Indian Journal of ChemistryVol. 32A,June 1993, pp. 472-477

Papers

Transfer enthalpies of tert-butyl chloride in some aquo-organic solventsMira Datta (nee Sarkar), Mohon L Das, Jayati Datta & Kiron K Kundu*

Physical Chemistry Laboratories, Jadavpur University, Calcutta 700 032, India

Received 15 September 1992; revised and accepted 8 February 1993

Transfer enthalpies, IlH:}, of tert-butyl chloride (r-BuCl) from water to aqueous mixtures of vari-ous cosolvents viz., protic ethanol (EtOH), tert-butanol (t-BuOH), ethanediol (EG) and methoxyetha-nol (ME), aprotic 1,2-dimethoxyethane (OME) and 1,4-dioxane (0) and dipolar aprotic N,N-dime-thylformamide (OMF) and dimethyl sulphoxide (OMSO), have been determined at 25°C by measur-ing the partial molal heats of solution at infinite dilution (IlH~) with a rapid response Tronac (model458) recording titration calorimeter by avoiding the uncertainties from hydrolysis of t-BuCI in thesolvents. These IlH~ values on extrapolation to zero cosolvent composition in each of the aqueouscosolvent systems lead to a common value for that of wat~ (willI)}), which is equal to 6.27± 0.25kJmol-I. IlH~ values of the initial state (IS) of z-BirCl (= ,IlH~}-wIlH?)on subtraction from the corres-ponding transfer activation energies iJIlH" (= sllH'" - wllHt) as obtained from parallel kineticstudies of the hydrolysis/solvolysis of t-BuCI in the solvents at different temperatures, and reportedelsewhere, helped determine the' transfer enthalpies IlH:)(TS) of the transition state (TS). A compa-rative view of the composition profiles of IlHl, as well as of iJIlH" and IlH:) (TS) in the solventsystems reflects that while the variations in IlH? values are guided by complex structural and inter-action effects in the respective solvents, f).iI~(IS) values are fairly small as compared to iJIlH" va-lues. Evidently, iJIlH" values are guided by the contributions of IlH:} of TS rather than that of IS,contrary to what has been concluded by Arnett et al. from the corresponding studies only in aque-ous ethanol system.

Kinetic solvent effects (KSE) of a key SN'-typereaction, viz. hydrolysis/solvolysis of tert-butylchloride (t-BuCI) in different media were longsince rationalized in the light of general parame-ters such as solvent polarity which summed up allthe specific and non-specific interactions of themedia with the initial state (IS) and transition state(TS). Unfortunately, quantitative measure of thisproperty was always empirical. .Consequently alarge number of solvent polarity scales are avail-able!". Because of this recent years have wit-nessed increasing efforts+" for the rationalizationof KSE's on this reaction from a thermodynamicapproach. These essentially entailed determiningthe KSE's by splitting the activation parametersinto contributions of IS and TS and hence attem-pting to correlate and understand their behaviourin the light of physico-chemical properties of thesolvents. Despite extensive studies on the KSE'sof the hydrolysis/solvolysis of I-BuCI in the differ-ent aqua-organic solvents 13·1 h, understanding ofKSE in the light of IS and TS solution yet remainslargely unsolved because solvation data for the ISor TS were apparently hard to obtain, especially forl :BuCl, due to its rapid hydrolysis in aquo-organicsolvents.

Recently, however, it has been observed by USl7

that heats of solution of IS (/-BuCI) are directlymeasurable by a commercially available rapid re-sponse Tronac (model 458) recording titrationcalorimeter not only in aqueous ethanol, as hasbeen done by Arnett et al.5 with their self-de-signed calorimeter, but also in a host of aquo-or-ganic solvents. Besides, the transfer free energiesof TS of t-BuCl: (CH3)3C+ox ... Cl-o.x (Ref. 18),could be tentatively estimated by determining thesame for a salt viz. (CH3hNHCl and evaluatingthe corresponding quantities of its ion-pair(CH 3hNH +Cl- (ref. 17). In fact, this salt becauseof 3(CH, -) groups instead of 4(CH1-) groupsof (CH,)~NCl salt used by Arnett et aP in thecase of aqueous ethanol and Abraham et al.'a.19 insome pure solvents, will be the nearest model ana-logue of the transition state TSspecies of t-BuC!.This- is of course possible, because of our knowl-edge on the single ion energetics based on widelyused tctraphenylarsoniurn tetraphenylborate(TATB) reference electrolyte assumption" in dif-ferent aquo-organic solvents.

Therefore, as a part of comprehensive studieson the KSE's of the hydrolysis of t-BuCI, we haverecently determined 17 the rate constants and the

DATI A et al.: TRANSFER ENTHALPIES OF t-suet 473

u

~~~========~======~~~~i=

.:.-::-...---·-Hydrolysis storIed

mV-

Fig. 1 - (A) Typical heater run; (B) typical burette run.

related transfer energies of activation ~ ~X" )[= ,~X" - II ~X "] of the reaction in several aque-ous mixtures of a number of organic solvents viz.protic ethylene glycol (EG), glycerol (GL) andmethoxyethanol (ME), aprotic dimethoxyethane(DME) and dioxane (D) and dipolar aprotic N,N-dimethylformamide (DMF) and dimethyl sulpho-xide (DMSO). In the present paper we are pres-enting the transfer enthalpies ~H~ of t-BuCl in allthese aquo-organic solvent systems save that ofglycerol, as obtained by measuring the partial mo-lal heats of solution, ,~H~lof t-BuCI at infinite di-lution at 25°C from direct calorimetry. The corre-sponding data were also obtained for the aqueousmixtures of ten-butanol (t-BuOH) as well as etha-nol (EtOH), the latter for the sake of comparisonwith the corresponding data reported previouslyby Arnett et al", Notably, heats of solution wereobtained from heat changes recorded within 5-10s of addition of t-BuCI i.e. well before the onsetof hydrolysis.

Materials and MethodsThe quality and the, methods of purification of

the organic co solvents used were as describedcarlier!":'. Solutions of different compositions viz.10, 30, 50 and 70 wt % of the co solvents werefreshly prepared using triply distilled water. t-BuCI (AR, BDH) was first purified by keeping in-to a bed of baked/dry AlcO, taken in a stopperedbottle for absorption of water and then by distilla-tion. The cut with proper refractive index was

used. The stock sample was kept away from lightand stored in a desiccator. Partial molal heats ofsolution of t-BuCl at infinite dilution, !!.H;l,weremeasured with a rapid response TRONAC Iso-perbiol Recording Calorimeter (model 458) inwhich error from the heats of solvolysis could beeliminated. The heat capacities (llmV) of the sys-tem before and after the experiments were deter-mined and the mean was used in the calculationof enthalpy of solution.

Fig. 1a shows a raw-data plot for a typical hea-ter run. Fig. 1b shows a raw-data plot for a typi-cal burette run, which indicates that the initial so-lution slope takes a turn when the heats of hydro-lysis becomes effective to change the direction ofthe base line. Evidently, the initial solution slopecould be reasonably assigned to that for the heatof solution of the added t-BuC!.

Heat change for the solution was given by Eq.( I)

(A)

(8)

Q,oln = Cp.i x Atlsoln slope - initial slope] ... (1)

where C . is the heat capacity of the system inp.r

llmV before adding t- BuCl and M is the time forwhich heater is on. Since very little amount of thesubstrate (t-BuC!) was added from the burette forthe present study (vide Table 1) the final and in-itial heat capacities were found to be equal i.e..Cp.i= CpJ' Therefore, the change in enthalpy dueto solution per mole, tacitly taken to be as thepartial molal heats of solution at infinite dilution',H~l(IS). is given

~H;l= Q,o'n/no. of moles of (-BuC! added (Jz'mol)

... (2)

Since the involved error depends upon variousfactors such as the amounts of the added sampledictating the size of the deflection and the heat ofsolution, specific heat of the systems as well asthe instrumental factors, no single estimate of er-ror could be adduced to over experimental re-sults. However, the reproducibility of the data isof the order ± 0.25 kl mol- 1 and our experimen-tal results compared fairly well with the literaturedata in aqueous ethanol system.

Results and DiscussionThe relevant calorimetric data for the partial

molal heats of solution at infinite dilution !!.H~':IS) for the initial state (IS= r-BuCl) are presentedin Table 1. Those in EtOH-H,O mixtures have al-so been included in the Table for the sake ofcomparison with those of Arnett et al.» data'.

474 INDIAN J CHEM, SEC A, JUNE 1993

Table 1 - Calorimetric data for the partial molal enthalpy of solution at infinite dilution l1il:' of I-BuCI and the values of I1H:' (IS),al1H~ and I1H:'(TS) in kJ mol' at 29g.15 Kin different compositions of different aquo-organic solvents

Wt% mol % Sp. heat Initial Slope Soln Slope time No. of moles I1H~'(IS) I1H:'(IS) osn: I1H:'(TS)cosolvent cosolvent (Cp) (S,) (S",'nJ (t) of

j rnv x to! mY see x 10-'mY see see I-Bue) x 10"

Et-OH-HP mix.

0 0 6.27"15 6.4 2.91· 3.26 114.0 2.5 1.09tl 7.36 1.09 - 1O.67h -9.5825 11.5 2.83 4.07 195.7 2.5 I.09R 12.36 6.09 - 12.96b -6.R730 14.4 2.76 8.70 313.1 4.0 1.758 15.42 9.1535 17.4 2.75 4.35 221U 4.2 l.tl45 15.27 9.00 -13.30b -4.3660 37.0 2.33 3.95 156.5 2.1 0.923 8.07 1.80 -6.29" -4.49

TBA-Hpmix.

10 2.6 2.91 3.91 146.g 2.0 0.tl79 9.48 3.21 -3.67" -6.46

20 5.7 2.52 3.55 Ig2.6 2.0 0.879 11.41 5.14 - 21.47" -16.33

30 9.6 2.73 5.22 130.0 2.5 1.09g 7.78 1.51 -1O.07b -8.5850 19.5 2.45 2.90 58.0 2.7 1.186 3.05 -3.22 -0.83" -4.05

EG-H20 mix.

10 3.2 2.63 3.65 98.0 3.0 1.318 5.64 -0.64 -9.69 -10.3330 11.1 2.46 3.56 93.0 2.0 0.879 5.02 -1.26 -28.05 - 29.31~O 22.5 2.51 4.74 65.0 2.0 0.879 3.46 -2.82 '. -8.88 - 11.16

70 40.4 2.18 5.02 ra.o 2.0 0.879 4.12 - 2.16 1.82 -0.34

ME-Hpmix.

10 2.6 2.84 4.15 66.0 3.0 1.318 3.97 -2.30 -36.4 -38.74

30 9.2 2.52 7.17 4g.0 3.0 I.3lg 2.30 -3.97 - 27.33 - 31.30

50 19.2 2.27 4.66 23.0 2.4 1.054 o.ss -5.29 10.44 5.1570 35.6 2.07 2.33 35.0 2.0 0.879 1.53 -4.74 -1.00 -5.74

DME-H!O mix.

10 2.2 2.69 3.26 65.0 2.8 1.230 3.80 -2.48 -13.95 -16.43

30 7.9 2.65 7.39 15.0 2.2 0.967 o 4tl -5.tlO -15.61 - 21.41

50 16.7 2.34 2.83 14.0 2.3 1.011 0.65 -5.63 - 57.94 -63.57

70 31.g 1.95 O.oI 49.0 2.4 1.054 2.17 -4.11 - 13.51 -17.62

D-Hpmix.

10 2.2 2.57 3.59 78.0 2.0 0.879 4.65 -1.63 -39.06 -40.69

30 8.1 2.34 6.52 29.0 3.1 1.362 1.19 -5.10 -15.07 - 20.17

50 17.0 2.16 z.rs 9.0 2.0 O.tl79 0.32 -5.95 - 21.tll - 27.76

70 32.0 1.89 2.17 43.0 2.0 0.ll79 I.7tl - 4.50 -12.28 - 16.7ll

DMSO-HP mix.

10 2.4 2.70 4.19 81.0 3.1 1.362 4.10 -2.17 -19.39 - 21.56

30 9.0 2.51 3.01 65.0 2.1 0.923 3.55 -2.72 -12.76 - 15.48

50 18.7 2.19 4.89 ss.o 2.0 0.tl79 4.64 -1.63 - 22.39 -24.02

70 35.0 1.98 3.91 91.0 1.9 0.835 3.94 -2.33 -6.55 -8.88

DMF-H!O mix.

10 2.7 2.79 1.70 54.0 2.3 1.011 3.34 -2.94 -2.55 -5.49

30 9.5 2.59 6,52 57.0 2.4 1.054 2.97 -3.30 - 29.69 - 32.99

50 19.8 2.23 4.7g 97.0 2.6 1.142 4.72 - 1.55 - 21.92 - 23.47

70 36.5 1.96 3.26 75.0 2.5 1.09ll 3.19 -3.0ll -6.1ll -9.26

a Extrapolated data for pure water as obtained in different types of aquo-organic solvent systems [vide Fig. 2 (A-H)I"As computed at 25°C by using the coefficients A B. C in the expression log", k ; = Ay I + B log '" T + C as in references 5 and 14

10 20 30

Ie) ME-H20 ",ix

-40;L::o 10 40 60

DATTA et al.:TRANSFER ENTHALPIES OF t-'SuCi 475

40

lOr IBIEG-H,O Mi•.0V'/.- -,6"H"-10 • \,6H;nS)

-ll

-30 /

-40

t~':~<"""01010304050

50

mol ·1. r csctvent -

o-10

IE) Dioxane-H20 miX

6l>H~/6Ht{TS)

" /,tf\'6,. -:

T_oE~~<,

'"'"Ee8.

;\ -.-~6H,(IS)

o0!:---2:':-0----'40,-~60

-2016 .

1:/\4' ~6H,(IS)

°O~~~10~L-2~O~~~

oIF) DMSO-H,O mi •.

66H',/t:.Hf(TS)

-40

2

°O~-~20----'4~0 --'60

(HI DMF-H,O mix

-40

6 ,

:~6H:(IS)

o 20 40

mol % cosolvent --

Fig. 2-(A) Comparison of I1H;' (IS) (kJ moll) of I-SuCI inEtOH-H"O mixtures of 25°C with literature data [Arnett etat's data (0) and present data (e)] and in t-SuOH-H,O solu-tions (11). (B-D) Variation of enthalpy parameters 11!r:' (IS),al1H" and I1H:1 (TS) for solvolysis of I-SuCi with mole frac-tion ot cosolvent in different aquo-organic solvents at 29g.l5K. (E-H) Variation of enthalpy parameters I1H~' (IS), esn:and I1H:' (TS) for solvolysis of I-SuCI with mole fraction of

cosolvent in different aquo-organic solvents at 2,}8.15 K.

These l!:.H~' (IS) helped determine the corre-sponding values of transfer enthalpies of IS, l!:.H:'(IS), by subtracting the corresponding values,"l!:.H~, in pure water. Since the latter value couldnot be obtained from direct measurements, it hasbeen obtained by extrapolating l!:.H~ (IS) values atthe initial cosolvent systems to zero cosolventcomposition (X, = 0). These l!:.H~ (IS) values wereused to determine transfer enthalpies of the trans-ition state (TS), l!:.H~ (TS) from the correspondingvalues of i)l!:.H # ( = ,l!:.H # - wl!:.H # ).

... (3)

60

All these quantities are also presented in TableI.

The variation of these molal heats of solutionof IS, l!:.H~1(IS) and the transfer enthalpies of acti-vation (i)l!:.H # ) and of TS, l!:.H:1 (TS), with coso 1-vent compositions are illustrated in Fig. 2 (A - H).The l!:.H~1(IS)-composition profiles, on extrapola-tion to X, = 0 gives wl!:.H~1(IS) value as 6.27 ± 0.25kJ mol- I, while the literature values vary fromI.OS (ref. 21) to 12.00 (ref. 22) kJ mol- I.

From Fig. 2 it is also interesting to note thatl!:.H~1data in the aqueous ethanol compare fairlywel1 with the corresponding data by Arnett et al.,and both sets of data exhibit maximum aroundthe same composition. This indicates that the ob-served l!:.Fi~'(IS) data in all the aqueous cosolventsystems are fairly reliable and meaningful. More-over, the fact that the maximum in l!:.H~' (is) inaqueous (-BuCl or TBA appears at still lowercosolvent composition (Fig. 2) also substantiatesthe same, since the magic composition:" arisingfrom the effect of packing imbalance is less forlarger sized I-BuOH than than for smaller sizedEtOH.

A perusal of the l!:.H~' (IS) - composition pro-files for all the cosolvent compositions exceptingthose for I-BuOH and ElOH indicates that themagnitudes of l!:.H~' (IS) arc fairly smal1 and liewithin 0-6 kJ mol'. Moreover, while this profilepasses through an endothermic maximum at wa-ter-rich compositions in aqueous I-BuOH systemlike that in aqueous EtOH system, those in mostother solvent systems pass through an endother-mic minimum around 10 to 20 mol% cosolventsfollowed by a small endothermic maximum athigher compositions in some of the aqueous co-solvent systems.

If one recalls the work of Arnett et al.<; for theexplanation of the distinctive minimum i)l!:.H t forthe solvolysis of r-Bn'Cl at 0.8-0.9 mole fractionof water in aqueous EtOH system one would note

476 INDIAN J CHEM:SEC A,JUNE 1993

that the reponed I1H~l values for a numher ofcarefully chosen salts and non-electrolytes passthrough extrema in the same solvent compositionrange. These show endothermic maxima whereosn: for solvolysis of f-BuCI usually gives mini-ma and other relevant properties show extrema.The size of the minima depends primarily on thevolume which solvent molecules occupy. Further-more, the size is greater for non-electrolytes thanfor salts of comparable size. This led them pro-pose an empirical explanation of theoliH ~- composition minima which arc observedfor reactions in which non-polar ground stategoes to a polar transition state. Data were citedfrom a rcvicw:' to support the idea that additionof alcohols to water increases the structurcdnessof the solution to a maximum in the region notedabove. Addition of a small amount of a thirdcomponent to highly aqueous ethanol solution ap-pears to result in less structure formation thanwhen addition is made to pure water. Thus theycontended that the endothermic minima referred10 above result either because the solute is a moreeffective structure maker in water than in the hi-nary solvent or a more effective structure break-er in the latter. The behaviour of ions carryinglarge non-polar groups will he determined hymixing of these effects.

If Arnett et al.s arguments are taken to he true,the observed minima in water-rich compositionsof EG and ME-water systems and that followedby maxima at higher compositions in some otheraqueous cosolvent systems like 0, ME, DMF andDMSO should appear to suggest that all these co-solvents at initial compositions induce hreakdown of 3D structure of water which will he fol-lowed by an intermediate region where the struc-turedness will increase either by the formation of3D-water structure or l-l-bonded intercomponentcomplexes. But various properties·~:i do not unam-higuously substantiate the same.

More importantly however, since the enthalpyterm is the sum of free energy and entropy termsand since entropy term rather than enthalpy term,is a better indicator of solvent structural effect, itis difficult to understand IiH~l(IS) values unambi-guously in terms of solvent structural effect unlessIiCI: (IS) values are available. The latter valuesare however, difficult to determine directly be-cause of fairly rapid hydrolysis of r-BuCl, thoughshown feasible in some pure non-aqueous sol-vents-". That is why attempts have been made todetermine the same by an indirect way, as wouldhe discussed in our suhsequent papcr".

Referring to the compositron profiles of flH:'\TS) (vide Fig. 2) in these present aqueous cosol-vent systems as well as in aqueous I-BuOH sys-tem, as obtained from literature data for oliH" 17,

we find that these profiles are more or less exactreplica of the corresponding oliH" -compositionprofiles in the respective solvent systems. In otherwords, it is evident from Fig. 2 that IiH\1 (IS) be-ing relatively small, variation of esn= with com-position is dictated chiefly hy that of IiH~1(TS).Evidently, Arnett's' contention that the initialstate solvation is the root cause of the well knownendothermic minima in oliH'" -cornposition pro-files for the solvolysis of I-BuCl in aquo-organicsolvents is hardly tenable.

ReferencesI Reichard! C. (a) ,I/lgell" C!II'I/I. 1111Ed ~ (l'Hl:;) :!9-~();

(b) 18 (1979) 98-JlO; (c) Pure appl. Chern. 58 (1982)1867-84.

::! Koppel I A & Palm V A in Advances ill linear free energyrelationship (Plenum, New York) 1972.

3 Abraham M H. (a) Pure app/ Chern, 59 (1985) 1055 andrelevant references therein: (b) Abraham M H. Taft R W& Kamlet M J. J org Chern. 46 (1981) 1655; (c) AbrahamM H, Doherty R M, Kamler M J, Harris J M & Taft R W,J chem Sac Perkin Trans ll, (1987) 93, 1097; (d) Abra-ham M H. Grellier P L. Nasehzedeh A & Walker R A C.J chem Sac Perkin Trans II. (198H) 1717.

4 Leffler J E & Grunwald E, Rates and equilibria of organ-ic reactions (Wiley, New York) 1963.

:; Arnett E M. Bentrude W C. Burke J J & Duggleby P. 1Am chem Sac. 87 (1965) 1541.

6 Abraham M H, Prog phys org Chern, II (1974) I.7 Parker A J. Adv phys org Chem, 5 (1967) 173; Chern

Rev. 69 (1969) I .. \Iexander R A. Parker A J & BroxtonT. J Am chem Sac. 90 (1966) 504<).

H (a)Blandamer M J & Burgess J, Chern Sue Rev, 4 (1975)55; B1andamer M J, Adv phys org Chern; 14 (1977) 203and references therein,

9 (a) Engberts J B F N in Water. a comprehensive treatise.edited by F Franks (Plenum, New York) 6, 1<)76;(b) Pureappl Chern, 54 (1982) 1947,

10 Kondo Y. Ittoh M & Susabayeshi. J chem Soc. FaradayTrans I. 7H(19H2) 3793.

II Parker A J, Mayer D, Schmid R & Gutman U, J orgChern, 43 (1978) 1843.

12 Mandai U. Sen S. Das K & Kundu K K. Can J Chem. 64(1986) 300; 7638.

13 (a) Fainberg lA H & Winstein S, J Am chem So~, 78(1956) 2770; (b) Winstein S W & Fainberg A H, J Amchem Soc. 79(1957)5937.

14 Robertson R & Sugamori S E, J Am chem Soc. 91 (1<)6<))7254; Can J Chem, 50 (1<)72) 1353 and the relevant ref-erences therein.

15 Haq R.Jchem Sac Faraday Trans-I,69(1973) 1195.16 Blandamer M J. Scott J M W & Robertson R M. Prog

phys org Chem; (John Wiley, Chichester) Vol. 15, 19H5and the relevant references therein.

17 Datta (nee Sarkari M. Ph D Thesis. Jadavpur University1992.

DATIA et al.: TRANSFER ENTHAI.PIES OF t-BuCI 477

18 Abraham M H & Abraham R J, J chem Sac, PerkinTrans-Il, (1974) 47.

19 Taft R W, Abraham M H, Doherty R M & Kamlet M J, JAmchemSoc,107 (1985) 3105.

20 (a)Bose K & Kundu K K, Call J Chern, 55 (1977) 3961;(b) Kundu K K & Das M N, J chem engng Data, 9 (1964)87; (c) Das K, Bose K & Kundu K K, Electrochim Acta,26 (1981) 479; (d) Bhattacharya A, Das A K & KunduK K, Indian J Chern, 19A (1980) 253; 20A (1981) 347;(e) DattaJ, Bhattacharya A & Kundu K K, Indian JChern, 21A (1982) 9; (f) Das A K & Kundu K K, J solnChern, 8 (1974) 299.

21 Gonclaves R M C & Somoes A M N, Call J Chern, 65(1987) 1474.

22 Gold Y, J chem Sac Faraday-I, 65 (1972) 1611.23 Bose K, Das K, Das A K & Kundu K K, J chem Sac Far-

aday Trans-I, 74 (1978) 1051.24 Franks F & Ives D J G, Quart Rev, 1 (1966) 20.25 Datta J & Kundu K K, (a) J phys Chern, 86 (1982) 4055;

(b) Can J Chern, 61 (1983) 625; (c) Can J Chern, 59(1981) 3141, 3149; (d) Guha P & Kundu K K, Call JChern, 63 (1985) 798, 806.

26 Datta (nee Sarkar) M & Kundu K K, Indian J Chern-(IC 7143).