# What is the mechanism for the following reaction?

Jun 27, 2018

Here's what I get.

#### Explanation:

The overall reaction

A ketone reacts with ${\text{PCl}}_{5}$ to form a gem-dichloride and ${\text{POCl}}_{3}$.

underbrace("C"_6"H"_5"C(=O)CH"_3)_color(red)("a ketone") + "PCl"_5 stackrelcolor(blue)( "0 °C"color(white)(mm))(→) underbrace("C"_6"H"_5"CCl"_2"CH"_3)_color(red)("a gem-dicloride") + "POCl"_3

The reaction mechanism

Step 1

The ${\text{PCl}}_{5}$ autoionizes into $\text{PCl"_4^"+""PCl"_6^"-}$

Step 2

The carbonyl oxygen attacks the $\text{P}$ atom of $\text{PCl"_4^"+}$ and forms a phosphonate complex.

Step 3

A $\text{Cl}$ atom from $\text{PCl"_6^"-}$ attacks the electrophilic carbonyl carbon of the complex.

Step 4

The new complex undergoes an intramolecular rearrangement to form the gem-dichloride.

An alternative to Step 4

Instead of being concerted, Step 4 could consist of three steps:

• Step 4a. A $\text{Cl"^"-}$ ion leaves the $\text{P}$ atom of the complex.
• Step 4b. ${\text{OPCl}}_{3}$ is an excellent leaving group. It departs in an ${\text{S}}_{\textrm{N}} 1$ ionization to form a resonance-stabilized carbocation.
• Step 4c. The $\text{Cl"^"-}$ ion attacks the carbocation and forms the gem-dichloride.