HALOGENATION OF PROPANONE

Review

Overall Reaction	Pk + I2 → PI + H+ + I-
Proposed Mechanism
Overall Rate of Consumption of Iodine	-d[I2]/dt = + k2[Pe][I2]
Steady State Approximation	d[Pe]/dt = 0
Overall Rate of Production of Pe	d[Pe]/dt = + k1[H+][Pk] - k-1[Pe][H+] - k2[Pe][I2]
From Applying the Steady State Approximation	[Pe]ss = + k1[H+][Pk] / (k-1[H+] + k2[I2])
Overall Rate of Consumption of Iodine	-d[I2]/dt = + k1k2[H+][I2][Pk] / ( k-1[H+] + k2[I2])

Simplifying the Expression for the Overall Rate of Consumption of Iodine

The overall rate of consumption of iodine has been found to be

-d[I₂]/dt = + k₁k₂[H⁺][I₂][P_k] / ( k_-1[H⁺] + k₂[I₂])

It has been previously stated that the reversible reaction

So comparing the terms (k_-1[H⁺] and k₂[I₂]) on the bottom of the rate equation would lead to the simplifying assumption that

k_-1[H⁺] is approximately equal to k₂[I₂]

k_-1[H⁺] is much larger than k₂[I₂]

k_-1[H⁺] is much smaller than k₂[I₂]

Nothing can be assumed about the relative magnitudes of k_-1[H⁺] and k₂[I₂]