Dec., 1951
INFLUENCE OF
[CONTRIBUTION FROM
THE
CF3 GROUP ON
AN
ADJACENT DOUBLE BOND
5527
DEPARTMENT OF CHEMISTRY AT THEOHIO STATE UNIVERSITY]
Influence of a CF3 Group on an Adjacent Double Bond. 111. Free Radical Additions’ BY ALBERTL. HENNEAND MAXWELL NAGER Under free radical conditions, CF,CH=CH* accepts HBr to yield CF3CH2CH2Brexclusively: the direction of addition is thus the same as under polar conditions. Consistent with this trend, CF3I or CC13Br are accepted t o yield CF3CHICH2CFa or CFaCHBrCH2CC13. All additions are slower than with propylene. The observed direction of addition is considered in terms of the currently proposed factors: (1)center of electron density, (2) stability of the intermediate radical and (3) steric considerations.
Under polar conditions CF3CH=CH2 accepts a reagent A+B- to give CF3-CHA-CH2B, exclusively ; by electrophilic attack, HBr yields CF3CH2CH2Br2and by nucleophilic attack an alcohol yields CF~CHZCHZOR.~ The polarization of the -6
+6
double bond is thus clearly shown as CF~-CH=CH~, and the sluggishness of addition indicates its deactivation by the CF3 group. Free radical addition proceeds in the same direction. Hydrogen bromide was added in the vapor phase and with ultraviolet irradiation to yield only one product, CF3CH2CH2Br. In the absence of irradiation, no addition occurred. In ultraviolet light, the addition proceeded to quantitative completion, but was not instantaneous as the addition of HBr to p r ~ p y l e n e . ~Addition of CFJ was brought about by heat or by irradiation, to yield a mixture of products formulated as CFB-CH-CHZ-CF~
I
( CHr-CH-CF3)n
the solvents by the intermediate free radical CFaCH-CH2-CXs. Three theories are current to account for the orientation in free radical addition to a double bond. (I) The attacking radical is electrophilic and, therefore, moves to the carbon with the higher electron density.6 (11) The point of attack depends upon the relative stability of the two radicals which may be formed.? (111) Steric factors, e.g., accessibility, determine the point of initial attack.* Our experimental results contradict the first theory, as an attack on the more negative of the two carbons would require the formation of a secondary bromide instead of the primary bromide actually obtained. They are acceptably accounted for by the second theory which would require the formation of an intermediate CFa-CH-CH2Br
I
f)
I
where n = 0, 1, 2, 3; the rate of addition was not improved by the presence of mercury, the catalytic effect of which had been observed in the addition of CF31to CFz=CF~.6 The thermal reaction gave principally a product where n = 1, while the irradiated runs gave mostly CF3-CHI-CH&F3, where n = 0. CC13Br was added with ultraviolet irradiation, and gave as main product CF3-CHBr-CH2CCla; in the absence of irradiation, no addition took place. The reaction between HBr and CF3CH=CH2 can be accounted for by a bromine atom attack in a chain reaction as follows hu HBr + H . Br. Br. CF3CH=CH2 +CF3-CH-CHzBr HBr --t CF3--dH-CH2Br CF3CH2CH2Br Br., etc.. . .
+
+
+
+
Similarly, the addition of CF31 or CC13Br can be represented by an attack by a CF3. or a CCk free radical, followed by abstraction of 1. or Br. from (1) Presented a t the 119th Meeting of the American Chemical Society, Cleveland, Ohio, April, 1951. (2) A. L. Henne and S. Kaye, THISJOURNAL, 73, 3369 (1950). (3) A. L. Henne, M. A. Smook and R. L. Pelley, ibid., 72, 4756 (1950). (4) W. E. Vaughan, F. E. Rust and T. W. Evans, J. Org. Chem., 7 , 477 (1942). ( 5 ) R. N. Haszeldine, J. Chem. SOC.,2856 (1949). In a private communication, Haszeldine informs us t h a t he has also found addition 0 and n = 1, in photochemical as well as thermal products with I reactions and has proven their structure.
-
CFs-CH=CHBr f, H H CF~-CH=CHB~
of lower energy level than CF3-CHBr-CH2 ++ CF3H CBr=CH2. They are also acceptably explained by the third theory, as the end carbon would be more accessible than the middle carbon. We can thus reject the first theory, but cannot choose between the second and third, as both could be effective. Mayo and Wallings state that in all known cases, the second and third theories operate in the same direction. In the case of additions to CFCl=CF2 however, it has recently been showngthat CC13Br gives CClrCFC1-CF2Br; a CClg radical attack would be a reaction a t the least accessible carbon, and the formation of an intermediate CC13-CFClCF2. of lower energy level than CCI&FZ-CFCI. could be considered as a more probable directing factor. Resonance in the lirst of these intermediates could be represented as CCl,-CFCl-C
