Positive isotope effect

Halogenation of 2-methylcyclohexanone and of its analogue deuterated at the α position, with rate constants kH and kD

C–D bonds are slightly stronger than C–H bonds. When this bond is broken in the rate-limiting step, the reaction is somewhat more difficult for the deuterated substrate and slows down (kH/kD > 1). This is a positive isotope effect.

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