Document Detail

A kinetic and mechanistic study of the amino acid catalyzed aldol condensation of acetaldehyde in aqueous and salt solutions.
MedLine Citation:
PMID:  18314969     Owner:  NLM     Status:  MEDLINE    
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. Our results show that the rate constant for the self-condensation of acetaldehyde in aqueous atmospheric aerosols (up to 10 mM of amino acids) is identical to that in sulfuric acid 10-15 M (kI approximately 10-7-10-6 s-1) clearly illustrating the potential importance of amino acid catalysis in natural environments. This work also demonstrates that under usual laboratory conditions and in natural environments 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 concentrations of amino acids.
Barbara Nozière; Armando Córdova
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't     Date:  2008-03-04
Journal Detail:
Title:  The journal of physical chemistry. A     Volume:  112     ISSN:  1520-5215     ISO Abbreviation:  J Phys Chem A     Publication Date:  2008 Apr 
Date Detail:
Created Date:  2008-03-27     Completed Date:  2008-06-16     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9890903     Medline TA:  J Phys Chem A     Country:  United States    
Other Details:
Languages:  eng     Pagination:  2827-37     Citation Subset:  IM    
Department of Meteorology, Stockholm University, SE-106 91 Stockholm, Sweden.
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MeSH Terms
Acetaldehyde / chemistry*
Aldehydes / chemistry*
Amino Acids / chemistry*
Models, Chemical*
Salts* / chemistry
Spectrophotometry, Ultraviolet
Reg. No./Substance:
0/Aldehydes; 0/Amino Acids; 0/Salts; 0/Solutions; 107-89-1/3-hydroxybutanal; 75-07-0/Acetaldehyde

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