. It now appears reasonably certain that in many compounds the stable conformations are those derivable from the conformation (la) established for acetaldehyde 5 and propene 6 by means of microwave spectroscopy. The rotamers of monosubstituted propenes may thus be depicted as (lb), (lc) and (lc').High resolution n.m.r. has proven a useful tool in exploring rotarner equilibria and rates of interconversion in unsaturated 8,9 and saturated 10-12 systems. Its application rests on the theoretically predicted 13 and experimentally observed 14 dependence of proton-proton spin coupling constant on H 4 -C -H dihedral angle. This dependence appears to be of essentially the same form whether the adjacent protons are both on tetrahedral carbon, or whether they are on adjoining tetrahedral and trigonal carbons.15The purpose of the present work is to explore the application of this tool to the study of rotamerism in other allylic compounds, namely allyl chloride, allyl bromide, allyl iodide and allyl benzene. EXPERIMENTAL MATERIALSSamples of coinmercial provenance were purified by distillation and gas chromatography (VPC). VPC showed no detectable impurities in the purified allyl chloride and allyl bromide, 0.6 % of unidentified material in allyl benzene, and 1-7 % unidentified material in allyl iodide which, however, could not be removed by further distillation, and presumably represented thermal decomposition products of the allyl iodide. CCb was commercial reagent grade, and showed no impurities detectable by VPC.Samples (pure, or in 10 % v/v solution in CCb) were introduced into precision ground 5 mm cells and degassed on the vacuum line ; 3 % (v/v) of tetramethylsilane was added ;

Vicinal proton coupling constants and the electronegativity of substituents: evidence for an effect alternating with the number of intervening bonds

“…t Due to j v (cis) and j~v (trans) increasing in the 3-halopropenes in going from I to CI it has been remarked [8] that the chloromethyl appears to have a lower electronegativity than the iodomethyl group. $ Solvent effects [9] or differences in experimental and analytical methods may be important.…”

Long-Range Spin-Spin Coupling in Alkylfluorobenzenes. The Stereochemical Requirements for Coupling of Fluorine and Hydrogen Separated by Five Bonds1

“…(1) where JHX is the observed coupling constant, is the fractional population in form Ia, Jhx1 and JHxg are the (H,X) coupling constants characteristic of form Ia and lb (or lb'), respectively. Solving for pa, and substituting in AF = -RTIn Keq = -RTIn 2pj(lpf) (2) one obtains AF = -RT In 2(JHx -Jnxs)l(Jnxt ~Jhx) (3) Similarly AF = -RT In (Jhy1 + JHYg ~2JHy)/(JhY -JHYg) (4)…”

Rotational isomerism and barriers to internal rotation in 3-halopropenes byscf-mo methods

“…From all these curves, it is clear that for all halides, the c/s form is energetically more favourable than the gauche form. But the high resolution proton NMR spectral studies (Bothner-by and Gunther 1962;Govi11966) show that in 3CIP itself, there is some preference for the gauche form rather than the cis form and this tendency is much pronounced in the case of 3BrP and 3IP. This fact has been confirmed quite recently from electron diffraction study in the gas phase on 3BrP (Schei and Shen 1982) wherein it is reported that the population of the gauche form is as high as 95 %.…”