n-Najah J. Res. Vol. 1( 1989) Number Mohammed A. AL-Nud Sh rt C n t n
SOLVENT EFFECTS ON VALUES OF THE HAMMETT' EQUATION FOR THE HYDROLYSIS OF BENZYLIDENE BENZOYLHYDRAZONES
Mo amme A A - u i
, ) 7 I c ; ( 34i i ;i -;
. a I i
Abstract
e e ec o so e s o a ues o e amme equa io w ic c a ac e i es e y o ysis o e y i e e e oy y a o e ( a i s e i a i es (m— O m—C 3 m—OC 3 a m—C a e ee s u ie wi ou so e s (MeO E (1 — uO a MSO y U — s ec o o ome y e e ec s o so e s o a ues o e o e amme equa io a ame e s a e ee e o e a iscusse
A t r f. , p rt nt f Ch tr , lt f S n . An j h t n l Un v r t .
51 S lv nt Eff t On
Introduction
The effects of solvents on the p values have been discussed by Wynne — Jones , ( who have reported the relative acid strengths of some acids in some hydroxylic solvents as water , methanol , and ethanol ; he suggested the inverse proportionality of P values to the dielectric constants of the solvent used . Furthermore , Kilpatrick and his co-workers (3 confirmed Wynne-Jones's results on the dissociation of acids in aqueous dioxane ; however , they indicated that this relationship was limited to solvents with dielectric constants larger than n, 25 .
Tokura et al . ( have treated the effects of solvents on p values as a two — variable problem within a linear free — energy relationship framework . They assumed that the standard activation free energy is a continuous function of o , a substituent variable , and , a solvent variable . it is proposed that the solvent effects on p values can be explained in terms of the interaction or cross-term of a a
The present work deals with the solvent effects on the Hammett constants of the hydrolysis of benzylidene benzoylhydrazone .
The existence of a linear dependence of a values , a substituent constant , on the rate constant , log kik° , and the mechanism of the hydrolysis of benzylidene benzoylhydrazones have been reported .
Experimental
Reagents and apparatus :
The benzylidene benzoylhydrazones (X — BBH , where X = H , m — CH 3 , and m NO were prepared and purified by the m — OCH 3 — C conventional method (.51 Si ick 0.001 M solutions were prepared in pure solvent . The modified tadversal I lifer series of Britton and Robinson ( were used hyrolysis media .
A Pye-Unicam Sp 8 —100 spectrophotometer equipped with a thermostated cell holder ± (0.1°C) was used for the kinetic study .
5 An-Najah J. Res. Vol. 1 ( 1989 ) Number M h d A. A r
a e Measu eme s
e y a o e a e u e we e mi e a oom em e a u e o kee e co ce a io o e y a o e equa 5 1 -5M i e ese ce o 5% (v/v) so e e cou se o e eac io was o owe o eac so e a eac y a o e y measu i g e ec ease i e a so a ce o e y a o e a a ious imes a a e ma imum wa e e g co es o i g o eac y a o e
X -BBH X max(nm)
X = H 295
m-CH 295 3
m-NO, 280
m-OCH 3 292
m-Cl 290
esu s a iscussio
As founded previously ( 1 the rate of hydrolysis of X -BBH was found to be first order as indicated from the polt of k n (A - A ) versus time . It can be shown from Table (1) that the rate constants of the y o ysis of BBH and its derivatives are affected by the nature of the substituent as well as the nature of the solvent used . In fact , the electron releasing substituents stabilize the transition state and increase the values of the rate constants , meanwhile , the electron withdrawing substituents destabilize the transition s a e a ec ease the values of the rate constants . It can be noticed from Table ( 1 ) that for e same su s i ue e a e constant increases with increasing the polarity of the medium .
- -
S lv nt Eff t On
e o se a io a e a e co s a o MSO is owe a i Me ( a oug i s ie ec ic co s a is ig e ca e a i u e o e ac a eac ime y su o i e mo ecu e coo i a es wi wo wa e mo ecu es y y oge o i g
The polt of cr , a substituent constant , and log kik , for the hydrolysis of BBH and its derivatives in four different solvents , is shown in the Figure . A straight line is obtained with a slope representing a value . This plot implies that as the solvent polarity decreases , the role of the substituent increases and values increase , and this is consistent with Tokura's results . ( 4 It can be noticed from Table ( 2 ) that the polarity of solvent increases with an increase in the percentage mole ratio of water . It means that 5 v % n — BuOH is the most polar aqueous medium among other aqueous media .
It is so interesting that the rates of hydrolysis of the hydrazones in DMSO — water follow the same trend ; i.e , similar to other hydroxylic solvents . Table ( 2 ) shows that the polarity of the medium goes in the following order:
5 v% n — BuOH > 5 v% DMSO > 5 v% EtOH > v% Me0H
Table (3) has shown ( that as the percentage of water decreases , the dielectric constant values decrease . The mechanism of hydrolysis of unsubstituted and substituted benzylidene benzoylhydrazones that has been postulated (9 1 is written as follows :
C = — —C
(I
+ A +
S
( II )
a i 1+ (II — C — — — ( 3
54 An- j h l l ( 8 b r 6 M h d A A - r
C"
_ + C (2 C
It has been shown , from the previous mechanism , that the attack of water is involved in the rate determining step . Therefore , as the concentration of water increases there is a decrease in the partial positive charge on nitrogen in the transition state which causes a decrease in values and hence , a decrease in the role of the substituent . This confirms the fact that as the solvent polarity increases , there is a decrease in p values and a decrease in the role of the substituent and vice versa .
Acknowledgement
I would like to thank Dr. Ali Abu Zuhri , Dr. Foad Mahmoud , and Dr. Maher 4bu-Eid for their worthy discussion during this work .
References
1— Abu-Eid M.; Shraydeh B. ; Abu-Zuhri A. ; and Mohmoud F., An-Najah J. Research , Vol. I ( 6 ) , 23 ,( 1989) . 2 — Wynne-Jones W. P. K. , Proc . Roy. Soc. ( London) , A 140 , 440 ( 1933 ) . 3 — Elliot J. H. ; Kilpartick M. , . h . Chem . , 45 , 454 ( 1941) . 4 — Kondo Y. ; Matsui T. ; Tokura N. , Bull.Chem . Soc. ( Japan ) , 42 , 1039 ( 1%9 ) . 5 — Sturve G. , . r t . Ch . , 2 , 50 ( 1894) . 6 — Britton H.T. , " Hydrogen Ions " , Chapman and Hall , London ( 1952 ) . 7 — Yagil G. , J. Phys . Chem . , 73 , 1610 ( 1969) .
8 — Timmermans J. , " Physico Chemical Constants of Pure Organic
55 Solvent Effects On .....
Com ou s " E se ie u is i g Co Ams e am e e e a s ( 195
9 — o Ama a ; Sa s o m W A ; a Co es E Am C em Soc 5 ( 19
1 — ei ma E ; e cks W Am C em Soc 39 3 ( 19 11— Ma c " A a ce O ag ic C emis y eac io s Mec a ism a S uc u e " McG aw — i ook Co ew Yo k ( 1977
igu e
A o o og k/k s a o e y o ysis o e y i e e e oy y a o es i ou i e e so e s a = 1 9 ( 7 °C
a - u y a co o ime y su o i e
c e y a co o me y a co o
a e (1 a e co s a s o e y o ysis o a i s e i a i es a P a ues i a ious so e s a = 1 9 ( 7°C 0 —C = — — C
= - m- O = m-C - m-C 3 = m-oc 3 So e 1 3 k/s - 1 1 3 k/s -1 1 3 k/s-1 1 3 k/s- 1 1 3 k/s-
Me a o 3 5 1 - 3
E a o 7 1 3 1 7 3 1 -
ime y 1 1 97 - 5 Su o i e
- u a o 1 1 1 55 1 7 - 7
6 An- j h l ( 8 b r 6 M h d A A - r
Table (2)
Percentage mole ratio of H 2O , P values for different solvent mixtures
So e % % mo e a io o 2O
- u a o 8 —0 2 0
ime y su o i e 8 68 —0 42
E a o 8 42 —0 600
Me a o 2 —0 8 0
'Fable ( 3 )
Effect of percentage of alcohols on the
dielectric constants of the aqueous
media at room temperature .
So e %
Me a o 20 0
40 6 4
E a o 20 2 0
40 66 0
57 S lv nt Eff t On
Table ( 4 ) Values of cr- for the groups used in this (11) study
Su s i ue CM
C - 7 3
OC 1
C 37
O 71
58