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This question is purely out of curiosity and to help me remember the "why" of a mechanism to help me remember the "what/how".

In organic chemistry we recently studied the Sn2 Substitution of Primary Alcohol with PBr3:

Originally from Wikipedia PBr3_alcohol_rxn.jpg

My question is, why is it favorable for PBr3 to act as a Lewis acid (i.e. accept electrons from the oxygen to form a bond) when it already has a full octet and no formal charge?

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    $\begingroup$ As an off-topic side note, I thought of a joke while studying this mechanism: a man wants to buy beers for himself and two of his pals, walks up to a bartender and says, "I'll have PBR—three please." He and his friends die of internal caustic wounds. $\endgroup$
    – kindrobot
    Mar 6, 2015 at 16:10
  • $\begingroup$ Thanks @DelPate, I was aware of that, but I'm still unclear why the phosphorus PBr<sub>3</sub> is attacked by the oxygen in the primary alcohol above. We're taught that "something negative attacks something positive," but while the oxygen can clearly act as a nucleophile (because of its lone pairs), it's unclear to me why the phosphorus would "want" to act as an electrophile. That is, how is it the "something positive" in this mechanism when it's already an overloaded octet? $\endgroup$
    – kindrobot
    Mar 6, 2015 at 17:12
  • $\begingroup$ While I was typing out that clarification question in the comment above, something occurred to me. Is it because of a dipole moment between the phosphorus and the bromines that makes the phosphorus slightly positive? $\endgroup$
    – kindrobot
    Mar 6, 2015 at 17:14
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    $\begingroup$ Could be a reason . But I feel its more about the molecule than the P atom . Since Br is a good leaving group , P doesnt mind a nucleophilic attack . And the ultimatum is " Nature loves stability " , P-O bond is stronger than P-Br . $\endgroup$
    – Del Pate
    Mar 6, 2015 at 17:39
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    $\begingroup$ What do you mean by 'overloaded' octet? P has eight electrons in PBr3. $\endgroup$
    – jerepierre
    Mar 6, 2015 at 19:15

1 Answer 1

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The why of a mechanism is always the best approach.

The reasons for this attack are:

  • $\ce{P}$ can expand its octet due to presence of vacant $d$-orbitals.
  • The $\ce{P-Br}$ bond is polar due to the electronegativity difference between the atoms and hence P develops partial positive charge favouring a nucleophilic attack on it.
  • Looking at the molecule as a whole, $\ce{Br}$ is a good leaving group so $\ce{P}$ doesn't mind a nucleophilic attack on it by the lone pair of oxygen.
  • "Nature loves stability" and $\ce{P-O}$ bond is stronger than $\ce{P-Br}$ so that might be the driving force for this reaction mechanism.
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    $\begingroup$ No, phosphorus can not expand its octet and its d orbitals are energetically too far removed to take part in bonding. $\endgroup$
    – Jan
    Nov 12, 2017 at 15:49

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