Carbon-deuterium bond

This means that the ionization and rearrangement need not be concerted and that symmetrical protonated ethylene can not be a major intermediate in the reaction. A similar experiment with isobutylamine and nitrous acid in heavy water gave products that contained no carbon-deuterium bonds. Since it is known that the -complex formed from isobutylene and acid is in rapid equilibrium with protons from the solvent, none of this can be formed in the nitrous acid induced deamination. This in turn makes it probable that the transition state for the hydrogen migration is of the sigma rather than the -bonded type.261... 

A primary isotope effect results when the breaking of a carbon-hydrogen versus a carbon-deuterium bond is the rate-limiting step in the reaction. It is expressed simply as the ratio of rate constants, i wlky,. The full expression of k /kn measures the intrinsic primary deuterium isotope for the reaction under consideration, and its magnitude is a measure of the symmetry of the transition state, e.g., -C- H- 0-Fe+3 the more symmetrical the transition state, the larger the primary isotope effect. The theoretical maximum for a primary deuterium isotope effect at 37°C is 9. The less symmetrical the transition state, the more product-like or the more substrate-like the smaller the intrinsic isotope effect will be. 

Carbon-deuterium bonds normally are broken more slowly than carbon-hydrogen bonds. This so-called kinetic isotope effect provides a general method for determining whether particular carbon-hydrogen bonds are broken in slow reaction steps. 

D. Carbon hydrogen bond is stronger than the corresponding carbon deuterium bond. 

B is correct. The question stem presents a mechanism for an elimination reaction (the product gains a double bond) that relies on a rapid C—H bond dissociation as the rate-limiting step. When the heavier deuterium (D) is used instead of a pure hydrogen atom, the reaction rate decreases because of a stronger carbon—deuterium bond. 

Because deuterium is heavier than hydrogen, the carbon-deuterium bond has a lower vibrational frequency than the carbon-hydrogen bond. This difference in frequency makes the carbon-deuterium bond slower to react. 

AC — D (carbon-deuterium) bond is electronically much like a C — H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however. 

We have seen that a carbon-deuterium bond is broken more slowly than a carbon-protium bond, and a carbon-tritium bond more slowly yet. How then, are we to interpret the fact that there is no isotope effect here If the rates of replacement of the various hydrogen isotopes are the same, it can only mean that the reactions whose rates we are comparing do not involve the breaking of a carbon-hydrogen bond. 

A second piece of evidence in support of the E2 mechanism is provided by a phenomenon know ii as the deuterium isotope effect. For reasons that we won t go into, a carbon-hydrogen bond is weaker by about 5 kj/mol (1.2 kcal/mol) than the corresponding carbon-deuterium bond. Thus, a C-H bond is more easily broken than an equivalent C-D bond, and the rate of C-H bond cleavage is faster. For instance, the base-induced elimination of HBr from l-bromo-2-phenylethane proceeds 7.11 times as fast as the corresponding... 

However, experiments have shown that toluene-d8 (completely substituted with deuterium) gives exactly the same smog chamber behavior as toluene itself under our standard conditions and measurements. Since a carbon-deuterium bond is stronger than a carbon-hydrogen bond, this lack of a deuterium isotope effect suggests that a carbon-hydrogen bond is not broken in the rate-determining steps of toluenes reactions. 

The addition of water or hydrogen halide to carbon-carbon double bonds is normally almost valueless as a method for formation of new C-H bonds. However, for formation of carbon-deuterium bonds it is purposeful, as deuterium is a stable component of a molecule only when bonded to carbon whereas the deuterium of an OD group is labile and thus removable. 

The fourth edition has been expanded by a chapter on the Formation of Carbon-Phosphorus Bonds and by another on the Formation of Carbon-Deuterium Bonds . The chapter on Alteration of Nitrogen Groups in Carbon-Nitrogen Compounds has been substantially expanded, but the remaining parts of the book have merely been completed by inclusion of preparative processes discovered in recent years. 

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