Characterisation of BAc2 and E1cb routes of ester hydrolysis by measurement of activation volumes

Physica 139 & 140B (1986) 701-704 North-Holland, Amsterdam

CHARACTERISATION OF BAc2 AND Elcb ROUTES OF ESTER HYDROLYSIS BY MEASUREMENT OF ACTIVATION VOLUMES Neil S. ISAACS and Tariq N A J E M

University of Reading, Reading, Berks, England

The common BAc2 route to ester hydrolysis, R~//O

oH

/O, R--C ~-- OH \ OR' s~ow \ OR'

~O , R--C \ + OROH

is accompanied by a negative volume of activation, AI~"~ = - i 0 to - 2 0 cm 3 mol-~ in accordance with an associative slow step. By contrast, the E1cb route, available to esters which possess an acidic c~-proton or one in a homologous position, shows positive volumes of activation consistent with a dissociative rate-determining step. The following case studies are reported:

a) p-hydroxybenzoates ( + 1 6 ) in contrast to the meta-analogue ( - 1 9 ) or the p-methoxy analogue ( - 1 5 ) (fig. 1).

:~

~

~OAr

~

GOAt

AV

~

AV ~

"OH

+OAr

p-OMe~

o. 4~

~Vt/cm3mo1-1 -19 -15

m-Oil

L K

0. O~

R

L

~

- o , 2 -.~

p-Oil

. ~ - ~ "

~

~_~

÷ 16

-t - o . 4--j

-o. I~

.

.

.

.

I

O

.

.

.

.

.

.

.

.

.

I

SOB

.

.

.

.

.

.

.

.

.

r

.

.

.

.

1000

P/BAR

Fig. 1. Pressure-rate relationship for the hydrolysesof dinitrophenyl benzoates, p-hydroxy, m-hydroxyand p-methoxy analogues. 0378-4363/86/$03.50 (~) Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

N.S. lsaacs and T. Najem / Characterisation of BAt2 and E~cb of ester hydrolysis

702

b) Malonates and acetoacetates with at least one a - H hydrolyse by the Elcb route ( + 9 , +1)) (figs. 2, 3) in contrast to the aa-dialkyl substituted malonates and acetates which must use the BAc2 mechanism.

H \c/GO.OAr R/

\ co.x

/CO.OAr

B:

• ' R/

[R--C--C----O" 'L

c \ co.x

H\

cI o,x

/ COOH

' R /C\co.

x

(X = R, OR) 0,8-~

&V~

/cm3mo1-1

-19

pCO.O-DNP Me

2

*

C ~CO.OEt

CO 0

ON|)

CO.OEt

,.2-~ 0.0--

~le CO.O~ONP , ~Ct H/ XCO.OEt -o. 2-~

+10

-0.4I

.

.

.

.

.

.

.

.

.

0

I

I

800

. . . .

1000

P/IAR

Fig. 2. Pressure-rate relationship for some malonate ester hydrolyses ( D N P = 2,4-dinitrophenyl, O N P = PNP = paranitrophenyl).

orthonitrophenyl,

,~v

N~-~CH~C

0

~

*

- 2O

O N II C7 2 ~ ) C 2 "~-PNI'_9

~

tO /C~{!_HNp

~ .

.

.

.

i o

.

.

.

.

.

.

.

.

.

i 500

.

.

0.

.

.

.

.

Cll 3

.

.

+ 9

.

;

.

.

.

.

-

lo00

e/aAR

Fig. 3. Pressure-rate relationship for the hydrolyses of some acetate and acetoacetate esters ( M N P =

metanitrophenyl).

N.S. lsaacs and T. Najem / Characterisation of BAc2 and Etcb of ester hydrolysis

703

c) Fluorene-9-carboxylic esters show a break in their pH-rate profiles (fig. 4) which points to a change in mechanism. The part below pH = 10.5 shows the rate proportional to O H - concentration typical of the BAt2 route while above this pH, the rate is independent of base strength and assumed to be E~cb. In the pH-dependent region accordingly the value of ALP" is large and negative, - 2 2 c m 3 mol -I but in the pH-independent region it is less negative but not positive, - 1 3 c m 3 mol-1 (fig. 5). The reason why this example does not exhibit a positive volume of activation has not been explained but may lie in a considerable and negative value of the volume of ionisation ALPfor this system unlike the other studied. Volumes of proton transfer between two oxygen bases with charge conservation, for example would be -2,11-

I

-It, I1~

. . . . .

ii

r

lO

T

11

11

PH

Fig. 4. pH-rate relationship for the hydrolysis of p-chlorophenyl fluorene-9-carboxylate showing a mechanistic change.

°,!

/

pHolo.o

0,4~

q

AVt : - 2 2 cm~mol " I

J

?

.

.

~

"

~

&V~

13 c 3mo]

1

0.2!

i ? o

500

looo

P/BAm

Fig. 5. Pressure-rate relationship for the hydrolysis of p-chlorophenyl fluorene-9-carboxylate at three values of pH.

704

N.S. lsaacs and T. Najem / Characterisation of Bat2 and Etcb of ester hydrolysis

very small. Overall however, the pressure dependence of rate is a useful guide to the differentiation between the two mechanisms of ester hydrolysis. Table I Summary of activation volume measurements for ester hydrolyses BAc2

acetates p-nitrophenylacetate (onp) (pnp) benzoates p-methoxybenzoate (dnp) m-hydroxybenzoate (dnp) malonates dimethylmalonate (Et, dnp)

fluorene-9-carboxylate (pcp) pH = 10.0 pH = 11.3 pH = 12.9

AV"

Elcb

AV ~

- 19 -15

p-hydroxybenzoate (dnp)

+16

-20

malonate (Et, pnp) methylmalonate (Et, onp) acetoacetate (mnp)

+ 10 + 10 +9

-9 -20

-22 13 -13