Photo-Fries Rearrangement of Aromatic Esters. Steric and Electronic

PHOTO-FRIES REARRANGEMENT OF AROMATIC ESTERS

cision between the possibilities that (1) the anion is hydrated or (2) the ion pair is linked together by a water molecule; although we favor the first possibility. We can rule out that the ammonium cation alone is hydrated. Exactly the same conclusions have been reached by Mohr, Wilk, and Barrowz6from an infrared absorption study of solutions of the tetrabutylammonium halides in CCI,. In conclusion, it can be said that both the dielectric constant and the chemical nature of the diluent affect the degree of extraction of the acid and the extent of aggregation of the resulting ammonium salt. In general, the higher the dielectric constant and the stronger the chemical interaction of the diluent with one or both of the ions of the salt, the better is the extrac-

3479

tion and the smaller the degree of aggregation, at least for ammonium salt concentrations below a few tenths molar. When, in a ‘(poor” diluent of low dielectric constant, the salt concentration becomes appreciable, determinations of the degree of aggregation by extraction methods, whether by radioactive tracer, pH determinations, or two-phase titrations, yield impossibly high results. A reason for this behavior has been suggested, and, indeed, direct osmometric measurements do give lower and more reasonable values, which, however, are still subject to the errors inherent in such measurements at concentrations in the tenths molar region. (25)

S. C. Mohr, W. D. Wilk, and G. M. Barrow, J. Am. Chem. Sac.,

87, 3048 (1965).

Photo-Fries Rearrangement of Aromatic Esters.

Role of

Steric and Electronic Factors

by G. M. Coppinger and E. R. Bell Shell Development Company, Emeryville, California (Received February 3,1966)

The rates of the light-induced rearrangement of substituted phenyl 3,5-di-t-butyl-4hydroxybenzoates to the corresponding o-hydroxybenzophenones have been measured. The reaction is first order in ester. All substituents decrease the quantum efficiency relative to hydrogen. The maximum and minimum quantum efficiencies vary by a factor of 20. Both increased molecular complexity and electron withdrawal from the phenyl ring depress the quantum yield. The results are discussed in terms of electron distribution in the excited state and of a quasi-equilibrium model of energy distribution in the excited state. A combination of a linear free energy equation with classical unimolecular rate theory satisfactorily correlates the observed quantum efficiencies.

Introduction The light-induced rearrangement of phenyl esters to hydroxy ketones has been described by Tamblyn and co-workers,’ Andenon and Reese,2 Finnegan and c+workers,4 and Kuo and co-workers.6 This photochemical to the Fries rearrangement appean to be a general reaction of phenyl esters.

This work presents some rate measurements of rearrmgement of VarioWlY substituted Phenyl esters (1) J. H. Chaudet, G. C. Newland, H. W. Patton, and J. W. Tamblyn, SPE Trans., 1, 26 (1961); G. C. Newland and J. W. Tamblyn, J. Appl. Polymer Sei., 8 , 1949 (1964). (2) J. D. Anderson and C. B. Reese, Proc. Chem. SOC.,217 (1960); J. Chem. sot., 1781 (1963).

Volume 70,Number 11

November 1966

G. M. COPPINGER AND E. R. BELL

3480

and three pyridyl esters. The substituent effects on quantum efficiency are discussed both in terms of electron distribution in the excited state and of a quasiequilibrium model of energy distribution in the excited state.

Technique and Results The technique used for rate measurement of rearrangement deserves description in some detail because it has general applicability for quantitative photochemistry. Esters were milled with powdered polyethylene, and the polyethylene was molded into thin films at 120". The films were placed between flat quartz plates, and the edges of the plates were sealed with paraffin to exclude oxygen from the system. (Oxygen has no effect on the rate of rearrangement. This precaution is to minimize oxidation of the polyethylene.) The plates were placed with fixed geometry beneath a Hanovia Model 3620 ultraviolet lamp fitted with a nickel oxide filter transmitting from 2450 to 4000 A. The photorearrangement was followed by measuring the change in the spectra as a function of time in the ultraviolet region directly from the film samples. Microthene was chosen as the polymer because the modest amount of branching minimized the crystallite content of the film and, consequently, minimized the amount of light dispersion in the spectral measurements. The polymer film was transparent to ultraviolet light in the frequencies employed. It was a medium of high purity because of the high molecular weight and, consequently, low concentration of end groups. There was no danger of change in concentm tions through volatility of the solvent or through physical manipulation. In the absence of oxygen, the polymer showed no damage from the ultraviolet radiation. With the assumptions that the photorearrangement is first order in ester and only light absorbed by the ester is photocheinically active, the rate equation was derived in the following way. The instantaneous change in composition at time t and at distance 2 from the front surface of the film is given by

where (C1),,, is the concentration of reactant at x at time t, @ is the quantum yield, and E1 is the molar absorbtivity of the reactant. (Note that we use E in natural logarithm units rather than log units.) Because both the reactant and product absorb light, however, the instantaneous light intensity at z is given by The Journal of Physical Chemistry

where Ez and Czare, respectively, the molar absorptivity and concentration of the product. It was assumed and confirmed experimentally that ( C I ) ~ (CZ),= (C,),, the initial reactant concentration at z. Combination of (1) and (2) provides

+

Table I: Rate of Rearrangement of Substituted Phenyl-3,5-di-t-butyl-4-hydroxybenzoates" Substituent

*Iov moles om-2 sec-* X l o s s

P-H p-CHa p-OCHs p-c1 p-i-CsHn p-C& p-CN p-CHO p-COGHs p-NOz m-CHa m-OCHa m-C1 m-CFa m-F rn-CN m-I m-NO2 o,o-Dimethyl ZPyridyl 3-Pyridyl PPyridyl

213.3 f 5.6" (211.2,211.0)* 150.8 f 5 . 8 108.6 f 3 . 9 109.6 f 3 . 2 (106.4)b 68.7 f 2 . 4 75.0 f 2.2 67.4 f 3 . 2 32.0 f 1 . 0 22.0 f 0 . 6 (21.0)b 15.1 f 4.2" 125.0 f 3 . 5 52.7 f 1 . 4 81.7 f 3 . 2 (83.3)b 42.7 f 0.7d 114.2 f 5.3' 43.7 f 1 . 4 57.9 f 3.6' 9.82 f 0.73 No rearrangement No rearrangement 99.2 f 10.3d 187.0 f 38.2d

a Standard deviation of fitted value. * Replicate runs. Rearrangement product was not isolated; for the corresponding hydroxy ketone was calculated from the experimental spectra after a minimum of 5 half-lives. Isosbestic points were well defined in all cases. Run with a different lamp. Observed values normalized to the others by equating values obtained for the unsubstituted ester. The terminology rate is used because of the dimensions of the data calculated. The term quantum efficiency used in the text is intended to have the same meaning as rate.

(3) H. Kobsa, J . Org. Chem., 27, 2293 (1962). (4) R. A. Finnigan and J. J. Matice, Tetrahedron, 21, 1015 (1965); R. A. Finnegan and W. W. Hogen, Tetrahedron Lettera, 365 (1963). (5) C.H. Kuo, R. D. Hoffsommer, H. L. Sla,es, D. Taub, and N. L. Wender, Chem. I d . (London), 1627 (1960).

3481


Equation 3 does not have a closed solution except for the special case when the product does not absorb a t the wavelength of excitation. It was solved numerically for +Io by nonlinear regression on an IBM 7040 by arithmetically dividing the film into ten differential thicknesses and proceeding from the initial conditions with the known molar absorptivities to obtain the best fit to the observed average concentration of reactant as a function of time. Tests of this procedure with five, ten, and twenty differential slabs indicated that the absolute values of +Io obtained would be altered less than 2% by using an infinite number rather than ten differential thicknesses. (The authors are indebted to Mrs. Jody Schlaff for carrying out these and later IBill7040 computations.) The quantum efficiency, +Io,obtained in this fashion is tabulated for the esters in Table I. Replicate rate measurements through the period of use of the same lamp are indicated. The value of E at 2537 A was used in these calculations because this was the principal exciting line within the absorption envelope. It was possible to establish that no rearrangement occurred with light above 2800 A by use of an appropriate filter, Hanovia, Pyrex 7740. Typical kinetic data are shown in Figures 1 and 2.

Discussion

I .z

CH,

I

C=O

I

QH

I

F=O

CH, No para migration occurred with esters of 3,5-dit-butyl-4hydroxybenzoic acid. Spectral and glpc examinations of the products in all cases indicated only a single isomer as product. I n all cases we have confidently assigned the o-hydroxybenzophenone structure

"";6j?JCH'

B\

c -0 II

t\ -

C L

0.6

-

0 E

0

0.4

-

0.2

-

O\

Anderson and Reese2 have reported that catechol monoacetate and phenyl acetate rearrange in both examples to a pair of isomers.

9

1.4

0

\

4

20

by -Calculated Regression Analysis

0 Experimental

40

60

80

IO

T i m e , minutes

Figure 1. Rearrangement of p-tolyl 4hydroxy-3,5-di-t-butylbenzoate.

to the product of rearrangement. When the starting material was a meta-substituted phenyl ester, we have assumed that rearrangement has proceeded to a 1,2,4substituted product rather than a 1,2,3-substituted product solely on the premise that the former alternative is more reasonable from steric consideration; however, this assumption is not important. The presence of isosbestic points in the spectra during rearrangement excludes the existence of both isomers. Confidence in these conclusions was assured by the presence of isosbestic points in all of the spectra and by the fact that long-wavelength absorption (30004000 A) by the hydroxybenzophenones at experimental infinite time was, in all cases, within 5% of that obtained from spectra of the pure o-hydroxybenzophenones whose comparison with the o-hydroxybenzophenone could be made. Kobsa3 has examined this photorearrangement with a number of esters substituted in the para position of the phenol ring with t-butyl and in the para position of the acid with various substituents. Volume 70,Number 11

November 1966

3482


Wavelength ( A )

Figure 2.

j C

I

4

Table I1 : Rates of Rearrangement of t-Butylphenylbenzoates @Io, moles cm-2 para-Substituent

6

see-'

X 108

46.6 f 2 . 2 158.8f18.0 61.1 f 5 . 1 135.6 6 . 1

C(CHah H

c1

CN

C(CHJJ

The form of KObSa'S data prevented comparison with the date of this work. TO circumvent this dilemma, the rates of rearrangement of four of the esters used by Kobsa were measured by the film technique (vide infra)' These are listed in 11. The Journal of Physical Chemistry

Kobsa has concluded that this rearrangement is entirely the free-radical type with scission of the ester carbon-oxygen bond and recombination of the radical fragments within a solvent cage.316 This suggestion (6) M. T&+,-Erhen and

(1955).

S. Bywater,

J. Am. Chem. Sot., 77, 3710


3483

appears to be supported by his observations that t butylphenol is observed as a product even when benzene is used as solvent. A free-radical mechanism is not quite so evident from our results with the esters in Table I ; the material balance of product and starting material was above 95% ; the nitro-substituted compounds, the slowest, were accounted for at the extent of 80%. No fragmentation was detected. We prefer the concept of a molecular rearrangement as better describing the reactions. (Figure 2 is an illustrative example of the spectral course of the rearrangement of compounds listed in Tables I and 11. Well-defined isosbestic points are apparent throughout the course of the rearrangements (CesteJI (eketone) I = (C,,,,) initlol. This indicates that a single product is formed and is stable.) Further, the lack of any reaction of the o,o-dimethylphenyl ester strongly implies involvement of the entire excited molecule rather than localization of energy in an isolated bond for homolytic bond cleavage. The striking difference in the pyridyl esters also militates against simple homolytic bond cleavage. When the results listed in Tables I and I1 are compared, it is apparent that the order of the effects on the rate of rearrangement by substitution in the ring bearing the carboxyl group are the reverse of the order of the effects when the substituents are in the phenolic ring. These observations in themselves cast doubt on a homolytic carbon-carbon bond cleavage implied in photodissociation since the order of the substituent effect should be the same regardless of which ring bears the substituents. Satisfactory correlation of the data can be made if one considers the effect generally of electron flow in the excited states of these esters prior to bond cleavage and regardless of subsequent transformation. Therefore, to understand this photochemical rearrangement, the electron density in the photoexcited state should be known. Since this information is not yet available, the less satisfactory method of examining the effect of substituents on the transition dipoles of the molecule has been employed.

When X is an electron-donating group, the effect on the dipole is to increase electron density about the carboxyl carbon; when X is electron withdrawing, the effect is to pull electrons away from the carboxyl carbon. In the second ring, the electrons on the phenolic oxygen tend to flow into the ring system in the excited state,’ increasing the electron density in the ring. When Y is electron donating, the electron flow is also into the ring, further increasing the electron density. When Y is electron withdrawing, the tendency is to reduce the electron density in this ring. If, then, one assumes that the excited state leading to rearrangement is a T-T* transition, the experimental observations, generally, fit within this explanation. By analogy with the Fries rearrangement, the requirements for photorearrangement are that the carboxy carbon be electron deficient and the ortho position of the phenol ring be electron rich. Now, for example, when X, the substituent on the benzoate ring, is CN, electrons are withdrawn from that ring system making the carboxyl carbon more electron deficient than it would be when X is tbutyl, which will tend to force electrons toward the carboxyl carbon. The rate of rearrangement of the ester with X as CN should be faster than that with X as t-butyl. With X as C1, the rate should be intermediate between the two. This argument is admittedly empirical. As one reviewer has pointed out there are an insuffcient number of esters differently substituted in the benzoate ring to establish a quantitative comparison between the data of Table I and Table 11. The point to be made is that with respect to substituent X the comparative quantum efficiencies are CN > C1 > C(CH& and with respect to Y the comparative quantum efficiencies are CH:, > C1 > CN. These differences are interpreted as the result of substituent effects, and the argument is an attempt to define the nature of the effect. The argument does not consider hydrogen to be a substituent and the consequences of the argument do not apply. This problem is considered in the discussion of energy distribution in the excited state. Some consideration has been given to the relationship between transition dipole and intensity of absorption. para-Substituted phenols exhibit both increased absorption intensity and shifts to the blue as the substituent increases the transition dipole. Applying this argument to the substituent Y, the substituent on the phenol ring, the rate of rearrangement of the ester with Y as CHs should be faster than

+

4 6 c=o I Y

(7) T. Forster, 2. Elektrochem., 54, 42 (1950); A. Weller, ibid., 56,

662 (1962).

Volume 70,Number 11

November 1Q66


3484

with Y as CN because in the former case the electron density in the ring and, in particular, at the ortho position, is higher. The maximum efficiency in terms of quantum yield among those esters measured was with the phenyl ester. This, too, is consistent with the effect of substituent Y on the transition dipole. Any substituent Y in the phenol ring, even OH, will increase the transition dipole. The long-wave absorption for phenol18 p-cresol, p-methoxyphenol, and hydroquinone1° are, respectively, 2720,2850,2920, and 2930 A. This treatment of substituent effect implies that interaction of the substituent group with the center of electron excitation is predominantly inductive. The effect of meta substituents in the phenyl ring is consistent with this treatment. The general effect of a meta substituent is to decrease the quantum efficiency with respect to the para substituent. Further, there is no reversal of the order of effect among substituents which would be anticipated if resonance interaction were dominant. It is of interest that there was no inhibition by product of rearrangement even beyond 90% reaction. The hydroxybenzophenones are extraordinarily efficient in quenching phosphorescence states of other moleculesll and yet the rates of rearrangements were independent of concentration of hydroxybemophenones. Even though collisions in polyethylene are improbable, the rearrangement in isooctane solution 0

PH

II

IC-?

PH

PH

followed the same kinetic order, and the rate of rearrangement was not affected by the presence of hydroxybenzophenone. These observations are consistent with a rearrangement proceeding directly from an excited singlet without involvement of a triplet state.12

Quasi-equilibrium Model The rates of decomposition of electronically excited ions in mass spectrometers’* and in radiation chemi ~ t r y have ’ ~ been treated quantitatively with considerable success by the use of the “quasi-equilibrium” model. This subject has been reviewed re~ent1y.l~ The basic postulates’6 of this model are as follows. (1) After excitation, very rapid internal energy transfer processes are assumed to lead to an essentially uniform The Journal of Physical Chemistry

distribution of excited ions among all accessible quantum states of the ions compatible with energy and angular momentum restrictions. (2) A certain fraction of the quantum states correspond to so-called activated complexes. It is further assumed that the fraction of ions which are activated complexes is at all times equal to the fraction of quantum states which represent activated complex states. (3) The rate of reaction of the ions is then given by the fraction (concentration) of activated complexes multiplied by the average rate at which the activated complex crosses the potential barrier. The classical form for the unimolecular rate constant13is

k(€) = sv(l -

y-l

(4)

where s is a steric factor in the range of 1-10-3, v is a reaction frequency of the order of vibrational frequencies, e* is the activation energy of the process, e is the total energy content, and S = 3N - 6, the number of internal degrees of freedom in an N-atomic molecule. In the present case of photorearrangement of excited molecules rather than ions, the principal difficulty with rationalization of the effects on quantum yield by substituents solely from the standpoint of electronic effects is that, if electron withdrawal in the phenol ring depresses the quant,um yield, why does electron supply not increase it? Combination of a Hammett-type treatment of electronic effects with the quasi-equilibrium model appears capable qualitatively of explaining the depression of quantum yield by alkyl substituents as well as by electron-withdrawing substituents. Although the activation energy may be lowered by electron supply of the alkyl groups, which would tend to enhance the quantum yield, the additional degrees of freedom lower the fraction of total quantum states which are activated complexes, which will have the opposite tendency of lowering the quantum yield. Implicit in such a (8) R. Passarini, Boll. Sci. Fac. Chim. I d . Bologna, 12, 71 (1954).

(9) This work. (10) R. Adams and J. L. Anderson, J . Am. Chem. Soc., 7 2 , 5154 (1950). (11) E.J. Smutny, H. R. Lukens, and G. J. Jaffe, these laboratories, private communication. (12) G. M. Coppinger and G. W. Cushman, unpublished. (13) H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and H. Eyring, Proc. Natl. Acad. Sei. U.S., 38, 667 (1952). (14) D.P. Stevenson, Radiation Res., 10, 610 (1959). (15) H.M. Rosenstock and M. Krauss in “Mass Spectrometry of Organic Ions,” F. W. McLafferty, Ed., New York, N. Y., Academic Press Inc., 1963, pp 1-64.


model is the assumption of rapid conversion of electronic to vibrational energy. The lack of fluorescence" in these ester3 supports such an assumption. The following is an attempt to treat this model semiquantitatively. In the photorearrangement reactions in polymer films, the excellent product balances, and lack of fluorescence indicate that the quantum yield is determined by only two competing reactions, namely, energy loss to the polymer matrix or rearrangement to product. Assume that energy loss is first order and the same for all substrates regardless of substituent, given by

(5) where e is the energy at time t after excitation, T is a characteristic relaxation time for energy loss, the same for all substrates, and eo is the energy of the exciting 2537-A line and again the same for all substrates. By substituting (5) into (4) we obtain (6), the rate constant of rearrangement as a function of time after excitation e = eoe-t/7

where s and v are assumed the same for all substrates since the reaction center is unchanged by substitution, but e* and S -- 1 are characteristic of each substrate with its different substituent. The total probability of reaction prior to degradation of the total energy to below the activation energy will be the integral rate of reaction of the photoactivated molecule and is given by

3485

Then

As shown in Figure 3, In [(l - z ) ~ - ' / x ] is linear in (S - 1)x over the significant range of variables, for S very large (50-70). Therefore X

where a and b are constants, and b then that

II

1.35. It follows

e-b(s-l)zdx = csvrea

-b(S - 1)

(e-

'('-

')')I1

(14)

e*/@

b(S - 1) (15) or In 9 = In m

- b(S - 1)-€ *

- In b(S

-

€0

1) (16)

From the Hammett equation the relation between up may be derived as

E and

(7) Since the quantum yield is given by (S), where only rearrangement or deactivation occur, if the rate of rearso

rangement is small relative to the rate of deactivation the quantum yield will be given by

AEH* - AE 2.303RT

(19)

= up

or

AE

=

AEH* - 2.303RTup

(20)

If we then modify the quasi-equilibrium model (16) with the Hammett concept (20), we obtain From (4) and (5:)

In

= In m'

-

b(S

- 1)

(BH*

- 2.303RTup)

-

€0

In (S

Now let

- 1) - In b

(21)

or Volume 70,Number 11

November 1966

3486


2.303RTup

(S

- 1 ) u - In b

to

(22)

Put in the usual form and with the proviso that our derived values are for +Io,(23) follows.

In +Io + In ___

s-1 = In @HIOSH - 1

€H*

b-(S €0

- SH)

+ 2.303R Tup(S - 1).

(23)

60

Since, in the rearrangement a substituent para to the phenolic oxygen is also meta to the position of the entering acyl group, the choice of u is ambiguous. We present here the results obtained by orienting the substituents to the phenolic group since tests of other assignment showed little change in over-all correlation. We may clarify this ambiguity as to orientation of substituents to some degree by describing the assumed transition state. We assume excitation to a T* electronic state followed by rapid (t < lo9 sec) internal conversion to a very high vibrationally excited ground state. That conversion is rapid is supported by lack of fluorescencell and that it is not conversion to a triplet state is supported by lack of phosphorescence" and lack of products resulting from attack on the solvent. The vibrationally excited molecule in the electronic ground state then either is deactivated by energy transfer to the matrix or is transformed to a transition state. We can describe this state only to the extent of assuming that bond breaking of the carbon-oxygen bond precedes any significant bond making, that some strong polarization of the bond occurs, and that this is the rate-limiting step. Thus, the orientation of the substituents is clear, assignment of only one vibrational degree of freedom to the reaction coordinate is indicated, and description of the path of hydrogen is unnecessary. Our kinetic treatment needs no accounting of the subsequent events. We feel, however, that in our system collapse of the transition state to an o-dienone is the only result. Those who work in mobile solvents observe also a high percentage of rearrangement but to both ortho and para products. Kobsa, furthermore, does observe some phenol from attack on the solvent. We infer from his high percentage of simple rearrangement that the route from the transition state to product does not involve separation of a phenol fragment from the acyl fragment, as does Kobsa. We feel, however, that the two portions remain associated at all The Journal of Physical Chemistry

0

(S - 1)z.

Figure 3. Logarithmic dependence of (1

- x)S-'/x

on (S

- 1)z.

stages not because of a cage e$ect but because of attractive forces between them. During this stage the intermediate might be called a charge-transfer complex in an excited state. Both Kobsa and we have observed effects of substituents which indicate that some degree of charge separation is involved in the intermediates. As suggested by Finnegan14one competitive pathway in ethanol or other nucleophilic solvents may be solvolysis of the excited intermediate which results in formation of phenol, acids, esters, etc. Rather than S = 3N - 6 , we will, in accordance with Rosenstockl16use S = 0.4 (3N - 6 - R ) where R is the number of rotational degrees of freedom. As discussed in that reference, the classical equation (4) overestimates the number of degrees of freedom because of the use of an integral rather than a sum. Furthermore, since the quanta involved in internal rotation are small, they should be weighted less than truly vibrational quanta. Table I11 presents the parameters used. With these parameters the coefficients for eq 23 for best fit of the experimental data were obtained by

PHOTO-FRIES IXEARRANGEMENT

OF

AROMATIC ESTERS

Table IV: Calculated and Experimental Rate Constants, Relative Quantum Yields for Rearrangement of Substituted Phenyl Esters of PHydroxy-3,5-di-t-butylbenzoic Acid

Table 111: Parameters Used for Correlation of Quantum Yields of Photorearrangement of Substituted Phenyl Esters of 4-Hydroxy-3,5-di-t-butylbenzoic Acid Substituent

H p-C& m-CH3 p-CH30 m-CHaO p-c1 m-C1 p-CsHs p-CsHn p-CHO p-CO&eHa p-CN m-CN pN02 m-NOn m-I m-CFa m-F

S

52.8 56.0 56.0 56.8 56.8 52.8 52.8 64.5 78.4 55.2 62.0 54.0 54.0 55.2 55.2 52.8 56.0 52.8

a H. H. Jaff6, Chem. Rev., 53, 191 (1963). taken as the value for t-butyl.

3487

.P

0.000 -0.170 -0.690 -0.268 0.115 0.227 0.373 0.009 -0.120b 0.216 0.522 0.628 0.678 0.778 0.710 0.352 0.415 0.337

* This

10wr

,--

Substituent

value is

H p-CHa m-CHa p-CHaO m-CHsO p-c1 m-C1 pc6HS CHI CH3 I I p-CHa-C-CHz-C I I CH3 CH3 pCHO I)-COzCzHz p-CN m-CN p-NOz m-NOz m-I m-CFa m-F a

By eq 24.

By eq 25.

Exptl

Calcd"

Calcdb

CalodC

213.3 150.8 125.0 108.6 52.7 109.6 81.7 75.0

128.5 154.9 123.4 182.9 79.3 81.1 60.3 63.6

154.5 140.4 113.2 155.5 67.9 97.9 73.0 45.4

168.7 168.3 168.3 99.8 99.8 64.1 64.1 49.4

68.7

56.1

79.9

54.0

32.0 22.0 67.4 43.7 15.1 9.8 57.9 42.7 114.2

64.1 21.6 32.6 29.5 21.5 24.7 63.1 43.6 65.0

67.4 15.6 35.1 31.7 20.7 23.8 76.1 40.2 78.4

54.6 35.6 27.9 27.9 23.3 23.3 67.6 47.9 70.3

E

By eq 26.

stepwise regression on an IBM 7040 computer. The equation becomes

4.856 - (0.0408 f O.O212)(S - S,) (0.391 O.O072)(S - 1)u

(24)

This equation accounts for 66% of the variance of the left-hand side while a simple Hammett equation accounts for only 50%. See Table IV and Figure 4. From the coefficients of eq 24 we may obtain p = -2.43 f 0.45 and e ~ *= 3.5 f 1.8 kcal/mole. The p value is on the high side for free-radical reactions while the activation energy seems somewhat surprisingly low. Alt#hough the high value of p might be taken as further argument for molecular rearrangement and against a free-radical pathway, its value as well as that of EH* is dependent on the value of the number of degrees of freedom. Thus it is better, perhaps, to regard the values of p and eH* as reasonable and of some assurance of the validity of the model. As in correlation of the rates of the Claisen rearrangernent,lBit might be argued that the correlation of the data is improved by the use of a four- or six-parameter equation, eq 24,rather than the ordinary two-parameter Hammett equation. The use of a conventional four-parameter Hammett equation in an attempt to

account for the influence of the substituent on both the leaving and entering positions still accounts for but 50% of the variance of the quantum yield and more importantly does not predict the drastically reduced quantum yield of p-octyl-substituted ester. Other approaches to accounting for steric effects may be used. Equation 25, which is a slight but important variant of (24) and is obtained from (22) In @Io = 104.8 - (0.0389 f O.O067)(S - 1). (31.04 f 17.34)In (8 - 1)

+

(0.4385 f 0.2778)(S - 1) (25) by allowing the computer to find the coefficient of the In (S-1) term (to which the model gave the value of l), accounts for 74% of the variance of In @Io using the u and S values of Table 111. Equation 26 represents a very naive "no model" equation allowing for both electronic and steric effects which accounts for 65% of the variance of In @Iousing only urn values of Jaffe and the parameter S, for degrees of freedom. Calculations from eq 24-26 are presented in Table IV. (16) (a) W.N.White, D. Gwynn, R. Schlitt, G. Girard, and W. Fife, J . Am. Chem. Soc., 80, 3271 (1958); (b) H.L. Goering and R. L. Jackson, ibid., 80, 3277 (1958).

Volume YO, Number 11 November 1066

G. M. COPPINGER AND E. R.

3488

x

-20

0

20

40

60

(5-I)o

Figure 4. Correlation of quantum yields of photorearrangement of substituted phenyl esters of Phydroxy-3,5-di-t-but~ylbenzoates.

(0.0567

f

0.0219)S (26)

These results of attempts to correlate the variation of quantum yield do show that complexity of the reacting molecule does depress the quantum yield of this photorearrangement, that this effect may be expected to be general, that quantum yields may be successfully correlated by a combination of the Hammett equation with the quasi-equilibrium model of first-order reactions, and that even a "no-model" linear combination of electronic effects and complexity improves correlation. This treatment in terms of the quasi-equilibrium model would be on firmer ground if we had determined absolute quantum yields instead of relative quantum yields. Then we would know whether our simplification of the mathematics accomplished in eq 8 by assuming low quantum yields was valid. If it was not, then the correlation equation should be of a much more complicated form. We hope that others will be stimulated by this moderate success to test further this approach to correlation of substituent effects on photoreactions. The Journal of Physical Chemistry

BELL

Experimental Section Preparation of Substituted Hydroxybenxophenones. The appropriate ester (2 g) was dissolved in 200 ml of isooctane. The solution was irradiated with an AH-4 lamp (General Electric) in a reactor which consisted of a cylindrical Pyrex vessel into which fitted a doublewalled quartz thimble. The ultraviolet source was placed in the interior well of the vessel with water circulated around the well for temperature control. The entire reactor assembly was so arranged that the contents could be isolated from the atmosphere, and the photolysis was carried out under nitrogen. The course of the rearrangement was followed by examination of the ultraviolet spectra of aliquots from the reaction. When the reaction had reached some arbitrary degree of completion, which depended upon the ester undergoing rearrangement, the photolysis was terminated. I n general, the products were isolated by removal of solvent in vacuo and recrystallization from methanol until the product has constant and sharp melting points. The m-methoxyphenyl and m-cyanophenyl ester rearrangement products were isolated by glpc separation on an Apiezon column at 200". The samples were trapped in Teflon tubing attached to the exit of a Wilkins glpc instrument. The products were intended as reference compounds for photorearrangement rate measurements, and no deliberate attempt to achieve maximum yield was made. The yields on the small scale used ranged from 5% with m-cyanophenyl ester to 60% with p-methoxyphenyl ester. Carbon-hydrogen analysis did not serve as a useful identification measure of the products since this ratio would necessarily be identical with the starting material. Three product compounds were examined by this method to determine the validity of this assumption. Anal. (1) Calcd for 3,5-di-t-butyl-4-hydroxy-2'hydroxybenzophenone (CzlHasOs): C, 77.5; H, 9.7. Found: C, 77.3; H, 9.7. (2) Calcd for 3,5-di-t-butyl4-hydroxy-2'-hydroxy-5'-phenylbenzophenone (C2,H3003): C, 80.6; H, 7.5. Found: C, 80.4; H, 7.6. (3) Calcd for 3,5-di-t-buty1-4-hydroxy-2'-hydroxy-5'carboethoxybenzophenone (C24H3006): C, 72.4; H, 7.6. Found: C, 72.0; H, 7.4. The melting points of the benzophenones, 3,5-di-tbutyl-4-hydroxy-2'-hydroxy-2'-benzophenone, were for the following 3: H, 119"; 4'-chloro, 162.3'; 5'-chloro, 164-165" ; 4-methyl, 126" ; 5'-methyl, 148-149" ; 4'-cyano, 225-226" ; 5-cyano, 190-193" ; 4-methoxy, 137-138" ; 5'-methoxy, 142-143" ; 5'-carboethoxy, 130192" ; 5'-phenyl, 194-195" ; and 5-carboxaldehyde, 180-183'.


Preparation of Aromatic Esters. The esters were prepared by reaction of 3,5-di-t-butyl-4hydroxybenzoyl chloride with the appropriate phenol in pyridine solvent. The reaction mixture was extracted with ether, washed with aqueous HC1 to remove pyridine and with aqueous carbonate solution to remove unreacted starting materials. The ether was removed, and the products were recrystallized from methanol. The melting points and analyses are listed in Table V. ~~

Table V : f-Phenyl3,5-Di-tbutyl-~hydroxybenaoates --Analysie-CalcdC H

-FoundC

H

m-CH3 p-CH3

130-133 133 137-138 158-159 154-155

77.5 69.8 69.8 74.5 74.5

9.7 7.2 7.2 8.3 8.3

77.5 69.7 69.7 74.3 74.5

9.5 7.2 7.0 8.3 8.3

m-CN Q-CN m-NOz p-NOz p-Cs&

152 164-165 198-199 231-233 190-191

75.0 75.0 69.0 69.0 80.9

7.4 7.4 6.7 6.7 7.5

75.1 74.8 69.2 69.0 80.5

7.2 7.3 6.7 6.9 7.4

P-COzCzH5 p-CHO m-CF3 m-F

m-I

154-155 192-193 171-172 147 178-179

72.3 74.5 67.0 73.1 55.6

7.5 7.3 6.3 7.3 5.6

72.1 74.6 66.7 73.0 54.9

7.5 7.2 6.0 7.5 5.2

m-OCH3 p-OCH3 o,o-Dimethyl 2-Pyridyl 3-Pyridyl 4-Pyridyl

131-133 138-139 168-169 181-183 176-177 186

74.5 74.5 78.0 73.5 73.5 73.5

7.6 7.6 8.5 7.5 7.5 7.5

74.3 74.2 77.7 73.0 73.1 73.4

7.8 7.6 8.6 7.4 7.3 7.5

Substituent 2

H m-C1

Q-cl

bfP, "C

The general criteria for identity of products were spectroscopic: the absence of ester carbonyl at 5.95 p and the presence of benzophenone carbonyl perturbed by hydrogen bonding to the o-hydroxyl at 6.2p in the infrared and the characteristic three absorption peaks in the ultraviolet. Irradiation Procedure. Polymer for Film. Powdered polyethylene, Microthene, was extracted with benzene and acetone to remove any additives which might have been present. The polymer was freed of extraction solvent in vacuo and stored in the dark under nitrogen. F i l m Preparation. The polymer powder was mixed with each ester, and a film formed from this mixture in

3489

standard fashion at 120". The concentration of ester was adjusted so that the adsorptivity of the solution in film was between 0.7and 0.9 density unit at the wavelength of maximum adsorptivity in the ultraviolet in a film of 10-mil thickness. Compensation was provided by a film blank of the same thickness. The films containing ester were mounted between polished plates of quartz 1 X 3 X '/I6 in., sandwiched tightly to exclude air with edges sealed by paraffin wax. Irradiation was accomplished by placing the film containing the esters in duplicate on an aluminum foil shield" with fixed geometry beneath a Hanovia lamp, Model 3620. A nickel oxide filter passing light of 4000-2450 A was employed. The films were irradiated for fixed time intervals and the course of rearrangement followed by measurement of the ultraviolet spectrum in a Cary Model 14 spectrophotometer. The rates of rearrangement were measured by one of two methods. In the first the concentration of either the ester or the hydroxybenzophenone was determined by measurement of the ratio of absorption at some arbitrary fixed wavelength to the absorption at a wavelength where there existed an apparent isosbestic point. This method worked very well for those esters with fast rates of rearrangement. The material balances for ester and hydroxybenzophenone were 93104%. The second method involved measurement of rate of increase of absorption of a band associated with the hydroxybenzophenone around 3500 A; the esters were transparent at this wavelength. When the molar absorbance of the benzophenone was known, the concentration of the benzophenone was calculated at the ester concentration determined by difference. I n those examples where the molar absorbance was unknown an arbitrary infinite time defining complete rearrangement was employed for measurement of molar absorbance of hydroxybenzophenone. The various methods appeared to be equivalent. Acknowledgment. The authors are pleased to express their gratitude to Dr. D. J. Meier and Dr. D. P. Stevenson for extensive help with the mathematics and valuable discussions of this and alternative approaches. They also are pleased to acknowledge their indebtedness to Dr. R. S. Miller and Mrs. Jody Schlaff for advice on and the programming and carrying out of the IBM 7040 computations. (17) The use of aluminum foil is, in principle, an error in technique. However, experimentally, rates of rearrangement with an aluminum shield and a totally absorbing shield were indistinguishable.

Volume 70,Number 11

November 1966

Photo-Fries Rearrangement of Aromatic Esters. Steric and Electronic

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