/sup 18/O isotope effect in carbon-13 nuclear magnetic resonance spectroscopy. Part 8. Oxygen exchange of 2,4,6-trimethylpyrylium cation

Journal Article · Wed Dec 12 00:00:00 EST 1984 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00337a030· OSTI ID:5781562

Risley, J M; Van Etten, R L; Uncuta, C; Balaban, A T

The rate of the oxygen-exchange reaction in an aqueous solution of a heterocyclic oxygen compound, 2,4,6-trimethylpyrylium perchlorate, was quantitated. The /sup 18/O isotope effect in /sup 13/C NMR spectroscopy affords a direct, continuous, and relatively simple analysis of the reaction and was used to obtain data on the incorporation of /sup 18/O from solvent H/sub 2//sup 18/O into the pyrylium salt. Due to the low solubility of this salt in aqueous solutions, 2,4,6-trimethyl(2-/sup 13/C)pyrylium perchlorate and 2,4,6-trimethyl(2,6-/sup 13/C/sub 2/)pyrylium perchlorate were synthesized to facilitate the acquisition of the data on an NTC-200 spectrometer operating at 50.31 MHz. Upon incorporation of /sup 18/O into the pyrylium salt, the /sup 13/C NMR signal of the directly bonded carbon atom is shifted upfield 0.038 ppm. A /sup 13/C isotope effect is also detected in the doubly /sup 13/C-enriched pyrylium salt where the /sup 13/C NMR signal for /sup 13/C-O-/sup 13/C appears 0.011 ppm downfield from the /sup 13/C NMR signal for /sup 12/C-O-/sup 13/C. The rate of the oxygen-exchange reaction was studied as a function of pH (2.0-4.8), buffer concentration, added electrolyte, and temperature. A positive salt effect is observed. The oxygen-exchange reaction is subject to specific hydroxide ion catalysis and to general base catalysis by buffers such as acetate. The /sup 18/O-exchange reaction is compared to the other reactions of the pyrylium salt including hydrolysis and /sup 2/H exchange. The apparent energy of activation for the oxygen-exchange reaction at pH 3.1 in the absence of buffer and electrolyte is approximately 25 kcal mol/sup -1/ while at pH 4.7 in 50 mM sodium acetate, ..mu.. = 0.2 M with NaCl, it is 10 kcal mol/sup -1/. 36 references, 2 figures, 1 table.

• Research Organization: Purdue Univ., West Lafayette, IN
• OSTI ID: 5781562
• Journal Information: J. Am. Chem. Soc.; (United States), Vol. 106:25
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CARBON 13
ISOTOPE EFFECTS
HETEROCYCLIC OXYGEN COMPOUNDS
CHEMICAL REACTION KINETICS
OXYGEN 18
ISOTOPIC EXCHANGE
AQUEOUS SOLUTIONS
DATA ACQUISITION
EXPERIMENTAL DATA
HYDROLYSIS
NMR SPECTRA
PERCHLORATES
PH VALUE
TRACER TECHNIQUES
CARBON ISOTOPES
CHEMICAL REACTIONS
CHLORINE COMPOUNDS
DATA
DECOMPOSITION
DISPERSIONS
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
HALOGEN COMPOUNDS
INFORMATION
ISOTOPE APPLICATIONS
ISOTOPES
KINETICS
LIGHT NUCLEI
LYSIS
MIXTURES
NUCLEI
NUMERICAL DATA
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
OXYGEN COMPOUNDS
OXYGEN ISOTOPES
REACTION KINETICS
SOLUTIONS
SOLVOLYSIS
SPECTRA
STABLE ISOTOPES
400302* - Organic Chemistry- Isotope Effects- (-1987)
400303 - Organic Chemistry- Isotope Exchange & Isotope Separation- (-1987)