J Phys Chem A. 2008 Apr 3;112(13):2827-37. Epub 2008 Mar 4.
A kinetic and mechanistic study of the amino acid catalyzed aldol condensation of acetaldehyde in aqueous and salt solutions.
Nozière B, Córdova A.
Department of Meteorology, Stockholm University, SE-106 91 Stockholm, Sweden. barbara.noziere@misu.su.se
The amino acid catalyzed aldol condensation is of great interest in organic synthesis and natural environments such as atmospheric particles. However,
kinetic and mechanistic information on these reactions is limited. In this work the kinetics of the aldol condensation of acetaldehyde in water and
aqueous salt solutions (NaCl, CaCl2, Na2SO4, MgSO4) catalyzed by five amino acids (glycine, alanine, serine, arginine, and proline) at room
temperature (295 +/- 2 K) has been studied. Monitoring the formation of three products, crotonaldehyde, 2,4-hexadienal, and 2,4,6-octatrienal, by
UV-vis absorption over 200-1100 nm revealed two distinct kinetic regimes: at low amino acid concentrations (in all cases, below 0.1 M), the overall
reaction was first-order with respect to acetaldehyde and kinetically limited by the formation of the enamine intermediate. At larger amino acid
concentrations (at least 0.3 M), the kinetics was second order and controlled by the C-C bond-forming step. The first-order rate constants increased
linearly with amino acid concentration consistent with the enamine formation. Inorganic salts further accelerated the enamine formation according to
their pKb plausibly by facilitating the iminium or enamine formation. The rate constant of the C-C bond-forming step varied with the square of amino
acid concentration suggesting the involvement of two amino acid molecules. Thus, the reaction proceeded via a Mannich pathway. However, the
contribution of an aldol pathway, first-order in amino acid, could not be excluded.
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