HALOGENATION OF PROPANONE

Introduction

Propanone reacts in aqueous, acidic, solution with chlorine, bromine and iodine to produce the monohalogen derivative. If the propanone and acid are in considerable excess the rate of reaction in any given case is independent of the concentration or the nature of the halogen (i.e. the reaction is zero-order with respect to the halogen).

Overall, the reaction (using Iodine) is:

CH₃COCH₃ + I₂ → CH₃COCH₂I + H⁺ + I^-

The following facts have been determined about the system:

• The halogen (I₂) is capable of reacting extremely rapidly with the enol form of propanone (CH₃C(OH)=CH₂) to produce iodopropanone (CH₃COCH₂I).
• The enol form of propanone is present in ordinary propanone in very small proportion in equilibrium with a large proportion of the keto form (CH₃COCH₃).
• In the presence of halogen the equilibrium between the enol and keto forms of propanone is disturbed by the rapid removal of the enol form.

Using the above facts two mechanisms have been proposed, the first of which (A) is as follows:

The aim of the Year 2 Halogenation of Propanone experiment is to determine the dependence of the reaction rate on the concentrations of iodine, iodide, acid and propanone and to determine the relevant rate constants for the reaction at 25^oC and 35^oC. The consistency of experimental observations with the proposed mechanisms, needs to be examined and explanation(s) for any deviation observed provided.

In order to determine the consistency of the experimental observations with the proposed mechanisms, appropriate rate expressions for both the mechanisms will need to be derived.

In this tutorial, the procedure to obtain an appropriate rate expression for the first of the proposed mechanisms (A) will be worked through.

Note: For the remainder of this worked example the keto form of propanone (CH₃COCH₃) will be represented by P_k, the enol form of propanone (CH₃C(OH)=CH₂) will be represented by P_e and iodopropanone (CH₃COCH₂I) will be represented by PI, i.e.

Overall Reaction	Pk + I2 → PI + H+ + I-
Proposed Mechanism

Elementary or Composite?

If the reaction

P_k + I₂ → PI + H⁺ + I^-

-d[I₂]/dt = -k[PI][H⁺][I^-]

-d[I₂]/dt = +k[PI][H⁺][I^-]

-d[I₂]/dt = -k[P_k][I₂]

-d[I₂]/dt = +k[P_k][I₂]