Electronic absorption spectroscopic studies of enolimine-ketoamine equilibria in Schiff bases

(1)

Prec. Indian Acad. Sei. (Chem. Sci.), Voi. 90, Number 6, Deeember 1981, pp. 519-526.

(~) Printed in India.

Electronic absorption spectroscopic studies of enolimine-ketoamiae equilibria in Schiff bases

A S N M U R T H Y * and A R A M R E D D Y

Department of Chemistry, Indian Institute of Technology, New Delhi 110 016, India MS received 10 August 1981

Abstract. The enolim;,ne-ketoamine equilibria in a variety of alkyl anti arylsali- eylMd.imines have been studied in 1,2-diehloroethane, methanol, ethanol, iso-pro.

panel and t-butanol by eleGtronie absorption spectroscopy. The equilibrmm depends on the nature of the alcohol and. the strength of hydrogen bond formed with the ketoarnine. In arylsalicylaldimines, the equilibrium is sen.oitive to the nature of the ,.ubstituentr.

Keywords. Sehiff base equilibria ; electronic absorption spectra.

1. Introduction

T h e ~.-hiff bases derived f r o m the condensation o f salicylaldehyde with all~yl and arylamines, l~nown as N-alhyl or arylsalicylaldirrdne,% are considered as suitable models for pyridoxal and, in general, B6 vitamirts. The salicylaldimines are planar intraraoleeularly hydrogen g o a d e d structures. P r o t o n tautomerism can occur in such structures leading to an equilibrium between the neutral intramolecularly

0~I

oryl

~ H C ~ N . . . . R R: alkyl .

hydrogen b o n d e d form (enolimine tautomer, .In) a n d the p r o t o n transferred

(~o) (lb)

(]~etoamine tautomer, Ib) structLlre. Evidence in favour of such a tautomerism in selected solvents has been shown by electronic absorption (Ledbetter 1966), infrared (Ledbetter 1977) and NMR, (Dttdel~ and D u d e k 1966) spectroscopy. The

effect o f solvents on the electronic absorption spectra o f $chil~ bases has received

* To whom correspondence should be made.

519

(2)

520 A S N Murthy and A Ram Reddy

considerable attention in recent years. Alexander and Sleet (t970) h~.ve examined the ultraviolet absorption spectra of monosalicylaldirnines ofethylamine, ethanG- lamine and n-hutylamine in various solvents lithe water, absolute ethanol, dioxan and cyclohexane. In eyelohexane, two maxima arottnd 255 and 320 nm were observed. In eth~.nol, in addition to, these, new bands at ,~280 and ,-~400 nm were noticed. A comparison with the spectra of N-methylamines o f m- and p-hydroxy benzaldehydes in the same solvents h~s shown th~.t o- and p-hydroxy- benzylideneimines exist mainly as dipolar ketoamines in polar, hydrogen handing solvents and m-isomers exist mainly as the enolimine tav.tomeric form in all the aolvents. The former set of hands in cyelohexane have been assigned to enolimine tautamer and the latter to the l~etoamine tautomer of Schiff bases. Polar hydrogen bond forming solvents thus seem to favour the formation of ketoamine farm.

Ch~.rette et al ([964) have also cxamined a series of N-alkyl Szhiff bases hy ultraviolet ahsorption spectroscopy in diff,:rent solvents. The changes in the spectra when inert solvents are replaced hy hydrogen bond forming solvents have been interpreted in terms of salvation eqttilihria. The interaction of enolimine with a hydrogen hand forming solvent (alcohol) would presttm,-.hly rcdo.ce the O-It bond strength and facilitates proton transfer to the nitrogen centre. S e l l . o r (1,977) ha~

demonstrated th.~t the ratio of enolimine ta l~eto~mine forms of N-ethylsali- eylaldimine varies roughly linearly with the number of aliphatic carbon atoms o f a series of straight chain alcohols used as solvents ~ obviously, this is related to the pro, ton donating ability o,f alcohols.

Thus, the date. so far reported in the literature presents only a qualitative picture o f the nature of $chiffhases in various solvents. No attempt has so far been made to obtain thermodynamic data for the equilihrium (10. It would be interesting to determine the. specific role o,f an alcohol in determining the relative amounts of tautomers in the eqttilihrium. More interesting would he to determine the amount of l~etoamine a, a function of the concentration of alcohol in a ternary system.

The influence of the structure of alcohol could easily be rationalized from such a study. In this paper, we report our results on the electronic absorption spectra of the following ~Ichiff hases in 1,2-dichloroethane, methanol, ethanol, iso-propanol and t-butanol.

(i) N-galieylideneethylamine (NE) ~ g = C2H5 (iS) N-S~alicylidene n-butylamine (SB) ~ g = n-C,Ho (iii) N-$alicylideneaniline (SA) ~ R = C~H5-

(iv) N-Sialicylidene p-toluidine (SPT) ~ R = CHa C6Hs - (v) N-Salicylidene p-nitroaniline ($PNA) ~ R = NO,. CsH4-

(vi) Bis (N-~alicylidene)-l,2-diaminoethane (BSE) ~ R = CH2 CH2 NI-I2

2. Experimental

All the S2hiffhases were prepared by methods described in the literature (Alexander and Sleet 1970 ; Selisier 1977 ~ Decoene and Teyssie I962). SE was prepared by reflttxing eqttimolar amounts of salicylaldehyde and anhydrous ethylamine in methanol for two hours. After evaporating the residual ethylamine and methanol.

the yellow liquid prodttet was dried over anhydrous sodium sulphate and vacuum

(3)

Electronic absorption spectra of Schiff basis 52t distilkd (B.P. 8;7-88~ SB was prepared by condensing equimolar amounts of salicylaldehyde arzd n-butylamine in dry methanol at room temperature. Excess meth~..nol was removed by vacuum evaporatioax av.d water hy treatment with anhydrous sodium sulph~.tc. The residue was fractionally distilled (B.P. 140 ~ C/1 ram). SA was preparcd hy refluxing equimolar amounts of salicylaldehyde andaniline in dry methanol for about 189 h. The solid SA was filtered and reerystaUizcd twice from methanol (M.P. 47 ~ SPT and SPNA were prepared in a manner similar to that of ~A and th,~" solids recrystallizcd flora methanol (M.P. 98 ~ C and 152 ~ C respectively). BSE was prepared hy the cos,Men- sation of salicylaldehyde (0.2tool) with ethylenediamine (0.1 tool). A yellow solid separated o u t and was recrystallized from meth_~.nol (M.P. 123 ~ C).

All the solvents were purified by standard methods (R.idd;.ck and Burger 1,970).

Electronic absorption spectra were recorded hy a Pye-Unicam SP700 spectro- photometer fittcd with a SP770 constant temperature cell holder and SP 775 electrical controller. The equilihrium constants (K) were evaluated by the method of B~.ba and Suzuki

(1,960.

The enthalpics ( A H ~ were obte.inr d by determining K at two or more temperatures.

3. Results aud discussion

Tile electronic absorption spectrum of SB in 1,2-dichloroetharte and methanol are shown in figure 1 (a). The spectra, in 1,2-dichloroethane show intense bands at 256 and 31,4 nm. Following SelisJ~ar (1977) the 256 and 314 nm bands may be assigned to 3 ' A ' ~ 1 ' A ' and 2 ' A ' ~ l ' A ' transitions (~ ~ zc*) rcspectively.

The 25a am hand undergoes a small blue sh~ft and tile 31,4 nm band either remains the same or undergoes a small red shift in methanol. The intensity, however, increases appreciably. On the other hand, the 254 and 325 nm bands in arylsalicylaldimines undergo large red shifts in alcohols. This is probably due to the weak tntramolecular hydrogen bond present in arylsalicylaldimines. On solvation, the red shift can be attributed to the major contributions o f polar

§ H H H

o'..,, $_.,

"-'-"~c~"R ~ , ~ . C # N . . . ~ 4-,. ~ . . / % C ~ . " R

H H H ~ H H

(, o) ^{( l~)IR=}^o~y,) ^(,^{b )}

structures (IIa and IIb) to the excited state. These structures are stabilized hy solvent polarity, resulting in a red shift of the bands. The progressive addition o f methanol in the concentration range 1.7247.35 tool. dm -3 to a dichloroethane solution of SB has an interesting effect. While in t,2-diehloroethane, SB has no absorption at ~-~ 400 nm, the progressive addition of methanol causes a band to appear in the 400 nm region, whose intensity increases with increase in the concentration of methanol. At high methanol concentrations, noticeable but smaller shifts of the ,~ 400 nm hand to. lower wavelengths h.~,s been observed. Similar changes occur in the spectra of arylsalicylaldimines in presence of various alcohols.

Following Ledhetter (1,96 0 and Alexander and Sleet (1,970), the ~ 400 nm hand

(4)

522 A S N Murthy and A Ram Re&ly 0 . 8

2 0.6

(b)

9 ac~ O.4

,q:

0.2

0 ,[ I_ I I I

250 3 0 0 350 5 5 0 4 0 0 4 5 0

Wovelength (nm)

Figgre 1. (a) The electronic absorption spectrum of N-salicylidene-n-butylamine (SB) in, 1 ; 1,2-d.ichloroethane and 2 ; methanol. (b) Effect of methanol on the electronic absorption spectra of SB in 1,2-dichloroethane in the 350-450 nm range 1 ; 1'90 xl0-4mol dan -a of SB; 2 to 9 correspond to 1.90 x 10 -~mol dm -a cf SB in 1,2-dichloroethane plus 1.69, 3.0, 3.7, 4'2, 4.5, 5.9, 6.6 and. "/.35 reel dm -a of methanol.

was assigned to the ketoamine form. The formation o f the t~eto.amine is prohahly due to solvation a n d resonance stahilizatio.n. The formation o f (Ih) requires the loss o f a large a m o u n t o f resonance energy (D~tdck 1963), which is only possible i f the ground state is stabilized by extensive contribution o f polar reso-

H

I1 b l llcl

nonce structures, such as (Ic). The structure (Ic) g~.ts stabilized in polar solvents therehy increasing the concentration o f the lcetaamine tautomer. The depen- dence o f l~etoamine formation on the concentration o f alcohol and the presence o f isohestic points a r o u n d 380 nm indicates that there is a definite equilihrium between enolimine a n d keto.amine forms ann that alcohol molecules are involved in this equilibrium as in

Enolimine + R O H ~ l~etoamine. (2)

The 400 nm b a u d was not, however, observed in acetonitrile. This clearly means that a polar hydrogen h o n d i n g solvent is necessarily involved in this equili- h r i u m process. The spectral b a n d positions are shown in tame 1. The variation in intensity o f the 400 n m b a n d with the concentration o f alcohols in 1,2-diehloro-

(5)

Electronic absorption spectra of Schiff basis 523

0"8

0 6

Q) O r

~D

sO.4

0 . 2

2 5 0 5 0 0 35O 350

Wavelength (nm)

9

8

I 4 0 0

Figure 2. The electronic absorption spectrum of N-salicylidene-aniline (SA, 7.36

• 10 -r' reel cm -~) in 1 ; 1,2-d.ichloroethane, 2 ; cyclohexane, 3 ; t-butanol, 4 ; ethanol and 5 ; mettianol. 6-9 correspond to 1.47 x 10 -a reel d.m -~ o f SA in 1,2 diehloroethane, t-butanol, ethanol and. methanol respectively.

Table 1. Electronic absorption spectral data o f Schiff bases9 Absorption maxima ( n m ) i n

Schiff base

1,2-Dichloro- Methanol Ethanol t-Butanol ethart~

SI~I 256 254 254 255

315 315 315 315

9 402 403 405

SB 256 254 254 255

314 316 316 315

9 400 401 402

SA 254 270 270 270

325 338 337 335

9 430 435 437

SPT 267 270 . . 270

335 335 .. 335

.. 428 . . 437

SPNA 268 27t3 (Sh) . . 270 (Sh)

335 355 .. 358

.. 442 ( v w ) . . 445 (~rW)

Sh : Shoulder VW : Very weak

(6)

524 A S N Murthy and A Ram Reddy

Table 2. Tbgrmodyiaamic data o f er~olimine-ketoamine equilibria o f Schiff bases in various alcohols.

SI. Schiff Solvertt K -- /~ H ~

No. base d m ~ reel - t kJ mol - t

1 SE lViethanol 0' 247 21' 8

E t h a n o l 0" 217 2l" 4 t-butanol 0" ] 65 17" 9

2 SB M~thaltel 0" 36 27" 7

t-butanol 0" 195 18" 1

3 S A M e t h a n o l 0" 124 18' 4

t-butanol 0' 067 13" 4

6 SPT Methanol 0" 152 20" 1

Ethaxtol 0" 132 19" 4

t-butanol 0" 074 15" 0

5 S P N A M e t h a n o l 0"067 lZ' 6 E t h a n o l O" 060 11- 7

6 BSE M~thanol 0' 184 33" 4

isa-propanol 0-154 29" 3 t-batartol 0" 103 21" 3

eth~rte has been quantitatively treated and equilibrium data obtained as shown in table 2. A typical plo, t fo.r the calculation of equilibrium constant is shown in figure 3. It is interesting to examine the effect of varying R in R OI-I on the equilibrium (2). For SB, while the equilibrium constant and enthalpy with methanol are 0.36 dm 3 reel -1 and 27.7 la~ reel -1 respectively, they are 0.195 dm S reel -1 and 18.1/~J, reel -1 respectively with t-butanol. Per Sehiff bases under investigation, the equilibrium constants and enthalpies vary in the order : meth~.nol

> ethanol > iso-propanol > t-butanoL Methanol is a better proton donor than t-butanol (Mttrthy and Rao t96g ;. J o e s t e n and 8~haad 1974) and so one would expect the former to favour the ketaamine than the latter. With methanol, the equilibrium constants and enthalpies for S:hitt bases SB and SE vary in the order SB > SE. This trend is consistent with the fact that the nitrogen atom becomes more basic when R = n~C4H9 than when R - - C 2 I - I 5. Interestingsuhstituent effects can be seen in the speetral and equilibrium data of arylamirte ~ h i f f basis.

In the spectra ofpara-sustituted arylsalieylaldimirtes in various alcohols (figure 4), the 400 nm band is more intense in ,qPT and less intense in 8PNA than SA ~ in t-butanol, the. 400 nm hand in SPNA could hardly be seen. In 8A, SPT and SPNA where the suhstimertts in the para position of aniline ring are I-I, CI-I a and NO2 respectively, the equilibrium constants and enthalpies with any alcohol are in the order SPT > SA > SPNA. Thus, while the electron donating e l i a group

(7)

Eleatronic absorption spectra of Schiff basis 525

50%

5 - -

#2

1

1 .. . . J

0 - - - - ~

0 . 2 0 . 4 0"6 0 . 8

Figure 3. Typical plot on the oalculation of equilibrium constant for the N-sali- cylideneethylamine-naethaaol system in 1,2-diohloroethane.

0 . 8

0 ' 6 - -

5

~176 I

0.2 [

O R

4 0 0 4 5 0

Wavelength (nm)

Figure 4. The electronic absorption spectra of para-substituted arylsalicylaldimines in different alcohols. Spectral curves 1-3 correspond to N-salicylidene p-toluidiae in methanol, ehtha~lol and t-butanol and curves 4-6 correspond to that of N-salicylidene p-nitroanilirte in methanol, ethanol and t-butanol respectively.

(8)

526 A S N M u r t h y and A R a m R e d d y

favours l~etoamine form, the electron withdrawing NO2 group does not. In case o f BSE, although t t e equilihrium constants are n o t larger than the remaining S : h i f f hoses, the enth~.lpies o f tautomerism are largest. This is prohahly due to the fact that two enolimine moieties per mole o f BSE are involved in the equili- b r i u m .

The results ohtained in this study indicate that alcohol molecules hreak the intramolecular hydrogen h o n d and weaken the O - H h o n d o f enolimine therehy facilitating proton transfer to the nitrogen atom. At the same time alcohol molecules also form hydrogen b o n d with the oxygen atom o f the keto, amine form, thereby sh:fting the equilibrium to the right. M e t h a n o l forms a stronger hydrogen b o n d th~.n t-butanol a n d hence the formation o f l~etaamine is more favourahle in the former solvent than in the latter. The differing strength o f hydrogen bonds also explains why the characteristic h a n d o f keto,amine occurs at a longer wavelength in t-butanol thxn in methanol (table 1). Thus, hydrogen h o n d i n g plays a major role in determining the relative amounts o f enolimine a n d ketoamine form~

in the S : h f f base eqttilihria in alcohol solvents. Similar conclusions have been arrived at in the laeto-enol equilibria in other systems ( M u r t h y et al 1962).

Acknowledgements

One o f the authors (All,) is th~.nl~fttl to the authorities o f the Indian Institute o f T e c h n o l o g y , New Delhi, for the award o f a fellowship.

References

Alexander P W and Sleet R J 1970 Aust. J. Chem. 23 1183 Baba H and Suzuki S 1961 J. Chem. Phys. 35 1118

Charette J J, Falthanal G and Teyssie P 1964 Spectrochim. Acta 20 597 Deeoenne C and Teys ie P 1962 J. Polym. Sci. 57 121

Dudek G O 1963 J. Am. Chem. Soc. 85 695

Dudek G O and. Dudek E P 1966 J. Am. Chem. Soc. 88 2407

Joesten M D and Schaaa L J 1974 Hydrogen bonding (New York : Marcel Dekker) Ledbetter J W 1966 J. Phys. Chem. 70 2245

Ledbetter J W 1977 J. Phys. Chem. 81 54

Murthy A S N and Rao C N R 1968 Appl. Spectry. Revs. 2 69

Murthy A S N, Balasubramanian A, Kasturi T R and Kao C N R 1962 Can. J. Chem. 40 2267 Riddick J A and Burger W B 1970 Technique of chemistry. Organic solvents (New York : Wiley,

Interscience) Vol. 2

Seliskar C J 1977 J. Phys. Chem. 81 1331