Dipole Benzene, Moments of Molecular Complexes Of

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

Dipole Moments of Molecular Complexes of Acrylonitrile with Benzene, p-Xylene & Mesitylene & of Acetone, Acetaldehyde & Ethyl Methyl Ke one with Benzene

R. 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 equilibrium quotient values, Kx, for the respective complex formation have been used to calculate, r, the separation between the dipolar centre of the polar solute and the point of induction in the aromatic solvent. An attempt has been made to rationalize the r-values so obtained with the gas phase value of the dipole moment of the polar solute and the steric environments of the complexing species.

Thas been suggested- that when a polar solute nitrile-p-xylene, acrylonitrile-mesitylene, acetone- (A) is dissolved in a reactive non-polar aromatic benzene, acetaldehyde-benzene and ethyl methyl Isolvent (B) a molecular complex of the type ketone-benzene. I-'sol, 1-'1nert and the gas phase A + B ~ A .... B involving dipole-induced dipole values were obtained from literaturev" (Table 1). interaction is formed. In most of the studies on K" values for the complex formed between acrylo- molecular complexes, spectroscopic methods- have nitrile and aromatic solvents were obtained from an been employed. However, it has also been shown> earlier NMR study of Homer and Yadava" and that dielectric studies can be used to obtain signi- K., values of the remaining complexes were obtained ficant information regarding molecular complexes. from a study' of the distribution of solute between The dipole moment of a solute as obtained from a aqueous phase and a non-aqueous phase consisting solution, I-'sol, may not be the same as its gas phase of different proportions of the reactive solvent and value or that measured in an inert solvent, 1-'1nert. the inert solvent. K" values are those for the condi- In a previous study! the authors determined the tion when the number of moles of the inert solvent in values of 1-'801 of some polar aliphatic solutes in the reaction mixture vanishes, i.e. ns = 0, and reactive aromatic solvents, and 1-'1nert, in cyclo- were obtained by extrapolation. hexane. From the relation I-'Ind = I-'soJ - 1-'1nert For a binary system (A + B) as employed in the semiquantitative inferences were drawn about the dipole moment determinations, involving the inter- nature of the interaction and the role of steric action of the type specified, K" is given by the factors therein. However, the value of 1-'801 does not expression (1) with ns = 0. represent the dipole moment in the fully complexed state, I-'complex. I-'sol can be equal to I-'complex K., = nAB (nA + nB - nAB\ ••. (1) only when. all the solute molecules are complexed (nA - nAB) (nB - nAB whereas at equilibrium conditions only a portion of In Eq. (1) n»: , nB and ns are, respectively, the number these exist in the complexed state. A knowledge of of moles of the polar solute, reactive aromatic I-'complex can furnish better quantitative information solvent and inert solvent taken initially, and nAB is about the molecular complex. In the present study the number of moles of the complex formed when it is proposed to calculate ",complex for the molecular equilibrium has been attained. If the fraction of the complexes formed between some polar aliphatic solute (A) complexed is represented by a, i.e. nAB solutes and reactive aromatic solvents from a know- = a ns , then ledge of "'sol, ",Inert and the equilibrium quotient values, K",. Polar solutes with different gas phase K., = a(nA + nB - anA) (2) values of dipole moment and solute and solvents (J - a) (I1B- anA) •.. with similar structures but differing progressively At infinite dilution of A in B, ne > n« and Eq. (1) in steric environments have been selected for the becomes present study to rationalize the results. nAB a K", ,..., - ... (3) Data & Methods of Calculation ns; - nAB 1- a Polar solute-aromatic solvent systems selected for Since, a fraction, (I-a), of the solute has been the investigation were acrylonitrile-benzene, acrylo- assumed to exist in the uncomplexed state with

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YADAVA & YADA VA: DIPOLE MOMENT OF MOLECULAR COMPLEXES

dipole moment f4inert, and a fraction, a, to be are related to, 1', the separation between the dipolar complexed, with dipole moment centre of the polar solute and the point of induction in the aromatic molecule, by the relations (10) fLcomplex = fLinert + fLaro •.• (4) and (11). where fLaro is the dipole m.oment induced in the aro- matic by the solute in the complex, the total orien- E", = flinert (3 cos? e - 1)/ €r3 ••• (10) tation polarisation of the soluble, Po,total, can be E1/ = 3/kinert (cos e sin e)/Er3 ... (11) represented by Eq. (5) In Eqs (10) and (11) e is the angle between the Po, total = a Po, complex + (1 - a) Po, inert ••. ~) . , solute dipole axis and the radius vector from the Since, Po = 4rcN fL2/9 kT, Eq. (5) becomes solute point dipole to the point at which the dipole is 2 2 induced. Combining Eqs (8) (11), we get fL soi = a fL comPlex + (1 - a) fL2inert ... (6) _ rt.fLinert (e 2) 2 I) so that, ( + (3cos e - fLaro)z - 3 E 1'3 , ... (12) /k2SOl - (1 - a) fL2inertJ112 ,ucomplex = a ... (7) [ _ rt./kinert (€ + 2) cos e sin e (13) ( fLaro ) II - E 1'3 ••• The proportion, a, of the solute in the comolexed state at infinite dilution can be found from the value It is evident that by employing Eqs (12) and (13), of K", and Eq. (3). The value of a is then substi- the distance, r, for each complex studied, can be calculated. tuted in Eq. (7) to obtain fLcomplox. The values of o. fLcomplex obtained by this method for various sys- tems are given in Table 1, together with the values It has been shown by Homer and Cooke'? that the of ,uaro deduced from Eq. (4). interaction energy for a dipole acting along the six- As pointed out previously, for all the systems under fold axis of symmetry of the aromatic is about twice consideration the interactions are of dipole-induced that when acting in the plane of the ring. Therefore, dipoje type. Frank" has shown that, in general, the most favoured structure for dipole-induced dipole /karo is related to the distance from the point dipole complexes is the one in which the most electropositive in the solute to the point at which it induces a dipole, end of the solute tends to be located above the plane 1'. This relationship can be described in the following of the ring adjascent to its six-fold axis of symmetry way: and with its dipolar axis coincident (or nearly so) A polar solute of moment, flinert placed in aro- with the symmetry axis as shown in Fig. L This has matic medium of polarisability rt. induces" a moment been substantiated by the NMR study" of such inter- ,uaro such that actions using a solute with non-equivalent protons. fLaro = rt.. F ... (8) In 'the above circumstances, e = O. Thus fro~ Egs (12) and (13), where F is the electric field acting on the aromatic molecule. In addition 2rt. .L,uInert (E + 2) (fLaro ) z = ...(14) F _ E(E + 2) 3 E 1'3 - 3 ... (9) (f4aro)v = 0 ... (15) where E is the total electric field in the medium of dielectric constant E. For the symmetrical aromatic where ~.L is ~he perpendicular polarisability of the molecules employed in the present studies; E can be aromatic. Using the value of f4aro (Table 2 anid resolved into two components, namely E", and EI/' ,uinert (Table 1) and various parameters recorded n Ez is the component of. the field in the direction of Table 3, the values of I' (assuming that the soluta the dipole axis of the solute, whereas E1/ is that at npoint dipole is acting at the ring centre) for vio s right angle to this direction. These field components complexes have been evaluated using Eq. (14). The results are given in Table 2.

TABLE I-DIPOLE MOMENT VALUES AT 307.4K OF POLAR I I SOLUTES IN DIFFERENT MEDIA I I I [Values in debye] Solute Gas Cycle- Ben- p-Xylene Mesi- ij phase hexane zene tylene c (a) (b) (b) (b) (b) /:~ Acrylonitrile 3.90 3.13 3.50 3.37 3.33 Hf : c;~ I Acetone 2.88 2.61 2.75 2.64 2.61 I Acetaldehyde 2.69 2.33 2.42 2.70 2.35 I Ethyl 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. Fig. 1

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electrostatic and the steric factors compete with TABLE2 - PARAMETERFSOR VARIOUSPOLAR SOLUTE-AROMATICeach other in the formation of the complex. SOLVENTMOLECULARCoMPLEXESAT 307.4K It is seen from the data in Table 2 that for a common System Kx (.tcomplex (.taro (.tlnd r reactive aromatic solvent benzene, the higher the gas (debye) (debye) (debye) (At phase value of the solute dipole moment, the larger the value of f'aro. Hence it is possible to conclude Acrylonitrile-benzene 1.507" 3.73 0.60 0.37 3.64 Acrylonitrile-p-xylene 0.929" 3.60 0.47 0.24 4.47 tentatively that for all these systems the interactions Acrylonitrile-mesi- are taking place through dipole-induced dipole tylene 0.736" 3.58 0.45 0.20 4.76. mechanism. However, a lower value of f'aro for

Acetone-benzene 6.000b 2.77 0.16 0.14 5.32 ethyl methyl ketone-benzene complex and the prog- Acetaldehyde-benzene 2.460b 2.46 0.12 0.09 5.57 ressive increase in the value of r (Table 2) for acetal- Ethyl methyl ketone- debyde, acetone, and ethyl methyl ketone seems to be benzene 5.280b 2.81 0.10 0.09 6.21 a clear manifestation of the blocking effect. (a) Ref. 6; (b) ref. 7. A perusal of Table 2 shows that, for a common solute acrylonitrile, the value of f'aro for the three aromatics benzene, p-xylene and mesitylene decreases TABLE3 - DIELECTRIC CONSTANTVALUES AT 307.4 K AND inspite of the fact that their polarisabilities increase. POLARISABILIT(YALONG SIX-FOLDSYMMETRYAXIS) VALUESFOR Here again, theblocking effect due to progressive sub- VARIOUSAROMATICSOLVENTS stitution of methyl groups in the aromatic molecule Aromatic solvent E(a) IXl. /1030 m8(b) seems to dominate the electrostatic effect. The fact that the substituents are obstructing the approach Benzene 2.2556 7.33 0,£ the solute to the reactive aromatic solvent is p-Xylene 2.2473 10.75 Mesity1ene 2.2629 12.47 evident from the progressive rise in the value of r (Table 2). (a) Ref. 4; (b) ref. 12. Acknowledgement Discussion The authors are thankful to Prof. R. P. Rastogi Neighbouring polar solute molecule polarises the for his keen interest in the work and facilities and to aromatic molecule through its n-electron cloud result- the UGC, New Delhi for financial assistance. ing in a dipole moment being induced in the latter. This process is then followed by electrostatic attrac- References tion between the two reacting species. At the same 1. BRIEGLEB, G., Electronen-dollor-acceptor-komplexes time, the solute experiences the resultant dipole (Springer-Verlag, Berlin), 1961. moment leading to an increases in f'complex, 2. FOSTER, R., Organic charge-transfer- complexes (Academic Homer and Yadava", have emphasized that there Press, New York), 1969. are two main factors governing the formation of 3. (a) EARP, D. P. & GLASSlOJ'..'ES.,, J chem. Soc., (1935), 1709 dipole-induced dipole complexes, namely electrosta- (b) HUKE, P. J., Studies on molecular interactions ill solu- tion, Ph. D. Thesis, University of Aston in Birmingham tic attraction and steric effects. The electrostatic (1968). factor, obviously, depends on .the polarising and 4. YADAVA, R. R. & YADAVA, S. S., Indian J. Chem. polarisability properties of the solute and the aromatic 16A (1978), 826. molecules respectively and for a common solute it 5. MCCLELLAN,A. L., Table of experimental dipole moments should increase on going up the aromatic series. This (Freeman, San Francisco), 1963, 64, 76, 85. 6. HOMER, J. & YADAVA, R. R., J. chem. Soc., Faraday should lead to closer approach of the solute to the Trans-I, (1974), 611. . aromatic and therefore an increase in f'aro' The 7. YADAVA,R. R. & YADAVA,S. S., Unpublished results. steric factor consists of two contributions, first a 8. FRANK, F. c., Proc. r. Soc. (Lond.), 152 (1935), 171. negative contribution due to methyl groups on the 9. FROHLICH, H., Theory of dielectrics (Oxford University aromatic solvent blocking the approach of the solute, Press, London), 1949. 10. HOMER, J. & CooKE, M. c., J. chem. Soc. (A), (1969), thus reducing the value of f'aro and a positive contri- 773. bution due to the solute being trapped by the aromatic 11. HOMER,J. & YADAVA,R. R., Tetrahedron, 29 (1973), 3853. methyl groups. Substituents on the solute molecule 12. LE FEVRE, C. G. & LE FEVRE, R. J. W., Rev. pure appl. may also exhibit similar steric effects. In fact, the Chem., 5 (1955), 261.

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