The C—Br bond dissociation energy in substituted benzyl bromides

1951 ◽  
Vol 209 (1096) ◽  
pp. 97-110 ◽  
Keyword(s):  
Bond Energy ◽  
Benzyl Bromide ◽  
Frequency Factor ◽  
Unimolecular Dissociation ◽  
Bond Dissociation Energies ◽  
Kcal Mole ◽  
Bond Dissociation ◽  
Dissociation Energies ◽  
Rate Determining Step

The ‘toluene-carrier’ technique has been used for the determination of the C—Br bond dissociation energies in the substituted benzyl bromides: p -, m - and o -xylyl bromides; p -, m - and o -chlorobenzyl bromides; p - and m -bromobenzyl bromides; p - and m -nitrobenzyl bromides; and p - and m -nitrilebenzyl bromides. The rate-determining step of the decompositions of all these compounds is represented by the unimolecular dissociation processes ( s ) Ph s . CH 2 . Br → Ph s . CH 2 • + Br, ( s ) where Ph s . CH 2 . Br refers to the substituted benzyl bromide. Assuming that the frequency factor of the decomposition of each benzyl bromide is equal to the frequency factor of reaction ( u ) Ph . CH 2 . Br → Ph . CH 2 • + Br, ( u ) the differences in activation energies between E u and E s were calculated using the relation E u ─ E s = RT In ( k s / k u ); (I) k s and k u denote the unimolecular rate constants of reactions ( s ) and ( u ) respectively. Since E s and E u are equal to the C—Br bond dissociation energies in the substituted benzyl bromides and benzyl bromide itself, equation (I) yields the differences, ∆ D’ s, between D ( Ph . CH 2 —Br) and the values for D ( Ph s . CH 2 —Br). The calculated differences in the C—Br bond dissociation energies are listed below: substituted ∆ D substituted ∆ D benzyl bromides (kcal. /mole) benzyl bromides (kcal. /mole) o -chloro 0·9 m -methyl 0-0 m -chloro 0·1 p -methyl 1·4 p -chloro 0·4 m -nitro 2·1 m -bromo 0·3 p -nitro 1·1 p -bromo 0·3 m -nitrile 1·4 o -methyl 2·0 p -nitrile 0·7 The significance of these findings is discussed, and the effect of substitution on a bond energy is contrasted with the effect of ionic reactions.

The C—Br bond dissociation energy in halogenated bromomethanes

1951 ◽  
Vol 209 (1096) ◽  
pp. 110-131 ◽  
Keyword(s):  
Bond Dissociation Energy ◽  
Dissociation Energy ◽  
Methyl Bromide ◽  
Frequency Factor ◽  
Unimolecular Dissociation ◽  
Bond Dissociation Energies ◽  
Kcal Mole ◽  
Fast Reaction ◽  
Bond Dissociation ◽  
Dissociation Energies

The pyrolyses of methyl bromide and of the halogenated bromomethanes, CH 2 CI. Br, CH 2 Br 2 , CHCl 2 .Br, CHBr 3 , CF 3 Br, CCI 3 . Br and CBr 4 , have been investigated by the ‘toluene-carrier' technique. It has been shown that all these decompositions were initiated by the unimolecular process R Br → R + Br. (1) Since all these decompositions were carried out in the presence of an excess of toluene, the bromine atoms produced in process (1) were readily removed by the fast reaction C 6 H 5 .CH 3 + Br → C 6 H 5 . CH 2 • + HBr. Hence, the rate of the unimolecular process (1) has been measured by the rate of formation of HBr. The C—Br bond dissociation energies were assumed to be equal to the activation energies of the relevant unimolecular dissociation processes. These were calculated by using the expression k ═ 2 x 10 13 exp (- D/RT ). The reason for choosing this particular value of 2 x 10 13 sec. -1 for the frequency factor of these reactions is discussed. The values obtained for the C—Br bond dissociation energies in the investigated bromomethanes are: D (C—Br) D (C—Br) compound (kcal./mole) compound (kcal./mole) CH 3 Br (67.5) CHBr 3 55.5 CH 2 CIBr 61.0 CF 3 Br 64.5 CH 2 Br 2 62.5 CCI 3 Br 49.0 CHCl 2 Br 53.5 CBr 4 49.0 The possible factors responsible for the variation of the C—Br bond dissociation energy in these compounds have been pointed out.

The pyrolysis of benzoyl bromide and the determination of the relevant bond dissociation energies

1952 ◽  
Vol 214 (1117) ◽  
pp. 273-280 ◽  
Keyword(s):  
Least Square Method ◽  
Heat Of Formation ◽  
Least Square ◽  
Bond Dissociation Energies ◽  
Kcal Mole ◽  
Bond Dissociation ◽  
Rapid Decomposition ◽  
Dissociation Energies ◽  
Rate Determining Step

Pyrolysis of benzoyl bromide in the presence of excess of toluene has been investigated. It has been shown that the rate-determining step is the unimolecular dissociation C 6 H 5 . CO. Br → C 6 H 5 . CO + Br, followed by the rapid decomposition of benzoyl radicals C 6 H 5 . CO → C 6 H 5 ⋅ + CO. Bromine atoms and phenyl radicals seem to be removed from the system by the reactions C 6 H 5 . CH 3 + Br → C 6 H 5 . CH 2 ⋅ + HBr and C 6 H 5 . CH 3 + Ph ⋅→ C 6 H 5 . CH 2 ⋅ + C 6 H 6 . The activation energy of the rate-determining dissociation process has been estimated using the least square method at 57⋅0 kcal/mole and has been identified with D (C 6 H 5 ⋅ CO-Br). Thus, having D (C 6 H 5 ⋅ CO-Br) = 57⋅0 kcal/mole, the heat of formation of benzoyl radicals has been calculated at ∆ H f (C 6 H 5 . CO) = 15⋅6 kcal/mole, and consequently the values for various bond dissociation energies of the type D (C 6 H 5 . CO- X ) have been derived.

Studies of the variations in bond dissociation energies of aromatic compounds - II. Substituted bromobenzenes

1953 ◽  
Vol 219 (1138) ◽  
pp. 353-366 ◽  
Keyword(s):  
Benzene Ring ◽  
Bond Dissociation Energy ◽  
Aromatic Compounds ◽  
Frequency Factor ◽  
Bond Dissociation Energies ◽  
Kcal Mole ◽  
Bond Dissociation ◽  
Dissociation Energies ◽  
Tentative Explanation ◽  
The Difference

The paper reports the effects of substituents on C— Br bond dissociation energy in substituted bromobenzenes. The following substituents introduced in various positions of benzene ring were investigated: F, Cl, Br, CH 3 , C 6 H 5 , CN and OH. In addition, the studies were extended to bromopyridines and bromothiophene. Assuming that the frequency factor is constant for the series of decompositions, the following values were obtained for the difference ∆ D = D ( Ph — Br) - D ( Ph s — Br), where Ph s Br denotes a molecule of substituted bromobenzene: substituent ∆D (kcal/mole) substituent ∆D (kcal/mole) p -F 0-5 m -C 6 H 5 0-8 p -Cl 0-6 o -C 6 H 5 2-7 m -Cl 1-0 p -CN 0-3 o -Cl 1-2 m -CN 0-8 p -Br 0-3 o -CN 0-6 o -Br 1-8 p -OH 3-9 P -CH 3 0-2 o -OH 3-8 m -CH 3 0-2 3-bromopyridine -5-0 o -CH 3 0-8 2-bromopyridine -0-6 p -C 6 H 5 0-2 2-bromothiophene 2-4 The significance of these results is discussed, and a tentative explanation of the observed effects is proposed.

Experimental methods for measurement of bond dissociation energies and heats of formation of radicals

1951 ◽  
Vol 207 (1088) ◽  
pp. 5-13 ◽  
Keyword(s):  
Experimental Methods ◽  
Experimental Results ◽  
Heats Of Formation ◽  
Bond Dissociation Energies ◽  
Carrier Gas ◽  
Bond Dissociation ◽  
Dissociation Energies ◽  
Kinetics Of

The paper describes a pyrolytic method of investigating the kinetics of gaseous reactions in which toluene is used as a carrier gas. It is shown that the method is particularly suitable for the determination of bond dissociation energies. The scope of the method is illustrated by various examples. A list of bond dissociation energies obtained is given. The manner in which the experimental results obtained can be cross-checked, is indicated and illustrated by examples. The effects of various constitutional factors on the bond dissociation energies are discussed.

H—N3 and CH3—N3 bond dissociation energies

1968 ◽  
Vol 46 (24) ◽  
pp. 3785-3788 ◽  
Author(s):  
Mervyn Chiang ◽  
Robert Wheeler
Keyword(s):  
Temperature Variation ◽  
Thermal Emission ◽  
Bond Dissociation Energies ◽  
Kcal Mole ◽  
Bond Dissociation ◽  
Dissociation Energies ◽  
Heated Filament

The thermal emission of negative azide ions from a heated filament operating in gases of both HN3 and CH3N3 has been studied in a magnetron cell and the temperature variation of this current used to deduce the C—N bond dissociation energies in both molecules. Results indicate higher values than previous estimates. They are: D0(H—N3) = 90 ± 8 kcal/mole and D0(CH3—N3) = 88 ± 8 kcal/mole at 0°K.

Sign in / Sign up

Export Citation Format

Share Document