BONDDISSOCIATION ENERGIES
IN
BROMOPROPADIENE AND 3-BROMOPROPYP\’E
1533
Bond Dissociation Energies and Conjugation Effects in Bromopropadiene and 3-Bromopropyne
by J. J. Throssell Department of Chemistry, We‘estfield College, Hamstead, London, N . W . S , England (Received J u n e i d ? 1965)
Dissociation energies have been measured for the carbon-bromine bonds in the isomeric molecules 3-bromopropyne and bromopropadiene which yield identical radicals in. the unimolecular fission process. The rate constants for the fission processes are k = 1012.15 exp[- (46,800 f 2000)/RT] and 12 = l O I 3 expl- (50,000 f 2000),IRT],respectively. An attempt is made to correlate the difference in the dissociation energies with the difference in the degree of through-conjugation between the bromine atom and the n-electron system of the hydrocarbon radical.
Introduction The strength of a chemical bond in a molecule R-R1 must be interpreted in terms of the structures of the radicals R and R1, into which the molecule dissociates when the boind is broken, and the structure of the parent molecule. Thus, it follows from the definition of the bond dissociations energy D(R-Rl) that D(R-RI)
=
AHf(R)
+ ANf(R1) - AHr(RR1)
(1)
where AHr(X) is the heat of formation of the species X. Consider two pairs of isomeric molecules R’R1, R”R1 and R’Rz, R”:Rz, in which the components of each pair yield the same radicals R* and R1 or Rz, i.e.
+ R1 ---zR ” + R1
R’-R1 ---zR* R’’-RI
(The structures of the radical R* and the fragments R’ and R” in the molecule are related in such a way that the structure of R* is a resonance hybrid to which the canonical structures of R’ and R” make the principal contributions.) ‘Then
D(R’-R1) - D(R”-Ri) = AHf(R”R1) - aHf(R‘R1) (2)
It has been shown by Szwarc and Taylor’ how the difference in the nature of the two bonds may be under-
stood by considering in a qualitative way the various contributions to the electronic energies of the molecules and of the dissociation products. Thus, if E(R’Rl) represents the total electronic energy of the R’-R1 molecule E(R’R1) = E,bl(R’)
+ Enb(R1) + U(R’R1)
+ n(R’R1)
(3)
where E,b’(R’) = energy of the electrons localized in the fragment R’ of the molecule R’R1 and not involved in the R’RI bond, the superscript 1 denoting the fact that the fragment R’ is attached to R1, u(R’R1) = energy of the u-electrons, and a(R’RI) = energy of the a-electron, in the R’R1 bond. If E(R*) and E(R1) represent the corresponding total electronic energies of the dissociation products, then
D(R’-Ri)
=
E(R*)
+ E(R1) - E(R’R1)
(4)
Similar equations to (3) and (4) may be written for the isomer R”R1, and thus, (2) may be expressed in the form D(R’-Rl) - D(R”-RI)
E(R”R1)
-
=
E(R’R1)
=
[Enb’(R”) -
Enbl(R’)]- [u(R’RI) - rr(R”Ri)] -
Ia(R’R1) - n(R”Ri)]
(5)
(1) M. Sswarc and J. W. Taylor, Trans. Faraday Soc., 47, 1293 (1951).
Volume 68, Number 6
J u n e , 1964
J. J. THROSSELL
1534
For the second pair of isomers R’R2, R”R2 D(R’-R2) - D(R”-Rz)
=
[Enb2(R”) - Enb2(R’)I [a(R’Rz) - a(R”Rz)] [n(R’Rz) - r(R”Rz)l
(6)
Now consider the difference AD
[D(R’-Ri)
- D(R”-Rl)]
-
[D(R’-Rz) - D(R”-Rz)]
(7)
If it is assumed that Enbl(R’) = E’nb2(R’) and E n b l (R”) = Enb2(R”),ie., that differences in the energies of the electrons isolated on the fragments R’, and on the fragments R”, in the two sets of molecules R’R1, R’R2 and R”R1, R”R2, may be neglected, together with differences in the corresponding u-electron energies, then (7) becomes simply AD
=
T(R”R1) - T(R’R1)
(8)
?\Tow if the structure of R’ is such that ?r(R’R1) may be neglected, for example, in the case of an R’-halogen bond when the carbon atom to which the halogen is attached is saturated, then AD = n(R”RI), and thus comparison of the dissociation energies of the two pairs of isomers may be related directly to the 7r-contribution to the bond. I n the present investigation C-Br dissociation energies in 3-bromopropyne CHrC-CH2-Br and bromopropadiene CH2=C=CH-Br were measured by a kinetic method. These molecules both give a Br atom and the C3H3radical on dissociation; the structure of the radical formed may be regarded as a resonance hybrid to which the canonical structures CH= C-CH2. and CH-C=cH make the principal contributions. The results are compared with those obtained by the electron impact method for the corresponding hydrocarbon in an attempt to illustrate the contribution of through-conjugation between the 7relectron system of the radical and the vacant p7r-da hybrid orbitals2of the Br atom.
Experimental The bond dissociation energies were determined by the kinetic method using the toluene carrier technique. The apparatus used was essentially the same as that used in previous investigations, and has been described fully elsewheree3 For completeness sake, and to give a check on the reliability of the experimental procedure, the C-Br bond dissociation energy in allyl bromide was determined. The pyrolysis of this compound has been The Journal of Physical Chemistry
Investigated previously, a and the results obtained in the present investigation agree well with the earlier ones. The sample of allyl bromide used was a commercial sample obtained from Eastman Kodak Co. and purified by distillation on a small column. The sample of 3-bromopropyne was obtained from General Aniline and Film Corp. and was purified by distillation. The sample of bromopropadiene was prepared by the isomerization of 3-bromopropyne, by refluxing over cuprous b r ~ m i d e . ~
Results The pyrolysis of bromopropadiene was investigated between 513 and 586”. In a typical experiment a t 537” a total pressure of 10.16 mm. was used with a bromide partial pressure of 0.023 mm. and a contact time of 0.3 sec. The reaction was established as kinetically first order since it was observed that alteration of the partial pressure of the compound had no effect on the rate constant. The rate constant was found to be given by the expression k = 1013 exp(50,000 f 2000)lRT. The pyrolysis of 3-bromopropyne was investigated between 469.5 and 564”. Again the reaction was found to be first order and the rate constant to be given by k = 1012.15 exp( -46,800 f 2000)lRT. The pyrolysis of allyl bromide was studied between 525 and 578”. The rate constant is given by the expression k = 1012.6 exp( -47,000 f 2000)IRT which may be compared with the value obtained by Szwarc, Ghosh, and Sehon, k = l O l 2 . 7 exp(-47,500 f 2000)/ RT. In all these pyrolyses the extent of the decomposition was determined by analysis of the HBr formed, assuming in the usual way that the only reaction producing HBr is Br
+ CBH6CH3-+ HBr + CeH6CH2
The fate of the C3H3 radical was not investigated but it appeared from the negligible quantities of noncondensible gases formed during the reaction that no “cracking” of this radical occurred. (Reactions of the C3H3radical are currently under investigation in this laboratory.)
Discussion The C-Br bond dissociation energy in 3-bromopropyne has previously been reported by Farmer (2) J. R. Hoyland and L. Goodman, J . Phys. Chem., 64,1816 (1960). (3) M. Sswarc, B. N. Ghosh, and A. H. Sehon, J . Chem. Phys., 18, 1142 (1950). (4) T. L. Jacobs and W. F. Brill, J . A m . Chem. Soc., 75, 1315 (1953).
BONDDISSOCIATION ENERGIES IN BROMOPROPADIEKE; AND 3-BROMOPROPYNE
and Lossing,6 who found a value of 57.9k cal. mole-’ by the electron impact method, which may be compared with the present value of 46.8 kcal. Collin and Lossing* have also reported the appearance potentials of the C3H3+ ion from CHF=C=CH, and from CH=C-CH3 as 12.00 f 0.05 and 12.02 f 0.03 e.v., respectively, = which correspond to values D(CHZ=C=CH-H) 81.2 f 1.1 kcal. and D(CH=C-CH2-H) = 82.8 f 0.7 kcal. Taken together with the known heats of formation’ of the hydrocarbons, these give a mean value of 75 kcal. mole-’ for the heat of formation of the the C3H3. radical. Unfortunately, no value is available for the heat of formation of 3-bromopropyne from which an independent estimate of the C-Br dissociation energy may be obtained. I[n Table I values of bond dissociation energies in the propynes with groups substituted in the 3-position are compared with those in the corresponding propenes. In both series of compounds the bond which is broken links a group X with a saturated carbon a,tom. In two cases D(C3H6-X:) ~
Table I 3-Substd. propwe
C3H3Br CaHa-H CaHa-CHs CaHa-1
DiCaHaX)
(57.9) (46.8) 82.8 67.5 46.0
Ref.
5
3-Substd. propene
D(CaHsX)
Ref.
CsHs-Br
(47.5) (46.7) 77.0 61.5 (35-37) (33.6)
3”
a
6 6 5
CsHs-H CaHE-CHa CSHS-I
6
’
b Calculated by means of the bond a Present investigation. additivity rules of Benson, el al., J . Chem. Phys., 29, 548 (1956); 36, 3464 (1962). c A. €I. Sehon and M. Szwarc, Proc. Roy. SOC. (London), A202, 263 (1950); M. Szwarc, Chem. Rev., 47, 75 (1950).
is less than D(C3H3-X) by about 6 kcal., whereas the difference in the case where X = B r is about 10 kcal. if the electron impact value is accepted. On the other hand, the kinetic result appears to be too low since it shows that D(C3Hb-X) D(C3H3-X). It is interesting here to compare these values with those predicted by the empirical equations of Errede.* Taking the quoted E values the C3H3-X dissociation energies are predicted to be 88, 72, 57.4, and 43.4 kcal. mole-l for X = H , CH3, Br, I, respectively. It appears probable that the t value for the CH=C-CH2 group was cal-
-
1535
culated from the available data for 3-bromo- and 3iodopropyne. If the electron-impact dissociation energies for C3H3-H and C3Hz-CH3 are taken as correct then a new value E = 0.88 may be calculated for the propynyl group which gives D(C3H3-Br) = 53.5 kcal. and D(C3H3-I) = 42.5 kcal., which are to be compared with the values for D(C&-Br) = 47.5 kcal. obtained experimentally and 46.7 kcal. obtained from the bond additivity rules, and for D(C3H6-I) = 35-37 kcal. obtained experimentally and 33.6 kcal. obtained from the bond additivity rules. In the case of the bromo compounds the difference is again about 6 kcal., while for the iodo compound the difference may be between 5 and 9 kcal. It is not possible a t present to say which value for D(C3H3-Br) is the more correct, and while these considerations are in no way conclusive about the actual magnitude of this quantity they do indicate that the electron impact value may be too high while the kinetic value is too low. It does, however, appear reasonable that D(CH2= C=CH-Br) should be greater than B(CH=C-CH2Br), (a) because of the use of the carbon sp2 orbital as compared with sp3, and (b) because of the degree of n-bonding between the n-electron system of the hydrocarbon fragment and the vacant p,-d, orbitals of the Br atom. Using eq. 5 a mean value for the quantity a(CH,= O=CH-Br) may be calculated as -4.8 kcal. mole-’ using the value for D(CH=C--CH,-Br) obtained in the present investigation. Because of the uncertainties in the experimental values for the C-Br dissociation energies, it cannot be said a t this stage that this is due mainly to the throughconjugation effect, but it does illustrate how bond dissociation energy data may be used in the analysis of such an effect in certain cases. Acknowledgment. This work was carried out under the direction of Prof. M. Szwarc in the State University College of Forestry, Syracuse, N. Y.; the author wishes to acknowledge Prof. Szwarc’s encouragement and to thank him for suggesting this problem. Thanks are also due to the National Science Foundation for a grant. ~~~
~~~
(5) J. B. Farmer and F. P. Lossing, Can. J . Chem., 33, 861 (1955). (6) J. Collin and F. P. Lossing, J . Am. Chem. Soc., 79, 5848 (1957). (7) F. D. Rossini, et al., “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds,” American Petroleum Institute, Project 44, 1953. ( 8 ) L. A. Errede, J . P h y s . Chem., 64, 1031 (1960).
Volume 68, Number 6 June, 1964
