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Solid-state decomposition of para-substituted salicylic acids.

P Pothisiri, J T Carstensen

    Journal of Pharmaceutical Sciences
    |December 1, 1975
    PubMed
    Summary

    Para-substituted salicylic acids decompose via Bawn-type kinetics, with rates following a Hammett-type relation. Their decomposition mechanism is intramolecular, occurring within the dimer unit.

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    Area of Science:

    • Organic Chemistry
    • Chemical Kinetics
    • Crystallography

    Background:

    • Para-substituted salicylic and benzoic acids exhibit decarboxylation upon heating.
    • Crystalline structures of these acids reveal dimerization.
    • Previous studies suggest varying decomposition mechanisms for related compounds.

    Purpose of the Study:

    • To investigate the decomposition kinetics of para-substituted salicylic acids.
    • To determine if their decomposition adheres to established kinetic models like Bawn-type kinetics.
    • To elucidate the specific mechanism (intramolecular vs. intermolecular) of their decarboxylation.

    Main Methods:

    • Synthesis and characterization of para-substituted salicylic acid derivatives.
    • Monoclinic crystallization and structural analysis.
    • Kinetic studies monitoring decarboxylation rates under controlled conditions.
    • Application of Bawn-type kinetic models and Hammett-type relations to experimental data.

    Main Results:

    • All studied para-substituted salicylic acids crystallized monoclinically and decomposed via decarboxylation.
    • Decomposition kinetics followed Bawn-type behavior.
    • Decomposition rate constants correlated with a Hammett-type relation, yielding a reaction parameter of -8.
    • Para-substituted salicylic and benzoic acids exist as dimers in the solid state.

    Conclusions:

    • The decomposition of para-substituted salicylic acids follows Bawn-type kinetics and a Hammett-type relationship.
    • The decarboxylation mechanism for these acids is intramolecular, occurring within the dimer.
    • This contrasts with a speculated intermolecular mechanism for para-substituted benzoic acids, highlighting substituent and structural influences on reaction pathways.

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