Secondary. cap alpha. -deuterium isotope effects for the cleavage of formaldehyde hemiacetals through concerted and specific-base-catalyzed pathways

Journal Article · Wed Oct 08 04:00:00 EDT 1980 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00541a016· OSTI ID:6644990

Palmer, J L; Jencks, W P

The observed secondary ..cap alpha..-deuterium isotope effects for catalysis by acetate ion of the cleavage of formaldehyde hemiacetals increase from k/sub 2H//k/sub 2D// = 1.23 to 1.28 to 1.34 with decreasing pK of the leaving alcohol in the series ethanol, chloroethanol, and trifluoroethanol. The pH-independent reaction shows a smaller isotope effect of 1.15 to 1.14 for the ethyl and chloroethyl hemiacetals. These reactions involve general-base catalysis of alcohol attack in the addition direction and the kinetically equivalent cleavage of the hemiacetal anion with general-acid catalysis by acetic acid or the proton in the cleavage direction. The results indicate that the amount of C-O cleavage in the transition state increases with decreasing pK of the alcohol and increasing pK of the acid catalyst, corresponding to a negative coefficient p/sub yy'/ = par. delta rho/sub n//-par. delta pK/sub 1g/ = par. delta ..beta../sub 1g//-par. delta sigma and a positive coefficient p/sub xy/ = par. delta rho/sub n//-par. delta pK/sub HA/ = par. delta ..cap alpha../par. delta sigma. These results provide additional support for a concerted reaction mechanism with an important role of proton transfer in the transition state. Qualitative and semiquantitative characterizations of the transition state are presented in terms of reaction coordinate diagrams that are defined by the structure-reactivity parameters. The properties of the transition state suggest that the reaction is best regarded as an electrophilic displacement on the oxygen atom by the proton and by the carbonyl group in the cleavage and addition directions, respectively. The large secondary isotope effect of k/sub 2H//k/sub 2D/ = 1.63 for cleavage of the chloroethyl and trifluoroethyl hemiacetals catalyzed by hydroxide ion indicates a late transition state for alkoxide expulsion from the hemiacetal anion. 7 figures, 4 tables.

• Research Organization: Brandeis Univ., Waltham, MA
• OSTI ID: 6644990
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 102:21; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400301* -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
ACETALS
ACETATES
ALCOHOLS
ALDEHYDES
ALPHA REACTIONS
CARBOXYLIC ACID SALTS
CATALYSIS
CHARGED-PARTICLE REACTIONS
CLEAVAGE
CRYSTAL STRUCTURE
DEUTERIUM
ETHERS
FORMALDEHYDE
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
HYDROXY COMPOUNDS
ISOTOPE EFFECTS
ISOTOPES
LIGHT NUCLEI
MICROSTRUCTURE
NUCLEAR REACTIONS
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
ORGANIC OXYGEN COMPOUNDS
OXYGEN COMPOUNDS
STABLE ISOTOPES
WATER