3572
Structural Studies by Nuclear Magnetic Resonance. XXI. Conformational Analysis of Dichloroacetaldehyde and Dibromoacet aldehyde Gerasimos J. Karabatsos, David J. Fenoglio, and Sheldon S. Lande Contribution from the Department of Chemistry, Michigan State University, East Lansing, Michigan 48823. Received September 11, 1968 Abstract: The following conclusions were reached from the solvent and temperature dependence of the vicinal spin-spin coupling constants of dichloroacetaldehyde and dibromoacetaldehyde. (I) The data are consistent with a threefold barrier to rotation about the carbon-carbon bond. (2) In nonaromatic solvents whose dielectric constant is less than six, AGO for XIII $ XIV, is positive, i.e., the rotamer with the C-H bond eclipsing the carbonyl group is of lower energy than the others. (3) In nonaromatic solvents whose dielectric constant is higher than 7, AGO is negative. (4) Both free energies and enthalpies for XIII XIV, vary over a range of about 2.5 kcallmol, as the dielectric constant of the solvent varies from about 2 to 45.
I
n a recent publication, we concluded that rotational isomerism about the carbon-carbon bond of chloroacetaldehyde and bromoacetaldehyde was best described in terms of a threefold barrier to rotation, with A H " for I e I1 ranging from -300 cal/mol (chloroacetaldehyde) and 0 cal/mol (bromoacetaldehyde) in
dihedral angle of 30" (V) for I11 best agrees with the experimentally determined vibrational frequencies. It was pointed out' that in cases where the assignments were made from nmr studies such distinctions in
v I I1 the least polar solvent trans-decalin, to - 1500 cal/mol (chloroacetaldehyde) and - 700 cal/mol (bromoacetaldehyde) in the most polar solvents formamide and dimethyl sulfoxide. Several other compounds containing a single halogen atom at the a-carbon have also been found to exhibit a threefold barrier to rotation about the sp2-sp3 carbon-carbon bond. For example, A H " for I11 $ IV is - 560 cal/mol, - 500 cal/mol and 0 cal/mol for ethyl fluoroacetate, chloroacetate, and bromoacetate, respectively;2 it is - 1000 cal/mol and
III
dihedral angles could not be made. The only related a-substituted halo compound, where a twofold barrier to rotation about the spz-sp3 carboncarbon bond has been found,g is fluoroacetyl fluoride, whose A H " for VI eVI1 is -9lOcal/mol.
VI
VI1
The analogous dihalo compounds differ from the monohalo compounds in two respects. (a) The A H " IX are much more positive than the values for VI11
IV
- 1900 cal/mol for bromoacetyl chloride and bromoacetyl bromide;3and it is - 100 cal/mol, 100 cal/mol, and over 100 cal/mol, respectively, for 3-fluoropropene,4,5 3-~hloropropene,~ and 3-bromopropene.' Although in most cases the data have been interpreted in terms of perfectly eclipsing conformations, i.e., dihedral angles of zero between planes HCC and CCZ in I11 and between planes XCC and CCZ in IV, for chloroacetyl chlorides and bromoacetyl chloride3 a
+
+
(1) G. J. Karabatsos and D. J. Fenoglio, J . Ant. Chem. Soc., 91, 1124 (1969). (2) T. L. Brown, Spectrochim. Acfu, IS, 1615 (1962). (3) S . Mizushima, T. Shimanouchi, T. Miyazawa, I. Ichishima, K. Kuratani, I. Nakagawa, and N. Shido, J . Chem. Phys., 21, 815 (1953). (4) H. Hirota, ibid., 42, 2071 (1961). (5) A . A. Bothner-By, S . Castellano, and H. Giinther, J . A m . Chem. Soc., 87, 2439 (1965). (6) A. A . Bothner-By, S . Castellano, S . J. Ebersole, and H. Gunther, ibid., 88, 2466 (1966). (7) A . A . Bothner-By and H. Gunther, Discussions Furuduji Soc., 34, 127 (1962).
Journal of the American Chemical Society
91:13
VI11
IX
corresponding ones for I11 e IV. In all cases the more stable rotamer is the one with the carbon-hydrogen bond eclipsing the double bond. For example, A H " (VI11 $ IX) for dichloroacetyl chloride'o is +200 cal/mol, it is +500 to +1400 cal/mol for 3,3-difluoropropene5 and 800 cal/mol for 3,3-di~hloropropene,~ (b) The ethyl dihaloacetates, in contrast to the ethyl monohaloacetates, exhibit twofold barriers to rotation, with A H o for X $ XI being +25 cal/mol and 0 cal/mol for ethyl difluoroacetate and ethyl dichloracetate, respectively.
+
(8) Y. Morino, I 6.83 cps, if they have the same sign; and J , < 2.17 cps and Jt > 11.17 cps, if they have opposite signs. We are now faced with the question of whether such relative values of Jt and J,, especially those of dichloroacetaldehyde, are reasonable. From valence-bond theory, the contact interaction term describing the dihedral-angle dependence of vicinal proton-proton coupling is approximatedl'j by eq 6. The relative magnitude of Jt and J , depends on the values of A , B, and C. For JHH =
A
+B
COS
4
+C
COS
24
(6)
ethane (both carbons sp3 and C-C distance 1.54 A), A = 4.22, B = -0.5, and C= 4.5 cps, the treatment predicts Jt = 9.22 cps and J , = 8.22 cps. For ethylene (both carbons sp2 and C-C distance 1.35 A), it predicts Jt = 11.9 cps and J , = 6.1 cps. Experimentally determined Jt and J, values of ethylenic compounds agree fairly well, if not always in absolute value at least in the relative magnitude of the two coupling constants, with the predicted values. There are no experimental Jt and J, values for systems with one carbon atom sp2 hybridized and the other sp3 with which to compare our values. Some values are available for systems with both carbon atoms sp2 hybridized, where the carbon-carbon length is between those of ethane and ethylene. The& of 1,3-butadiene17 and J, (single bond) of 1,3-cyclohexadiene1* are 10.41 and 5.14 cps, respectively. For cY,P-unsaturated aldehydes (malondialdehyde and acetylacetaldehyde) the analogous coupling constants have been estimated l9 by nmr to be about 7.7 and 2.8 cps, respectively. If one were to consider that the 0.9 cps value of J, of dichloroacetaldehyde is an upper limit, as it is based on the assumption (incorrect) that at - 30°, in N,N-dimethylformamide, dichloroacetaldehyde exists exclusively in conformation XV, then one would conclude that a threefold rather than a twofold barrier to rotation best fits the experimental results. The discussion just concluded, having left unanswered the question of whether a twofold or a threefold barrier to rotation best fits the experimental results, cogently illustrates the major weakness of nmr in rendering an unambiguous verdict in such cases of rotational isomerism. Irrespective, however, of whether one chooses to interpret the results in terms of a two- or a threefold barrier to rotation, the conclusion that XI11 is the most stable rotamer in the low dielectric constant solvents,
(14) R. J. Abraham and I
