| Abstract
| - The amino acid catalyzed aldol condensation is of great interest in organic synthesis and natural environmentssuch as atmospheric particles. However, kinetic and mechanistic information on these reactions is limited. Inthis 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) atroom 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 kineticregimes: at low amino acid concentrations (in all cases, below 0.1 M), the overall reaction was first-orderwith respect to acetaldehyde and kinetically limited by the formation of the enamine intermediate. At largeramino 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 withthe enamine formation. Inorganic salts further accelerated the enamine formation according to their pKb plausiblyby facilitating the iminium or enamine formation. The rate constant of the C−C bond-forming step variedwith 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 inamino acid, could not be excluded. Our results show that the rate constant for the self-condensation ofacetaldehyde in aqueous atmospheric aerosols (up to 10 mM of amino acids) is identical to that in sulfuricacid 10−15 M (kI ∼ 10-7−10-6 s-1) clearly illustrating the potential importance of amino acid catalysis innatural environments. This work also demonstrates that under usual laboratory conditions and in naturalenvironments aldol condensation is likely to be kinetically controlled by the enamine formation. Notably,kinetic investigations of the C−C bond-forming addition step would only be possible with high concentrationsof amino acids.
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