Propanone, $$\(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)$$, reacts with iodine, $$\(\mathrm{I}_{2}\)$$, in the presence of an acid catalyst. $$\[ \mathrm{CH}_{3} \mathrm{COCH}_{3}+\mathrm{I}_{2} \rightarrow \mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{I}+\mathrm{H}^{+}+\mathrm{I}^{-} \]$$ The rate equation for this reaction is shown. $$\[ \text { rate }=k\left[\mathrm{CH}_{3} \mathrm{COCH}_{3}\right]\left[\mathrm{H}^{+}\right] \]$$ A four-step mechanism is suggested for the overall reaction. $$\[ \mathrm{CH}_{3} \mathrm{COCH}_{3}+\mathrm{I}_{2} \rightarrow \mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{I}+\mathrm{H}^{+}+\mathrm{I}^{-} \quad \text { rate }=k\left[\mathrm{CH}_{3} \mathrm{COCH}_{3}\right]\left[\mathrm{H}^{+}\right] \]$$ Part of this mechanism is shown. $$\[ \begin{array}{rlrl} \text { step 1: } & \mathrm{CH}_{3} \mathrm{COCH}_{3}+\mathrm{H}^{+} & \rightarrow \mathrm{CH}_{3} \mathrm{C}^{+}(\mathrm{OH}) \mathrm{CH}_{3} \\ \text { step 2: } & & \mathrm{CH}_{3} \mathrm{C}^{+}(\mathrm{OH}) \mathrm{CH}_{3} & \rightarrow \mathrm{CH}_{3} \mathrm{C}(\mathrm{OH})=\mathrm{CH}_{2}+\mathrm{H}^{+} \\ \text {step 3: } & & \rightarrow \\ \text { step 4: } & \mathrm{CH}_{3} \mathrm{C}^{+}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{I} & \rightarrow \mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{I}+\mathrm{H}^{+} \end{array} \]$$ Identify one conjugate acid-conjugate base pair in the mechanism. conjugate acid conjugate base

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