Rotation about Aromatic amide Bonds A Computational Project Evan Grassi and Donald D. Clarke Department of Chemistry, Fordham University James B Foresman, Department of Physical Sciences York College of Pennsylvania July 26, 2014 Mercury Conference

Preparation of substituted benzenes is a frequent experiment in organic lab; an example of this is 4-bromo-2-nitroacetanilide. It was stated in textbooks of 1970’s vintage that it can be made only by indirect means. A microscale procedure was described in J. Chem. Educ. 1994, 71, A144-5.

6H 5H H3

4-Br-2-NO2-acetanilide in CDCl 3 NH H6 H3H5CHCl 3

The 1 H NMR spectrum of 4-bromo-2- nitroacetanilide was first reported in J. Org. Chem. 1968, 33, by a group from Temple University and Sadtler Research Labs. They noted that additivity rules failed to predict the observed spectrum in CDCl 3. Such a 4-substituted derivative allowed clear distinction of the proton at H-3 which has only m-coupling from that at H-6 which has both o- and p-coupling.

Note: H-E at 7.15 ppm is missing

The 1 H NMR spectrum of 2-nitroacetanilide is still reported in databases [e,g. SDBS] as well as in a more recent publication [Z Naturforschung 2006, 61a, 59] using the misassignment of H-3 and H-6 predicted by additivity rules. The correct assignment mentioned above is ignored in the literature. Previously we observed that the 1 H NMR spectrum of 2-nitroacetanilide was quite different in CDCl 3 and DMSO-d 6. This is true of 4-bromo-2-nitroacetanilide or other 4- substituted 2-nitroacetanilides, e.g. 4-CH 3 and 4-Fluoro.

4-Br-2-NO 2 -acetanilide in DMSO NH H3 H5 H6

The Sadtler group explained the deviations from additivity as due to strong H bonding between the nitro and NH groups. The NH shift moves from 7.5 ppm in acetanilide to 10.3 ppm in 2-nitroacetanilide confirming the strong H bonding. This is not changed in going from CDCl 3 to DMSO, hence H bonding doesn’t explain the deviations from additivity. Rather rotation about the amide bond does.

H3H4H5H6 2-Nitroacetanilide in DMSO

Calculation of the change in chemical shift of the aromatic ring protons as the amide group of 2-nitroacetanilide is rotated using Gaussian is shown above. H6 changes greatly as a result of such rotation while the other protons change minimally. The angle of the acetamido group to the plane of the ring is near 50 o in CDCl 3 while it is close to 90 o in DMSO. This explains the reversal in the order of shifts of H3 and H6 in going from CDCl 3 to DMSO as solvent.

It is apparent that acetyl is a useful reporter group for studying aromatic amines. The free amines show very little difference in 1 H chemical shifts between spectra taken in CDCl 3 and DMSO. We first became aware of this type of behavior when studying 4-halo-2- hydroxyacetanilides [Monatsh Chemie. 1993, 124, 367].

Because the light absorption spectrum of 2- nitroacetanilide changes markedly on hydrolysis, it has been used as a popular substrate for assaying the aryl acylamidase activity of butylcholinesterase [EC ] since More polar solvents decrease the intensity of its absorption indicating a decrease of electron release by the lone pair of electrons on the N atom to the  system of the ring as the amide group moves closer to 90 o to the ring plane. This agrees with the conclusion from the NMR studies above.

Acknowledgment We gratefully acknowledge financial support from the National Science Foundation’s Division of Undergraduate Education through Grant DUE # for the NMR spectrometer at Fordham University as well as Faculty Research grant. Also we wish to thank our colleagues in organic lab James Ciaccio, Amy Balija, and Shahrokh Saba for their help in connection with this project.