Reactions of acrolein, crotonaldehyde and pivalaldehyde with Cl atoms: structure–activity relationship and comparison with OH and NO3 reactions

Abstract

Rate coefficients for the reaction of acrolein (prop-2-en-1-al), crotonaldehyde (but-2-en-1-al) and pivalaldehyde (2,2-dimethylpropanal) with chlorine atoms were determined. The resulting rate coefficients were (1.8 ± 0.3) × 10⁻¹⁰, (2.2 ± 0.4) × 10⁻¹⁰ and (1.2 ± 0.2) × 10⁻¹⁰ (cm³ molecule⁻¹ s⁻¹) for acrolein, crotonaldehyde and pivalaldehyde, respectively. Rate coefficients for chlorine atom reaction with propanal, butanal, 2-methylpropanal and trans-but-2-ene were determined to be (1.2 ± 0.2) × 10⁻¹⁰, (1.5 ± 0.3) × 10⁻¹⁰, (1.5 ± 0.3) × 10⁻¹⁰ and (3.0 ± 0.6) × 10⁻¹⁰ (cm³ molecule⁻¹ s⁻¹), respectively. The relative rate technique was used with propene as the reference compound. The experiments were carried out at 297 ± 2 K and 1020 ± 2 mbar using a 0.153 m³ borosilicate glass reactor with long-path FTIR spectroscopy as the analytical tool. Synthetic air and nitrogen were used as bath gases. Literature values of the corresponding hydroxyl and nitrate radical rate coefficients were confirmed. The chemical characteristics of the organic substances have a limited influence on the reactivity with Cl, a larger effect in the OH-case but are decisive for the NO₃ reactions. Introduction of an aldehydic carbonyl group into an unsaturated compound reduces the reactivity of a neighboring double bond for reaction with all three radicals. The unsaturated aldehydes reacting with NO₃ show a rate coefficient that is lower than both the corresponding simple alkene and aliphatic aldehyde, indicating that also the reactivity of the aldehydic hydrogen atom is affected. The results show that during the morning hours, Cl atoms may be the most significant oxidising agent for organic substances in urban coastal air.