Ionization constants and reactivity of isomers of eugenol

JOURNAL OF RESEARCH of the National Bureau of Standards— A. Physics and Chemistry
Vol. 68A, No. 6, November-December 1964

Ionization Constants and Reactivity of Isomers of Eugenol

G. M. Brauer, H. Argentar, and G. Durany

(July 15, 19G4)

To determine the scope of the reaction of zinc oxide with isomers of eugenol, the effect
of changes in the position of the substituents in the benzene ring on the ionization constants
and reactivity of these isomers has been studied.

The ionization constants of eugenol isomers as well as those of newly synthesized allyl-
and propenylbenzoic acids were determined by spectrophoto metric and potentiometric
techniques. The influence of inductive, resonance and steric effects of the substituents on
the ionization constants has been discussed and the substituent constant for the Hammett
equation pK — pK=<rp has been calculated. For the 4- and 5- substituted allyl and propenyl
derivatives, the Hammett equation is valid.

Vicinal trisubstituted isomers do not harden readily with zinc oxide due to the steric
hindrance of the side chain. The unsymmetrically trisubstituted derivatives react rapidly
in the presence of zinc acetate. Besides the steric effects of ths substituent groups the rate
of the chelation reaction is to a lesser degree dependent on the ionization constants as in-
dicated by the shorter setting time of chavibetol-zinc oxide slurries compared to those
containing eugenol.

1. Introduction

Mixtures of zinc oxide and 4-allyl-2-methoxy-
phenol (eugenol) harden to form a product that
consists of zinc oxide embedded in a matrix of zinc
eugenolate chelate [l]. 1 To determine the scope of
this reaction it appeared of interest to study how
changes of the position of substituent groups in
eugenol affect the behavior of the isomers as evi-
denced by their ionization constants, absorption
spectra, hydrogen bonding and reactivity with zinc
oxide. The absorption spectra studies will be dis-
cussed in a subsequent paper.

2, Experimental Procedures and Results

2.1. Materials

The eugenol isomers that are commercially avail-
able — 4 - allyl - 2-methoxy phenol, 2-methoxy - 5-pro-
penylphenol (isochavibetol), and £r(ms-2-methoxy-4-
propenylphenol (/raws-isoeugenol) — were redistilled
or recrystalized ; the final boiling or melting points
agreed with those given in the literature.

The synthesis of 3-allyl-2-methoxyphenol, 3-allyl-
5-methoxyphenol (chavibetol) , 6-allyl-2-methoxy-
phenol (o-eugenol), and 2-methoxy-4-propenylphenol
has been described previously [2]. ^-Allylphenol
(chavicol) and p-propenylphenol (anol) were syn-
thesized from ^-allylanisole (estragole) and ^?-pro-
penylanisole (anethole) by demethylation with
methyl magnesium iodide. The procedures were
similar to those employed by Schopf and coworkers
for the demethylation of 4-allyl-l,2-dimethoxy-
benzene (methyleugenol) [3]. 2-Methoxy-6-pro-
penylphenol (o-isoeugenol) was prepared by the
method of Pal'gi [4]. p-Allylbcnzoic and p-pro-

1 Figures in brackets indicate the literature references at the end of this paper-

penylbenzoic acids were synthesized from 2>dibromo-
benzene through a series of reactions described by
Quelet [5, 6].

The m-allylbenzoic acid was synthesized by modi-
fying the procedure given by Quelet for the para-
isomer [5, 6]. To 5.8 g (0.24 gram atom) of mag-
nesium and a few crystals of iodine, 50 ml of ether
was added and the solution was refluxed until the
purple color changed to pale green (approximately
30 min). Then 50 g (0.21 mole) of m-dibromoben-
zene in 50 ml of ether was added and the solution
refluxed for 2 hr when most of the magnesium
had reacted. Allyl bromide (20 ml, 0.24 mole)
dissolved in 25 ml of ether was added drop wise with
cooling to keep the temperature of the reaction
mixture between 5 and 15 °C. After refluxing for
18 hr, the Grignard reagent was decomposed with
saturated ammonium chloride adjusted to pll 8 with
NH 4 OH, the ether was evaporated, and the residue
was steam distilled. Sodium chloride was added to
the distillate, the product was separated from the
water and dried over calcium chloride. The m-
bromoallylbenzene containing traces of dibromo-
benzene distilled at 76-78 °C/6 mm, n D 30 = 1.549.
Yield: 22 g (47%).

The acid was prepared from m-bromoallylbenzene
as follows: To 0.92 g magnesium (0.038 gram atom)
in 25 ml of ether a few crystals of iodine were added
and the mixture refluxed until the iodine color
disappeared. After addition of 7.0 g (0.0355 mole)
m-bromoallylbenzene dissolved in 25 ml of ether,
the mixture was refluxed for 24 hr, cooled to 5 °C,
and dry ice was added over a 4-hr period. The
mixture was poured onto crushed ice, 100 ml of 10
percent HC1 was added, and the ether layer was
separated and extracted with dilute sodium bicarbon-
ate. The extract was acidified and the precipitate
extracted with ether. The yellow solid obtained on
evaporation of the ether was recrystallized from

743^261^64-

619

water and dilute ethanol yielding 0.9 g (16%) of
m-allylbenzoic acid, mp 61-62 °C.

Anal: Calcd. for C 10 H 10 O2: C, 74.05; H, 6.2; neut.
equiv. 162.2. Found: C, 74.2; H, 6.3; neut. equiv.
161.1.

By modifying the synthesis given for the para
isomer [5, 6], ra-propenylbenzoic acid was obtained
through the following reaction steps :

The m-bromophenylmagnesium bromide was pre-
pared as described above. The mixture was cooled
to 5 °C and 14 g (0.24 mole) of propionaldehyde in 50
ml of ether was added dropwise with stirring to keep
the temperature of the highly exothermic reaction
between 5 and 12 °C. The mixture was kept over-
night at room temperature and was decomposed
with crushed ice. Then 100 ml of 10 percent HC1
was added. The water layer was separated and
extracted with ether. The ether extracts and
original ether layer were combined, dried with
anhydrous sodium sulfate, and the ether was evapo-
rated off. On distillation at 123-128 °C/6 mm there
was obtained 20 g (44% yield based on m-dibromo-
benzene) l-(m-bromophenyl)-l-propanol, u 2 d= 1.5580.

Anal: Calcd. for C 9 H u OBr: C, 50.3; H, 5.2.
Found: C, 50.9; H, 5.1.

On dehydration of 20.7 g (0.096 mole) l-(m-
bromophenyl)-l-propanol by refluxing for 19 hr
with 9 g (0.063 mole) P 2 5 in 150 ml of benzene (dried
over sodium) there was obtained 6.83 g (36%)
l-(m-bromophenyl)propene, bp 92-93 °C/6 mm,
rt?= 1.5855.

Anal: Calcd. for C 9 H 9 Br: C, 54.85; H, 4.6.
Found: C, 54.7; H, 4.4.

The l-(m-bromophenyl)propene (3.0 g, 0.015
mole) was converted to the acid by refluxing it with
0.39 g (0.016 gram atom) magnesium and 25 ml of
ether for 3 days and subsequent addition of dry ice.
The m-propenylbenzoic acid was recovered from the
reaction mixture as described in the preparation of
m-allylbenzoic acid. After recrystallation from
acetic acid-water and ethanol-water the colorless
needles melted at 104.5-105.5 °C. Yield: 0.5 g
(21%).

Anal: Calcd, for C 10 H 10 O 2 : C, 74.05; H, 6.2; neut.
equiv. 162.2. Found: C, 74.0; H, 6.3; neut. equiv.
163.1.

2.2. Ionization Constants

The thermodynamic ionization constants of euge-
nol isomers and related phenols were determined
spectrophotometrically according to the procedure
of Robinson and Biggs [7]. This method depends
upon the fact that the ultraviolet absorption spec-
trum of a weak acid is often markedly dependent on
pTH] that is, in an alkaline solution one obtains the
spectrum of the negatively charged anion of the acid
whereas in an acidified solution one measures the
spectrum of the uncharged molecule of the weak
acid. Thus a range of wavelengths can be found in
which the anion is highly absorbent and the un-
charged molecule shows little if any absorption. All
measurements were made with a Beckman DU
spectrophotometer thermostated at 25.0 ±0.1 °C.

The optical density at a specific wavelength was
studied in acidic (O.liV HC1) and alkaline (0.1 JV
NaOH) media as well as solutions that had been
buffered. Buffers used were equimolar mixtures of
0.25M or 0.01M sodium carbonate and sodium
bicarbonate (pH = 10.020 and 10.112 [8, 9]), 0.1M
and 0.01939M solutions containing sodium acid
succinate and sodium chloride (^)H=4.802 and 4.853
[10] and 0.06M sodium acetate and 0.14iW acetic acid
(pH = 3.875 [11]) at 25 °C. These buffers were
chosen since the pH of the resulting solutions ap-
peared to be in the neighborhood of the expected pK
values of the acids.

The negative logarithm of the thermodynamic
ionization constant is given by:

pK=pU— log -—^ log y A -.

1 — a

The degree of ionization can be calculated from
the optical density D of the buffered, acidic, and
basic solutions.

OL= (^buffer — -£'acid)/(M)ase — ^acid) •

The values of y A ~ } the activity coefficient of the
anion, were obtained from the equation

-log 7 A- = [0.5115/ 1 / 2 /(l + i rl/2 )]-0.2/,

where I— ionic strength [12]. The values of —log y A ~
were 0.102 and 0.077 for the 0.25M and 0.01 Jlf
carbonate and 0.217 and 0.0418 for the 0.100M and
0.01939M sodium acid succinate-sodium chloride
buffer solutions.

The concentration and wavelengths that would be
most favorable for the determination of the ioniza-
tion constant were obtained by scanning the absorp-
tion curve of the acidic and basic solution of each
phenol in the 270 to 330 m/z region and of the acids
in the 230 to 330 m/x region. The concentrations
employed varied from 7X10; 5 to 3.5X10~ 4 M.

The propenylbenzoic acids were insoluble in
O.liVHCl and thus could not be determined by the
spectrophotometric method. The apparent and
thermodynamic pK values of the propenyl- and
allyl-benzoic acids were therefore determined poten-
tiometrically at 25.0±0.1 °C. A 40 ml 1:1 (by
volume) abs. ethanol-water solution of the acid of
2.75 to 5.50X10 -3 molar concentration was titrated
in a nitrogen atmosphere with carbonate free
(approximately 0.025AO 50 percent ethanolic sodium
hydroxide employing an automatic constant rate
microburette (Sargent, Model C). The change in
pH was followed using a Radiometer pH meter
(Model 22) with scale expander which was stand-
ardized each day against aqueous 0.05M potassium
acid phthalate (pH=4.010) and Beckman con-
centrated buffer solution diluted 24 to 1 (pH=7 .000) .
The pH was measured in the buffer region where the
degree of ionization is between 25 and 75 percent and
also in the vicinity of the end point. The precise
end point was obtained by plotting ApH/A ml versus

620

ml of NaOH added and taking as the end point the

where

number of ml of NaOH at which A pH/A ml is a

T= absolute temperature

maximum. The thermodynamic pK values were

and

calculated using the following equal ion :

e = dielectric constant of the solvent which was

obtained by interpolation from the values given by
Akerlof [14].

„*r .ii,^ C-[Na + ]-[H + ], ^VlNa+] + [H+]

/ DlV = 7)xi-f-l0£ — T^r^ — , ., , r .„ , .,

A Royal computer was used to calculate the pK
values. The standard deviation was about 0.004

V* p 1-1 g [Na+]+[H+] l 1+jB> / [Na+] + [ H+]
where

pK units for one titration and the standard deviation

[Na + ]= concentration of the sodium ion

between different runs was within 0.02 pK units.

C= [HA] + [A~] = total concentration of all

The degree of ionization and the thermodynamic

acid species

pK values obtained from the optical density measure-

[HA] = concentration of the undissociated acid

ments are given in table 1 . The pK values are

[-A~]= concentration of the anion of the acid.

considered to be accurate within ±0.02 pK units

except for ^-propenylphenol and m-allylbenzoic acid

The constants A and B depend on the solvent

since the latter compounds were not stable in the

composition and were calculated from the expression

buffered or basic solutions. The pK values of the

given by Bates [13].

allyl and propenyl substituted guaiacols increase

.A=1.825. 10 6 (eT)- 3/2

in the following order: 3-substituted <5-substituted

5-1.5 (78.3/e) 1/2

<4-substituted <6-substituted guaiacol.

Table 1. Thermodynamic ionization constants of alli/l- and propenyl substituted phenols and benzoic acids

Solvent: H 2 Temperature: 25.0±0.1 °C

Normality

Compound

Concen-
tration

Wave
length

of NajCOr

\:llir<>;

buffer

a

pK

Avg. pK

10-M

my.

Phenol

10.00[8]

p-Allylphenol

3. 00

300

0. 01

0. 462

10. 26

(Chavicol)

3. 00

300

.025

.452

10.21

10. 23

3. 00

290

.01

. 660

9.90

3-Allyl-2-methoxyphenol

3. 00

290

.025

.617

9. 92

9. 92

3.00

300

.01

. 653

9.91

3. 00

300

.025

. 598

9.95

4-AUyl-2-methoxyphcnol

2. 00

300

.01

.514

10.17

(Eugenol)

2. 00

300

. 025

.458

10. 20

10. 19

5-Allyl-2-methoxyphenol

3. 00

300

.01

. 640

10. 01

(Chavibetol)

3. 00

300

. 025

. 593

10.03

10.02

6-Allyl-2-methoxyphenol

0.70

300

. 025

. 363

10.37

(o-Eugenol)

3.50

300

.01

.374

10.41

10. 38

3.50

300

.025

. 361

10. 37

p-Propenylphenol

3. 00

320

.01

a. 438

a 9. 8

(Anol)

3. 00

320

.025

a. 335

a 9. 8

a— 9. 8

c 9. 824

0.70

320

.01

.679

9.86

2-Methoxy-4-propenylphenol___

.70

320

.025

.636

9.88

(Isoeugenol)

.70

325

.01

.685

9.85

.70

325

.025

.643

9.87

9.88

1.00

325

.01

.652

9.92

c 9. 875

1.00

325

.025

.644

9.87

2-Methoxy-5-propenyl phenol. __

3.00

325

.01

.665

9.89

(Isochavibetol)

3.00

325

.025

.621

9.91

9. 90

0.70

320

.025

.448

10. 21

2-Metlioxy-6-propenylphenol. _ _

2.00

320

.01

.489

10.21

(o-Isoeugenol)

2.00

320

.025

.453

10. 20

10. 20

2.00

325

.01

.491

10.20

2.00

325

.025

.459

10.20

Benzoic acid

4. 20[12]

2.00

255

b. 10

.782

4.36

p-Allyl benzoic acid

2.00

255

b. 01939

.797

4.34

4.34

2.00

260

b. 10

.800

4.32

2.00

260

b. 01939

.800

4.33

m-Allylbenzoic acid

0.08

240

f .06N
NaAc

a. 519

a 4. 33

a 4. 32

.08

245

\ +

.14N
llIAc

.53

4.31

« Approximate value (±0.1 pK units) since the optical density of the basic and/or buffered solutions

changes on standing.

b Sodium acid succinate— sodium chloride buffer.

c Since completion of this work these recently determined pK values have come to our attention [25].

621

The presence of an allyl group usually decreases the
ionization since this group furnishes electrons to the
benzene ring. However, the proximity of an allyl or
propenyl group ortho to the hydroxy group hinders
the removal of a proton. Thus 6-allyl-2-methoxy-
phenol and 2-methoxy-6-propenylphenol have lower
ionization constants than their respective position
isomers. The small magnitude of the inductive
effect of the allyl group is indicated by the very slight
change in the ionization of 5-ally]-2-methoxyphenol,
pK= 10.02 (where the allyl group is meta to the
hydroxyl group and exerts little if any steric and
resonance effects) as compared to guaiacol, pK=9.98.
3-Allyl-2-methoxyphenol (3-allylguaiacol) has a
slightly lower pK value of 9.92. Here again the
allyl group is meta to the hydroxyl group, but the
proximity of the allyl group to the methoxyl group
may cause some electronic interaction. In 4-allyl-
2-methoxyphenol the p-allyl group supplies electrons
to the conjugated system and the pK value of 10.19 is
larger than that of 5-allyl-2-methoxyphenol
(pK= 10.02).

Keplacement of the allyl group of the position
isomers of eugenol by the propenyl group lowers the
respective pK values by 0.12 to 0.31. In part the
acid strengthening characteristics of the propenyl
group (which has a w bond conjugated with the ben-
zene ring) especially in the para position are due to
the resonance interaction with the phenolic hydroxyl
group which increases the stability of the phenoxide
ion.

The apparent and thermodynamic ionization con-
stants of the m- and ^)-allyl and propenylbenzoic
acids in 50 percent ethanol-water are given in table
2. From these values the a values of the Hammett
equation log (k/k ) = <rp can be calculated. In this
equation, which relates the reactivity of the side chain
of an aromatic compound and the nature of the
substituent, k and k are rate or equilibrium con-
stants for reactions of the meta or para substituted
and unsubstituted compound respectively, a is the
substituent constant which depends on the nature
and position of the substituent and p is the reaction
constant which depends on the reaction and reaction
conditions.

Table 2. — Apparent ionization constants of allyl- and pro-
penylbenzoic acids in 50 percent ethanol

Temperature: 25.0±0.2 °C

pK*

Hammett

Acid

Concen-
tration a

Thermo-
dynamic

sigma constant

Benzoic Acid. __ -

5.705
5.787
5.812
5. 765
5.833

5.694 ±0.012°
5. 810 ±0. 006
5.813 ±0.005
5. 765 ±0. 020
5.844 ±0.004

m-Ally] benzoic Acid _ _

o- m = -0.08

p-Allylbenzoic Acid

<T P = -0.08

rw-Propenvl benzoic Acid, _ __ __

o- m =-0.05

p-Propenylbenzoic Acid__

o> = -0. 10

a pK (concentration) = —log

Ca-Ch+

Cha

b Average of two or more measurements.
c Standard deviation.

apparent pK values of the benzoic acid derivatives
using the p value of 1.522 given for the ionization of
benzoic acids in 50 percent ethanol [15]. The <r m
and a v values for the allyl group are close to those
reported for the C 2 H 5 (_r m =— 0.04, <r p =— 0.15) and
7i-C 3 H 7 ((Tp=— 0.13) groups [16]. Differences in the
sigma values for propenyl and allyl groups should be
a measure of the increased resonance interaction
contributed by the t bond of the propenyl substituent
which is conjugated with the benzene ring.

The (T p value of the allyl group (—0.14) was the
same when calculated from the thermodynamic pK
values of the respective substituted benzoic acids and
phenols in water. A slightly lower cr p value of
— 0.08 was obtained from the apparent ionization
constant in 50 percent ethanol. Solvation effects
may account for this difference of <r p in the two
solvents.

Generally, the Hammett equation is not applic-
able to substituents in the ortho position of the
benzene ring. However, previous investigations have
shown that for a few reaction series of unsym-
metrically trisubstituted compounds, such as 4- and
5- substituted toluic acids [17-19], with the same
substituent in the ortho position relative to the
reactive group throughout the series, the reaction
parameter is constant within experimental error.

Thus for the o-methoxyphenol (guaiacol) series the
Hammett equation can be written:

pK G -pK* G =p G <r

where pK G = — logarithm of the ionization constant

of guaiacol
pKa G =— logarithm of the ionization constant

for the substituted guaiacol
p G ___z reaction constant for the ionization of

the guaiacol derivative.
Although there is hydrogen bonding between the
hydroxyl and adjacent methoxyl group in guaiacol,
the influence of the ortho methoxyl group on the
ionization of the phenolic hydroxyl group is small
and of importance only when the coplanarity and
resonance of the molecule are concerned [20]. Thus
guaiacol, 4-allyl-2-methoxyphenol, and 2-methoxy-
4-propenylphenol are only slightly more acidic than
phenol, 4-allylphenol, and 4-propenylphenol, re-
spectively (table 3) . Furthermore, the sigma values

Table 3. pK values of substituted phenols and the correspond-
ing guaiacols

The substituent constants for m- and ^-allyl and
propenyl groups (table 2) were calculated from the

pK a

p# a Guaiaeol_p2Jr a Ph_nol

Sigma values

Substituent

Phenol

Guaiacol

Phenols
P = 2.29[26]

Guaiacols
P =2.2b

H

3-Allyl

4- Allyl

5-Allyl

4-Propcnyl

5-Propenyl

10.00 [8] a

9.98 [8]
9.92
10.19
10.02
9.88
9.90

-0.02

0.00

0.00
-.03

10.23

-.04

-.10

-.10
-.02

9.824 [25]

+.06

+.08

+.05
+.04

a Brackets indicate literature reference.

b p calculated from the pK values of phenol, guaiacol, 4-hydroxymethyl-[20] ,
4-allyl- and 4-propenyl phenol and guaiacol.

622

for the 4-allyl- and 4-propenyl group when calculated
from the pK values of the respective phenol and
guaiacol derivative and unsubstituted compound
agree within experimental error.

The above equation does not hold for 3-allyl-2-
methoxyphenol since the allyl group in the 3-position
will affect the interaction between the methoxyl and
the phenol group. The principle of additivity of
substituent effects for obtaining the change of the
free energies of ionization of 2,3-disubstituted
benzoic acids often leads to serious discrepancies
between the calculated and observed values [21].
Similar discrepancies should occur for 2,3-disub-
stituted phenols and even larger differences would be
expected for 2,6-disubstituted derivatives (6-allyl-
2-methoxyphenol). The propenyl group with a
7r bond conjugated with the double bonds of the
aromatic nucleus withdraws electrons from the ring.
In ^-propenylbenzoic acid the — R resonance effect
of the propenyl group is transmitted to the para
carbon atom of the ring by conjugation, but must be
relayed to the acidic —Oil group by induction. On
the other hand the 4-propeny] group is in direct
resonance interaction with phenolic OH.

H-0

CH=£H-CH 3

CH=£H-CH 3

Departures from the Hammett equation have
generally been observed for reaction equilibria
involving derivatives of anilines and phenols having
— R groups para to the reaction center. For such
reactions a second set of substituent constants
somewhat greater than the norma] <j values and de-
noted by 0% (the superscript c indicating direct
conjugation) has been suggested [22]. This agreement
of the sigma values derived from ^-propenylbenzoic
acid and from 4-propenyl-2-methoxyphenol should
not be expected.

2.3. Reaction of the Eugenol Isomers With Zinc
Oxide

The four eugenol isomers were mixed with zinc
oxide, and zinc oxide containing 1 percent zinc
acetate to determine their relative reactivities in the
chelating reaction with zinc. Final setting times
were determined at 37 °C and 100 percent relative
humidity according to American Dental Association
Specification No. 9 [23] . Results are given in table 4.

Slurries containing 6-allyl-2-niethoxyphenol do
not harden and those containing 3-ally]-2-methoxy-
phenol set only in the presence of zinc acetate.

5-Allyl-2-methoxyphenol mixes, especially those con-
taining zinc acetate harden faster than eugenol
mixes. The greatly reduced reactivity of the
vicinally substituted isomers as compared to the
unsymmetrically substituted ones shows that the
chelation reaction is greatly influenced by steric
hindrance of the bulky neighboring allyl groups.
Studies to determine if substitution of metals with
smaller atomic radii than zinc will reduce this
steric effect would be of interest.

Table 4. Reactivities of eugenol isomers with ZnO

Liquid B

Sotting time

Powder a

Initial

set h

Final
set

ZnO

hr
3.5

.18
G.5

.15

(•)

( c )
( c )
~90

hr
l) 3

ZnO+1% Zn(Ac) 2

ZnO

(eugenol).

4-allyl-2-methoxyphenol

5-allyl-2-methoxypheiiol

.35

99

ZnO+1% Zn(Ac) 2

ZnO-.

(chavibetol)

5-aUyl-2-methoxypheno]

6-allyl-2-me1 hoxyphenol

.19

( c )

ZnO+l%Zn(Ac)2.--
ZnO

ZnO + 1% Zn (Ac) 2

(i-all y 1-2- met lioxy phenol

3-allyl-2-me1 hoxyphenol

3-allyl-2-methoxyphenol

-170

11 Powder-liquid ratio: 1.3 g powder per 0.4 ml liquid.

fc> Initial setting time in hours is the time elapsed from starting the mix to the
time when the point of a penetrating instrument such as the point of a Gilmore
needle makes only a slight but visible indentai Ion after placing the needle on the
material for 5 see.

" Did not harden within 10 days.

Besides steric effects of the substituent groups,
the chelation reaction is also dependent to a lesser de-
gree on the ionization constants of the isomers. Thus,
4-allyl-2-methoxyphenol (eugenol) (pK=\0.19) is
somewhat less reactive than 5-allyl-2-methoxypheno]
(chavibetol) (pK= 10. 02).

For the reaction of the nucleophilic chelating
agents (eugenol or chavibetol) with zinc oxide the
reaction constant p is likely to be positive. The
increased rate of setting of 5-allyl-2-methoxyphenol
compared to 4-allyl-2-methoxyphenol would thus be
expected since in water the sigma value of the meta
allyl- is slightly larger than that of the para allyl
group [24] . Results of this study would indicate that
synthesis of new chelate cements should be directed
towards derivatives with unsymmetrically substi-
tuted groups (1,2,4- and 1,2,5- substituted benzene
derivatives) .

The authors thank R. A. Robinson for valuable
suggestions and R. W. Morris for assisting in some
of the experimental work.

3. References

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[6] Quelet, R., Ibid [4], 45, 255 (1929).

623

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[23] American Dental Association Specification No. 9 for
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[24] Brauer, G. M., H. Argentar, and G. Durany, (unpub-
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Paper 68A-309

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