Secondary Isotopes Effects.
●The b-isotope effects are greater when there is a greater carbocation character in the transition state.
• These effects are believed to be due to hyperconjugation.
• Due to this the effects are greatest when deuterium is anti to the leaving group.
• The secondary isotope effects can be transmitted through unsaturated systems.
• In some cases b-isotope effects are due to steric reasons.
• a-Secondary isotope effects are observed when hydrogen is replaced with deuterium at the carbon centre attached to the leaving group.
●The a-isotope effects range from 0.87 to 1.26
• Here also the reactions involving carbocation character have a greater isotope effects.
• The reasons for a-isotope effects is that C-H bond vibrations are affected by the substitution with D for H more or less strongly in the transition state than in the ground state ,resulting in decrease or increase in the rate of the reaction.
●Another kind of isotope effects are solvent isotope effects.
• They are observed when H in the solvent are replaced with D. e.g. D2O for H2O.
• The solvent isotope effects may be due to:
• Involvement of solvent (O-H) bond breaking in the rate degerming step.
• Solvent Is exchanging hydrogen in the substrate and then taking part in the rate determining step.
• Deuterated and non deuterated solvents may have different solvent-solute interactions changing the energy of the transition state.
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