Indi an Journal of Chemical Technology Vol. I 0. November 2003. pp. 684-693

Articles

Kinetic and mechanistic studies of saponification of industrially important esters viz. diesters in alcohol-water and dioxane-water moieties-A novel mathematical

approach for evaluation of concentrations of half-ester and end-products

B M Rao"* , K Gajanan" & T Raghunath Raoh

"Department of Chemi stry. Kavikuluguru Institute of Science and Technology, Ramtek 441 I 06. Nagpur, India

bDepartment of Mathematics. KITS. Ramrek. agpur. India

Received 26 August 2002; revised receil'ed 24 April 2003; accepted 23 May 2003

Kinetic and mechanistic studies of saponification of seven structurally related and industrially important diesters have been investigated. These saponification processes were followed as series first order reactions and a novel mathematical approach of determinantal method of evaluation of concentrations of half-ester and other end­products was applied. Time ratio method and Swain's standard data for series first order reactions have been utilized fot· the evaluation of rate data and thermodynamic parameters viz. llE"", -tJ-r, D.G"", tlS" and log;\ for hoth the steps which involve the competitive and consecutive saponification reactions. Further, these reactions indicate that the first step of saponification process is much faster than the second step and the tau rates are more saponifiable than stearates and oleostearates. The concentrations of half-ester and end-products could not be monitored experimentally, however, they are evaluated through the application of the above said mathematical appmach. The advantage of this novel mathematical approach is that the concentrations of half-ester and end-products thus evaluated are with in the mathematical accuracy of ±0.005 %.

The determination of rate constants from the ex perimental data in series first order reactions, was first explored in detail by Swain 1

• A number of worke rs2

·3 made detailed kinetic studies of alkaline

hydrolysis of diesters of different carboxylic acids in protic and aprotic solvents. Bruice and Fife4

forwarded logical explanation for a faster alkaline hydrolysis on the basis of internal solvation of the transition state followed by the attack of hydroxide ion (O W ) at the es ter carbonyl g roup. Several workers5

·7 made detailed studies on the kinetics and

kinetic laws, influence of temperature, variance of thermodynamic parameters in respect of substituents, solvents and their dielectric constants, structure and reactivity , influence of dipolar, aprotic-pi·otic solvents in the saponification processes of saturated esters and also aliphatic dicarboxylic esters.

Instances of the meaningful interpretations of activation parameters along with LFER relationships are also encountered in the literature8

-10 with

particular reference to alkali catalysed hydrolysis of mono and diesters . A critical analysis of the available literature' u s reveals that most of the work is confined to the alkaline hydro lysis or saponification processes

•'•For correspondence (E-mail: praf_madhav@yahoo.co.in ; gajanan_2k @ yahoo.com)

of e ither the norma l esters or the sy nthesized estn' with much emphasis on the study of corre l ation s hip~ .

pertinent to activation parameters or specific sol\'ellt effects or LFER relationships. However, very lillie work is encountered in literature , as regards. the detailed study of kinetics and mechanism nl saponification of industrially important es ter~ 1 n protic-aprotic solvent systems and si mu ltancnus l y evaluating rate data and thermodynam ic parametns. Further, in the foregoing literature survey. none h<t.'­ever attempted to apply any type of mathemal ica l or experimental approach for the evaluation o f the concentratio ns of half-ester and subseq uentl y the end ­products viz. diol and carboxylate ion. This paper ~tbn

furnishes a detailed novel mathematical approach for the evaluation of concentrations of half-ester and end ­products viz. diol and carboxylate ion.

Experimental Procedure

Theoretical treatment Consider the case,

k, Diester ______. Half- ester

kz Half- ester__. End- products (d io l and carboxylate ion)

Rao er al.: Kinetic and mechanistic studies of saponification of industrially important esters Articles

This mechanism is exemplified by certain hydrolyses, by radioactive series and by the reaction of potassium permanganate, manganese sulphate and oxalic acid as in the original work of Esson22

. Esson first integrated the differential equations which are as follows:

d [Diester] [ . ] ----"------=- = -k

1 Dtester

dt

d [Half -ester] ----=--------=- = k1 [Diester]- k2 [Half - ester]

dt

d [End- products] [ ] ----=------=- = k2 Half -ester

dt

The determinantal method23-

25 for the evaluation of concentrations of half-ester as well as end-products viz. diol and carboxylate ion in the saponification process is discussed as follows:

dA =-k A dt I

dB -=k A-k B dt I 2

dC =k B dt

2

... (l)

... (2)

.. . (3)

These equations can be written m the following matrix form

Let l-kl

k I

0

0

whece l ~ 1 " a column vectm and [M] ;s squace

matri x

Now, the characteri stic equation or detenninant ;d equation or secular equation of the matrix [M] is

IM-Ali=O

-k1 -A 0 0

IM -A II= kl -k2- A 0 0

0 k2 -A

A3 +A 2 (k1 +k2 )+A k/2 =0

Hence 1-..=0 and

... (4 )

Comparing the quadratic Eq. (4) with the genera l

algebraic equation having roots At, /...2 is

. . The Eigen values or characteristic roots o f tilL' determinantal equation are

A,. = 0, - k1, - k2 , where r = 1, 2, 3

Next, the Eigen vector expansion of the matri x I M l i ~

[M-/...I] X= 0

Substituting the Eigen values 111 the Eigen vect or expansion,

Case(l ): /... 1 = 0

[M -A I] x ~l ~' 0

k2

-k1x=O,. k1x-k2 y =O, Cr = O

Solving these equations, one gets x=O, y=O

Let z=k2

:. Eigenvector is [0 0 kJ

Articles

Solving these equati ons, one gets

:. Eigen vector is [ k2 - k1 k1 -k2 ]

Case (3): A.,=- k"

Solving these equati ons, one gets

X= 0, )' = k2 , Z = - k2

:. Eigen vector is [ 0 k2 -k2 ]

Modal Matrix [ P] = r ~ k 2

The particular solution of the first order matrix di fferenti al equation is

where [ e i.,r J is a diagonal matrix

To find P- 1

p-1 = ~ ~ ~ adj p

686

Indian J. Chem. Techno!. , November 200~

r e'' 0 0 r-k,k, k2 (k2- "J

X 0 e).,r 0 k; 0

0 0 /''/ - klk2 k2 ( k2 - kl )

0

ro k2 -kl

or = 1 0 k k 0 e- !,r

e (k -k) I

-~2 0 2 2 I k2 -k2

k2 ( k2- kl )

0

k2 ( k2- kl )

0

Now, substituting this matrix in Eq. (5)

0

k 2 (k - k )e-1' ' 2 2 I

e (k - k )e-Ll 2 2 I

Multiplying these matrices, one gets

k, (k ~- k ,)j

~ ,j e .

0 l ()

k ~ (i-.: 2 -k,)

Rao eta/.: Kinetic and mechani stic studies of saponification of industrially important esters Articles

:. A=Aoe-k'' [·: 8 0 =0, C0

=0]

8 - ki Ao [ - k,t -k,t J ---- e -e k~ -kl

C =A, [1 + k, ~ k, ( k,e-'•' - k,e-''')] [·: 80 =0, C0 =0]

k1 and kz are the rate constants obtained in these saponification reactions, which are necessarily series first order in nature. The initial conditions of A0 = [Diester] should invariably be known, while [80] = [ C0] = 0 at the start of any kinetic run .

In the investi gations here, the time ratio method 1 is adopted rather than Powell's graphical method26 since time ratio method yields comparatively more precise rate constants. In this method, times for 15, 35 and 70% of the reaction are recorded from a graph drawn on a large scale (curve-fitting programme) and the

coiTesponding rand K values are noted from Swain's modified table 1 for series first order reactions. From

the relation r =Bok 1t, the value of k 1 and from the other relation K=k21 kt. the value of k2 are evaluated. Thus, the rate constants k 1 and k2 for the two consecutive steps involved in saponification process at different temperatures as well as in different sovlent systems are calculated.

Materials and methods The liquid esters and semi-solid esters employed in

the present work were of extra pure variety (BDH/E. Merck) and were further purified by distillation or by crystallization from a suitable solvent before use. The physical data, viz. m.p./ b.p., saponification value and IR spectra for esters employed are in agreement with the data collected from literature. The rate studies were carried out over 0.3 to 0.7 of the life period of the saponification reaction. Requisite amounts of the reaction mixture (diesters and an excess of alkali which is twenty times over and above the stoichiometric equivalent concentration) were pipetted out at noted time intervals into a solution containing a known excess of potassium hydrogen phthalate, which served to arrest the reaction. Carbon dioxide was carefully removed from the original reactants; the solvents and the whole system was kept during the reaction as well as titration in a stream of nitrogen. The sodium hydroxide solution employed in the saponification process as well as in the titration was made carbonate-

free by the reaction of metallic sodium with conductivity water. The solvents-ethanol and dioxane used in the saponification process were purifi ed by repeated distillation with CaO and also by azeotropic distillation methods .

Treatment of data The concentrations of half-ester and end-product s

(diol and carboxylate ion) are evaluated employing determinantal method23

-25 as discussed earlier. The

initial conditions viz. A0 = [diester] should invariably be known while { 8 0 } = { C0 } = 0 at the start of any kinetic run. k1 and k2 are the rate constants of the first step and second step in these saponification reactions. Percentages of a saponification reaction for a particular kinetic run are plotted as a function of time 't' on a large scale. From a smooth curve drawn through these experimental points , the times for I 5. 35 and 70% of the reaction are recorded. From the time ratios, e.g. t3sltts or logt3sltt 5 the corresponding K

values and r values are noted from the standard tabl es furnished by Swain 1•

Using the following relations:

T = {30k1t

K = k2 I kl

the rate constants for the consecutive, competiti ve steps are evaluated . The rate constants obtained in these investigations represent an average of atleast three kinetic runs and are accurate within ±3 %- . Thermodynamic parameters, viz. energy of activation t:.F!, enthalpy of activation -t:.H*, entropy of

activation t:..~, Gibb's free energy t:.G* and frequenc y factor (A) with respect to these individual steps are calculated employing the necessary formulae. A summary of the rate constants, concentrations of half­ester and end-products evaluated through the nove l mathematical approach and also the thermodynami c parameters are presented in Tables 1-6.

Results and Discussion A number of instances concerning anchimeric

assistance are encountered in the literature. However. very little work is on record in respect of the saponification of diesters of glycols, glycerol and the application of a novel mathematical approach (determinantal method23

-25

) for the evaluation of concentrations of half-ester and end-products vi::. . diol and carboxylate ion. The advantage of th is mathematical approach is, that, the above results

687

Articles

obtained are with in the mathematical accuracy of ±0.005 %. In the saponification process of these esters, anchimeric assistance is provided by the neighbouring hydroxyl group located in close proximity to an ester bond. Further, the aliphatic hydroxyl group is found to facilitate the alkaline hydrolysis of the ester without participating itself as a nucleophile. Such anchimeric assistances are also encountered in a variety of biochemical transformations and this has promoted the study of the saponification of the above said industrially important diesters.

Anchimeric assistance from the neighbouring carboxy l group in the second stage of hydrolysis could be surmised, provided, the neighbouring carboxy l group remain intact with further stabilization or neutra li zation or linking to the glyceride molecule. Such a situation is difficult to visualise in the saponification process of industrially important diesters (oi ls and fats which are essentially -glycerides). Hence, anchimeric assistance from the neighbouring carboxyl group may not be possible. A meticulous critical analysis of the rate data furni shed in Tables 1-4, shows that the reactivity pattern in respect of saponification process of diesters into monoester/half-ester is more than that of monoester/half-ester hydrolytic reaction. This strange fi nding could be rationalized by involving the concept

16 h . of Kumuira et at. , that, the longer carbon c ams present in the alkyl groups of the fatty acid units of the diester provide such a difference in rates . A survey of literature also reveals that the transition state for med from the saponification of an ester has negative charge localized on the carbonyl oxygen atom. making this a good proton acceptor through a hydrogen bond formation 17. This explanation of the transition state was also supported by Haberfield et al. 18 by calorimetric determination of the relative en thai pi es of reactant and transition states. In the alkaline hydrolysis of an ester, the transition state resembles a species such as an alkoxide ion much more than a deloca lized anion hav ing a weak hydrooen-bondino interaction with the solvent. In the b b

present study, particularly in the saponification of distearate, o leostearate and dilaurate of glycerol the negative charge on the carbonyl oxygen atom in the trans ition state diffusion through intramolecular hydrogen bond formation as shown in the structure-!

As mentioned earli er the rate of sapon ification of diester is more than that of monoester/half-ester which could further be explained on the basis the transit ion state (structure II) , which is also formed

6R8

Indian J. Chern . Techno! .. November 2003

H 0

I II H -~-0-~:-R

H-C-0-C-R

where 'R' stands for

Laury!= C 11 H03

Stearic = C 11 H.,

I ~ Oleo = C 17H13

H-C-0-H I H

Structure I

from the intramolecular hydrogen bond formation of the monoester/half-ester and is less solvated by aqueous ethanol or aqueous dioxane and therefore. responsible for the lowering of the rate of saponification of the monoester/half-ester.

H OH.

I t where 'R ' stands for

H-C-0-T-R

I ?. Laury I= C11 Hn

H-c-o-H

I H

Structure II

A perusal of thermodynamic parameters (Tab les 5 and 6) indicates that in general the energ ies of activation are higher in laurates while, the entha l pi e~

of activation, entropies of activation and free energ ies of activation of dilaurates of ethylene g lycol and glycerol are relatively lower than the values of the corresponding individual di stearates of g lycols ancl g lycerol , with the exception of g lycery l dilaurate. Such a behaviour may be attributed to the presence o f Iaury! unit (C 11 H23 ) which has a shorter a lkyl group than the oleic/stearic unit (C17H33/C 17H3s). Moreove r. the vicinal hydroxyl group formed 1n the saponification process also plays a prominent ro le in the internal stabili zation of the transition state causi ng lower saponification rates. These thermodynamic parameters are also in consonance with any type of ion-dipole reactions .

The results of the present work also show th ~11

systems which involve a high degree of internal stabilization of the transition state are susceptible to the solvent influence and there is much vari ance in the thermodynamic parameters and also in the value of the rate constants for both the steps of diesters. The variab le susceptibility to po lar effec ts suggests <Ill

increased importance of transition state so l vat ion in dioxane. If pK value is equated wi th the degree of

::;--. :xJ --c

Name of ester

EGOS

PGDS

GDS

EGDL

PGDL

GDL

GOS

Name of ester

EGOS

PGDS

GDS

EGDL

PGDL

GDL

GOS

Table 1-Series first order reaction, saponification of stearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate

[011] = 0.02 M; [Cr] = 0.02M; [Ester]= O.OOIM; Temperature=30±0.05°C Alcohol-water= 0.445 mole fraction ; Dioxane- water= 0.352 mole fraction ; (v/v 72/28) EtOH-water and Dioxane-water Systems

Alcohol-water S;rstem Dioxane-water s;rstem k1102

, s-1 kz!Oz, s·' [8]60 [C]60 [8hooo XI0-3 [Chooox i0-3 k1102, s-1 k2102, s·' [8)6QX 10 3 [C]60x!0-3 [DbooXI0-3 [Clzi00x l0-3

I" step 2"d step I" step 2nd step

5.114 0.014 0.9943 0.0057 0.6569 0.3431 55.0 0.354 0.8136 0.1863 -0.000 1 0.9999

20.96 0.348 0.8237 0.1763 0.0001 0.9999 22. 15 0.425 0.7898 0.2101 -0.0002 0.9998

56.66 0.085 0.9876 0.0124 0.4656 0.5350 63.05 2.850 0.8999 0.1000 --Q.0226 0.9774

5.520 0.012' 0.9950 0.0050 0.6506 0.3494 65.35 2.925 0.1757 0.8242 --o.oooo 1.0000

6.060 0.0436 0.9812 0.0188 0.2688 0.7312 24.02 1.835 0.3600 0.6399 --o.oooo 1.0000

9.380 0.0028 0.9986 0.0014 0.9197 0.0803 94.16 0.196 0.8906 0.1093 --Q.Ol62 0.9838

4.657 0.001 0.9994 0.0006 0.9477 0.0523 30.05 3.250 0.1595 0.8484 --o.oooo 1.0000

Table 2-Series first order reaction, saponification of stearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate

[011] = 0.02 M; [Cr] = 0.02M; [Ester)= 0.001M; Temperature=40±0.05°C Alcohol-water= 0.445 mole fraction; Dioxane- water= 0.352 mole fraction; (vlv 72/28) EtOH-water and Dioxane-water Systems

~;-s=,- k2i02, s-1

Alcohol-water System Dioxane-water system [8]60 [C]60 [8)JOOOXI0-3 [ClJ000xx i0-3 k1102

, s 1 kzl02, s-T - -[[B]60x10-3 C]60x l0-3 (8lziooX 10-3 [ClziooXI0-3

I" step 2"d step I" step 2"d step

10.97 0.016 0.9918 0.0082 0.5589 0.4410 68.90 1.700 0.3696 0.6304 --o.oooo 1.0000

26.44 0.369 0.8124 0.1876 --o.oooo 0.9999 32.18 0.520 0.7438 0.2562 -0.0001 0.9999

70.88 0.149 0.9160 0.0840 0.0111 0.9888 88.00 3.256 0.6803 0.3197 -0.000 1 0.9999

21.96 0.016 0.9912 0.0080 0.5625 0.4374 83.15 3.755 0.1100 0.8899 - 0.0000 1.0000

19.44 0.145 0.9231 0.0769 0.0021 0.9978 35.00 2.333 0.2643 0.7357 - 0.0000 1.0000

14.35 0.011 0.9937 0.0063 0.6520 0.3479 125.0 0.920 0.5800 0.4199 - 0.0001 0.9999

7.780 0.002 0.9987 0.0013 0.9006 0.0994 40.00 5.170 0.0516 0.9484 - 0.0000 1.0000

;;v .., 0

~

~

0. :l

~ ;:;· ::.0 :l 0..

3 0 ()

::r ::.0 :l ~· ;:;· en 2 0.. n; · en 0 ..... en

"' -g :::! . ::n ()

~ c;· :l

0 ..... :l 0.. c en q §I -< §'

"0 0 ;:::.

"' ~ 0 en

~ en

> .., ..... ;:;· ;;-V':

0\ \0 0

Name of ester

EGOS

PGDS

GDS

EGDL

PGDL

GDL

GOS

Name of ester

EGOS

PGDS

GDS

EGDL

PGDL

GDL

GOS

> ..., ..... -· ~ ;;-"' Table 3-Series first order reac tion, saponification of stearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate

[OW] = 0.02 M; [Cr-] = 0.02M; [Ester]= O.OOIM; Temperature=50±0.05°C Alcohol-water = 0.445 mole frac tion; Dioxane - water= 0.352 mole fraction; (vlv 72/28) EtOH-water and Dioxane-water Systems

Alcohol-water System Dioxane-water system k11 02, s-1 k2I02, s-1

I" step 2"d step [8)60 [C]60 [8) J()()()X I0-3 [C]J000xx 10-3 k1102, s-1 k2102

, s-1 [[B]6QX I0-3 C]60xl0-3

1" step 2"d step [8boox 10-3 [CbooXI0-3

11.18 0.052 0.9736 0.0264 0.0148 0.9852 156.0 3.450 0.1290 0.8710 --0.0000 1.0000

32.58 0.684 0.6776 0.3234 0.0001 0.9999 45 .75 0.895 0.5959 0.4041 --0.0001 0.9999

72.87 0.160 0.9105 0.0895 0.0019 0.9980 163.0 5.241 0.5673 0.4327 --0.0001 0.9999

35.90 0.050 0.9717 0.0283 0.1854 0.8 146 176.0 4.880 0.0550 0.9451 --0.0000 1.0000

28.11 0.930 0.5921 0.4079 --0.0000 1.0000 126.0 2.603 0.2142 0.7858 --0.0000 1.0000

75.12 0.015 0.9912 0.0088 0.5327 0.4673 187.0 1.180 0.4955 0.5044 --0.0000 1.0000

10.91 0.013 0.9933 0.0067 0.5527 0.4472 150.0 13.65 0.0003 0.9997 --0.0000 1.0000

Table 4-Series first order reaction, saponification of stearates, dilaurates of glycol, propylene glycol, glycerol and glyceryl oleostearate

[OW]= 0.02 M; [Cr] = 0.02M; [Ester)= O.OO IM; Temperature=60±0.05°C Alcohol-water= 0.445 mole frac tion; Dioxane- water= 0.352 mole fraction; (v/v 72/28) EtOH-water and Dioxane-water Systems

Alcohol-water System Dioxane-water system k1102, s-1 k210\ s-1 [8)60 [C]60 [8hoooxl0-3 [CJJoooxx i0-3 k1l02, s-1 k2102, s-1 [[8)60x 10-3 C]6QXI0-3 [8booX 10-3 [Cl 2tooX10-3

I" step 2"d step I" step 2"d step I_ 57.66 0.190 0.8952 0.1048 0.0033 0.9966 272.0 6.200 0.0248 0.9752 -0.0000 1.0000

~ "' :l

75.90 2.884 0.1842 0.8158 --0.0000 1.0000 98.20 1.025 0.5463 0.4537 --0.0001 0.9999 ~ ()

146.4 2.143 0.2805 0.7195 - 0.0000 1.0000 215.0 7.3ll 0.4889 0.5111 -0.0000 1.0000 :T ... 3

144.0 0.150 0.9 149 0.085 1 0.0045 0.9954 303.9 6.750 0.0178 0.9822 - 0.0000 1.0000 -l ... 35. 12 2.060 0.3086 0.39 13 --0.0000 1.0000 142.0 3.002 0.1 686 0.8313 - 0.0100 1.0000

()

:T :l

132 .5 0.21~ 0.8779 0.122 1 0.0024 0.9998 314.0 3. 100 0.1572 0.8428 -0.0000 1.0000 2..

28. 56 0.159 0.9 136 0.0864 0.003 1 0.9968 2 11.2 19.05 0.0001 0.9999 - 0.0000 1.0000 z 0 < g o-"' I ·J

·:J

Rao er al.: Kinetic and mechani stic studies of saponification of industrially important esters Articles

Table 5- Series first order reaction, saponification of stearates, dilaurates of glycol, propylene glycol,g lycerol and glyceryl o l eostear~ll t:

[OW]=0.02 M [Ester] = 0.00 I M rcn =O.o2 M Alcohol-Water= 0.445 mole fraction (vlv: 72/28)

Name of t;./!a kcal!mol -t:;.FI kcal/mol t:;.s'l e. u. t:;.G# kcal/mol log A1 lng:l : ester t;./!al t;./!a2 -t:;.F/1 - t:;.F/2 t;.s'll t;.s'l2 /I.G#1 /I.G#2

EGOS 16.51 17.28 16.23 17.40 - 16.16 - 19.40 - 10.84 - 10.94 8.762 9.45')

PGDS 7.7 10 9.597 8.070 8.730 -8.680 -10.53 -5 .170 -5.220 7. 12 1 7.) 14

GDS 6.930 7.380 7.9 10 8.100 -6.470 -9.480 -5.808 -4.990 7.242 7.2X4

EGDL 20.42 28.96 18.09 29.17 -19.95 -20.30 - 11.33 -22.41 9.593 9.657

PGDL 12.25 16.08 12.62 18.34 -12.2 1 -21.30 -8.530 - 12.72 7.896 lJ .X 76

GDL 9. 120 15.20 4.980 5.700 -9.160 -9.630 -1.770 - 2.496 7.227 L\1 7

GOS 8.730 17.19 8.580 17.6 1 - 13.67 - 19.6 1 -4.050 - 10.08 8.2 16 9.506

Table 6-Series first order reaction, saponification of stearates, di1aurates of glycol, propylene glycol,g1ycerol and glyceryl oleostearat t:

[OW]=0.02 M; [Ester]= 0.001 M; [CI- ] = 0.02 M; Dioxane-Water= 0.352 mole fraction (vlv :72/28)

Name of t;./!a kcal/mo1 -t:;.F/ kcal/mol ester t;./!al t;. /!a2 -t:;.F/1 -t:;.F/2

EGOS 3.720 9.380 4.872 8.452

PGDS 3. 150 4.254 1.525 5.226

GDS 2.540 5.230 1.396 2.250

EG DL 5.804 10.29 12.44 14.23

PGDL 4.804 7.550 2.985 5.456

GDL 2.974 5.582 1.596 2.306

GOS 3.889 5.720 5.569 10.84

negative charge developed in the transition state or alternati vely the degree of 'tightness' of the transition state complex, implies that the attacking hydroxide ion and the carbonyl carbon are separated by a greater di stance in aqueous dioxane, than aqueous alcohol. While it is not permissible to make a quantitative assessment of the contribution of transition state solvation to the -/':.F term, the importance of the contribution shows that substitution of aqueous dioxane than in aqueous alcohol leads to an enhanced rate of reaction.

However, more importantly, as per the Hughes­Ingold1 9 theory, both the reduced enthalpy and entropy of activation support the involvement of a more highly solvated transi tion state of diester in aqueous dioxane. The state of anion solvation is frequently mentioned by Benson20 and Tommila et al. 21 as a contributory cause of reactivity. Thus, in aqueous dioxane system of the present study shows that the reactivity of hydroxide ion is dependent upon the following equilibrium:

t:;.s'l e.u. ~:;.eN kcal/mol log AI logA :

t;.s'll t;.s'l2 /l.cNI t:;.G#!

-2.620 -5.424 -14.77 - 16.64 14.45 14.56

-4.212 -5.425 -8.550 - 10.22 13.58 13.6X

-2.3 10 -5. 120 -7.42 1 - 10.32 18.34 I X.45

-7.230 - 8.250 -20.44 -24.72 16.2 1 16 .. 1 I

-4.063 -8.235 -15.55 -18.23 15.22 15 .32

-3.959 -6.175 -8.958 - 12.99 2 1.59 2 1 Xl)

-3.256 - 4.625 - 9.568 - 18.84 18.56 I:'U5

where n equals the maximum number of water molecules hydrogen-bonded to hydrox ide ion. With increasing dioxane content, the equilibrium would be shifted to right, resulting in a less solvating hydroxide ion .

To some extent it is unlikely that the an ion di ssolution is the major cause for the increase of rat e constant because,

(i) the activity of the hydroxide ion with increasing dioxane concentrations does not explain the dependency of medium effect upon substrate steric substituent constant or the presence of medium effect discontinuities.

69 1

Articles Indian J. Chern. Techno!.. November 100.'

Table 7-Effect of solvent composition, series first order reaction of saponification of EGDL

[0W]=0.02M; [CI-]=0.02 M; [Easter]=O.OOJM; Temp=50±0.05°C

Solvent compositi on Mole fraction k x 10-2, s- 1 k x 10-2, s- 1 E

alcohol/water w.r.t, %of alcohol 151 step 2"ct step

72/28 0.445 35.92 0.050 55.63 60/40 0.3141 28.36 0.021 41.34 50/50 0.2339 19.45 0.009 37.91 40/60 0.1 691 10.65 0.005 34.33 28172 0.1062 1.842 0.001 30.86

Table 8-Effect of solvent composition, series first order reaction of saponification of EGDL

[0W]=0.02M; [Ester]= O.OO IM; [Cr]=0.02M; Temp=50±0.05°C

Dioxane-water Mole Pseudo first order Pseudo first order Dielectric constant composition (vlv) fracti on rate constant rate constant (E)

k1 x I 02• s- 1 k2 x 102, s - J I " step 2"d step

72/28 0.3521 176.0 4.880 29.65

60/40 0.2408 40.04 2.880 20.97

50/50 0.1745 29.41 0.021 15.80

40/60 0.1235 23.37 0.017 11 .87

28172 0.0759 3.280 0.002 8. 11 9

(ii ) with a small ion such as hydroxide, It tS more realistic to treat the ion, plus the hydrogen-bonded water molecules as a single kinetic unit. If the anion desolvation mechani sm were operative, this would imply decreasing size of the nucleophile with increasing dioxane content in the solvent medium. Such a charge in the stearic bulk of the attacking reagent should be reflected on variation in the reaction constant parameter 8. However,

such responses are not generally recorded in 8, at higher mole fractions of dioxane (Tables 7 & 8). It is also obvious, that, systems which involve a high degree of internal stabilization of the transitio n state are susceptible to pronounced dipolar aprotic solvent influences. Further, on the basis of thi s observation, it would be seen that one can use di-polar aprotic solvent influences on reaction rates as a criterion in the assessment of anchimeric assistance in reactions involving internally stabilized transitiOn states. The concentrations of B=half-ester and C=end-product thus evaluated through the novel mathematical approach (determinantal method23-25

) reveal that all these industrially important esters are more saponifiable in dioxane-water system than alcohol-water system. Such a phenomenon may be due to the change of the system from protic to aprotic solvent sys tem. The laurates viz . ethylene glycol dil aurates , propylene glycol dilaurate and

692

glyceryl dilaurtate are much more sapo nifi ab le in dioxane-water system, moreover, for the same duration of time in any kinetic run , the concentrations of half-ester is much less in dioxane-water system than that in the a lcoho l­water system. This confirms the labile nature of hydroxide ion in dioxane-water sys tem (aprotic system) leading to more reactiv ity. Another important conclusion that could be drawn from the data furnished in Tables 1-4 is, that , the shorter the alkyl group in the ester linkage the more the saponification process. Such a pronounced activ ity is very much evident in dioxane-water system(aprotic system) at di lle rent temperatures.

Acknowledgements The authors wish to express their s incere gratitude

to Dr V. Ramchandra Rao, retired Prof. and Head. Department of Chemistry, VNfT, for his encouragement in the progress of this work. A ll th e authors are grateful to the Managemen t as we ll as Dr. G . Thimma Reddy, Principal KITS , Ramtek. One nf the authors (K. Gajanan) is thankfu l to Dr K Vijaya Mohan, Head , Department of Chemistry. KITS. Ramtek and also grateful to Wg. Cdr. K.R. Shanm and other family members for their forbearance and contin ued moral support.

Rao et al.: Kinetic and mechani stic studies of saponification of industrially important este rs Articles

Abbreviation used

Abbrev iations Name of Structure ester

EG OS Ethylene CH200CC11H3s glycol I distcarate CH200CC11H3s

PGDS Propylene .{,CH200CC11H3s glycol c 2 distearate '-' CH200CC,,HJs

GDS Glyceryl FH200CC11H3s distcaratc

CHOOCC11H3s I CH20H

EGDL Ethylene CH200CC,,H23 glycol I dilaurate CH200CC,,H23

PGDL Propylene i CH200CC11H23 glycol c 2 dilaurate '-' CH200CC11H23

GDL Glyccryl CH200CC11H23 dilaurate I

CHOOCC11 H23 I CH20H

GOS Glyceryl CH200CC11H33 oleostearate I

CHOOCC,,HJs

I CH20H

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