Oct. 20, 1964
COMMUNICATIONS TO THE EDITOR
fiYSi9Y7FlQ)JSi?Y-F15 is negative in agreement with the sign predicted by Pople and Santry.2a The relative sign of J s ~ z ~and - H ~JHI-C-S~-HI was obtained in a similar manner. In this case, however, the iowest Siz9frequency collapsed the highlfeld satellite pair indicating t h a t the signs were opposite. Since the vicinal H'-C-Si-H' coupling constant is almost certainly positive, i t follows that Jsi 5-HI is negative. If the negative magnetogyric ratio is taken into account, however, the reduced coupling constant KSiSg-Hi is positive in agreement with the signs observed for other X-H couplings. Acknowledgments.-The author is indebted to Drs. D . H. Whiffen, K. A. hlclauchlan, and ;1.D . Cohen of the Basic Physics Division for many helpful discussions and to Dr. E . -1. Ir. Ebsworth for kindly providing the samples investigated. This paper is published with the permission of the Director of the National Physical Laboratory, Teddington, Middlesex, England.
4505
methyl ( P D h l , VIII). This can be obtained in good yield by the reaction between perchlorodiphenylmethane (VII) and stannous chloride in ethyl ether or chloroform. Anal. Calcd. for C13Clll: C, 28.6; C1, 71.3; mol. wt., 546.1. Found: C , 28.6; C1, 71.4; mol. wt., 563 f 25. E.s.r. data6 show g-factors of 2.0062 (solid) and 2.0057 (in chloroform): N : 3.1 X loz3 spin mole (solid)'; hyperfine splitting: 2.5 f 0.1 gauss (in chloroform). No perchlorotetraphenylethylene is isolated. (C&15)gCCll 1.11
+ SnCl2
-
(CsClj)d2C1 YIII
(3)
P D M consists of orange-red, paramagnetic crystals melting about 190' dec. Under certain conditions, the description of which is not pertinent to this brief preliminary note, P D M can be oxidized to perchlorobenzophenone or reduced to aH-undecachlorodiphenylmethane. The facts given above show P D l l ' s structure as well DEPARTMENT OF CHEMISTRY STEVEN 8.DANYLUK as its being a true free radical even in solid form. This OF TOROSTO USIVERSITY is not unexpected for the following reasons. TOROSTO 5 , CASADA On account of the steric interactions among the atoms RECEIVEDAEGUST 31, 1964 around the a-carbon, VI1 is an extremely strained chlorocarbon. The formation of P D M occurs, therePerchlorodiphenylmethyl (PDM), a Carbon fore, with a great release of strain. PerchlorotetraFree Radical of Remarkable Stability' phenylethane would be, consequently, prohibitively Sir: strained since it would result from VI1 by substitution of the huge PD51 group for a chlorine. Within our program on aromatic and alkaromatic h remarkable property of P D M in solid form is its chlorocarbons, we showed a few years ago the formation stability toward oxygen. I t can be left for months in of perchlorobenzyl radical (11) in the reaction of perthe air without appreciable alteration, as ascertained by chlorotoluene (I) with iodide ion This radical dimeranalyses and spectra. There is no question t h a t this izes to perchlorobibenzyl (111) which undergoes imunique chemical inertness is due, a t least to a great exmediate dechlorination to czs- and frans-perchlorotent, to shielding of the central carbon atom and its lone stilbene ( I V ) * electron by the surrounding atoms (two sp2carbons and c6c!,ccI1 1- --f c6cIocclZ 1 2 five chlorines). I In forthcoming publications we shall describe and I1 1(1) cScljcc~2ccI9c6clS c~c~~cc~=cc~c~c~ discuss 5 in detail the preparation and properties of this 111 cas and trans and other stable free radicals.
+
+
J
IY
The analogous reaction with perchloro-p-xylene (V) gives perchloro-p-xylylene ( V I ) . The ultraviolet and infrared spectra of this chlorocarbon in solution ruled out the alternative biradical structure C1
c1
(6) T h e a u t h o r s a r e i n d e b t e d t o D r . A. Horsfield, Varian A . G . , Zurich. S w i t z e r l a n d , for t h e e.s.r. d a t a a n d c o m m e n t s . ( 7 ) Since t h e a c c u r a c y of absolute radical c o n c e n t r a t i o n d e t e r m i n a t i o n s is p r o b a b l y n o t b e t t e r t h a n i 5 0 L Z this d a t u m is consistent with P I I M being a 1007, f r e e radical
DEPARTAMENTO DE QU~MICA ORG~NICA PATRONATO " J U A NDE LA CIEKVA" VNIVERSITY OF BAKCELOSA, SPAIN
MANUELBALLESTER JCAX KIERA
RECEIVEDJCLY 20, 1964
Cl
'Cl
K ~ K for H Base Ionization of Mono-
VI
K7e now report the preparation of a stable, remarkably inert, carbon free radical, the perchlorodiphenyl(1) T h i s work h a s been sponsored b y t h e Office of Aerospace R e s e a r c h , U n i t e d S t a t e s Air F o r c e , t h r o u g h c o n t r a c t A F 61(052)-749. ( 2 ) hl. Ballester, C . M o l i n e t , a n d J R o s a , Telvahedvon, 6 , 109 (1959). ~ . , (3) M . Ballester a n d J . Castafier, Anales veal soc. e s p a f i . fis y Q Z L ~ 66B, 207 (1960). (1) I t s inertness c o n t r a s t s with t h e high r e a c t i v i t y of t h e p a r e n t h y d r o c a r b o n (p-xylylene) which oxidizes a n d polymerizes very readily (3) O v e r 100 highly chlorinated a n d perchlorinated benzene derivatives synthesized b y u s , inciuding PD.11, show t h e intense characteristic benzenoid b a n d g r o u p a t 7 . 5 $ which is a b s e n t in perchloro-p-xylylene a n d all n o n benzenoid derivatives available t o us. Also, t h e b a n d g r o u p a t 6 4-6.6 ii f o u n d in perchloro-P-xylylene indicates t h e presence of conjugated perchloroethylene groupings. T h e s e observations rule o u t t h e biradical (benzenoid) s t r u c t u r e for perchloro-p-xylylene. I t s ultraviolet s p e c t r u m is also con^ s i s t e n t with a polyene s t r u c t u r e of quinoid t y p e 3 F u r t h e r m o r e , perchlorop-xylylene is n o t p a r a m a g n e t i c in solid f o r m . W e a r e i n d e b t e d to a referee f o r suggesting this explanatory f o o t n o t e .
and Dimethylamine
Sir : The common tendency' to adopt the zero-point energy approximation2 in discussing kinetic secondary deuterium isotope effects was shown to be inadequate for the solvolysis of isopropyl-@-&halides and sulfonates,3and for a-phenylethyl-/%da chloride.4 For these examples &AF* = &AS*, approximately. Leffek, (1) ( a ) E . A. H a l e v i , "Progress i n Physical Organic C h e m i s t r y , " I n t e r science Publishers, I n c , N e w Y o r k , N.Y . , 1963, C h a p t e r 2 , ( b ) I. M e l a n d e r , " I s o t o p e E f f e c t s o n Reaction R a t e s " T h e Ronald Press, N e w Y o r k , N . I' , 1960. p p . 4 3 , 87 ( 2 ) A Streitwieser. J r . . K H . J a g o a , K C P a h e y . a n d S S u z u k i , J A m Cheai. S O L 8 0 , 2 3 2 6 (19583. ( 3 ) K T . Leffek, K E R o b e r t s o n . a n d S . Sugamori C a n . J . Chern., S9, 1989 (1961). (1) P. P a c e y , R . E R o b e r t s o n , a n d S. S u g a m o r i , unpublished work.
COMMUNICATIONS TO THE EDITOR
4506
et al., suggested that this unexpected result could be explained if there was a decrease in the barrier to torsional motion of the methyl group on deuterium substitution in the initial state and that this barrier became small in the transition state. In a more detailed theoretical analysis, Bartellj discussed the possibility that CY and 13 secondary deuterium isotope effects could arise from changes in nonbonded interaction accompanying the change from tetrahedral to trigonal configuration. IVolfsberg and Stern6 have shown that large normal temperature-independent isotope effects for such reactions can be calculated if force constants, which give rise to small frequencies (such as torsions), become larger in the transition state, and some large force constants (e.g., C-H stretching) simultaneously become smaller. Since the un-ionized isopropyl halides do not interact strongly with the solvent, nucleophilic interaction could provide just such a condition in the activation process for solvolytic displacement by an s N 2 mechanism. However, in addition to uncertainties with regard to the extent of such interaction and with respect to the degree of charge development, there must be added the possibility of anharmonicity, of tunnelling, and of possible variation in the transmission coeficien t . These kinetic uncertainties are in some measure removed and the position for argument and discussion of this surprising phenomena materially strengthened by the discovery that the secondary deuterium isotope effect for the thermodynamic dissociation constants in acid-base equilibria involving mono- and dimethylamine are temperature independent over 30-40" temperature range (Table I ) . TABLE I K D / R HFOR BASEIOSIZATION OF M O N O ASD DIMETHYLAMINES I N WATER
__ 7'. Y C
~
_____ KD
KR
hlonomethylamine
D
10 15 20 25 30 35
40 3 t e r n P!oc l o p / i h c m
81 I ) I ) t . \ e r e t t a n d \\
A177
i W (1'141
F
8 , 3 2 5 (1961)
R \ \ \ n i l e Joiiei P o i R O V 5 o r f I o n d o n )
Val. 8(i
ary halides but it will be obvious that the same type of calculation proposed by LTolfsberg and Stern6 will be expected to yield similar temperature-independent isotope effects for amine equilibria provided suitable changes in force constants are assumed.g In this connection it is significant that MacLean and Leffek'" have recently reported t h a t the inverse isotope effect associated with the displacement of I- from methyl iodide by amines in benzene is temperature independent. (9) hl. \\'olfsberg, p r i v a t e c o m m u n i c a t i o n . (10) J. W . hlac1,ean a n d K. T . L e f f e k , Spring M e e t i n g , C. I . C Kings t o n , 1964. (11) N a t i o n a l Research Council of C a n a d a Postdoctoral Fellow 1961-
1963.
S . R C. To. 8182 DIVISIOXOF PURECHEMISTRY SATIOSAL RESEARCH COUSCIL
WM. VAS
DER
LISDE"
R. E. ROBERTSON
OTTAWA, C A S A D A
RECEIVED M A Y13, 1964
Cyanogen Azide Sir: Cyanogen azide (1) has been synthesized in virtually quantitative yield from the reaction of sodium azide with cyanogen chloride in aprotic media.' This new Sa?;? C l C S --+ S a C S + Sac1
+
1
azide has a versatility and scope of chemical reactivity that is very broad and useful. Cyanogen azide, a colorless oil, detonates with great violence when subjected to mechanical or thermal shock, and great care should be taken in a n y work with this compound. It can be handled relatively safely in solvents where most of its properties have been studied. The half-life of a 2iyo solution of the azide in acetonitrile is 15 days a t room temperature, but this solution can be stored indefinitely without change a t 0 to - 20". The pure azide is too sensitive for combustion analysis; however, its niolecular weight (freezing point in benzene) is 69 (calcd. 68). The infrared spectrum of 1 in carbon tetrachloride shows absorptions a t 2230 (s), 2199 (vs), 2143 (s), and (associated with the nitrile and azide 2090 (s) stretching vibrations) and a t 1215 (vs) cm.-' (C-N stretching). In cyclohexane, 1 has two resolved absorptions a t 275 ( E 103) and 220 mb ( E 2157). The mass spectrometric cracking pattern of . shows a peak of 18%) relative abundance for the parent and is entirely consistent with the formulated structure. The synthesis of cyanogen azide is carried out by adding cyanogen chloride to sodium azide. Excess cyanogen chloride or anhydrous aprotic solvents may be used as reaction media. In a typical preparation, sodium azide was suspended in dry acetonitrile, and cyanogen chloride was distilled into the mixture a t a rate to maintain the temperature below 12". The solution was allowed to warm to room temperature and filtered to remove sodium chloride. The use of dry solvents is important to avoid the formation of explosive, solid by-products, and care also must be taken : 1 , T h e r e a r e several references t o cyanogen azide in t h e older l i t e r a t u r e . none n f which a p p e a r currect. For e x a m p l e , A I . G Ilarzens (( ontfil. I'rnd., 164, 1 2 3 1 (1912! ] o b t a i n e d a crystalline p r o d u c t incorrectly characterized a s SaCS, which was l a t e r suggested b y C 1.. H a r t [ J A m . C h p m .SOL., 6 0 , 1!221 i1028'11 t o he guanyl azide, a result we h a v e now confirmed.
