Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 67 / c

The natural mechanism is ionizing solvolysis: C-OTs cleavage gives a cyclopentyl cation/ion pair, capture by acetic acid gives the protonated acetate ester, and deprotonation gives product.

The $1.15$ effect from H/D at the leaving-group carbon is normal alpha-secondary: that carbon develops trigonal carbocation character. The $2.06$ effect from four neighboring labels is a combined beta-secondary effect. Adjacent C-H sigma bonds stabilize the developing empty p orbital by hyperconjugation; replacing H by D changes the vibrational stabilization and slows ionization. If the four contributions were equal and independent, each would be $2.06^{1/4}=1.198$.

None of those labeled C-H bonds is broken. The isotope effects support substantial carbocation character, although distinguishing a free cation from an intimate ion pair needs additional evidence.