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A Kinetic Study of the Reaction of Diols with Hydrogen Halides 49

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A Kinetic Study of the Reaction of Diols with Hydrogen Halides 49 A KINETIC STUDY OF THE REACTION OF DIOLS WlTH HYDROGEN HALIDES BY P. S, RADHAKRISHNAMURTIAND T. P. VISVANATHAN (Dr of Chemiatry, Khallikote College, Berhampur-Ganjam, Orissa~ India) Recr August 12, 1968 (Communicatr by Prof. S. V. Anantakrishnan, F.A.Se., F.N.I.) ABSTRACT The kinetics of th~ reaction of diols with vmious hydrogen halides is discmsed. The teaetioa ktvolves ndghbouring group participation as postulated earlier, the rate of substitution being faster fora heavier nueleophile. The reaetions are inert in aqueous systems as the X- nucleo- philicity becomes negligible as compared to the nucleophilicity of water. At and above 7.5 per eent. water these reactions do not oecur. Varia- tioa of dielectric constant also notably a#fects the process. INTRODUCTION IN our previous communieation1 we reported the kineties of this reaction with HBr and postulated that this reaction involves anchimeric assistance by the Hydroxyl group. Ir was found worthwhile to study the action of other hydrogen halides on diols. We report here certain interesting features of this reaction. EXPERIMENTAL Materials.DAll the substrates used were obtained from Fluka and checked for purity by the usual procedure. The hydrogen halides used were freshly digtiIlod azeotropic mixtarr The solvent, glacial acr acid, was purified by the usual procedure and redistilled br use. l(.inetic method.--The kinetics were followed by the procedure de- scribed r~arlier,a RESULTS AND DtSOJSSION The second ord~r tate constant,5_ ar ,ummarized in T~bles I Ÿ II. A referente to these tables shows that the reaction is fastest with hydriodie 47 48 P.S. RADHAKRISHNAMURTI AND T. P. VISVANATHAN acid and slowest with hydrochloric acid. The rate constants of first dis- placement only are computed for ethylene glycol, propylene glycol and 1 : 3 butane diol, as the roaction is very sluggish after first substitution. Frost- Schwemer treatment 2 could not be applied as the time ratios were very high. With 1:4 butane diol the rate constants of both the steps have been com- puted using Frost-Schwemer method, in the case of HC1 and HBr only. In the case of HI the reaction virtually stopped after the first displacement and hence the rate constant of the first displacement alone is reported. TABLE I Second order rate constants (I. moles -I min. -1) of first displacement in diols in glacial acetic acid at 80 ~ C. HC1 Concentration of diol HC1 Ethylene glycol .. 0.0525 M 0.0518 M 1-06 Propylene glycol .. 0-04986 M 0.0558 M 0.912 1 : 3 Butane diol .. 0-04991 M 0.0499 M 0.0238 HBr Concentration of diol HBr Ethylene glycol .. 0.04718 M 0.05845 M 5.93 Propylene glycol .. 0.05645 M 0.05770 M 2.35 1:3 Butane diol .. 0.03265 M 0.06465 M 0,2101 HI Concentration of diol HI kl Ethylene glycol .. 0.02634 M 0-05460 M 12-18 Propylene glycol .. 0-02556 M 0-05415 M 4-24 1 : 3 Butanr diol .. 0.02588 M 0'05370 M 0-678 A Kinetic Study of the Reaction of Diols with Hydrogen Halides 49 TABLE II Second order rate constants (10S• moles-l min. -1) of both the steps in 1 : 4 butane diol in glacial acetic ccid at 80 ~ C. Concentration of k~ k2 diol Hydrogen halide HC1 0.02529 M 0.05050~M 4.95 1.3 HBr 0.02996 M 0.05955 M 10.6 1.97 HI 0.02646 M 0.05320 M 42.1 Very slow The question of reversible hydrolysis does not arise as halide solvolysis is very slow in a low dielectric medium like glacial acetic acid. The Grun- wald-Winstein Y-value for acetic acid is -- 1.64 and the relative rate of hydrolysis in glacial acetic acid is only 0.023 as compared to the Y-value for water, + 3.56 and the relative rate, 3,650. The results can be rationalised thus: As thege are bimolecular substi- tutions the rate process is dependent upon the nature of the nucleophile. Ir is suggested that the heavier members ate more nucleophilic largely because their atoms ate more polarizable. It looks as ifthe polarizability phenomenon is only one of the factors for the higher reactivity of the heavier nucleophiles. During the activation process the solvation cage of the attacking nucleophile must be disrupted anda new ' cage' be formed around the activated com- plex. As the solvation energies of ions increase as their charge to size ratios increase, ir requires more energy to desolvate a smaller ion than a larger one having the same charge. Hence small attacking groups exhibit lower nucleo- philicity as there is difficulty in removing the molecules of the solvent from the attacking site. The ionisation potential and its solvation energy ate the most important factors which are responsible for the relative order observed in the reaction, I- > Br- > C1-. 8 There is no complication of estorification in t~ese reactions as hydrogen halŸ seem to function better as nucleophilic agents than as mere catalysts for esterification. This is quite evident from the tate constants for the respective processes with respect to methanol. 50 P.S. RADHAKRISHNAMURTI AND T. P. VISVANATHAbl k 1.moles-1 sec_ a Esteri¡ .. 6.6 • 10-n Substitution .. 1 58 • 10-4 Under our experimental conditions even this remote possibility is ruled out as the concentration of alcohols is very low untike the larger quantities used in synthetic processes. Another important feature that requires exptanation is the relative stow- ness of the second step with HI in the case of 1 : 4 butane diol. Table III shows that the size of the halogen atom that is present in the molecule after the first displacement seems to be important. The steric repulsion seems to be the highest with I- antt least with Ct-, quite in consonanee with their sizes. TABLE III kl/k~ ratios of I : 4 butane diol with hydrogen halides HCI HBr HI k,/kz 3-8 5.4 .. Effect of Addition of Water It is very interesting that HBr reaction does not take place beyond 7.5 per cent. water. As the system is beeoming aqueous the Br- does not function as an effective nucleophile and there seems to be a competition between HzO and Br- and hence there is no reaction as shown in Table IV. The same effect of water has been reported by us earlier in the reaction of HBr with esters, s A plot of log k vs. mole fraetion of water slaows that the rea~tion stops at mole fraction of water 0"206 (7"55 per cent. water). l/ariation of Dielectrie Constant Addition ofbenzene.--It is interesting to observe that increase in content of hydrocarbon solvent like benzene is aceelerating the rate with all the diols (Table V). A plot of log k vs. 1/D in all the cases shows faix linearity. Decrease of dielectric constartt favouring the reaction is also similar to what we observed with ester cleavages with HBr. 6 .4 Kinetic Study of the Reaction of Diols with Hydrogen Halides 51 TABLE IV Second order rate constants (kx L moles -1 min_ 1) for ethylene glycol with HBr at 80 ~ C. in acetic acid-water mixtures Mole Solvent fraetion kl of water 1007o acetie aeid .. 0 5-93 97" 5% aeetie acid-2.5~ water (v/v) 0"0753 1 "0 957o aeetie aeid-5% water (v/v) 0.1432 0-06 92" 5% aeetic acid-7.5% water (v/v) .. 0"205 No reaction TABLE V S~cond order rate constants (L moles -1 min.-1) for diots with HBr at 80 ~ C. in acetic acid-benzene mixtures (v/v) 100% AcOH 10%benzene 20% benzene 90~ AcOH 80~ AcOH Ethylene glycol .. 5.93 8.27 10.91 Propylene glycol .. 2.35 5.33 8.05 1:3 Butane diol .. 0.2101 1.00 1.47 1 : 4 Butane diol*.. 0.0106 0.0138 0.0184 * First displacement. ,4ddition offormamide.--The reaction does not take place even at 2.5% of formamide and 97.5% of acetic acid (v/v). This observation along with the observation that the reaction does not take place with 7.5% water and 92.5% acetic acid (v/v) deserves comment. It appears tliat even smaU percentages of liquids of high dielectric constant are totally suppressing the nueleophilicity of X- thus stopping the reaction. 52 P.S. RADHAKRISHNAMUR'I-1 AND T. P. VISVANATHAN Mechanism We postulated that in these reactions there is neighbouring participation by the hydroxyl group leading to acceleration as evident from the rates of the first displacement. Progressively there is a decrease in the rate of first displacement as the position of OH's is varied. It was postulated that the reaction is composed of two factors: (1) the OH displacing the OH + forming the epoxide and (2) a relative fast cleavage of the epoxide by the nucleophile. It is evident from the activity of the nucleophiles--I, Br and Cl--that the second factor also is important in addition to the first one. Ir is not incom- patible with the mechanism postulated that both the factors are important as similar observations have been made in elimination reactions, especially El. It is seen E1 is composed of two steps, n slow HCR2--CR2--X -- ~ HCR2--CR ++X- (1) fast HCR2--CR + ~-~- H ++CR2=CR2 (2) Apparently though the rate-determining step is the first one the second factor is also taken irtto consideration for determining the overall rate of production of olefin. Similar reasoning is enough to explain the relative activity of the nucleophile though they are participating in a relatively fast step as compared to the step of epoxide formation.
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