Bond Dissociation Energies in the Phenyl Benzoate Molecule and in

21 Bond Dissociation Energies in the Phenyl Benzoate Molecule and in Related Free Radicals

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0075.ch021

P E T E R GRAY Department of Physical Chemistry, Leeds University, Leeds 2, England

The standard heat of formation of phenyl benzoate as the gaseous species at 25°C. has been determined as: ∆ H ° ( P h C O P h ) = - 35 ± 1 kcal. per mole. Seven distinct bond dissociation energies are immediately related to this reference basis. Values for these, together with values for the heats of formation of related free radicals, are discussed, and a provisional set is presented. They include the following estimates (kcal. per mole): O(PhCO —Ph) = 94; O(Ph—CO Ph) = 96; O(·CO —Ph) = 62; O(PhCO-OPh) = 64. Errors are likely to be around 5 kcal. per mole. f

2

2

2

2

>Tphis paper extends previous discussions (6,7,8) of the thermochemistry of oxygenated free radicals to the strengths of bonds i n phenyl benzoate and i n related free radicals and the heats of formation of these free radicals. It is based on our recent redetermination ( 1 ) of the heats of combustion and formation of phenyl benzoate—measured principally to establish reliable thermochemistry for the decomposition and explosion (5) of dibenzoyl peroxide, which yields phenyl benzoate as a major product. Enthalpy of Formation of"PhenylBenzoate Standard procedures were followed, and experimental details w i l l be published elsewhere. The mean value found for AU was —31.803 kjoules per gram, with a standard deviation of 0.015%. Washburn cor rections and conversion to constant-pressure conditions yielded ΔΗ = -1506.5 ± 0.5 kcal. per mole, and AH ° = - 5 7 . 7 ± 0.5 kcal. per mole. C

Ό

0

f

282 Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

21.

GRAY

283

Bond Dissociation Energies

Errors quoted are twice the standard deviation, and the new value repre sents a significant shift from Stohmanns (1892) measurement of 1510.4 kcal. per mole for —AH °, on which heats of formation i n the literature have been previously based. This value relates to the pure crystalline ester, and to discuss bond dissociation energies it is necessary to have a value for the heat of forma tion of gaseous phenyl benzoate. The latent heat of sublimation at 25 °C. may be derived from separate values for fusion and vaporization. W e have measured the latent heats of fusion at 70 °C. as AH = 7.0 ± 0.3 kcal. per mole (both electrically and from determining the cryoscopic constant). A n average value of the latent heat of vaporization, A f f = 14.2 ± 0.2 kcal. per mole, may be evaluated from existing (17) vapor pressure data between 106° and 314°C. T o correct these values to 2 5 ° C , estimated specific heats must be used. Fortunately, the corrections to AH and Δί/ are in opposite directions, and tend to cancel each other; the standard heat of formation of gaseous phenyl benzoate is: C

fus


v a p

ναρ

fus

ΔΗ/ (PhC0 Ph) = - 3 5 ± 1 kcal. per mole. 2

Bond Dissociation Energies and Radical Thermochemistry Seven distinct bond dissociation energies are relevant to aromatic esters. They are not independent, and their interrelations arc shown below.

PhC0 R 2

(D + D ) = ( D + D ) 1

2

3

4

(D + D ) = (D + D ) 8

e

5

7

Oxygen-Phenyl Bond in Phenyl Benzoate. B y using the new value for the enthalpy of formation of phenyl benzoate together with the most recent values (80 and —21 kcal. per mole, respectively) for the enthalpies of formation of the phenyl (3) and benzoate (11, 12) radicals, we obtain for the strength of the oxygen-phenyl bond i n phenyl benzoate a value of

Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

284

OXIDATION OF ORGANIC COMPOUNDS

1

D ( P h C 0 — P h ) = 94 ± 4 kcal. per mole. The strength of this bond is less than that of the corresponding bond in phenyl acetate D ( M e C 0 — Ph) = 105 kcal. per mole [a value derived from the enthalpy of hydroly sis (18) of phenyl acetate, AH = —6.86 kcal. per mole and the pres ently accepted (8) value for the heat of formation for the acetate radical A H ( M e C 0 - ) = —45]. A difference of 3 to 5 kcal. per mole might be expected to arise from the different inductive effects on the oxygen-phenyl bond in the two parent molecules. It is too great to be readily explicable in terms of superior derealization in the product radicals, and it contrasts with the similarity found for Ο—Ο bond dissociation energies (11) in the two parent p e r o x i d e s — P h C 0 C 0 P h and M e C 0 C 0 M e . In effect, it casts doubts on the literature values (11) for the heat of formation of the P h C 0 - radical. x

2

2

hydv

2


2

2

2

2

2

Phenyl-Carbon Bond in Phenyl-, Methyl-, and Ethyl Benzoate. F u l l experimental data that would permit the strength of the phenyl-carbon bond in phenyl benzoate to be determined independently are lacking. A n estimate can be based on data for methyl benzoate and ethyl benzoate. Values for D in these compounds may be derived from their heats of formation (AH (PhC0 Me) = - 7 2 kcal. per mole; A H ( P h C 0 E t ) = —79 kcal. per mole) and from the heats of formation (7) of the methoxycarbonyl and ethoxycarbonyl radicals. W e take A H / ( - C 0 M e ) ^ —52 kcal. per mole, and A H / ( - C 0 E t ) — —57 kcal. per mole, revising the original values (2) upward in step with the "high" values (12) for the corresponding alkyl carbonyl radicals. The bond dissociation energies D ( P h — C 0 M e ) and D ( P h — C 0 E t ) are thus ca. 99 kcal. per mole and 101 kcal. per mole, respectively. In the benzoate esters, inductive effects analogous to those discussed above might be expected to produce a 3 to 5 kcal. weakening from D ( R — C 0 P h ) to D ( P h — C O o P h ) ; on this basis, the strength of the phenyl-carbon bond in phenyl benzoate is estimated to be about 96 kcal. per mole. Although it is not possible to estimate reliably the error i n D ( P h — C 0 P h ) , it is not likely to be very large. Phenyl-Oxygen Bond in the Phenoxycarbonyl Radical. From the relationship D ( P h — C 0 P h ) + D ( C 0 — P h ) = 102 kcal. per mole and the value derived above for D of 96 kcal. per mole, it follows that in the phenoxycarbonyl radical the strength of the bond joining the phenyl group to the O C O group, D ( C 0 — P h ) = 102 - 9 6 = 6 kcal. per mole. In turn, the heat of formation of the phenoxycarbonyl radical given by ΔΗ^ΡηΟΟ,Ρη) - AHf(V\\ - ) + D ( P h — C 0 P h ) is A H , ( C 0 P h ) = —19 kcal. per mole. Useful comparisons may be made here with the corresponding alkoxycarbonyl radicals, C 0 M e and C 0 E t , for which values (6) for D of around —10 kcal. per mole have been advanced. These radicals C 0 R , 3

f

/

2

2

2

2

3

3

3

2

3

2

2

3

2

3

2

4

2

3

4

2

3

2

2

2

2

4

2


21.

GRAY

285


like the isomeric R C 0 - family, contain an "endothermic" bond. Both P h C 0 · and · COoPh require energy for dissociation. 2

2

Some data exist in the literature for the hydroxycarbonyl radical, • C 0 H . Back and Sehon (2) concluded that A H , ( - C 0 H ) was —62 kcal. per mole, and D ( C 0 - H ) was 20 kcal. per mole. This bond strength, which would place ( C 0 H ) beyond the ( * C 0 A r ) family thermochemically speaking, seems exceedingly high. 2

2

2

2

Back and Sehon based their deductions on a kinetic study of the pyrolysis of gaseous phenylacetic acid, which they found to be a homo geneous process obeying first-order kinetics with a velocity constant given by (fc/sec." ) = 10 · exp ( - 5 5 0 0 0 / R T ) . They considered bond fission into benzyl and hydroxycarbonyl radicals to be the rate-determining step and identified their experimental activation energy with the bond disso ciation energy D ( P h C H — C 0 H ) . Their other thermochemical assump tions were an old experimental value for the heats of formation of the parent acid, A H / ( P h C H C 0 H ) = —75 kcal. per mole and an "inter mediate" value for the benzyl radical A H / ( P h C H - ) = 42 kcal. per mole. The former is consistent with empirical estimates based on the heats of formation of toluene and acetic acid, but even if the latter value is revised (3) nearer A H ( P h C H ) = 45, the essential difficulty remains. The mechanism of pyrolysis seems to merit further investigation. 1


2

12

9

2

2

2

2

2

2

Phenoxyl-Carbon Bond in Phenyl Benzoate. To derive a value for the dissociation energy of the phenoxy-carbon bond in phenyl benzoate, D ( P h C O — O P h ) , the heat of formation of the benzoyl and phenoxyl radicals must be known. The currently recommended (12) value for benzoyl is A// (PhCO) = 16 kcal. per mole. F o r phenoxyl, an estimate can be based on the heat of formation (9) of gaseous phenol (—23.05 kcal. per mole) and kinetic experiments on the abstraction of hydrogen atoms. Work with methyl radicals (10,15,16) suggests that the dissocia tion energy in phenol is markedly less than in the alcohols. A n upper limit of ca. 95 kcal. per mole is placed by Lossings electron impact studies. New work on substituted phenols (13) suggests that i n them D is ca. 88 kcal. per mole. A similar value has been proposed for phenol by Benson (4) on the basis of evidence from inhibition studies involving a chain of resemblance linking D ( P h O — H ) to D ( H 0 — H ) and D ( M e C — H ) . If the strength of the phenoxyl-hydrogen bond is 88 kcal. per mole, the heat of formation of phenoxyl is 13 kcal. per mole. The strength of the phenoxyl-carbon bond in phenyl benzoate may now be calculated: 5

/

2

3

D ( P h C O — O P h ) = A H ( P h O ) + A H ( P h C O ) - Δ Η , ° (PhC0 Ph) 64 kcal. per mole 5

/

;

2

It is the weakest bond i n the ester.


=

286

OXIDATION OF ORGANIC COMPOUNDS

1

In the phenoxycarbonyl radical, the same bond is further weakened by the electronic rearrangements accompanying the formation of the products: CO—OPh) = 7 kcal. per mole

D( 6

Reaction Thermochemistry and Mechanisms for Phenyl Esters Table I summarizes the values recommended here and indicates the magnitude of their uncertainties. These values have many implications outside the chemistry of phenyl benzoate itself. For example, some bond dissociation energies (kcal. per mole) in other species are: D ( P h — C O P h ) = 85; D ( P h C O — C O P h ) = 62; D ( P h C 0 — C O P h ) = 77; D ( P h O — C 0 P h ) — 62; D ( P h C O — N H ) = 84.


2

2

Table I.

2

Recommended Values for Bond Dissociation Energies and Radical Thermochemistry in Aromatic Oxidation

Bond Dissociation Energies Bond

D, (kcal. per mole)

PhC0 —Ph Ph—C0 Ph PhCO—OPh PhO—CO OCO—Ph Ph—C0 Ph—CO 2

2

2

94 96 64 7 6 7 30

± ± ± ± ± ± ±

4 5 5 5 5 3 5

Heats of Formation (g) at 25°C. Species

AH , (kcal. per mole) f

PhC0 Ph PhC0 •C0 Ph PhCO PhOPh-

-35 ± -21.4 -19 16 ± 13 ± 80 ±

2

2

2

1 ± 3 5 3 2

Warbentin (19) has indicated how the radical thermochemistry in volved can assist in assigning a mechanism for the thermal decomposition of phenyl oxalate. Exclusive initial fission of either the one C — C or the two C—Ο bonds would lead, via P h O C O or O C C O intermediates, to high C 0 or high C O yields. In experiments lasting about 75 hours at 500°K. in diphenyl ether, comparable amounts of C O (13%) and C 0 ( 9 % ) are formed. The following steps, possibly concerted: 2

2

PhOCOC0 Ph-»PhO-

+ COC0 Ph

2

2

•COC0 Ph -> C O + 2

•C0 Ph-»C0 2

2

C0 Ph 2

+ Ph-

could afford a reasonable explanation. In phenyl benzoate itself, Miller (14) reported results of radiolysis yielding 10 times as much C O as C 0 , which is wholly consistent with predominant split at the weakest bond, P h O — C O P h . The situation is 2


21.


GRAY

287

similar i n phenyl acetate but reversed i n benzyl benzoate ( and i n benzyl acetate), again i n agreement with thermochemical requirements.


Conclusions A l l the radicals concerned are important i n oxidation processes of aromatic molecules, and this paper aims to offer a starting point for the thermochemical dissection of such oxidation processes. It is also hoped that it may stimulate further investigations of radical thermochemistry, especially i n the aromatic field. Areas for fruitful work on bond energies include the formate and carbonate esters, including phenyl formate and phenyl carbonate, and the bond strengths i n formic acid itself and i n benzaldehyde. Acknowledgments It is a pleasure to acknowledge the combustion calorimetry carried out by G . P. Adams, D . H . Fine, and P. G . Laye and the extremely useful criticism of S. W . Benson. Literature Cited (1) Adams, G . P., Fine, D . H., Gray, P., Laye, P. G., J. Chem. Soc. 1967, B720. (2) Back, M., Sehon, Α., Can. J. Chem. 38, 1261 (1960). (3) Benson, S. W . , Golden, D . M., Rodgers, A . S., personal communication, 1967. (4) Benson, S. W., J. Am. Chem. Soc. 87, 972 (1965). (5) Fine, D . H . , Gray, P., Combust. Flame 11, 71 (1967). (6) Gray, P., Thynne, J. C. J., Nature 191, 1357 (1961). (7) Gray, P., Thynne, J. C. J., Shaw, R., "Progress in Reaction Kinetics," Vol. 4, p. 63, Pergamon Press, New York, 1967. (8) Gray, P., Williams, Α., Chem. Rev. 59, 239 (1959). (9) Green, J. H . S., Quart. Rev. Chem. Soc. 15,125 (1961). (10) Herod, Α. Α., Ph.D. Thesis, Leeds, 1967. (11) Jaffe, L . , Prosen, E . J., J. Chem. Phys. 27, 416 (1957). (12) Kerr, J. Α., Chem. Rev. 66, 465 (1966). (13) Mahoney, L . R., Da Rooge, Ν. Α., "Preprint, International Oxidation Symposium," Vol. II, p. 585, 1967. (14) Miller, Α. Α., J. Phys. Chem. 69, 1077 (1965). (15) Mulcahy, M. F. R., Williams, D . J., Australian J. Chem. 18, 20 (1965). (16) Shaw, R., Thynne, J. C. J., Trans. Faraday Soc. 62, 104 (1966). (17) Stull, D . R., Ind. Eng. Chem. 39, 533 (1947). (18) Wadsö, I., Acta. Chem. Scand. 14, 561 (1960). (19) Warbentin, J., personal communication, 1967. RECEIVED

October 9, 1967.


Bond Dissociation Energies in the Phenyl Benzoate Molecule and in

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