المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 372-361 ,38
Thermodynamic Study on tautomerism reactions of some benzoin compounds by halogen titration method
A.S.P. Azzouz, M.A. Hussin and M.G. Al-Dabbagh Chemistry Depatrtment, College of Education, Mosul University, Iraq Keywords: benzoin, tautomerism, enediol, keto, halogen titration and thermodynamic
(NJC)
(Recevied on 12/4/2009) (Accepted for publication 20/4/2010)
Abstract The study is started with the synthesis of benzoin, 4-methoxybenzoin and 4- nitrobenzoin by well known method. Their structures are confirmed by physical methods such as m.p., UV and IR spectra. The study deals with the tautomerism reactions of type enediol keto form by simple halogen titration method. The results show that the keto forms in compounds stated are predominant in benzoin and 4-methoxybenzoin shows a similar result at only 298K, and reverse results at other higher temperatures. Finally, the calculated thermodynamic parameters were .estimated, namely Gْ, Hْ and Sْ support, the tautomerism reactions stated الخلةصة ابتدأت الدراسة بتحضير مركبات بنزوين، 4-ميثوكسي بنزوين و 4-نتروبنزوين وبطريقة معروفة. تم تثبيت الهيئات التركيبية للمركبات تحت الدراسة بالطرائق الفيزياوية مثل درجات النصهار واطياف U.V و I.R. يشتمل البحث على دراسة تفاعلت التوتوميرية من نوع enediol keto بطريقة التسحيح الهالوجيني والبسيطة. ذللت النتائج على ان اشكال هيئات الكيتو في المركبات تحت الدراسة هي السائدة في مركبي بنزوين و 4-ميثوكسي بنزوين وفي مديات درجات الحرارة المحصورة بين 328K-298. اما مركب 4-نيتروينزوين فقد اظهر نتائج متشابهة فقط بدرجة حرارة 289K ونتائج معاكسة بدرجات الحرارة العلى واخيرا فقد تم حساب المتغيرات الثرموداينميكية والمشتملة على حساب G ، ْ Hْ و ْ Sوالتي ادعمت تفاعلت التوتوميرية المذكورة في اعله.
Introduction melting point or boiling point, vapour The enolization(1) reactions in a pressure and the different forms of simple carbonyl compounds having spectra. Enolization(1) generally is a alpha hydrogen are the most discussed slow process, but it is catalysed by examples of prototropic tautomerism(2) acids and bases. reactions. These tautomerism reactions The concept of tautomerism of affect the physical properties of keto carbonyl(1, 3), analide(3) and Schiff(4-6) and enol forms, such as solubility, bases had been extensively studied.
105 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
The methods(7) were used for the Benzaldehyde was purified by normal determination of keto-enol equilibrium distillation. Pure sample has a B.P. of constants can be classified as chemical oC. methods and physical, particularly Synthesis of 4-methoxybenzoin spectroscopical, methods. The extent This compound was prepared of enol-keto tautomerism study in by a similar method(16). Transfer 1.25g phenol(8) or phenolic Schiff bases(4, 6, 9) of KCN dissolved in about 10ml of had received a great deal of attention water into 100ml round bottom flask. by many workers. NMR(10) and UV(6, 11- To the last solution, add 6.8g of 4- 12) spectroscopical methods had been methoxybenzaldehyde, 5.3g of pure used for the evaluation of tautomerism distilled benzaldehyde and 18ml of equilibrium constants in these phenolic ethanol. The mixture was refluxed for compounds. Infrared spectroscopic 1.5 hour. Steam was then passed measurements had been used to study through the solution until all ethanol enolization of simple ketones(13-14). The and nearly all unchanged aldehydes tautomerism of deoxybenzoin and were removed. Cooling and filtration some related compounds in the of the mixture. Dry sample has a m.p. gaseous state had been measured by of 106oC. the integrated ion current curve by Synthesis of 4-nitrobenzoin mass(7) spectroscopy. The final method A similar(16) procedure was was also used for tautomerism(15) followed to 4-methoxybenzaldehyde reaction of benzylidene o- by replacement of 4- hydroxyaniline in the gaseous state. A methoxybezaldehyde by 4- direct and indirect halogen titration nitrobenzaldehyde. Pure sample has a method(1) had been applied for the m.p. about 300oC. determination of enol contents in Tautomerism study(1) on benzoin and carbonyl compounds containing alpha 4-methoxybenzoin hydrogen atom. As a 1. 10ml of 0.025N of each continuation of tautomerism reactions compound was placed in 50ml were studied(6-7, 11-12, 15) in our conical flask. The last was laboratory. This work describes the immersed in a water bath until influence of temperature on equilibrium is attained at 25oC. equilibrium constants of tautomerism 2. Add 2ml of 5% v/v bromine reactions of some benzoin compounds, solution in methanol, which is by using the indirect halogen titration previously thermostated at method. The influence of nitro or 25oC. the mixture is left 30 methoxy substituent group in benzoin minutes for completion of on equilibrium constant values of reaction of bromine with enol tautomerised reactions, beside their contents in the sample. thermodynamic parameters are 3. Add a few milliter of 5% v/v considered. aqueous formic acid solution to Experimental the final mixture, until the red Materials and Methods colour of bromine is The following chemicals are completely disappeared, used as supplied without any further followed by addition of 10ml of purification. They are, benzoin, 10% w/v aqueous potassium methanol, bromine, 4-methoxy- iodide. benzaldehyde,4-nitrobenzaldehyde, 4. The last mixture is left for potassium cyanide, formic acid and about 30 minutes and titrated sodium thiosulphate penta hydrate.
106 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
-3 with 10 N Na2S2O3.5H2O. Record the volume of titrant. 5. Calculate the % of enol content in any sample from the neutralization equation: meq of enol = meq of I2 = 2 meq of titrant 6. Repeat steps (1-5) at other temperatures 35, 45 and 55oC and evaluate the thermodynamic parameters of tautomerism in benzoin compounds. Instrumentation 1. The desired temperatures under study were fixed by using a water Julabo thermostat model SW 23. 2. The IR spectra of solid benzoin compounds are measured by KBr disk method and by using a computerized Bruker Tensor- 27. 3. The UV spectra of 10-5M ethanolic solutions of benzoin compounds were measured by a double beam computerized UV 1601 Shimadzu spectrophotometer and by using a matched 1×1×3cm3 silica cells.
107 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
Table (1): Melting points, IR and UV spectral data for benzoin, 4-mehoxybenzoin and 4-nitrobenzoin
IR spectra cm-1
UV spectra λnm (εmax) Symbol Structure m.p. °C in ethanol - OH C=O Arom. -OCH3 NO2
OH O 206(16600), I C C Ph 133-135 3416.08 (m) 11680.16 (s) 1598.08 (s) 248(13900), 290(900) H
OH O 211(580), 252(6100), CH O C C Ph II 3 95-97 3478.81 (m) 1665.36 (s) 1600.03 (s) 1175.08 (s) H 281(5900)
OH O 221(28300), 270(7300), O N C C Ph III 2 300 3374.99 (m) 1725.05 (s) 1606.25 (s) 1572.72 (s) H 341(1800)
m = medium s = strong
108 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
Results and Discussion methoxy group as compared with nitro A direct and indirect halogen group results in a minimum melting titration methods(1) had been used for points value 95-97oC. The IR spectra the determination of enol and content for all benzoin compounds observed in in carbonyl compounds, containing Table 1, shows the following wave alpha hydrogen atom. In the indirect numbers: method, a solution of the substance to 1. The sticking frequencies for all OH be investigated is titrated with a groups of benzoin compounds have a standard solution of bromine. The enol medium absorption range between present reacts rapidly with bromine, (3375-3479)cm-1. yielding a bromo ketone, whereas the 2. Similarly, the carbonyl and direct bromination of keto form is very aromaticity stretching frequencies for slow. The problem of such method all benzoins have a ranged of values might result in too high enolic values between (1665-1725)cm-1 and (1598- and rather unsharp and point. This was 1600)cm-1. avoided in a modification often 3. Additional strong absorption referred to as indirect method, which frequencies are observed for p- was applied to our benzoin compounds methoxy and p-nitro groups in p- under investigation. Actually, before methoxybenzoin and p-nitrobenzoin at studying the tautomerism of benzoin, a wave numbers 1176.0cm-1 and p-methoxybenzoin and p-nitrobenzoin, 1572cm-1 respectively. by the indirect halogen titration, it was The UV absorption spectra for decided to study their chemical a benzoin compounds are seen in Fig. 2 structures by physical method. The last and is summarized in Table (1). This includes melting points, IR and UV shows three absorption bands at spectra of benzoins mentioned as an wavelengths (nm) and molar available facilities in our laboratory, as extencition coefficient values of max shown in Table 1. The melting points (Liter.mole-1.cm-1). Benzoin shows a of benzoin compounds are increased in longer absorption band at 290.0nm the following order: with max value of 900 for n→* p-nitrobenzoin>benzoin > p-methoxybenzoin transition(19). The other two absorptions The increases in melting point at wavelengths of 248nm and 206nm of p-nitrobenzoin as compared with with max values of 13900 and 11500 others can be explained by two fold respectively for →* transition(19). p- reasons: Methoxybenzoin and p-nitrobenzoin 1. The relative increase in show a similar absorption transition molecular weight of p- bands for benzoin. All UV and IR nitrobenzoin as compared(17) absorptions confirm the chemical with benzoin or p- structures of benzoin compounds under methoxybenzoin. This is in a study and are agreed with other good agreement with other compounds in literature(20). This studies observed previously. encourage the workers to deal with the 2. The greater extents of p- influence of temperatures on the nitrobenzoin to polymerize, possibly equilibrium constants values of types by intermolecular(18) hydrogen enediol keto tautomerised bondings. This produces a long chain reactions in benzoin compounds, of molecules of relatively high beside their thermodynamic molecular weights, and agrees well parameters, surely, the temperature as with the previous paragraph. The known has a great influence on many reverse electronic behaviours of p-
109 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
chemical reactions, as tautomerism of included Gْ, Hْ and Sْ deoxybenzoin(7) and some related thermodynamic parameters estimated compounds in the gas phase, phenolic from equations 1-3 of the forms: (Schiff base(21) in solution and gaseous Gْ = -R T lnK… (1 states, some aromatic mono and bi Hْ Schiff bases(22), formation of hydrogen lnK = constant – RT … (2) (23) (24) bonds , pKa of benzaldoxime and (other(25). Gْ = Hْ – T Sْ … (3 The equilibrium constants for The signs of Gْ are negative tautomerism reactions of benzoin for benzoin, 4-methoxybenzoin at all under study of type enediol keto, temperature. This means that their shown in Table 2, have estimated tautomerism reactions are spontaneous. between temperatures 298-328K. A Similar sign is observed for 4- maximum value of 80.53 is observed nitrobenzoin at 298K. At other higher for benzoin at 298K. Increasing the temperatures, their Gْ signs are temperature to a higher values are changes to a positive, due to the accompined by decreasing the values smaller equation values of lower than estimated. This means that the unity. The heats of tautomerism relationship between the equilibrium reactions Hْ in benzoin and its constant values and the temperatures 4-methoxy or 4-nitro derivatives are are inversely proportional. Introduction evaluated by plotting a graph between of electron donating group as methoxy lnK versus the inverse of absolute or an electron accepting group as nitro temperature using Excel program as in as in 4-methoxy benzoin or 4- Fig. 3. The plots show a straight lines nitrobenzoin respectively is resulted to R2 range values 0.9678-0.9971. They the relative decrease in equilibrium have a negative signs values, which constant value as compared to benzoin. means that heat of tautomerism is The equilibrium constant exothermic. In addition to that, the Hْ values seen in Table 2, have a ranged values are comparable with of values between 80.53-0.07, literature(1, 10). The Sْ values of depending on the chemical structure of entropies of tautomerism in benzoins benzoin or its 4-methoxy or 4-nitro are depend on their chemical derivatives and the ambient structures. Their signs are positive in temperature. Those values of greater benzoin and 4-methoxybenzoin. This than unity as observed for benzoin and means that their tautomerism reactions 4-methoxybenzoin at all temperatures , are directed toward the more random. these mean that the keto forms are This can be expected if intermolecular predominant or they are more stable. 4- hydrogen bonding for the double Nitrobenzoin shows a similar result hydroxyl groups in enediol tautomer is only at 298K. At other higher greater than its keto tautomer. temperatures, the equilibrium constant have a values lower than unity or the Conversely, the Sْ signs for the favoured tautomer and the more stable 4-nitrobenzoin are negative or they one is the enediol. have an opposite interpreted to those The thermodynamic of just mentioned before. Those tautomerism in compounds under study tautomerisms under study are in a good represent the various forms of energies agreement with other tautomerism studies as deoxybenzoin(7), phenolic associated with enediol keto (21) reversible reactions. These are Schiff bases and aromatic mono and bi Schiff bases(22).
110 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
111 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
Table (2): Equilibrium constants of tautomerism of benzoin compounds at various temperatures and their thermodynamic parameters. ml of Temp. (T) Enol Keto Gْ Hْ Sْ Compound Na S O ln K K 2 2 3 mqeq/10 ml mqeq/10 ml kJ.mole-1 kJ.mole-1 kJ.mole-1 10-3 M 298 6.2 0.00065 0.05235 4.39 -10.88 -6.57 +14.46 308 7.3 0.00077 0.05223 4.22 -10.81 -6.35 +14.48 Benzoin 318 7.7 0.00082 0.05218 4.15 -10.97 -6.37 +14.47 328 8.3 0.00088 0.05212 4.08 -11.13 -6.38 +14.48 298 10.7 0.00129 0.05921 3.83 -9.49 -2.45 +23.62 4-methoxy 308 11.2 0.00136 0.05914 3.77 -9.65 -2.38 +23.60 benzoin 318 11.5 0.00139 0.05911 3.75 -9.91 -2.41 +23.58 328 12.0 0.00145 0.05905 3.71 -10.12 -2.37 +23.63 298 1.0 0.00013 0.00018 0.32 -0.79 -59.39 -196.64 4-nitro 308 1.8 0.00023 0.00008 -1.05 +2.69 -57.87 -196.62 benzoin 318 2.0 0.00025 0.00006 -1.43 +3.78 -58.75 -196.64 328 2.3 0.00029 0.00002 -2.66 +7.25 -57.24 -196.62
112 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
Benzoin
Methoxy benzoin-4
113 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
Nitrobenzoin-4
Benzoin
114
methoxy benzoin-4 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
Nitrobenzoin-4
References 6. A.S.P. Azzouz and A. B. N. 1. Ed.Z. Zabicky, The chemistry AL-Dabagh, National J. of carbonyl Group, 1970, 2, Chem., 2007, 26, 295. Interscience, London, 157-240. 7. A.S.P. Azzouz, Mutah J. Res 2. C.K. Ingold, Structures and and Stud., 1993, 8, 93. Mechanism in Organic 8. S.H. Pine, J. B. Hendrickson, Chemistry 1953, G. Bell, D. J. Cram and G. S. London, p. 529. Hammond, Organic Chemistry, th 3. G. Allen and R. A. Dwek, J. 4 ed., 1981, McGraw Hill, Chem. Soc., 1966,161. London, p. 183. 4. G.O. Dudek and E. P. Dudek, 9. M.D. Cohen, T. Hirshberg and J. Amer. Chem. Soc., 1964, 86, G. M. J. Schmidt, J. Chem. 2483. Soc., 1964, 2060. 5. A.S.P. Azzouz, National J. Chem., 2005, 22, 214.
115 المجلة القطرية للكيمياء-2010 المجلد الثامن والثلثون National Journal of Chemistry,2010, Volume 38
10. G.O. Dudek and E. P. Dudek, Identifica-tion of organic J. Amer. Chem. Soc., 1966, 88, compounds, 2nd ed., John 2407. Wiley, NewYork. 11. A.S.P. Azzouz, A.A. Rahman 21. A.S.P. Azzouz, Nat. J. Chem., and A.G. Taki, National J. 2006, 22, 214. Chem. 2005, 20, 568. 22. A.S.P. Azzouz and A.B.N. AL- 12. A.S.P. Azzouz, A.A. Rahman Dabagh, Nat. J. Chem., 2007, and A.G. Taki, National J. 26, 295. Chem, 2007, 26, 217. 23. S.K. Al-Dilami and A.S.P. 13. A.C. Cope and B.D. Tiffany, J. Azzouz, J. Edu. Sci., 1990, 10, Amer. Chem. Soc., 1951, 73, 45. 4158. 24. A. S. P. Azzouz and N.A. AL- 14. N.A. Abood and A.F. Ajam, J. Azzawi, J. Edu. Sci., 2002, 1, Chem. Soc., Pakistan, 1985, 7, 20. 1. 25. A.S.P. Azzouz and N.A. Al- 15. A.S.P. Azzouz and I.Z. Azzawi, 2002, 14, 20. Sulyman, J. Edu. Sci., 2004, 16, 125. 16. W.S. Ide and J.S. Buck, Organic reactions, 1948, 4, 269-81. 17. G.C. Pimental and A.L. Mecellellan, The hydrogen bond, Freeman, NewYork, 1960. 18. A.S.P. Azzouz and M.M.H. AL-Niemi, Z. Phys. Chem., 2005, 219, 1591. 19. A.S.P. Azzouz, A.A. Rahman and A.G. Taki, J. Edu. Sci., 2003, 15, 1. 20. R.M. Silverstein and G.C. Bassler, Spectrophotometric
116