SOME DEUTERIUM ISOTOPE EFFECTS: I. WATER SOLVOLYSIS OF METHYL-d₃ ESTERS

Abstract

The a-deuterium isotope effect has been examined for the solvolysis of a series of esters containing a fully deuterated methyl group. The possible sources of the effect have been divided into "thermodynamic" effects which appear to favor more rapid reaction of the protium compound and "zero point" effects where stiffening of out-of-plane vibrations may account for the direction of the observed isotope effects. It appears that the inverse isotope effect may be a measure of the spatial restrictions placed on the hydrogen atoms on the carbon atom in the activated complex.The substitution of deuterium for a-hydrogen in alkyl halides and substituted sulphonates has been shown to produce varying effects on reaction rates. Thus the solvolysis of methyl p-bromobenzenesulphonate in ~nethanol and the reaction with ~nethoxide ion are accelerated by a-deuteration ( 1 ) . Isopropyl bromide reacting with ethoxide ion is apparently unaffected by this substitution (2), and phenyl ethyl toluenesulphonate solvolyzes in formic acid a t a reduced rate when isotopically substituted a t the a-hydrogen (3). Johnson and Lewis systematized these changes, suggesting that they were the consequence of the sensitivity of the C-H bending vibrations to the presence of entering and leaving groups and noted a decrease in the isotope effect, kH/kD, with increasing necessity for nucleophilic attack from the rear ( 1 ) . Fro111 information available a t the time the); were unable to choose between several possible sources for the inverse isotope effect observed for nlethyl esters.Streitwieser and co-workers (4) have observed a reduction in the acetolysis rate for cyclopentyl p-toluenesulphonate followi~lg a-deuteration. They have attributed this to the change in vibrational frequencies involving the a-hydrogen, produced by the difference in hybridization between initial and transition states, together with the effect of the leaving group which was postulated to stiffen the out-of-plane vibrations of the hydrogen.Thus there appear to be three factors for consideration in assessing the zero-point contribution to the change in rate for nucleophilic displace~nents caused by a-deuterium substitution. These factors are: ( a ) the effect of the incoming group, ( b ) the effect of the leaving group, and (c) the change in hybridization.Factors ( a ) and (b) will be related, inasmuch as bulky entering or leaving groups will be expected to stiffen the out-of-plane C-H bending modes. This effect will be modified by distance, but clearly, stiffening will increase the disparity in the energy levels of deuterium and hydrogen compounds in the transition state, and will produce changes tending to reduce the free energy of activation for the deuteriu~ll compound, conlpared with that of the protiu~n compound. Reactions having the highest degree of nucleophilic interaction with large entering groups will presumably have the most stiffening.The change in hybridization will also affect the characteristic frequencies but presents a problem in that even a qualita...

Calculations of Kinetic Isotope Effects for the Iodide Exchange and for the Solvolysis of Methyl Iodide

“…In this ~vorli, these methods are applied to a theoretical study of soine nucleophilic substitution reactions of methyl iodide in order to gain an understanding of the causes of the isotope effect and to investigate the conclusions, with respect to the transition state, which may be drawn froin experimental isotope effect data. For these systems, results of some previous calculations have been reported which do not involve complete solutions of the secular equations of the 6, 7, or S atomic transition states (7,8).…”

“…For b and c the 13C isotope effect has been measured (9), and, for a and d the D effect (rate of CI-131 divided by rate of CD3I) is known (8,10). Experimental data on 14C isotope effects in four SN reactions of CH3I (11) are also available.…”

ISOTOPE EFFECTS IN CARBONIUM ION REACTIONS: V. SOLVOLYSIS OF METHYL AND ETHYL p-BROMOBENZENESULPHONATES

“…Among the data leading to these coilclusions were those showing solvent dependence of the effect of P-hydrogens on solvolysis rates (4). The reverse hydrogen isotope effects ( 5 ) in the reactions of methyl esters were attributed to increases in bending force constants resulting from steric interactions of the methyl hydrogens with the attaclting and leaving groups, and this suggestion was supported by more extensive studies on the reactions of methyl halides in water (6). Table I shows some rates and isotope effects for the solvolysis of nlethyl and ethyl fi-bromobenzenesulpho~~ates in two solvents of widely differing nucleophilic character.…”

SECONDARY KINETIC ISOTOPE EFFECTS IN BIMOLECULAR NUCLEOPHILIC SUBSTITUTIONS: II. Α-Deuterium EFFECTS IN MENSCHUTKIN REACTIONS OF METHYL IODIDE

“…According to LIeweIlyn, Robertson, and Scott (9), the isotope effect in the solvolyses of methyl halides depends upon the proxinlity of the entering group to the carbon aton1 a t \\;hich substitution is taking place. Thus they observed greater inverse isotope effects the greater the S,2 character of the solvolysis.…”