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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: tasnim rahman on May 02, 2012, 02:31:39 AM

Title: Mechanism of substitution of OH group with Cl using PCl5.
Post by: tasnim rahman on May 02, 2012, 02:31:39 AM

Does anyone know the mechanism for substitution of OH group in an alcohol or carboxylic acid with Cl using PCl5? I found the one for PBr3 (PI3 probably also has a similar mechanism) on the internet, but I didn't find this one.
Thanks in advance.

Title: Re: Mechanism of substitution of OH group with Cl using PCl5.
Post by: discodermolide on May 02, 2012, 02:58:31 AM

Quote from: tasnim rahman on May 02, 2012, 02:31:39 AM

Does anyone know the mechanism for substitution of OH group in an alcohol or carboxylic acid with Cl using PCl5? I found the one for PBr3 (PI3 probably also has a similar mechanism) on the internet, but I didn't find this one.
Thanks in advance.

I know it. What is your suggestion?

Title: Re: Mechanism of substitution of OH group with Cl using PCl5.
Post by: tasnim rahman on May 02, 2012, 10:31:57 AM

First the PCl5 dissociates on its own into PCl4+ and Cl-. This provides an attacking site for the lone pairs on oxygen in ROH, as well as produces Cl- ions. The oxygen in ROH behaves a nucleophile, and attacks the slightly positively charged phosphorus, donating one of its electron pairs to form a co-ordinate bond with it, to form RO(+)H-PCl4. The chloride ion then picks up the alkyl group(R) to form RCl and HOPCl4. The HOPCl4 then somehow becomes POCl3 and HCl. This is my take on it, but I am not sure.

Title: Re: Mechanism of substitution of OH group with Cl using PCl5.
Post by: discodermolide on May 02, 2012, 11:20:34 AM

Quote from: tasnim rahman on May 02, 2012, 10:31:57 AM

First the PCl5 dissociates on its own into PCl4+ and Cl-. This provides an attacking site for the lone pairs on oxygen in ROH, as well as produces Cl- ions. The oxygen in ROH behaves a nucleophile, and attacks the slightly positively charged phosphorus, donating one of its electron pairs to form a co-ordinate bond with it, to form RO(+)H-PCl4. The chloride ion then picks up the alkyl group(R) to form RCl and HOPCl4. The HOPCl4 then somehow becomes POCl3 and HCl. This is my take on it, but I am not sure.

What is the structure of POCl3?
Can you write a balanced equation for this reaction, incorporating all the components?

Title: Re: Mechanism of substitution of OH group with Cl using PCl5.
Post by: tasnim rahman on May 03, 2012, 11:19:19 AM

I took that picture of POCl3 from wikipedia.

The balanced equation is something like this:
PCl5 + ROH  :rarrow: RCl + POCl3.

The separate equations are probably:
PCL5 ::equil:: PCl4+ + Cl-.
R-OH + PCl4+  ::equil:: R(+)OH-PCl4
Cl- + R(+)OH-PCl4 ::equil:: RCl + H-O-PCl4.
H-O-PCl4 ::equil:: HCl + POCl3 (but I don' t know how this reaction occurs).

Title: Re: Mechanism of substitution of OH group with Cl using PCl5.
Post by: discodermolide on May 03, 2012, 11:54:22 AM

Quote from: tasnim rahman on May 03, 2012, 11:19:19 AM

I took that picture of POCl3 from wikipedia.

The balanced equation is something like this:
PCl5 + ROH  :rarrow: RCl + POCl3.

The separate equations are probably:
PCL5 ::equil:: PCl4+ + Cl-.
R-OH + PCl4+  ::equil:: R(+)OH-PCl4
Cl- + R(+)OH-PCl4 ::equil:: RCl + H-O-PCl4.
H-O-PCl4 ::equil:: HCl + POCl3 (but I don' t know how this reaction occurs).

In your equation you are missing HCL. The separate reactions are not equlibria, except for the first, because of the thermodynamic sink (see below).
POCl3 is formed because the P=O bond is one of the strongest known and in this case it's formation by loss of HCl is the thermodynamic driving force of the reaction.
Otherwise your suggestions seem reasonable to me.

Title: Re: Mechanism of substitution of OH group with Cl using PCl5.
Post by: tasnim rahman on May 03, 2012, 01:40:55 PM

Sorry, I forgot to include the HCl.
Thanks a lot.  ;D