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BACHA AND KOCHI
VOL.30
Polar and Solvent Effects in the Cleavage of t-Alkoxy Radicals JOHN D. BACHA AND JAY K. KOCHI Department of Chemistry, Case Institute of Technology, Cleveland, Ohio 44106 Received July 1, 1966 The homolytic decomposition of various tertiary alkyl hypochlorites in carbon tetrachloride has been examined. The j3 scission of the intermediate talkoxy radicals waa determined by quantitative analysis of the mixture of alkyl chlorides. This method, involving intramolecular competition, yields the relative rates of formation of alkyl radicals from the fragmentation of talkoxy radicals. These rates are compared with those obtained earlier from the nonchain decomposition of peroxides. Together, these results provide support for the assumptions made in the kinetic treatment. For primary radicals, the relative rates of cleavage decrease in the order: ethyl > r,r-dimethylbutyl > n-propyl, n-butyl > isobutyl > neopentyl >> methyl. The rates are slightly dependent on the structure of the alkoxy radical. The effect of structure of the alkyl moiety on the rate is reminiscent of carbonium ion type reactions. Hyperconjugative effects in the transition state for j3 scission are proposed. The roles of solvent in complexing and hydrogen bonding the alkoxy radicals and in the solvation of the tramition state for cleavage are discussed.
Tertiary alkoxy radicals undergo a variety of freeradical reactions in solution. A kinetically secondorder hydrogen transfer is the most common reaction (eq. 1). Other transformations include p scission'
homolysis of carbon-carbon bonds. Qualitatively, the relative ease of ejection of an alkyl radical from the fragmentation of t-alkoxy radicals decreases in the order: tertiary > secondary > primary > methyl.6J Effects of ring size and strain and other variables on RsCO. + SH +R&OH S* (1) these rates have also been examined. In an earlier study, unsymmetrical &alkoxy radicals to form an alkyl radical and ketone (eq. 2) and intrawere generated from the thermolysis of dialkyl perCHsCHzCHzCHz(CHs)zCO*+ oxides and the reduction of alkyl hydroperoxides with CHsCHnCHzCHz. + (CHs)zCO (2) reducing metal ions.6 I n the presence of hydrogen donors the radicals were converted to alkanes. Relamolecular hydrogen transferzI* to generate an isomeric tive rates of formation of n-alkyl radicals were dehydroxyalkyl radical (eq. 3). The latter reactions in termined by analysis of the mixture of alkanes. Alkoxy radicals are formed as intermediates in the CHaCHaCHzCHz(CHs)zCO- + CH~CHCH~CH~(CH~)~C (3) O H photoinduced decomposition of &alkyl hypochlorites.8 The relevant reactions involved in this sequence are solution have been commonly considered to be first 4 and 5 . The direct determination of cleavage rates, order. Pressure dependence and the unimolecularity k,, kb, and k,, depends on the analysis of the mixtures of the cleavage of t-butoxy radical in the gas phase have been disc~ssed.~ The relative rates of formation of alkyl radicals can be determined directly from the fragmentation of of alkyl chlorides. The two direct methods of examinunsymmetrical t-alkoxy radicals (eq. 4).6 These rates ing the rates of cleavage of alkoxy radicals differ kinetically. Peroxides as sources of alkoxy radicals ka Ra* 4-Rb k co Rb involve nonchain processes, in contrast to the efficient free-radical chain decomposition of alkyl hypochloRa Rb' -k R. k co (4) rites?& From both sources the direct determination of kc I_ k* Ra Rb co RO cleavage rates depends on the assumptions that alkanes (from peroxide) and alkyl chlorides (from hypochlorites) have also been evaluated from intermolecular comare formed in rapid, nondiscriminating steps, 1 and 5 , parisons by employing a competing hydrogen transfer respectively. The methods also demand that alkyl (eq. 1) as a monitoring reaction.6 Walling and Padwa radicals are not fractionated by extraneous reactions, found that relative cleavage rates obtained by the such as, combination, disproportionation, etc. direct and indirect methods differ. They attributed I n this paper we wish to establish that alkoxy radithis discrepancy to the inconstancy of hydrogen transcals generated from peroxide yield the same fragmentafer rate constants and of steric interactions with variation patterns as those derived from alkyl hypochlorites. tions in the structure of the alkoxy radical. Thus, I n addition, a thorough study of primary alkyl subfor quantitative determination the direct evaluation stituents would aid in discerning the relative imporof the relative cleavage rates represents the more retance of the factors influencing alternative fragmenliable method. tation paths, ie., the importance of the stability of the The @ cleavage of tertiary alkoxy radicals is a conejected radical and product ketone and the role of strain venient method of studying structural effects in the in the reactant alkoxy radical. Solvent studies should shed some light on the nature of the transition state, (1) P. Gray and A. William, Chem. Rm., 69, 239 (1959). the degree to which the carbon-carbon bond is broken, (2) F. D. Greene, et ol., J . A m . Chem. Soc., 88, 2196 (1961); C. Walling and the contribution from ionic structures. and A. Padwa, ibid., 88, 2207 (1961).
+
-@.
-1%
+
(3) M. Akhtar and D. H. R. Barton, ibid., 88, 2213 (1961); P. Kabasakalian, et al., ibid., 84, 2711,2718 (1962). (4) H. Herahenaon and S. W. Benson, J. Chem. Phys., 87, 1889 (1962). (5) J. K.Kochi, J. A m . Chem. SOC.,8 4 , 1193 (1962). (6) C. Walling and A. Padwa, ibid., 86, 1593 (1963).
(7) F. D.Greene, et ol., J . Ore. Chem., 38, 55 (1963). (8) (a) C. Walling and B. B. Jacknow, J. A m . Chem. Soc., 81, 6108,6113 (1960); @) C. Walling and W. Thaler, ibid., 88,3877 (1961).
SOLVENT EFFECTSIN THE CLEAVAGE OF ALKOXY RADICALS
OCTOBER1965
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TABLEI HOMOLYTIC DECOMPOSITION OF ALKYL HYPOCHLORITES I N CARBON !hTRACHLoRIDE -Alkyl
Relative RClb-
hypochlorite. E
7%
Total
RC1,C %
Relative rates?
Eo - Eb' EE - %' Rb c1 M Ab/Ao' h c1 %/Rb -0.50 0.45 0.565 36.1 33.4 0.66 63.9 0 0.726 0.629 61.4 38.6 35.3 25 0.659 39.7 36.4 60.3 40 25.6 0. 68gh 0.55 -1.5 0.53 67.4 74.4 0 0.597 0.750h 27.3 72.3 72.7 25 0. 783h 28.1 75.3 71.9 40 0.63 -3.5k 0.48 0.638i 27.7 20.3' 43.5 0 1.21 31.1 0.720 29.4 40.9 25 29.4 0.738 38.6 39.8 40 -3.4 0.46 0.519 34.1 0.79 9.87 0 1.08 65.9 15.4 37.0 0.587 63.0 25 0.621 38.3 20.0 61.7 40 0.32 23.1 0. 600h 0.90 -3.8 21.3 0 1.16 76.9 32.4 0. 690h 25.5 74.5 25 0.741h 41.4 27.0 73.0 40 -1.4' 0.431 0.30 1.10 30.1 34.8 0 1.07 69.9 0.516 34.0 41.8 66.0 25 0.575 44.2 35.8 64.2 40 0.82 0.63 -0.5"' 71.9 27.6 0.382 72.4 0 0.910 0.420 72.3 29.7 70.3 25 0.441 30.6 70.5 69.4 40 -1.2 0.62 91.1 -2.2 1.1 95.3 98.9 0 0.956 63.9 98.5 1.5 94.5 25 54.3 94.2 98.2 1.8 40 1.34 -3.3 1.9 -0.50 57.3 18.3 0 0.973 42.7 1.25 25.1 55.6 25 44.4 1.20 28.8 54.4 40 45.6 0.63 0.35 -0.8 0.909 47.6 82.6 52.4 0 0.513 0.881 46.8 83.9 53.2 25 0.838 84.7 45.6 54.4 40 2.11 -2.8 3.0 -1.0 32.2 40.0 67.8 0 0.770 1.81 64.4 35.6 48.8 25 1.67 52.3 62.5 37.5 40 Only traces of methyl chloride were observed. Total B-scission Droducts observed. d Molar a Molarity of alkyl hwochlorite. ratio of a k i l chloride products; probableerror (standard deviation), &2%. 0 Difference in activation energies in kcaloriea per mole; probable error, &5%, f Difference in activation energy for internal hydrogen abstraction (H) and ethyl or isobutyl cleavage (b). Per cent n-CaH&l (III),28.8 ( O O ) , 29.7 (25O), 30.8. fR./Rb Corrected for statistical factor. 0 Ratio of pre-exponential factors. Ea - E. (111)= -4.1 kcal./mole. 1 b = ethyl. Maximum value, determined a t 0 (111), 0.662 (OO), 0.726 (25O), 0.773 (40'). and 25" only. "C..
Results and Discussion
A series of unsymmetrical hypochlorites listed in Table I were prepared from the alcohols in carbon tetrachloride solutions. The conversion to alkyl hypochlorite was determined iodometrically. I n all cases, it was not less than 85% and it was usually greater than 92%. Aliquots were pipetted into glass tubes and the contents were thoroughly degassed by the standard freezethaw method. The decompositions were conducted in a thermostated water bath and induced photolytically. The decomposition of a t-alkyl hypochlorite [Ra(Rb) (R,)COCI] proceeding via the &alkoxy radical [R,(Rt,)(R,)CO~.] can exhibit three fragmentation patterns (eq. 4 and 5). The yields of alkyl chloride and ketone as products will depend on the relative rates of the respective first-order &scission processes, ka, kb, and k,. If it is assumed that the liberated alkyl radicals only yield alkyl chloride (eq. 5),9 the ratio of rate constants is given by eq. 6. Similar expressions
are applicable to k,. The incursion of other reactions such as hydrogen transfer (eq. 1) and rearrangement (eq. 3) which destroy alkoxy radicals does not effect these values of relative rates. Values for the relative rates were based on the yields of alkyl halides rather than those of the ketones. The ketone is more likely lost by subsequent chlorination (either heterolytically or homolytically) than the less reactive alkyl halide. In fact, chlorinated ketones have been reported as products in similar systems? However, when nbutyl chloride was deliberately added to a solution of methylethylpropylcarbinyl hypochlorite (I), it was recovered quantitatively after photolysis. The relative ratios of @-scissionproducts from I were also independent of initial hypochlorite concentrations (0.5-1.5 M ) , although the yields of 0-scission products varied slightly with concentration (See Table I X in the Experimental Section). These ratios were invariant when decompositions were carried out in the presence of acetone. Moreover, the same values were obtained when the decompositions were interrupted at intermediate conversions. Effect of the Structure of the Alkyl Radical.-The relative rates of cleavage of alkyl radicals from t alkoxy radicals obtained by intramolecular (direct)
BACHA AND KOCHI
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TABLE I1 RELATIVEINTRAMOLECULAR RATESOF CLEAVAGE OF ALKYLGRouPs FROM t-ALKoxy RADICALS AT 25' Radical
-Observed
relative ratea--
VOL.30
TABLEI11 COMPARISON O F DIRECTAND DERIVATIVE RELATIVE RATES OF CLEAVAGE OF ALKYLRADICALB~ Peroxideb
