Nov. 5, 1058
SUBSTITUTION AND ELIMINATION WITH DEUTERO-ISOPROPYL
BROMIDES
3285
[CONTRIBUTION FROM THE CONVERSE MEMORIAL LABORATORY, HARVARD UNIVERSITY ]
Substitution and Elimination Rate Studies on Some Deutero-isopropyl Bromides BY V. J. SHINER, JR.' RECEIVED JUNE 11, 1952 T h e rates of thc bimolecular substitution and elimination reactions of a-deutero- and p-hexadeutero-isopropyl bromides in These are compared with the rates observed for ordinary isopropyl bromide. The substitution rate is the same for all three compounds. The elimination rate is the same for the ordinary and the a-deutero compounds while the p-deutero one eliminates more slowly than the protium analog by a factor of 6.7. It is concluded that these results are in full accord with the predictions of the familiar s N 2 and E2 mechanisms. ca. 1 N alcoholic sodium ethoxide have been determined.
It has been proposed by Hughes, Ingold and coworkers2 that the bimolecular elimination reaction (E2) proceeds by a rate-determining attack of the base on a @-situatedhydrogen atom while the electronegative group X separates simultaneously with its bonding electrons Y: H-CI12-CRrX --f YH' f CR2CR2 f XThis scheme has enabled the considerable body of evidence collected on this reaction to be consistently explained. An obvious corollary of this mechanism is that the bimolecular elimination and the bimolecular substitution ( s ~ 2which ) generally accompanies it are two competing, concurrent, independent reactions. It appeared that interesting evidence concerning the validity of the E2 mechanism could be obtained by determining the isotope effect on the reaction rate when the a- or p-hydrogen atoms were replaced by deuterium. I t has been established from theoretical considerations and amply demonstrated by a number of experimental exa m p l e ~ ~that - ~ a reaction which involves the removal of a hydrogen atom in its rate-determining step will show a marked difference in rate for removal of protium, deuterium or tritium. This consideration arises from the difference in zero point energies of bonds involving the isotopically different atoms. Thus the E2 mechanism predicts that the rate of the elimination reaction of p-hexadeuteroisopropyl bromide will be slower than the corresponding rate for the protium analog. However, the rate effect of an a-deuterium substitution should be negligible. The accompanying substitution reaction rate should be essentially independent of any deuterium substitution. Acetone-ds was made by exchanging acetone with successive samples of deuterium oxide. This material was then reduced to p-hexadeuteroisopropyl alcohol and converted to the alkyl bromide. a-Deutero-isopropyl alcohol was made by the reduction of acetone with lithium aluminuni deuteride. From the analyses and the infrared spectra of the various compounds together with the kinetic behavior of the bromides, it seeins cer
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(1) d u Pont Postdoctoral Fellow, 1951-1952.
Now a t Department
of Chemistry, Indiana University, Bloomington, Indiana.
(2) For the latest discussion, with complete references, of t h e present state of elimination reaction theory see M. L. Dhar, E. D. Hughes, C. K. Ingold, A. M. M. Mandour, G. A. Maw and L. I. Woolf, J . Chem. SOL,2094 (1948). (3) 0 Reitz, Z. p h y s i k . Chcm., Al19, 119 (1937); A184, 429 (1939). ( 4 ) For an interesting example of a large negative isotope effect see €1. Gilman, G. E. D u n n and G. S. Hammond, THISJOURNAL, 1 3 , 4409 (105 1). ( 5 ) 12. Ii. Westheimer and N. Nieolaides, i b i d . , 11, 25 (1949).
tain that' no spurious isotope exchanges took place in the reduction and bromination reactions. The method of kinetic analysis was essentially that described by Dhar, Hughes and Ingold..6 The reaction was carried out in sealed tubes at 25.0'. The total reaction was followed by titration of developing bromide ion using the Volhard method. At the end of the reaction the olefin fraction was determined by the reaction with bromine in chloroform. The over-all bimolecular rate constant obtained for isopropyl bromide (2.82 X l./mole/sec.) agreed well with that observed by Dhar, Hughes and Ingold6 (2.95 X loT6). However the olefin fraction here reported (0.625) is definitely smaller than that observed by the above authors (0.82) although it seems to be more in line with the value obtained a t 55" (0.79) by Hughes, Ingold, Masterman and McNulty.' The correction factor for losses in the extraction analytical procedure for propylene was determined by analyzing sealed vials of known volume containing pure propylene gas. Some sealed vials containing propylene were analyzed also by direct reaction with bromine in chloroform. This method always gave within 1% of the value obtained from volume calculations. The correction factor for extraction losses was 1.07 which is essentially the same as that used by Dhar, Hughes and Ingold.6 Table I gives the deuterium analyses in atoms of deuterium per molecule for the isopropyl bromides and the intermediates isolated in their preparation. Experiment number 1 was done on only partially exchanged acetone, while in number 2 acetone which had been equilibrated six times with three molar equivalents of deuterium oxide was used. 1.n experiment number 3, ordinary acetone was reduced with lithium aluminum deuteride. TABLEI DEUTERIUM ANALYSES (IN ATOMSPER MOLECULE) P;xpl. no.
1 2 3
Acetone
3.02(?) 5.84
Isopropyl alcohol
Isopropyl bromide Corrected"
Uncor.
Position
3.30 3.40 5.54 5.91 p 0.977 0.977 Q 0.00 0.970 Corrected in cases 1 and 2 for the presence of a small Not corrected for the amount of ethyl bromide (see text). presence of a small amount of ethyl alcohol.
3.31* 5.24b
*
As is shown in Table I1 the kinetic runs gave good second-order rate constants. The zero point (6) M . L, Dhar, E. D. Hughes and C. K. Ingold. J . Cite?%.SOC. 2058 (1948). (7) E. I). Hughes, C. K. Ingold, S. Masterman aiid B. J. hlcNulty, i 6 i L 89U (1940).
V. J. SHINER, JK.
52%
Vol. 74
olefin fractions were reproducible to within & 2.76. It is obvious that within these limits the a-deutero compound gives the same results as ordinary iso()RDIh'.4RP A I D DEUrERA1.EU I S O P R O P Y L BROMIDES I N propyl bromide. Also, the substitution rate conAI.COHOI.IC SUDIIJM ETIIOXIDE stants are experimentally the same in all four cases 1 Run 1 :j 4 studied. I t is not thought that the apparent slight Sumher of atoms of I f :3.40 ,!!?I 008 IJOO effect of &deuteration on the substitution rate is Position of D 3 /j e ( N a O E t ) in niole/l. 1.fW J.Of! 1.11 11997 significant enough to allow speculation as to its origin. This very interesting question is one of the 1 .%j 1 . 10 2.7X 2 83 objects of further experiments now under way. On 1.91 1.20 2 . 7 6 2 . 8 3 the other hand it is clear that the elimination rate 1 . 9 2 1 . 2 6 2 . 7 3 2.8U constant is cut down severely by @-deuteration. 1 . 8 9 1.24 2.72 2 . 7 8 If one makes the assumption that each protium Total bimolecular reaction 1 1 . 9 4 1 . 2 1 2 . 7 6 2.72 and deuterium atom was eliminated a t its own rate constants (units, 2.05 1 . 2 3 2 . 7 7 2.88 characteristic rate regardless of whether the other l.,/rnole/sec.) for each point 1 . 9 7 1 . 2 3 2.76 2 . 7 8 /3-hydrogens were protium or deuterium, then it is u p t o 7 0 7 , completion 1.92 1 . 2 5 2 . 6 8 2 . 7 9 possible to calculate the elimination rates for I $17 l..'mmission Pu:,i-doctoral l ~ e l i o w .l ! V ~ I - (,j)I
