TH0500007 The Estimation of Enol Contents in a Ketone by Proton Nuclear Magnetic Resonance Spectroscopy

sfiurceV. mi?r mmitni12', isva Somsak Aramrueng'*. Damrat Supyen'*2, Weerachai Phutdhawong2 'Chemistry Department, Chiang Mai University, Chiang Mai, 20200 Thailand 2Chemistry Department, Maejo University, Sansai, Chiang Mai, 50290 Thailand

6 Kuwiitisfn^^Snau^BmntiiiBwen 2 vualu CDCI3 vi 25°c T«tnS 'H-NMR VJII 83.0% uas 1.6% i S lJ ttuf\f\t> mBiilaauwi'masaimilusniMBnflm'nBUBfi (CD3OD) tfiwfiJBsiwtaBsi^lflUYiimfl'UBatifta.ikil'u 65.2 % iafltiBs;iifl«iosi«Hia?iS8^0?imijwtilu 14.3 % lufhmwmu'uqiu (C6D6)

Abstract : Enol contents in CDCI3 at 25°C of 6 carbonyl compounds and 2 compounds containing electron withdrawing groups were determined using 'H-NMR. The results were approximately 83.0% and 1.6% enol contents for acetylacetone and ethyl acetoacetate respectively. In the other compounds enol forms appeared to be absent in the equilibrium mixture. When the solvent was changed from CDCI3 to denterated methanol (CD3OD ) for acetylacetone the enol content was lowered to 65.2%. On the contrary for ethyl acetoacetate it increased to 14.3% in deuterated benzene (Cyi^).

Methodology : The 'H-NMR spectra of the Following compounds (see table 1) were recorded in CDCI3 (-10 mg/0.5 ml) employing TMS as an internal reference. The machine was Hitachi 60 MHz and run at ~25°C. To see the effect of solvents, compound 1 was dissolved in deuterated methanol (CD3.OD ) (~10 mg/0.5 ml) and compound 3 in deuterated benzene (C6D6). Using TMS as an internal reference at ~25°C and the solutions were recorded to obtain the 'H-NMR spectra.

Results and Discussion : The percentage of keto and enol forms were estimated from the integration of acetyl methyl protons (COCH3) at 2.2 ppm and allyllic methyl proton (C=C-CHj) at 2.0 ppm. When there were no enol forms the peak at 5 - 6 ppm corresponding to vinyl proton would be absent. The results were shown in table 1. The effect of solvents was found as follows : compound 1 contained 65.2 % enol form in CD3OD at 30°C ; compound 3, 14.3 % in C,sD6 . This phenomenon was explained by the increasing of intermolecular hydrogen bonding between the solvent and keto-form in the case of acetylacetone. But in the case of ethyl acetoacetate intramolecular hydrogen bonding in the enol form was increased by denterated benzene which is less polar than CDCI3. The results were similar to those reported in the literature 1 except for compound 3. Table 1 Compounds studied and enol contents (%)

Compound no. names enol content(%) _ acetylacetone 83.0 2. acetone none 3. ethyl acetoacetate 1.6 4. methyl acetate none 5. nitro-methane none 6. acetonitrile none 7. phenylacetone none diethylmalonate none

Reference: 1. Goro, J. Org. Chem. 19,469,1960(1954). Keywords : NMR, Enol, Ketone