![[Halogenation of Propanone - Mechanism A]](../images/four1a.gif)
of Consumption
of Iodine
Approximation
Production of Pe
Steady State
Approximation
of Consumption
of Iodine
Assumption
(since the reversible reaction is slow and the iodination reaction is fast)
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 Assumption | k-1[H+] << k2[I2] (since the reversible reaction is slow and the iodination reaction is fast) |
Final Simplification of Rate Expression
Assuming that k-1[H+] << k2[I2], the k-1[H+] term on the bottom of the expression for the overall rate of consumption of iodine
-d[I2]/dt = + k1k2[H+][I2][Pk] / ( k-1[H+] + k2[I2])can be neglected, resulting in
-d[I2]/dt = + k1k2[H+][I2][Pk] / k2[I2]
This can be further simplified (by cancelling terms that appear top and bottom), giving the final expression (derived from Mechanism A) for the overall rate of consumption of iodine as