NOTES
2447
Table I : Rotational Diffusion Constants Di X
Sec-l
Competitive Chlorination of
1- and 2-Chloropropane Caloulated Compound
Acetonitrile-&
Methyl-&acetylene
Di
DI Dll Dii/Dl DL Dil DlilD~
Perrin
0.27 0.33 1.2 0.25 0.33 1.3
micro- Modified viscosity Hill
1.5 2.2 1.5 1.2 2.3 1.9
1.4 13.1 9.4 1.4 14.0 10.2
Experimentala
1.35 12.0 8.89 1.29 18.3 14.2
a Values for acetonitrile-da taken from ref 4; those €or methyl&-acetylene from ref 6.
by Carol C. Kelly and 11. H. J. Wijnen Hunter College of the City Unisersity of N e w Y o r k , N e w York, N e w York 100821 (Receiued January 17,1968)
We have used the gas-phase photochemical decomposition of IC1 in daylight to study the attack of C1 atoms on chloropropanes. The results indicate that the general reaction mechanism may be presented by
+ hv +I + c1 c1 + IC1 Clz + I C1+ RH R + HC1 R + IC1 +RCl + I IC1
correct. The microviscosity correction improves the the D, value, but leaves the ratio Dli/Dl still in error, as the anisotropy correction depends even in this approach only on the shape of the molecule. On the contrary, the modified Hill method gives reasonable agreement17 between the experimental and calculated rotational diffusion constants, considering the number of assumptions made. There is one feature of the Hill method which could be regarded decisive for this agreement, namely, the appearance of moments of inertia li in the final expression for Di (see eq 3). In spite of the agreement between the experimental D, and Dll and their calculated values using the modified Hill method, one has to be aware of serious shortcomings of this approach. This calculation predicts Dil/DL I J I l I independent of temperature which is contrary to the experimental results both for CDaCN6vB and CD3CCH;? however, this is not surprising in view of the approximative nature of the treatment. Also, it has been shown6s7that the use of the classical rotational diffusion mechanism is questionable for the description of the reorientation about the main symmetry axis in these symmetric-top molecules. To decide whether the agreement is partly fortuitous or whether it actually indicates a relative success of the Andrade theory, one should test this method on a larger number of molecules. The results of this study demonstrate that the recent interest in the role of inertial on reorientation in liquids is well-justified.
Acknowledgment. We wish to acknowledge the support of this work by the National Science Foundation. Our thanks are due to the referees for their valuable comments. (17) The disagreement between calculated Dil and the experimental Dll might be ascribed to questionable applicability of the rotational diffusion equation for the description of reorientation about the main symmetry axis (see discussion in ref 6 and 7 ) . (18) W. A. Steele, J . Chem. Phys., 38,2404,2411 (1963). (19) P.W.Atkins, to be published. (20) P. W. Atkins, A. Loewenstein, and Y . Margalit, to be published, (21) R. E. D.McClung and D. Kivelson, J. Chem. Phys., 49, 3380 (1968).
---f
--f
(1)
(2)
(3) (4)
Additional reactions may occur if the iodine atom and/ or Clz concentrations are allowed to build up to an appreciable extent. It is, therefore, necessary to keep the conversions small. Our experiments were carried out in a 1-1. flask attached to a vacuum system. The chloropropane was admitted, frozen into a cold finger, and degassed. The IC1 was then admitted from a reservoir, separated from the reaction flask by a Teflon stopcock, and frozen on top of the chloropropane. No attempt was made to measure the amount of IC1 admitted. Care was taken to admit sufficient IC1 so that its maximum vapor pressure of 29 Torr' at 25" would be reached upon evaporation. The pressure of chloropropanes in the reaction flask varied between 30 and 90 Torr. Most of our experiments were carried out at 25". Samples were taken at intervals of about 20 min and analyzed by gas chromatography. The reaction products of IC1 and l-chloropropane were 1,l-, 1,2-, and 1,3-dichloropropane. With IC1 and 2-chloropropane we obtained 2,2- and 1,2-dichloropropane. Typical results of experiments carried out at 25" are given in Table I. The conversion of chloropropanes did not exceed 2%. In addition to the relative amounts of products produced, we have also given in Table I the amounts of products formed per available hydrogen atom. These data indicate clearly that the C-H bond strength is influenced considerably by its environment. I n Table I1 we compare the reactivities of the various C-H bonds in 1- and 2-chloropropane with each other (taking the reactivity of the C-H bond in the end methyl groups as unity). It should be pointed out that we have studied the effect of IC1 upon a mixture of 1- and 2-chloropropane to determine whether the hydrogen in the CHI group of CH2ClCH2(1) J. Cornog and E. E. Bauer, J . Amer. Chem. SOC.,64,2620 (1942),
Volume 78, Number 7
July 1960
XOTES
2448 CH3 had the same reactivity as hydrogen in the CH3 groups of CH3CHC1CH3. We observed that, within experimental error, the rates of production per available hydrogen atoms for 1,2-dichloropropane from CH3CHClCWs and for 1,3-dichloropropane from CH2C1CH2CH3were equal. This indicates that C1 substitutions do not influence C-H bond strengths in neighboring carbon groups. Table I : Reaction Products of 1- and 2-Chloropropane with IC1 ----Rei,
Products
Expt 1
amt-----Rel. Expt 2
1,l-DCP 1,2-DCP 1,3-DCP
(A) CH2ClCH2CH3 29 82 68 176 29 74
2,2-DCP 1,2-DCP
80 46
(B) CHaCHClCHa 65 36
amt/avail. H--Expt 1 Expt 2
+ IC1 14.5 34.0 9.7
41.0 88.0 24.7
Acknowledgment. The authors are pleased to acknowledge support of this work by the National Science Foundation through Research Grant GP-6947.
65.0 6.0
(2) J. H. Knox and R. L. Nelson, Trans. Faraday SOC.,5 5 , 9 3 7 (1959). (3) G. C. Fettis, J. H. Knox, and A. F. Trotman-Dickenson, J . Chem. Soc., 4177 (1960). (4) J. H. Sullivan and N. Davidson, J . Chem. Phys., 17, 176 (1949).
+ IC1 80.0 7.7
than the one in >CHz. Thus, 2-chloropropane is much more vulnerable to H atom abstraction than l-chloropropane. Finally, we wish to mention that we have attempted to carry out chlorination reactions with IC1 at different temperatures. It was possible to obtain results at 0"; below this temperature the IC1 vapor pressure is too low to conduct meaningful experiments. At 70" we obtained many more products than those reported by us. This is probably caused by the thermal decomposition of ICl, producing large concentrations of I atoms, I, and Clz molecules, thus, complicating the system immensely. The use of IC1 to study competitive chlorination reactions seems limited to a temperature range from 0 to about 30".
Table I1 : Relative Reactivity of H Atoms in Chloropropanes H atom in
Rel. reactivity
-CH, -CH&l -CHZ-CHCl-
1.o 1.6 3.4 10.5
Oxygen-17 Hyperfine Splitting in an Iminoxy Radical
by Bruce C. Gilbert and Wilson 11.Gulick, Jr.' Department of Chemistry, Cornell Uniwrsity, Ithaca, New York (Receiaed January 28, 1969)
It is interesting to compare our data with values reported in the literature. Knox and Nelson,2studying the H-atom abstraction from propane by C1 atoms, report activation energies of 1.0 and 0.7 kcal/mol, respectively, for reactions 5 and 6.
+ CH3CH2CH3 +CHzCHZCH3 + HC1 C1 + CHaCH2CH3 +CH3CHCH3 + HC1
C1
(5)
(6)
This is in complete agreement with our observations indicating that the H atom in the CHZgroup is much more labile than the H atom in the CH3 groups. To our knowledge, no systematic study has been made of the reactivity of the various C-H bonds in chloropropanes. It has been reported earlier that the C-H bond is weakened considerably by the immediate presence of one or more C1 atoms as shown by the activation energies of 18.3, 14.5, and 9.3 kcal/mol for the H-atom abstraction reaction by bromine atoms from, respectively, CH4,3 CH&1,3 and CHCl3.4 No doubt, the highly electronegative C1 atom draws the electrons toward itself thus weakening the other bonds. Our results are in agreement with this observation. The C-H bond in -CHZCl is 1.6 times more labile than the one in -CH3 and the C-H bond in >CHC1 is about 3.4 times more labile The Journal of Physical Chemistry
We have observed the isotropic hyperfine splitting due to oxygen-17 in the esr spectrum of an isotopically enriched iminoxy radical (I, RR'C=K-0). The iminoxy radical was generated from the reaction of nitrogen dioxide with methyl ethyl ketone.2 Labeled NOz ( 2 mmol), prepared by reaction of nitric oxide with oxygen gas of composition 45.1 atom % 170and 42.5 atom % was condensed onto 5 mmol of freshly distilled methyl ethyl ketone. An esr spectrum characteristic of an iminoxy radical (aN N 30 G) was detected after the sample had been allowed to stand at room temperature for several minutes (see Figure 1). Traces of nitroxide radicals were also detectedS2 At high amplification, satellite lines due to 1 7 0 ( I = 5 / / 2 ) were detected and a. was determined to be 22.75 G. The iminoxy radical has the following character(1) Author to whom communications should be addressed a t The Florida State University, Tallahassee, Fla. 32306. (2) W. 31.Fox, J. McRae, and M. C. R. Symons, J . Chem. Soc., A , 1773 (1967). (3) The labeled oxygen was supplied by Miles Laboratories, Inc., Elkhart, Ind. The mass spectrum of the labeled NO2 was consistent with a statistical distribution of oxygen isotopes.
