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Anal. Chem. 1990, 62, 304-308
Kinetics of the Reaction between Aromatic Aldehydes and o-Dianisidine Mayda Lopez-Nieves,' Peter D. Wentzell,2 and S . R. Crouch* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
Kinetlcs studles were performed to invegtlgate the of Schiff base formation In the reaction between aromatic aldehydes and odianiskline. The studies were conducted at 40 O C with ethanol as solvent and acetic acid and stannic chloride as catalysts. The evidence supports a threepath mechanism. Two paths are catalyzed by acetic add and the other by stannic chloride. I n the acetic acid catalyzed pathways, It is proposed that the first step involves the reaction of the aldehyde wHh a solvated proton (Le., specific acid catalysis). DepenUlng on the ackllty of the medium, free or monoprotonated o-dianlsidlne attacks the carbonyl carbon to form a carbindamlne Intermediate whkh then dehydrates to form the product. I n the stannic chloride catalyzed path, It is proposed that the aldehyde Is first complexed to stannic chloride. The complexed aldehyde is then attacked by a 2:l o -dianisidine-stannic chloride complex to form a carbinolamine which continues to react to form the product. The rate law derlved from the proposed mechanism Is in agreement with the experimental data and observations. Analytical Implications of the kinetics studies are discussed.
of amines to aldehydes is proposed to occur by forming a carbinolamine intermediate which subsequently dehydrates to form an imine. The initial attack of the amine a t the carbonyl carbon has been reported to occur via specific or general acid catalysis depending on the basicity of the amine and the strength of the acid catalyst (14, 15). The kinetics studies reported here of the reaction of o-dianisidine with a representative aldehyde, 2,4-dichlorobenzaldehyde,in ethanolic media are interpreted in terms of a mechanism that is similar to those previously reported for aqueous reactions. Analytical implications of the results are discussed. PRELIMINARY CONSIDERATIONS The reaction of aromatic aldehydes with o-dianisidine can be described by the simplified equilibrium shown below 0
II
o -dianisidine
aldehyde
OCH,
INTRODUCTION There is currently a good deal of interest in the automated determination of aromatic aldehydes. It is important to determine trace levels of aldehydes in the air ( I ) , in water ( Z ) , in foods (3), and in many other samples (4). The most widely used instrumental methods for determining aromatic aldehydes involve UV-visible absorption spectrophotometry (4-6) and molecular fluorescence spectrometry (7). Unfortunately, many of these methods are subject to interferences, especially from ketones. Furthermore, many of the procedures are difficult to automate because they require somewhat extreme conditions, such as mixing with concentrated acids, high temperatures, or long reaction times ( 4 , 7). One potential reaction that has not been widely studied is the Schiff base formation reaction of o-dianisidine with aromatic aldehydes. This reaction was originally used as a spot test by Feigl (8) and has been reported by Wasicky and Frehden (9) to be sensitive and selective for aromatic aldehydes. However, the reaction has not been extensively applied, probably because of the reported instability of the colored product (10)and the positive, although lower, response of some ketones (11). Because of the potential analytical applications of the 0dianisidine reaction with aldehydes, we have undertaken a kinetics and mechanistic study. Addition reactions of primary amines to carbonyl groups (in water as solvent) have been studied extensively (12-15). In water as solvent, the addition
* Author to whom correspondence should be addressed.
Present address: University of Puerto Rico, Aguadilla Regional College, P.O. Box 160, Ramey Station, Aguadilla, PR 00604. *Present address: De artment of Chemistry, Dalhousie University, Halifax, NS, Cana& B3H 453.
1:l adduct
Preliminary tests indicated that ethanol was an excellent solvent because the equilibrium lies to the right and most aldehydes are soluble in ethanol. Stannic chloride and acetic acid were employed as catalysts. When they were used simultaneously, there was an interaction that led to a higher and more rapid response than obtained with either catalyst alone. A small amount of product decomposition was observed at high stannic chloride concentrations. For the kinetics studies, concentrations were chosen so as to obtain reasonable rates with minimal product decomposition. For solutions containing acetic acid, the solvated proton concentration, EtOH2+ here abbreviated as simply H+, was calculated from eq 2 which assumes that the dissociation of acetic acid in ethanol (K, = 5.92 X lo-" at 25 "C) is very small
where CHoACis the analytical concentration of acetic acid. Preliminary experiments were also performed to ascertain the conditions under which the 1:l adduct shown in eq 1was formed preferentially over a 2:1 aldehyde to o-dianisidine adduct. A 10-fold molar excess of o-dianisidine to aldehyde was found necessary to ensure formation of only the 1:l adduct. Hence, in the kinetics studies, the molar ratio of odianisidine to aldehyde was never lower than 10. EXPERIMENTAL SECTION Reagents. Commercial absolute ethanol was dried with 3-A molecular sieves (Davison Chemical, Baltimore, MD). After being decanted, it was filtered through a 0.40 pm pore filter (Millipore Corp., Bedford, MA).
0003-2700/90/0362-0304$02.50/00 1990 American Chemical Society
PrNALYTICAL
The o-dianisidine was recrystallized4 to 5 times from ethanol by using 50-200mesh activated coconut charcoal as a decolorizing agent (Fisher Scientific,Springfield, NJ). The final product was vacuum dried and stored in a desiccator in the dark. The net yield was approximately 38% of white to off-white crystals. These are stable for more than 6 months if all solvent is removed. Solutions of o-dianisidine must be prepared fresh daily. The 2,4-dichlorobenzaldehydewas of commercial origin. It was recrystallized from ethanol and then vacuum dried. NMR spectra and melting points were used to check identity and purity. Procedure. For the kinetics studies,the method of initial rates was employed in order to avoid complications from product decomposition at high catalyst concentrations. Absorbance (380 nm) versus time data were acquired with an IBM compatible personal computer (Bentley Model T, Round Rock, TX) interfaced to a thermostated, modular single-beam spectrophotometer (CGA McPherson, Acton, MA). Triplicate runs were made at each concentration. After data acquisition, initial rates were calculated by dividing the initial slope of the absorbance versus time plot by the molar absorptivity of the product (for the 1:l adduct of o-dianisidine and 2,4-dichlorobenzaldehydec = 1.34 X 104 L mol-' cm-l). Preliminary reaction orders were determined from slopes of log (initial rate) versus log (concentration) plots. The conM and that centration of o-dianisidine was fixed at 6.31 X M, unless otherwise of 2,4-dichlorobenzaldehydeat 6.00 x noted (e.g., when determining the order with respect to either of these reagents). Because of the difficulty in estimating ionic strength in ethanolic solutions, the influence of ionic strength was not investigated. For the kinetics experiments, the desired volume of o-dianisidine was first added into a 1.0 cm path length quartz cuvette with a micropipet. The aldehyde solution was then added and the solution mixed manually. The cuvette was placed in the spectrophotometer and allowed to equilibrate thermally for 5.0 min. The catalyst solution (0.50 mL) was then added with a syringe. Reaction temperature was maintained at 40.0 0.2 "C by circulating water from a constant-temperature bath around the spectrophotometer cuvette. All solutions were maintained at 40 O C for at least 15 min prior to use.
*
RESULTS Characterization of the Product. The molar ratio method of Yoe and Jones (16) suggested that both a 1:l aldehydeo-dianisidine adduct and a 2:l product could be formed. These studies indicated that the 1:l adduct forms exclusively a t low molar ratios of aldehyde-o-dianisidine (
