Which bond is easier to break C-H or C-D?
The energy is the same at the dissociation limit of C-H and C-D bonds ,But the bond energy is effectively higher for the C-D bond. In other words, the C-D bond is stronger. Therefore, any reaction in which the C-H bond is broken during or before the rate determining step will be slower if the hydrogen is replaced by a deuterium. The relative rates, kH/kD should be greater than 1. This is known as the normal primary kinetic isotope effect