Kinetics and mechanism of benzaldehyde Girard T hydrazone formation

Abstract

In aqueous solution, Girard T hydrazone formation from para-substituted benzaldehydes does not proceed to completion under the conditions used. The corresponding equilibrium constants were determined and employed to calculate the rate of the reaction at completion. For all the reactions studied, the pH-rate profiles, extrapolated to zero buffer concentration, show one break at pH 4-5, characteristic of a change in the rate-determining step from carbinolamine dehydration to carbinolamine formation upon going from pH 7 to pH 1. The formation of the carbinolamine is subject to catalysis by hydronium ion and by carboxylic acids present in the buffers used to maintain pH. Bronsted a values for this catalysis varied from 0.19 to 0.37. Rate constants for the cyanoacetic, chloroacetic, and formic acid catalyzed formation of carbinolamine from the para-substituted benzaldehydes correlate with + substituent constants, giving a value of p+ equal to 0.70. Rate constants for the hydronium ion catalyzed formation of the carbinolamine from the same benzaldehydes are, however, insensitive to the substituent effect. These observations lead to the conclusion that the reactions occur by different routes as a function of the catalyst. We suggest a stepwise preassociation mechanism for the reaction catalyzed by carboxylic acids and a concerted preassociation mechanism for that catalyzed by hydronium ion.

Kinetic Selectivity and Thermodynamic Features of Competitive Imine Formation in Dynamic Covalent Chemistry

“…[47] 1) For imine formation studies:AN MR tube was first charged with solvent. All reactions with aldehyde and amine were carried out at room temperature with af inal concentration of 20 mm and atotal volume of 600 mL.…”

“…As it is well known that both the rates and equilibria of imine formation in generala re pH-dependent, [44][45][46][47][48] experiments were performed under controlled pD conditions (pD5.0, 8.5, and1 1.4;t he pD value was calculated employing pD = pH + 0.4 where "pH" is the apparent pH value indicated by the conventional glass electrode [36,49] )u sing phosphate buffers (160 mm NaH 2 PO 4 /Na 2 HPO 4 )m ixtures.…”

New mass spectrometry techniques for studying physical chemistry of atmospheric heterogeneous processes

“…Figure 4(a) illustrates the reactive nano-DESI/HR-MS experiment performed for quantitative analysis of selected individual limonene SOA constituents using the GT reagent [70]. The GT reagent [79] [80]. Characteristic shifts in exact masses of peaks observed before and after doping the solvent with GT identify the reactant-product pairs produced during reactive nano-DESI experiments as indicated by black (reactants) and red (products) colors in the figure.…”

Cooperative, bottom‐up generation of rigid‐rod nanostructures through dynamic polymer chemistry

“…Indeed, the acylhydrazone bond serves as a reversible covalent intermonomer linkage, while hydrophobic interactions are thought to be responsible for the folding into well‐defined rod‐like secondary structure in water. By virtue of the lability of the acylhydrazone linkage under acid catalysis,71–75 the constituent monomers are demonstrated to undergo dynamic (reversible) covalent polymerizations below pH ≈ 4, but are constitutionally static under neutral conditions. Further, distinctly non‐statistical relationships between monomer concentration and weight‐average molecular weight are observed in a manner that is consistent with N–E behaviour 49–51.…”