Why does phosphorus tribromide act as a Lewis acid (electron acceptor)?

*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](https://i.stack.imgur.com/w4Qx0.png)


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?




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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