?;. SAQVI, E. L. AMMA,Q. FERTASDO AYD
218
R.LEVISE
T'ol. 65
ULTRAVIOLET SPECTRA AND ACID DISSOCIATIOX CONSTL4n'TS OF SOME PYRAZYLMETHYL KETONES BY
yz4SEEM NAQVI,'E.
L. AMMA,QUIETUSFERSU'ANDO~ B S D ROBERT LEVTNE
Contribution No. 1074 f r o m the Departmmt of Chemistry, University of Pittsburgh, Pittsburgh I S , PPnnsijloania Received June 10,1960
The ultraviolet spectra of phenyl pyrazylmethyl ketone (phcnacylpyrazine), and the three isomeric pyridyl pyrazylmethyl ketones have been determined in aqueous solutions of varying p H and in methanol. Empirical assignments of electronic transitions have been made and used to determine qualitatively the position of the keto-enol equilibrium in each of these compounds. The acid dissociation constants of these compounds have been determined spectrophotometrically and potentiometrically. The concentrations of the keto and enol tautomers of the 2, 3 and 4-pyridyl derivatives in aqueous solutions, in the p H range 1-12, have been calculated. The rate constants for keto and enol formation in neutral aqueous solutions have been determined for the three isomeric pyridyl compounds.
I n earlier work carried out in this Laboratory a series of ketones containing t,he pyrazylmethyl group was ~ r e p a r e d . ~Since t,hese ketones are st,ructurally analogous to P-diketones, it is not t,oo surprising that they form chelates with copper(I1). It can be seen from structures I and I1 t,hat a pdiketone I differs from a pyrazylmethyl ketone 11, in that a carbonyl group in t.he former class of compounds is replaced by an azomethine function in the latter. I n structure I1 where R is the 2-
and enol concentrations with solvent for each compound can be ascertained by a study of the ultraviolet spectra. Experimental
Preparation and Purification of Compounds .-A detailed description of the preparation of the compounds used in this work has been p ~ b l i s h e d . ~The pure enol form of phenacylpyrazine was prepared by dissolving the substance in 20% aqueous sodium hydroxide and neutralizing rapidly with hydrochloric acid to a p H of 7 . The freshly precipitated compound was dissolved in benzene, and the solution shaken with Florisil, filtered and cooled. The product thus obO H 0 tained had a melting point of 82-83'. R-)J-b--C-R/ I1 The 2-pyridyl and 3-pyridyl pyrazylmethyl ketones were recrystallized from a mixture of ether and petroleum ether I and had melting points 87-88" and 129-130°, respectively. H The 4-pyridyl compound was recrystallized from a mixture I of benzeFe and petroleum ether and had a melting point of 142-143 . A sample of methylpyrazine was kindly supplied by Wyandotte Chemicals Corporation, Wyandotte, Michigan. R = C6H6; -2, -3 and 4-CsH4N / " " H i Spectrograde solvents were used in obtaining the ultra\N/-y-C-R violet spectra. All other compounds used in this work were of reagent grade purity. H Ultraviolet Spectra.-The ultraviolet spectra of all soluI1 tions were recorded with a Cary Model 14 spectrophotometer pyridyl radical, it is possible for chelat,ion with a a t 25 & l o , using a pair of stoppered 1 em. silica cells. metal ion to take place between eit'her the pyridine Measurements were made on solutions ranging in concenfrom 3 X 10-6 to 5 X 10" M . Standard solutions nitrogen or a pyrazine nitrogen and t,he enolic tration of HC10, were used for solutions of pH less than 2 and carhydroxyl group. Two factors mould appear to bonate-free sodium hydroxide for solutions of pH greater influence whether the pyridine nitrogen or the than 11. In the intermediate range of p H , buffer solutions pyrazine nitrogen is involved in chelate format,ion. of constant ionic strength 0.1 were used. The buffer comwere CHICOOH and CH&OO?r'a, Na2HP04 and These are (1) a steric factor and (2) the basicities ponents KH2P04, H3B0, and NaOH, together with appropriate of the nitrogen atoms in t'he pyridine and pyrazine quantities of KC1. rings. It, was t,herefore of interest to determine the The pH values of the aqueous solutions were measured acid dissociation constants as well as t'he ultraviolet with a Beckman Model G pH meter equipped with an external calomel electrode pair and calibrated with spectra of the pyrazylmet'hyl ket'ones. I n t'his glass-saturated standard buffer solutions a t pH 4.00 and 7.00. work the various bands in the ult,raviolet spectra of Potentiometric Titrations.--A weighed quantity of the these compounds have been assigned empirically pyrazylmethyl ketone was transferred to a n-ater-jacketed to the appropriate molecular species present in vessel thermostated a t 25.0", and dissolved in a measured volume of standard HClO,. The resulting solution was solut,ion. Some iiiforma,tion concerning the na- degassed with nitrogen and titrated ITith carbonate-free ture of the bonding in the metal complexes of these sodium hydroxide in an atmosphere of nitrogen. A Beckcompounds could be obtained from a qualit'ative man Model G pH meter with a glass-saturated calomel interpretmattionof the ultraviolet spectra of t'he electrode pair, calibrated with buffer solutions at pH 4.00 for pH measurements. I n the case of ligand molecules along with that of the metal com- and 7.00, was usedthe titration had to be carried out in a plexes. For example, if T-bonding in t'he met.al phenacylpyrazine, 50% v./v. water-dioxane mixture, since the compound was complexes is a significant factor, its effect should be not sufficientlv soluble in aqueous solution\.
'
readily observable in the spectra of the complexes. Results It is t o be expect,ed just as in t,he case of the PUltraviolet Spectra.-The spectra of acetophediketones, tha,t the compounds having structure I1 none and methylpyrazine were obtained in various will exist in solut,ion as an equilibrium mixture of solvents and the results are given in Table IV. the keto and enol forms. The variation of keto The spectrum of acetophenone consists of two main (1) Abstracted from a thesis submitted by N. Naqvi in partial fulbands at -240 and 280 mp. I n addition there is a fillment of t h e requirements for t h e degree of Doctor of Philosophy. very lveak, broad band at 320 mfi. Methylpyrazine (2) All inquiries a b o u t this paper should be addressed t o this author. shows two principal regions of absorption, -270 (3) J. D. Behun a n d R. Levine, J . A m . Chem. Soc., 81, 5157 (1959).
Feh., 19G1
SPECTRA AID ACIDDISSOCI.\TIOI COXSTASTS OF PTRAZTLMETHTL KETONES
219
The ultraviolet spectrum of the 2-pyridyl derivative in various solvents and at various pH values h C I D D I S S ~ I C I . ~ T I O N( h X S T A N T S O F PYRAZYLMETHYL KEare given in Table VI. The spectra of phenacylT O N E S DETERMINED POTENTIOMETRICALLY AT 25" pyrazine and the 2-pyridyl derivative are very Compound pK' pK1' similar in 95% methanol and in 1.0 i1-I SaOH, but Phenacylpyrazine .. 12" it is of considerable significance that the spectra 2-Pyridyl pyra:iylmethyl ketone 2.98 10.21 are different in 0.1 III HC104 solution. Indeed a :3-Pyridyl pyrazylniethyl ket>onc 3.42 10.53 general similarity of the pyrazylmethyl ketones is 4-Pyridyl pyrazylmethyl ketone 4.I7 9 . 72 to be expected, and the purpose of examining phenDetrrmind in 50';; v . !v. n-T.nter-diosmc1. acylpyrazine vas t o gain insight into the more T A B L E 11 complex pyridyl derivatives having more than one basic center. ACID DISSOCIATIOS CONST.4NTS O F PYRIDYL PYRAZYLIn 95% methanol, 2-pyridyl pyrazylmethyl keMETHYL KETOXES DETERMINED SPECTROPHOTOMETRICALLY AT 25 & 1" tone exhibits four prominent bands at 234, -307 M e t h y l ketone Kt Kt' pKie PKik pK2e PKZk and 356 mp. The spectra of the 3-pyridyl derivative in 95% methanol and in solutions of pH 12. This is seen by the decrease in intensity of the 235, 310 and 360 mp bands, while the 270 m,u band increases in Intensity. Acid Dissociation Constants.-The substitution of the group R--CH=C(OH)(where R is a pyrazine ring) in the meta position in a pyridine ring decreases the pK, of pyridine from 5.16 to 3.5 (Table I1 pK1, values). Substitution of the same group in the ortho position in the pyridine ring causes a further decrease in the base strength of pyridine, and substitution in the para position results in an increase in the base strength of pyridine over the ineta substituted compound. These results can be accounted for readily in terms of an inductive (electron-withdrawing) effect, which decreases wii h incireasing distance from the pyridine nitrogen. If the substituent is a pyrazylmethyl keto group, the same trend is observed, the base strength of the pyridine nitrogen, pK1,, decreasing in the order 4-pyridyl > 3-pyridyl > 2-pyridyl pyrazylmethyl ketone. All pK1,values are smaller than the corresponding pK1, values, as expected, since a carbonyl group exerts a greater base-weakening effect on an acid-base center in an aromatic ring, than a -C(OH)= group substituted in the same position. The identical trends in pK1, and pK1, valueq of the pyridyl derivatives indicate that hydrogen bonding, steric effects and resonance interactions have only a minor effect on the base strength of the pyridine nitrogen. Furthermore, the values of the apparent dissociation constant pK' determined potentiometrically should also show the same trend as pK1,and pK1,. The apparent dissociation constants pKI1 ah ne11 as p K s , and pK2, decrease in the order 3pyridyl > 2-pyridyl > 4-pyridyl pyrazylmethyl ketone. If the benzene ring in phenacylpyrazine is replaced by a pyridine ring it is not surprising that there is a marked decrease in pKI1 as well as pKn, and pKzk. The pyridyl group can stabilize the molate ion to a greater extent, through resonance interactions, than a phenyl group. Moreover, this stabihzatioii takes place to a greater extent with an ortho mbstituted pyridine and a para substituted pyridine, the meta substituted pyridine showing the smallest tendency to stabilize the anion. It is of interest that other factors such as hydrogenbonding, steric and solvation effects seem t o have a negligible effect on these acid dissociation constants. Figure 1 shows the manner in which the sum of the concentratioos of the neutral enol Be, the pro-
223
1001
101 I
I I0
20
Fig. 1.-Variation
30
40
50
60
70 PH
80
of concentration of [Be with pH.
90
IO0
110
I20
+ Be" + B-]
tonated enol B,H+ and the enolate anion B- varies with pH for each of the three pyridyl compounds. These curves were calculated from the values of the equilibrium constants in Table 11. The curves in Fig. 1 confirm the assignments made for the enol bands in the ultraviolet spectra of the compounds, and also account for the observation that the total concentration of the enol forms is a minimum in neutral solutions and increases with the addition of either acid or base. The rates of enolization should depend on the acid strengths of the enol, if no other complicating factors are present. The stronger the acid, the more rapidly will enol formation occur, since stronger acids tend to stabilize the enolate anions to a greater extent. The values of the rate constants kz (Table 111), show that the 4-pyridyl compound which is the strongest acid enolizes most rapidly. But the 2-pyridyl compound, which is tt stronger acid than the 3-pyridyl compound has the slowest rate of enolization. This anomaly is possibly due to the steric effect of the pyridine nitrogen in the ortho position. The rate at which the enol form reverts to the keto form will depend to a great extent on the mechanism by means of which this prototropic reactions occurs. If me ignore the minor differences in kl, the rate constants for keto formation, it appears that the same mechanism is involved in all three cases. If the rate-determining step in keto formation is that in which the enolate anion picks up a proton from the solution, then the rate constant kl would be governed primarily by the extent to which the negative charge in the enolate is delocalized. It appears therefore that the delocalization of the negative charge 011 the enolate anion occurs to practically the same extent in all three compounds. If however we take into account the small differences in k l , then the rate constant kl parallels the pK2, value. Therefore the
DONALD W. MOOREANI) JAMES A. HAPPE
224
manner in which the base strength increases and the rate of keto formation decreases is: 3-pyridyl > 2pyridyl > 4-pyridyl pyrazylmethyl ketone.
Vol. 65
Acknowledgment.-One of the authors (Q.F.) is grateful to the National Science Foundation for supporting this work with a research grant G-9736.
THE PROTOX MAGNETIC RESOTU'AXCE SPECTRA OF SOME METAL VINYL COMPOUNDS BY
DONALD w.&TOORE
AND
JAMES A. HAPPIC
Michelson Laboratory, U.S. Naval Ordnance Test Station, China Lake, California Received June $Or 1960
The proton n.m.r. spectra of divinplmercury, tetravinyltin and trivinylaluminum etherate have been studied a t 40 mc. Spectral parameters have been determined by solution of the seculiar equations for an ABC-type spin system.
Introduction Proton magnetic resonance spectra of metalorganic compounds are often characterized by a reduction of the separation between functional groups found in ordinary organic compounds, since the metal tends to increase the electronic shielding around adjacent groups. It is even possible for a strongly electropositive metal to invert the usual order of chemical shifts in the n.m.r. spectrum of an alkyl derivative. The work of Raker1 with tetraethyllead and Karasimhan and Rogers2 with diethylmercury illustrate typical results encount'ered with metal alkyls. A second and often useful characteristic of metalorganic n.m.r. spectra is the apprearance of satellite lines owing to the normal abundance of magnetic isotopes present. In reference 2, the spectrum of diethylmercury exhibits Hg199 satellites which have the effect of expanding the CH2-CH3 chemical shift, permitting assignment of spectral parameters for the main spectrum by simple first-order treatment of the satellites. In this case and in tetraethylleadl the satellite spectra indicate two unexpected effects: the spin-coupling between CH, protons and the metal is stronger than that for the CH2 protons and, further, these two Jvalues are of opposite sign. In the work reported here on divinylmercury and tetravinyltin, the determination of precise chemical shifts and coupling constants was simplified by the presence of strong spin-coupling between all three vinyl protons and the magnetic isotopes Hgleg,Sn"' and Sn"9, all of spin I = Though the main spectra were badly collapsed owing to the electropositive screening influence of the metal, straightforward determination of approximate spectral parameters was made possible by first-order treatment of satellite lines. The treatment of the proton spectrum of trivinylaluminiim etherate was then simplified, since approximate coupling constants were known and chemical shifts could be roughly predicted. Precise determination of spectral parameters for the three metal-vinyl compounds was accomplished using an IBRT 709 computer program which rapidly calculates the required ABC-type (1) E. R. Baker, J . Chem. Phys., 16, 9GO (1957).
(2)
P.T.Narasimhan and M. T.Rogers, J . Am. Chem. SOL,82, 34
( 1060).
vinyl spectrum. Refinement of input 6- and Jvalues was done by trial-and-error until agreement between calculated and observed spectra was within the limits of observational error, about &0.3 C.P.S. Assignments of lines were based upon the presumed relationship between proton-proton spincoupling constants in vinyl groups: Jtrans> Jeie> JBem.Results indicate that the metal-proton spin-coupling constants do not necessarily obey this rule. They were, however, all of the same sign in the two cases studied. Experimental The n.m.r. spectra of all three materials were obtained from vacuum-distilled samples run a t 40 mc. A Varian V-4302 spectrometer was used and line position measurements were made by means of the audio sideband modulation technique. Chemical shifts relative t o cyclohexane were measured using an external concentric reference sample. The divinylmercury and trivinylaluminum etherate were prepared by the Chemistry Division, Kava1 Ordnance Laboratory, Corona, California. Tetravinyltin was obtained as a research sample from the Metal and Thermit Corporation, Rahway, New Jersey.
Results Divinylmercury.-The complete proton n.m.r. spectrum (Fig. 1) shows a strong central spectrum flanked by satellite systems which are typical 12line ABC-type spectra. The HgIg9isotope which produces the satellite lines has a normal abundance of 16.86%. The strong spin-coupling of Hg199 nuclei to the three vinyl protons has the effect of producing an expanded set of pseudochemicalshifts in the satellites, while the appearance of the central spectrum is obviously that of a collapsed vinyl system in which 6- and J-values are of comparable magnitude. The problem of assigning spectral parameters was thus simplified by first identifying the line groups in the two satellite systems. This was accomplished readily by invoking the accepted rule for vinyl proton spectra J~rans
> J e i s > Jgem.
The determination of accurate spcctral parameters was initiated with approximate 6- and J values obtained by first-order treatment, of the satellite sets. It was assumed that high- and lowfield systems were unmixed, that is, all metalproton J-v&ws are of the same sign. Sitbsequent refinement of parameters proved this assumption correct, as well as the assumption that all proton-
