Indian Journal of ChemistryVol. 18A, August 1979, pp. 120-122

DipoleBenzene,

Moments of Molecular Complexes of Acrylonitrile withp-Xylene & Mesitylene & of Acetone, Acetaldehyde &

Ethyl Methyl Ke one with BenzeneR. R. YADAVA* & s. s. YADAVA

Chemistry Department, Gorakhpur University, Gorakbpur 273001

Received 20 October 1978; revised 29 January 1979; accepted 12 February 1979

The dipole moment values ((LcomPlex) for the molecular complexes formed between acrylonitrlle-benzene,acrylonitriIe-p-xylene, acrylonitrile-mesitylene, acetone-benzene, acetaldehyde-benzene and ethyl methyl ketone-benzene have been calculated from the apparentdipolemoment values of the solutes in reactive aromatic solvent, (LBO!,

and those in an inert solvent cyclohexane, (Llnert. (Lcomplex values so obtained, in conjunction with equilibriumquotient values, Kx, for the respective complex formation have been used to calculate, r, the separation between thedipolar centre of the polar solute and the point of induction in the aromatic solvent. An attempt has been made torationalize the r-values so obtained with the gas phase value of the dipole moment of the polar solute and the stericenvironments of the complexing species.

IThas been suggested- that when a polar solute(A) is dissolved in a reactive non-polar aromaticsolvent (B) a molecular complex of the type

A + B ~ A .... B involving dipole-induced dipoleinteraction is formed. In most of the studies onmolecular complexes, spectroscopic methods- havebeen employed. However, it has also been shown>that dielectric studies can be used to obtain signi-ficant information regarding molecular complexes.

The dipole moment of a solute as obtained from asolution, I-'sol, may not be the same as its gas phasevalue or that measured in an inert solvent, 1-'1nert.In a previous study! the authors determined thevalues of 1-'801 of some polar aliphatic solutes inreactive aromatic solvents, and 1-'1nert, in cyclo-hexane. From the relation I-'Ind = I-'soJ - 1-'1nertsemiquantitative inferences were drawn about thenature of the interaction and the role of stericfactors therein. However, the value of 1-'801 does notrepresent the dipole moment in the fully complexedstate, I-'complex. I-'sol can be equal to I-'complexonly when. all the solute molecules are complexedwhereas at equilibrium conditions only a portion ofthese exist in the complexed state. A knowledge ofI-'complex can furnish better quantitative informationabout the molecular complex. In the present studyit is proposed to calculate ",complex for the molecularcomplexes formed between some polar aliphaticsolutes and reactive aromatic solvents from a know-ledge of "'sol, ",Inert and the equilibrium quotientvalues, K",. Polar solutes with different gas phasevalues of dipole moment and solute and solventswith similar structures but differing progressivelyin steric environments have been selected for thepresent study to rationalize the results.

Data & Methods of CalculationPolar solute-aromatic solvent systems selected for

the investigation were acrylonitrile-benzene, acrylo-

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nitrile-p-xylene, acrylonitrile-mesitylene, acetone-benzene, acetaldehyde-benzene and ethyl methylketone-benzene. I-'sol, 1-'1nert and the gas phasevalues were obtained from literaturev" (Table 1).K" values for the complex formed between acrylo-nitrile and aromatic solvents were obtained from anearlier NMR study of Homer and Yadava" andK., values of the remaining complexes were obtainedfrom a study' of the distribution of solute betweenaqueous phase and a non-aqueous phase consistingof different proportions of the reactive solvent andthe inert solvent. K" values are those for the condi-tion when the number of moles of the inert solvent inthe reaction mixture vanishes, i.e. ns = 0, andwere obtained by extrapolation.

For a binary system (A + B) as employed in thedipole moment determinations, involving the inter-action of the type specified, K" is given by theexpression (1) with ns = 0.

K., = nAB (nA + nB - nAB\ ••. (1)(nA - nAB) (nB - nAB

In Eq. (1) n»: , nB and ns are, respectively, the numberof moles of the polar solute, reactive aromaticsolvent and inert solvent taken initially, and nAB isthe number of moles of the complex formed whenequilibrium has been attained. If the fraction of thesolute (A) complexed is represented by a, i.e. nAB= a ns , then

K., = a(nA + nB - anA) (2)(J - a) (I1B- anA) •..

At infinite dilution of A in B, ne > n« and Eq. (1)becomes

nAB aK", ,..., - ... (3)ns; - nAB 1- aSince, a fraction, (I-a), of the solute has been

assumed to exist in the uncomplexed state with

r--YADAVA & YADA VA: DIPOLE MOMENT OF MOLECULAR COMPLEXES

dipole moment f4inert, and a fraction, a, to becomplexed, with dipole moment

fLcomplex = fLinert + fLaro •.• (4)where fLaro is the dipole m.oment induced in the aro-matic by the solute in the complex, the total orien-tation polarisation of the soluble, Po,total, can berepresented by Eq. (5)

Po, total = a Po, complex + (1 - a) Po, inert ••. ~). ,

Since, Po = 4rcN fL2/9 kT, Eq. (5) becomesfL2soi = a fL2comPlex + (1 - a) fL2inert

so that,

[/k2SOl - (1 - a) fL2inertJ112

,ucomplex = a

The proportion, a, of the solute in the comolexedstate at infinite dilution can be found from the valueof K", and Eq. (3). The value of a is then substi-tuted in Eq. (7) to obtain fLcomplox. The values offLcomplex obtained by this method for various sys-tems are given in Table 1, together with the valuesof ,uaro deduced from Eq. (4).

As pointed out previously, for all the systems underconsideration the interactions are of dipole-induceddipoje type. Frank" has shown that, in general,/karo is related to the distance from the point dipolein the solute to the point at which it induces a dipole,1'. This relationship can be described in the followingway:

A polar solute of moment, flinert placed in aro-matic medium of polarisability rt. induces" a moment,uaro such that

fLaro = rt.. F ... (8)where F is the electric field acting on the aromaticmolecule. In addition

... (6)

... (7)

F _ E(E + 2)- 3 ... (9)

where E is the total electric field in the medium ofdielectric constant E. For the symmetrical aromaticmolecules employed in the present studies; E can beresolved into two components, namely E", and EI/'Ez is the component of. the field in the direction ofthe dipole axis of the solute, whereas E1/ is that atright angle to this direction. These field components

TABLE I-DIPOLE MOMENT VALUES AT 307.4K OF POLARSOLUTES IN DIFFERENT MEDIA

[Values in debye]

Solute Gas Cycle- Ben- p-Xylene Mesi-phase hexane zene tylene

(a) (b) (b) (b) (b)

Acrylonitrile 3.90 3.13 3.50 3.37 3.33Acetone 2.88 2.61 2.75 2.64 2.61Acetaldehyde 2.69 2.33 2.42 2.70 2.35Ethyl methyl ketone * 2.71 2.80 2.77 2.75

(a) Ref. 5; (b) ref. 4.*Value not available in the literature but it is expected to be

slightly higher than that of acetone.

/

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are related to, 1', the separation between the dipolarcentre of the polar solute and the point of inductionin the aromatic molecule, by the relations (10)and (11).

E", = flinert (3 cos? e - 1)/ €r3 ••• (10)E1/ = 3/kinert (cos e sin e)/Er3 ... (11)

In Eqs (10) and (11) e is the angle between thesolute dipole axis and the radius vector from thesolute point dipole to the point at which the dipole isinduced. Combining Eqs (8) (11), we get

( _ rt.fLinert (e + 2) (3cos2e - I)fLaro)z - 3 E 1'3 , ... (12)

( ) _ rt./kinert (€ + 2) cos e sin e (13)fLaro II - E 1'3 •••

It is evident that by employing Eqs (12) and (13),the distance, r, for each complex studied, can becalculated.

o.

It has been shown by Homer and Cooke'? that theinteraction energy for a dipole acting along the six-fold axis of symmetry of the aromatic is about twicethat when acting in the plane of the ring. Therefore,the most favoured structure for dipole-induced dipolecomplexes is the one in which the most electropositiveend of the solute tends to be located above the planeof the ring adjascent to its six-fold axis of symmetryand with its dipolar axis coincident (or nearly so)with the symmetry axis as shown in Fig. L This hasbeen substantiated by the NMR study" of such inter-actions using a solute with non-equivalent protons.

In 'the above circumstances, e = O. Thus fro~Egs (12) and (13),

)2rt. .L,uInert (E + 2)

(fLaro z = ...(14)3 E 1'3

(f4aro)v = 0 ... (15)

where ~.L is ~he perpendicular polarisability of thearomatic. Using the value of f4aro (Table 2 anid,uinert (Table 1) and various parameters recorded nTable 3, the values of I' (assuming that the solutanpoint dipole is acting at the ring centre) for vio scomplexes have been evaluated using Eq. (14). Theresults are given in Table 2.

IIIII

ijc/:~

Hf : c;~III< >

Fig. 1

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rINDIAN J. CHEM., VOL. 18A, AUGUST 1979

TABLE2 - PARAMETERSFOR VARIOUSPOLAR SOLUTE-AROMATICSOLVENTMOLECULARCoMPLEXESAT 307.4K

System Kx (.tcomplex (.taro (.tlnd r(debye) (debye) (debye) (At

Acrylonitrile-benzene 1.507" 3.73 0.60 0.37 3.64Acrylonitrile-p-xylene 0.929" 3.60 0.47 0.24 4.47Acrylonitrile-mesi-

tylene 0.736" 3.58 0.45 0.20 4.76.

Acetone-benzene 6.000b 2.77 0.16 0.14 5.32Acetaldehyde-benzene 2.460b 2.46 0.12 0.09 5.57Ethyl methyl ketone-

benzene 5.280b 2.81 0.10 0.09 6.21

(a) Ref. 6; (b) ref. 7.

TABLE3 - DIELECTRIC CONSTANTVALUES AT 307.4 K ANDPOLARISABILITY(ALONG SIX-FOLDSYMMETRYAXIS) VALUESFOR

VARIOUSAROMATICSOLVENTS

E(a)

2.25562.24732.2629

IXl. /1030 m8(b)

7.3310.7512.47

Aromatic solvent

Benzenep-XyleneMesity1ene

(a) Ref. 4; (b) ref. 12.

DiscussionNeighbouring polar solute molecule polarises the

aromatic molecule through its n-electron cloud result-ing in a dipole moment being induced in the latter.This process is then followed by electrostatic attrac-tion between the two reacting species. At the sametime, the solute experiences the resultant dipolemoment leading to an increases in f'complex,

Homer and Yadava", have emphasized that thereare two main factors governing the formation ofdipole-induced dipole complexes, namely electrosta-tic attraction and steric effects. The electrostaticfactor, obviously, depends on .the polarising andpolarisability properties of the solute and the aromaticmolecules respectively and for a common solute itshould increase on going up the aromatic series. Thisshould lead to closer approach of the solute to thearomatic and therefore an increase in f'aro' Thesteric factor consists of two contributions, first anegative contribution due to methyl groups on thearomatic solvent blocking the approach of the solute,thus reducing the value of f'aro and a positive contri-bution due to the solute being trapped by the aromaticmethyl groups. Substituents on the solute moleculemay also exhibit similar steric effects. In fact, the

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electrostatic and the steric factors compete witheach other in the formation of the complex.

It is seen from the data in Table 2 that for a commonreactive aromatic solvent benzene, the higher the gasphase value of the solute dipole moment, the largerthe value of f'aro. Hence it is possible to concludetentatively that for all these systems the interactionsare taking place through dipole-induced dipolemechanism. However, a lower value of f'aro forethyl methyl ketone-benzene complex and the prog-ressive increase in the value of r (Table 2) for acetal-debyde, acetone, and ethyl methyl ketone seems to bea clear manifestation of the blocking effect.

A perusal of Table 2 shows that, for a commonsolute acrylonitrile, the value of f'aro for the threearomatics benzene, p-xylene and mesitylene decreasesinspite of the fact that their polarisabilities increase.Here again, theblocking effect due to progressive sub-stitution of methyl groups in the aromatic moleculeseems to dominate the electrostatic effect. The factthat the substituents are obstructing the approach0,£ the solute to the reactive aromatic solvent isevident from the progressive rise in the value of r(Table 2).

AcknowledgementThe authors are thankful to Prof. R. P. Rastogi

for his keen interest in the work and facilities and tothe UGC, New Delhi for financial assistance.

References1. BRIEGLEB, G., Electronen-dollor-acceptor-komplexes

(Springer-Verlag, Berlin), 1961.2. FOSTER, R., Organic charge-transfer- complexes (Academic

Press, New York), 1969.3. (a) EARP,D. P. & GLASSlOJ'..'E,S., J chem. Soc., (1935), 1709

(b) HUKE, P. J., Studies on molecular interactions ill solu-tion, Ph. D. Thesis, University of Aston in Birmingham(1968).

4. YADAVA, R. R. & YADAVA, S. S., Indian J. Chem.16A (1978), 826.

5. MCCLELLAN,A. L., Table of experimental dipole moments(Freeman, San Francisco), 1963, 64, 76, 85.

6. HOMER, J. & YADAVA, R. R., J. chem. Soc., FaradayTrans-I, (1974), 611. .

7. YADAVA,R. R. & YADAVA,S. S., Unpublished results.8. FRANK, F. c., Proc. r. Soc. (Lond.), 152 (1935), 171.9. FROHLICH, H., Theory of dielectrics (Oxford University

Press, London), 1949.10. HOMER, J. & CooKE, M. c., J. chem. Soc. (A), (1969),

773.11. HOMER,J. & YADAVA,R. R., Tetrahedron, 29 (1973), 3853.12. LE FEVRE, C. G. & LE FEVRE, R. J. W., Rev. pure appl.

Chem., 5 (1955), 261.