Eric Dammer Dr. Jose Barreto Organic Chemistry 2 July 12, 2001 Product determination of azo dye synthesis through qualitative inclusion as an application of molecular orbital theory regarding solvatochromatic effects leads to theoretical speculation on the possible calculation of absorbance as an environmentally dependent colligative property of such dyes. Abstract Azobenzene dyes are synthesized by coupling phenyldiazonium species to other substituted benzenes in vitro at the millilit er scale. First, thermal environment of the synthesis and later, branching of the product azo dyes are varied across different experimental groups. Branching is varied through the selection of alternate substituted benzene substrates in the later synthesis process. At this point, more than one such reagent is introduced to yield a set of one to potentiall y numerous products with the intention of determining these products after the fact. A probable array of products from predictive synthesis schemes is narrowed down and verified by qualitatively matching physical properties measured in the lab with theoretical predictions. This retroactive determination of product(s) is a puzzle pieced together primaril y in the discussion section with regard to solubilit y, reactivity, and absorptivity in the visible spectrum.

High water solubilit y and reactivity of the potential substituents of the dye products provide some evidence for a narrow range of particular products, while absorbance data is explained with qualitative reference to molecular orbital theory. This nicely explains the solvatochromatic nature of the dyes synthesized by this group. Quantitative application of these concepts is referenced in recent papers1,2 that directly calculate absorbance peaks of substituted azo dyes using computer-modeling software (Dalton), which employs refinements of second-order molecular perturbation theory, specificall y second-order polarization propagator approximation (SOPPA). These calculations in particular are beyond the scope of this research, but could possibly be used to further quali fy the products predicted in discussion here.

Notwithstanding, what is especiall y interesting about this implementation of the refined MP2 model is that it has been applied to indicate ideal wavelengths for excitation of diazo-coupled benzenes to induce maximum E-Z isomerization in the presence of polarized monochromatic light (laser diodes in CD-writers, e.g.), effectively predicting what wavelength or energy of light best alters the common planar geometry of azo-coupled benzene rings to achieve these transition states. While transition and solvent effects definiti vely cause shifted spectral properties, a dependence of the λmax calculation on known molecular geometry may allow for a computational framework in which systematic seeding of bond angles in the predicted (or initiall y ideally known) dye structure leads to a computed λmax that aligns with the spectral intensity and energy measurements of the dye in any particular (solvent) environment. If parameters discarding less li kely equivalent geometries can be specified, the program may lead to a mathematical “black box” system that would be capable of uncovering geometric variations of such a dye due to solvents already indexed in conjunction with empirical data. Likewise, geometric conformation due to a mixture of interacting dye molecules, or as a generali zed refinement to theory might effectively provide a mathematical approach towards more accurately defining and predicting colligative spectral properties of a single pure dye. It is not clear if such a system would duplicate or fall short of an existing one—for example, density functional theory. At least for this reason, this side note is not the focus of the presentation of data and discussion that follows.

1 Roos, Björn O. Theoretical Studies of ElectronicallyExcited States of Molecular Systems Using Multiconfigurational Perturbation Theory. American Chemical Society Website. Nov. 13 1998. http://pubs.acs.org/isubscribe/journals/achre4/jtext.cgi?achre4/32/i02/html/ar960091y.html; see also Acc. Chem. Res., 32 (2), 137-144, 1999. 2 Per-Olof Åstrand, P.S. Ramanujam, S ren Hvilsted, Keld L. Bak, Stephan P.A. Sauer. Ab Initio Calculation of the Electronic Spectrum of Azobenzene Dyes and Its Impact on the Design of Optical Data Storage Materials. American Chemical Society Website. Mar. 25, 2000. http://pubs.acs.org/CHECKCCIPP-994731372/isubscribe/journals/jacsat/jtext.cgi?jacsat/122/i14/html/ja993154r.html#ja993154rb00017; see also Jrnl ACS, 122 (14), 3482-3487, 2000.

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Data The procedure to synthesize and analyze dyes was carried out over multiple days by various groups. In an initial trial synthesis, all groups tested identical reagents in controlled concentration while temperature of the reaction was experimental. In summary, the procedure required a test tube with 2 ml deionized water, 100 µl 6M sulfuric acid (6 x 10-4 mol), and 100 µl of a saturated, low concentration, aqueous anili ne solution at 25° C. After addition to acid, anili ne solubili ty temporarily would have increased as the amino group became protonated. The increase, however, would be less dramatic than for a non-aromatic amine, because the charged nitrogen would be partially neutralized through inductive withdrawal of pi electrons from the benzene ring. In the other tube involved in this procedure, 40 mg sodium nitrite (MW=69, 5.8 x 10-4 mol) were added to 2 ml deionized water. The contents in turn were added to the acid-adjusted tube, which was chill ed in an ice-water bath and remained chill ed until this and other groups had a thermal environment prepared. Nonetheless, the reactions were all initiated in the 0° C bath as salicylic acid was added to the reaction tube. This initiated the following reactions (figure 1), which were drawn forward to completion by the formation of phenyldiazonium ion, capable of irreversible decomposition into nitrogen gas and benzene, only. This decomposition was observed as slow but consistent bubbling when the acidic solution was removed from the bath. Thus, somewhat less than 0.0006 mol of phenyldiazonium could have been formed, while it is certain that some unsubstituted benzene was present after this bubbling.

O-

ON

H+

O ON

HO

N+

N

H

H H

HH

ONH 3

+H

H H

HH

Figure 1: Phenyldiazonium formation. Some diazonium formation may have continued after excess salicylic acid was added, but probably ceased as the temperature was allowed to increase, in this group, to a range of 45-50° C. At this point, rate of formation of nitrogen as the most kinetically stable product must have jumped, and exceeded the thermodynamic impetus of higher reaction temperature against exothermic reaction, i.e., exergonic reaction was not slowed sufficiently by a Le Châtelier equili brium shift at 45-50° C, based on the observed consistent rate of bubbling at that time. If temperature and perhaps pressure were increased arbitrarily high, decomposition of this coupling agent could be avoided and dye yields improved. All groups did get results of an orange dye in a range from zero to 50 degrees, so it would realistically be a high temperature and pressure like that used in the Haber process that would also improve yield in this synthesis. Regardless, at the point salicylic acid was added, electrophili c aryl substitution of hydrogen on its activated carbons by phenyldiazonium ion would have been underway. Kinetics in the still -acidic tube resulted in a yellowing solution. As bubbling continued, salicylic acid tended to float and mix above the yellow solution, forming an orange-brown solid. Later, some to most of this solid was dissolved through vigorous agitation. At this time, a total of 1.2 ml of 1M sodium hydroxide was added in 100 µl increments. The solution became wispy or cloudy after 0.5 ml were added while the solution incrementally visibly shifted from yellow to orange in increments likewise. The solution

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cleared only between 1.1-1.2 ml (~0.00115 mol* ). The solution at this time was a tangerine color, which it remained for the next and last hour of observation. Groups generally obtained similar colors ranging from yellow to red. Interestingly, while adjusting pH, after addition of 0.0008 mol hydroxide, red was seen at the interface of cloudy solution and brownish effluent. Some of this brown remained above, while the red dissipated in the case of this group. Perhaps a particular hydroxide addition rate or other adjusted conditions short of protecting groups on the substrate could have encouraged an ortho- (sterically hindered) isomer of the resulting dye, which would be considered red (less dominant in our case).

Upon examination of the experimental evidence, though, it is more likely that another substrate coupled to give the second color component of the solution. In this line of reasoning, it seems very plausible that temperature altered the amount of unsubstituted benzene produced as a product of diazonium decomposition. Though there was salicylic acid present in excess—in terms of solubili ty, at least—and salicylic acid is the more activated substrate by virtue of its hydroxy group, it appears that there is a possibili ty at a particular pH range for increased rate of the diazonium-benzene coupling to occur. *0.0008 mol hydroxide ion is able to only partially neutralize 0.0006 mol of the diprotic sulfuric acid, leaving 0.0004 mol proton concentration due to the strong acid, i.e. the solution effectively was approaching 3.4 pH. At this pH, it would seem that two things were likely happening: (1) salicylic acid would have been near equili brium between its protonated and deprotonated form (at or near pKa of this particular supply of acid as observed in a later lab), and (2) solubili ty of salicylic acid would have increased from this point onward as pH increased and salicylic ion would be forming, albeit not necessarily by leaps and bounds. This is at least some empirical evidence that at the pH during which the red was observed in vitro, the conditions were as good as they would be for formation of the dye with benzene substrate. Perhaps, had there only been benzene and diazonium coupler in the solution, a purely monochromatic dye could have formed at the higher pH, but it is clear from the comparative reactivity of salicylic acid that whatever successfully dissolved would preferentially couple with diazonium until completion of the reaction.

Now the question as to which dye is red and which is yellow remains. Observations and the preceding argument would lean towards azobenzene, the benzene-anili ne-coupled dye, being red. Molecular orbital theory supports this claim: this simplest of two azo dyes, having less conjugation, has a greater difference between π and π* . This difference is the excitation energy, which would be less for the salicylic-substrate dye that has conjugation of four, instead of three bonds—thus stabili zing the molecule electronically, while respectively shifting absorbance to prefer smaller, or redder, energy quanta. So, it becomes apparent that an earlier assumption is incorrect. Excess salicylic acid is excess not because of its low solubili ty, but because benzene is a preferred substrate in solutions with pH <3.4. Thus, carboxyl group on an ortho- carbon with respect to a hydroxy group overall is a deactivating arrangement on a substituted benzene substrate in protonated solvent. Either as the hypothesized benzene is consumed or as pH increases does it only become more likely that coupling with salicylic acid occurs, based on the foregoing logic. The result is an incrementally redder solution, leading to an orange mixture of product dyes; this later-formed component must therefore be deprotonated red 1-hydroxy-5-[(E)-(phenyl)-azo]-benzoic acid.

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The brown solid, on the other hand, is deduced to be an anhydrous version of this same dye. The color brown is considerably more complex than monochromatic red in terms of an explanation with MO theory. One explanation of a polychromatic multiplier effect that skirts calculation is that each molecule of solid is being differently effected by varying and somewhat random intermolecular electrostatic interactions, likely in a molecule with both electron -withdrawing and -donating groups arranged without opposing symmetry. The randomized shifts of non-bonded electron clouds on the hydroxy group, for example, would lead to a wide swath of equally intense absorption. Such a multiplied absorption peak would generally distribute the area under the absorbance curve so that intensity would decrease, or perhaps it could enclose a wider area including the non-colli gative λmax while there would be less peak definition and lower intensity. Different interactions of the dye with water apparently are less random because of solvent effects, but this underscores the fact that it is unclear whether an absorption maximum measured of a dye in polar solvent, especially, would coincide with a theoretical peak of the dye, disregarding any colli gative properties, i.e. physical properties arising from same-molecule intermolecular interactions, in solution or solid mass. One thing is certain: chemists do not agree on the preferred geometry of trans-azo benzene, though over the years, it has been shown that “planarity is probably sensitive to the surroundings of the molecule.2” A SOPPA calculation in table two of this reference for π to π* excitation is 3.71 eV, or 5.94 x 10-19 J (1 eV=1.602x10-19 J), which is just slightly higher than 500 nm in wavelength of maximum absorbance. This indicates theoretical color prediction of the simple azobenzene dye centrally in the red, according to Bruice, p 520. This is additional evidence for the findings of relative reactivity and formation of both dye products.

In the second trial of the lab, the same procedure was followed, though no

additional heat was added to the reaction tube, which was allowed to complete reaction approaching room temperature. Para-aminophenol replaced the anili ne that was activated to phenyldiazonium, thus leading to para-hydroxydiazonium coupler. Diazonium decomposition in this case would therefore have yielded some phenol that would have been an ideally activated substrate for para- addition in the reaction. Other substrates used are among the reagent molecules shown and named in the following lineup (figure 2).

ON

+N

H

H H

HH

H

O

Cl

HH

H

HH

H

HO

O

N

H

HH

H

H

H

H

H

H

O

HH

H

H

H

H

HH

HH

H

1) para-hydroxydiazonium 2) 4-chloro-3-methylphenol 3) 4-acetamidophenol 4) 2,4,6-trimethylphenol (<0.1092 g) (0.0900 g) (** << 0.1010g) (0.102g) MW 121.117 MW 142.583 MW 151.163 MW 136.191 0.009016 mol 0.006312 mol << 0.006682 mol 0.007431 mol

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Figure 2: Reagent molecules in second trial of azo synthesis. (Preferred substitution-directed hydrogens in blue, with disregard for steric hindrance in the amide, #3.)

There is reason to believe that the amide would have undergone hydrolysis in the presence of sulfuric acid. The end result, surprisingly, is formation of #1, and acetic acid:

H

O

O

N

H

H

H

H

H

H

HH

OH

O

O

S OH

NH3+

H OH

H

H

H+

OO

H

HHH

O

N+

N

H

H

HH

H

O

N

H

HH

H

OH

H

HH

H

Figure 3: Formation of additional para-aminophenol coupling candidate through hydrolysis of 4-acetamidophenol. Tautomerization to ethanimidic acid through proton exchange is shown.

It is not clear how much additional coupling agent would have formed in the time

prior to coupling. It is tempting to say that the strongly acidic conditions would have driven the reaction nearly to completion; however, the hydrolysis kinetics may have been pushed to a temperature range above 0-25°, as the (E)-N-(4-hydroxyphenyl)ethanimidic acid tautomer shown above would have increased thermodynamic stabili ty arising from the additional conjugation with aryl pi bonds.

After or while stabili zation of the reagent molecules in acidic solution was taking place, coupling was consuming these, likely at differing rates. This is primarily due to reactivity enhanced to varying degrees by activating substituents in all the reagents shown in figure 2, as well as para-hydroxydiazonium in figure 3.

O

N

N

O

ClH

HH

H

HH

H

HH

HH

(2-1) 5-chloro-2-hydroxy-4-[(E)-(4-hydroxyphenyl)-azo]-toluene

ON

N

H

H H

HH

O

O

N

H

H

H

H

H

H

HH

(3-1) N-(4-hydroxy-2-[(E)-(4-hydroxyphenyl)diazenyl]phenyl)acetamide

Dammer, 6

O

N

N

H

HH

HH

O

HH

H

H

H

H

HH

HH

H

(4-1) 2,4,6-trimethyl-3-[(E)-(4-hydroxyphenyl)-azo]-phenol

O

NN

O

Cl

N

ON

H

H

H

H

HH

H

H

H

H

H

H H

HH

H

O

NN

O

Cl

H

HH

H

HH

H

H

H

H

(2-1-1) 5-chloro-2-3’ -4’’ -trihydroxy-4-methyl meta-diazobenzene

ON

N

H

H H

HH

O

O

N

H

H

H

HH

H

HH

O

NN

HH

H

H

H

O

O

N

H

HH

HH

H

H

O

N

N

H

H

H

H

H

(1-3-1) 5’ -acetamido-4-2’ -4’’ -trihydroxy para-diazobenzene

O

N

N

H

HH

HH

O

HH

H

H

H

H

HH

HH

H

O

N

N

H

H

H

H

H

ON

N H

H H

H

O

HH

H

H

H

H

HH

HH

H

(4-1-1) 3-4’ -4’’ -trihydroxy-2-4-6-trimethyl meta-diazobenzene

Dammer, 7

All molecules shown involve coupling with excess para-hydroxydiazonium coupler (in figures 2(1), and 3) would have consumed this coupler at varying rates due to steric hindrance. With consideration of this effect, it becomes obvious that the para-aminophenol monomer may very well have self-coupled, because it has the least bulky and/or fewest substituents of all candidate reactants. It should be noted that the decomposition product phenol show in figure 2 would likely have been completely consumed by the coupler for the same reason and incorporated as a random copolymer into a totally conjugated network of azo-coupled rings. General patterns of dually coupled aryl groups would suggest a preference for any additional coupling in a pattern of arcing linear linkage at lower temperatures, and a zig-zag pattern allowed for by more active kinetics, capable of overcoming steric effects that hinder ortho- coupling to an already coupled hydroxyazobenzene.

N

H O

H

HH

N

N

O

H H

N

HH

H O

H

HN

O

H

N

HH

H

H

H

H

N

O

H

HN

H

H H

N

O

H

H

N

H

H

N

O

H

H

N

HH

O

HH

H

H

H

Figure 4: Pure polymerization geometries likely part of an actual hybrid in self-coupled para-aminophenol networks. The zig-zag arrangement on left would have increased network characteristics, and been responsible for areas of greater density, as additional coupling ortho- to hydroxy groups would then be sterically favored. Original locations of diazonium groups that have left are shown in magenta. Polymerization may have terminated as diazonium groups on the monomer could more readily decompose into nitrogen as conjugation of the molecule(s) increased, rendering stabilization due to additional conjugation with the diazonium group less consequential overall. In basic conditions, the result may have been a resonance-stabilized positive charge on the terminal monomer ring. This, in turn, would have enhanced the random intermolecular electrostatic interactions affecting overall structural geometry, and hence greatly randomized visible wavelength excitation energy across the entire visible spectrum, resulting in a black solid.

Note that phenol, as a minor copolymer in this network, would have resulted in meta- diazo- coupled aryl groups in the above-proposed networks, as in the arc pattern; neighboring aminophenol substituents coupling afterwards would have inverted the growth pattern at these sites. There is some evidence for such a network forming in that solids the solids obtained were not particularly soluble in water and not the least bit soluble in hexane. One inference is that the solid is likely a macromolecule, network, or other assembly of strongly interacting products. For the reasons mentioned above, colligative properties of this solid, particularly its indeterminate dark green or black

Dammer, 8

color, require considerable theoretical explanation beyond the scope of this research. One general explanation for the broad and near-total visible absorption of the black solid is given in the caption of figure 4.

If additional experiments were carried out, it would be very interesting to see the effects of increased temperature and phenol concentration. It is possible the amount of network character and density of the compound could be altered by these means. If so, the reasoning for primary formation of such a product in this synthesis could be reinforced.

The coupling of para-aminophenol with itself and phenol would have to approach a second order rate, and so proceed to form a network more rapidly than coupling with uniquely substituted aryl reagents. But, there is evidence that one or two of the two- and three- ring coupled products shown in the figures on pages 5 and 6 formed. The solution color and measured absorbance spectra in various solvents indicate that there were two distinct absorbance peaks in the filtered aqueous solution, where the synthesis was carried out: one at 450-455 nm (yellow to slightly orange), and another at 630 nm (blue). Assuming separate molecules contributing to this spectrum is justified because all possibilities for coupling shown lead to increased conjugation, not localized areas of conjugation which could give a larger molecule with distinctly separate areas of conjugation two unique absorption peaks.

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

400 450 500 550 600 650

λ λ (nm)

Ab

sorb

ance

(ar

bit

rary

un

its)

WaterAcetoneIsopropylEther

Graph 1: Solvatochromatic Shifts of Synthesized Dye(s) in Varying Solvents

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Confirmation of distinct products was not achieved in attempts at thin layer chromatography on sili ca gel (polar) or paper (less polar) plates; however, informal separation of blue from a yellow green was found accidentally on perforated paper towels. This indicates (1) that the blue unknown was a unique molecule, and (2) it interacts more strongly with some separation environments of less polarity. It is tempting to say that the remaining dye or dyes were yellow, but no pure yellow color was isolated throughout the analysis of the product solutions. If there was a yellow product, it may have strongly interacted with the blue dye such that TLC was difficult in the plethora of running solvent-plate combinations tried: sili ca-isopropanol, sili ca-ethyl acetate, sili ca-toluene/methylene chloride, sili ca-methylene chloride/isopropanol, sili ca-tertbutyl methyl ether/isopropanol/ethyl acetate; paper-water, paper-isopropanol, and paper-ethyl acetate. In the last environment mentioned, a 15-minute run across a paper plate resulted in a yellow penumbra below a red-brown streak (Note, the colors changed in the running solvents, and remained so after drying).

The fastness of shifted color in the ester solvent is as interesting as the slight separation finally achieved there. Small esters are more polar in comparison to most of the other aprotic solvents tried, perhaps they interacted strongly enough with the dye to remain in the deposits due to intermolecular electrostatic and/or geometric compatibili ty, as water hydrates many ionic crystals. An ester scent remained detectable on this plate. This also explains why the dyes were not particularly amphipathic—solvent effects in a polar solvent were required to carry the dye. Because there is an unusually high correlation of running rates in most every running solvent, this may be some evidence for the idea that terminal coupling to a free diazonium monomer by an existing diazonium ion as explained in figure 4 may have resulted in an aromatic-stabili zed charge that might not have been able to find a proton in the basic solution that resulted after the procedural addition of excess hydroxide (1.5 ml or 0.0015 mol in this case). There is only one pattern of coupling among any of the potential reagents that results in diazonium groups available for decomposition, abstracting aryl bonding electrons and effectively leaving a charge on the terminal phenol, which remains stabili zed through conjugation with to all other coupled aryl groups in the molecule. This positive charge could have been distributed through the conjugated bond network of any particular size and deposited on atoms most suited to carry the charge, when influenced by the presence of anions in solution (acetate in particular, shown protonated in figure 3).

Discussion

At this point, speculative arguments on structure and solvent-dye interaction are being built upon prior speculation as to the formation of acetate and accounting of hydroxide with only one significant figure. Nonetheless, it is very interesting to speculate. Perhaps molecules of dye primarily made up of para-aminophenol and phenol with a total of 3, 4, or maybe more aryl groups intermittently remained intact. At this point, there would presumably be acetate ions, interacting with the partial charge on the dye, perhaps in charge transfer excitation like that found in chelated complexes. It seems more likely that such initial interaction would result in a stable bond, perhaps with some partial or, more easily explained, ionic character. Electronegativity of oxygen and nitrogen make the phenol hydroxy and diazo nitrogen atoms the most likely candidates

Dammer, 10

for localization of the formal positive charge, balanced in vitro by strong (ionic) interaction with acetate ions.

The variable mobili ty of the presumed closely related family of diazo-phenol networked dyes in the ester may have been due to exchange of acetate groups at varying rates, depending on variable atoms in the dye molecule responsible for ionic bonding with acetate. Those bonded at greater distance from the formally charged aryl carbon would probably have rearranged in the ester to form a bond closer to the formal charge source. The yellow dye perhaps is due to electron withdrawal which favored more separation between formal positive charge and the preferred ionic bonding site, presumably due to a variable branched group (#2 or #4 in figure 2) on the far end of the thermodynamically preferred multi-ring chain. Continuing this extrapolation to a logical conclusion, one of the variations must have a significantly higher excitation energy, or acetate ion-exchange activation energy, due to the difference in molecular orbital geometry thus effected by solvent with a sufficient dielectric constant; opposing this effect, perhaps, is solvent effect as gauged by comparison of dielectric constants (table 2)—the more planar, hence less polar conformation of any diazo structure is more stable, other things equal2. The direct ionic or electrostatic interaction between aryl carbon and any polar or charged species would probably account for this lower energy (blue) dye.

λ Water Acetone Isopropanol Ether

Original 400 nm 0.835 0.424 0.618 1.440

1 Week Later

400 nm 0.782 0.388 0.576 1.720

Adj. Factor 0.936527 0.915094 0.932039 1.194444 Table 1: Product dye absorbance shifts over time.

Solvent Water Acetone Isopropanol Ether

Dielectric k 79 21 25>k>11 4.3>k Table 2: Dielectric constant ranges of solvents used. (Bruice 385).

Loss of color over a week in the protic solvents justifies a slow kinetic

progression towards equili brium with a less intensely colored, or uncolored, product (table 1). One reason may be that extensively conjugated systems have a low pKa, and are thus relatively stable compared to aliphatic alkoxides, especially considering a high pH in the hydroxide-adjusted synthesis product solution (Bruice 646). It seems that protonation of some of the yellow dye in isopropyl alcohol, water, and acetone took place slowly, but no water, not even ambient humidity would have entered the immiscible, non-polar ether phase (which, inversely increased dye concentration through high solvent vapor pressure).

Canceling the charge of the molecule may have decreased intensity not only by eliminating the absorbance of ion-exchange excitation energy (if this is even possible), but also potentially through formation of a decoupled product, which makes sense in that nitrogen would be most able to hold onto electrons after hydrolysis and protonation to give an amine, before protonation of the formally positive aryl carbon. In relatively basic solution, the nonbonding electrons on the amine product would also have increased contributions to lambda max in the UV. This decrease in conjugation is expected to shift

Dammer, 11

Addition of a drop of the dye to the acetone solvent resulted in a blue shift for the first 1-2 minutes, followed by an almost complete loss of color, with a faintly detectible pink or grey tint. In the search for an acid-catalyzed mechanism, it seems that this combination may have resulted in availability of one proton from the equilibrium product of equivalents of acetone and dye, while the azo nitrogen-nitrogen bond in this case may have remained intact. The proposed mechanism is formation of an imide with the carbonyl carbon of acetone, followed by a withdrawal of charge into the oxygen of phenol. The likelihood of such a shift is not known, but the mechanism fits with projections for loss of color in acid, while

the pKa would only have to be lowered enough to donate the exchangeable proton to excess hydroxide carried with the dye into acetone (figure 5). The structure in the center would likely be an equilibrium product. Apparently, a similar (but somewhat slower)

Figure 5: Yellow solvent-dye interaction with acetone. Note, this simplest product does not have a charge because diazonium decomposition yielding phenol occurred in acidic conditions.

λmax to higher energy, i.e. greater difference between pi and excited pi transition states, precisely because there is less stabilizing conjugation. Because color decreased overall, this shift would have reached into the UV, which is at least characteristic of a single ring dye without azo chromophore. So, some of the visible dye was apparently made up of two azo-coupled rings. Perhaps this minor synthesis product is, by virtue of minimal steric hindrance purely a product of coupling between para-aminophenyldiazonium and phenol. It would likely be the yellow product due to less extensive conjugation (higher energy λmax). An estimation of the decomposition kinetics of this hypothetical product, (E)-azodiphenol could be gauged more accurately by the rate of subsidence of the 450 nm peak of this dye. Confirmation of the mechanism implied would be loss of color in an acidic solution.

N

O

H

H

N-

H

H

H

H

O+

HH OH

H

H

HH H

H

O-

H

H

H

H

H

H

N O

HH

N

HH

H

OH

H

HH

H+

H

H

H

H

H

N O

HH

N

HH

H

OH

H

HH

H O-

disconjugation of the more stable (blue-lower energy absorption) system of 3 or more rings also underwent this interaction with the solvent within the first minutes after addition, and before absorbance spectra were measured.

Apparently, in polar solvents like the basic aqueous solution that was the experimental synthetic environment and also the much more dilute (neutral) solution prepared for spectral analysis, there is no mechanism that eliminates an additional hydrogen atom to yield product with an additional pi bond on the aryl carbon missing a substituent, though it would fit with expectations. Again, as in the argument against abstraction of hydrogen, pKa of the resonance-stabilized cation is such that it does not eliminate hydrogen, either. In the ether, however, it would make sense that the unusually

Dammer, 12

strong and consistent slope upwards towards the UV energies is evidence for such a neutralizing elimination. The effect of this proton released into the ether may have also been to increase solubili ty of the reagent, charged dye, which might otherwise have been effectively insoluble in the ether—the solvent with lowest dielectric constant apparently caused the greatest pKa shift of the proposed diazonium-coupled cation. This fits with the observation that the dye only dissolved partially in the ether after agitation; filtering of a dark solid which congealed in the ether was necessary before absorption could be measured initially.

The explanation of the unusual absorption for ether is that one peak became converted into a flattening of slope in the 450 nm region, while the blue peak seen in water at 630 nm is absent altogether, indicating that this particular dye may not have been as reactive, which still fits with greater conjugation. The gross intensity of the UV peak that is implied must then be a result of a new difference in pi to pi-star transition, apparently brought about by an increase in the electron density of the entire conjugation network through the triple bond.

References

Bruice, Paula Yurkanis. Organic Chemistry. 3rd ed. Prentice Hall: Upper Saddle River, 2001.