Work out reasonable mechanisms for the reactions of phosphorus pentachloride and sulfur tetrafluoride with carbonyl groups. A reasonable first step with phosphorus pentachloride is dissociation into \(\mathrm{PCl}_{4} \oplus\) and \(\mathrm{Cl}^{\Theta}\).

As given in the problem, the first step for the reaction of phosphorus pentachloride with carbonyl group is the dissociation of PCl5 into PCl4+ and Cl-. \[ \mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{4}^{+} + \mathrm{Cl}^{-} \]

The nucleophilic oxygen of the carbonyl group attacks the electrophilic phosphorus center in PCl4+, forming a new bond between the oxygen and phosphorus atoms. \[\mathrm{O=C}\phantom{---}\mathrm{R} + \mathrm{PCl}_{4}^+ \rightarrow \mathrm{O}\mathrm{-P}\mathrm{Cl}_4 \rightarrow \mathrm{O}\mathrm{-P}(\mathrm{Cl})_{3} + \mathrm{Cl}^{-}\]

After the displacement of one Cl- ion from OP(Cl)3, the Cl- ion that was initially dissociated from PCl5 in step 1 can now reattach to OP(Cl)3 forming phosphorus pentachloride (PCl5) and a chlorinated carbonyl compound. \[\mathrm{O}\mathrm{-P}(\mathrm{Cl})_{3} + \mathrm{Cl}^- \rightarrow \mathrm{O} \mathrm{=} \mathrm{C}(\mathrm{R})\mathrm{-Cl} + \mathrm{PCl}_{5}\] The mechanism for the reaction of phosphorus pentachloride with the carbonyl group has the following three steps: 1. Dissociation of PCl5. 2. Nucleophilic attack of carbonyl oxygen on PCl4+ and displacement of Cl-. 3. Reformation of phosphorus pentachloride and formation of a chlorinated carbonyl compound. Now let's investigate the mechanism of the sulfur tetrafluoride reaction with carbonyl groups.

In the reaction of sulfur tetrafluoride with carbonyl groups, the nucleophilic oxygen of the carbonyl group attacks the electrophilic sulfur center in SF4, forming a new bond between the oxygen and sulfur atoms. \[\mathrm{O=C}\phantom{---}\mathrm{R} + \mathrm{SF}_{4} \rightarrow \mathrm{O}\mathrm{-S}\mathrm{F}_4\]

After the formation of the new bond, the fluoride ion transfers from the sulfur to carbon, releasing a molecule of SF3 and forming a fluorinated carbonyl compound. \[\mathrm{O}\mathrm{-S}\mathrm{F}_4 \rightarrow \mathrm{O} \rightarrow \mathrm{C}(\mathrm{R})\mathrm{-F} + \mathrm{SF}_{3}\] The mechanism for the reaction of sulfur tetrafluoride with the carbonyl group has the following two steps: 1. Nucleophilic attack of carbonyl oxygen on SF4. 2. Fluoride ion transfers from sulfur to carbonyl carbon, forming a fluorinated carbonyl compound, and releasing SF3. In summary, we have elucidated the reasonable mechanisms for the reactions of phosphorus pentachloride and sulfur tetrafluoride with carbonyl groups to form chlorinated and fluorinated carbonyl compounds, respectively.