What is the mechanism for the following reaction?

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1 Answer
Ernest Z.
Jun 27, 2018

Here's what I get.

Explanation:

The overall reaction

A ketone reacts with #"PCl"_5# to form a gem-dichloride and #"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 #"PCl"_5# autoionizes into #"PCl"_4^"+""PCl"_6^"-"#

PCl5

Step 2

The carbonyl oxygen attacks the #"P"# atom of #"PCl"_4^"+"# and forms a phosphonate complex.

Mechanism

Step 3

A #"Cl"# atom from #"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 #"Cl"^"-"# ion leaves the #"P"# atom of the complex.
  • Step 4b. #"OPCl"_3# is an excellent leaving group. It departs in an #"S"_text(N)1# ionization to form a resonance-stabilized carbocation.
  • Step 4c. The #"Cl"^"-"# ion attacks the carbocation and forms the gem-dichloride.
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