Can the hydroxyl group of 4-hydroxybenzoic acid react with phosphorus pentachloride?

I learned that, a phenol will not react (or react very slowly) with $\ce{PCl5}$ due to its stabilized structure.


How about the $\ce{-OH}$ group on the benzene ring of 4-hydroxybenzoic acid? Will that $\ce{-OH}$ group react with $\ce{PCl5}$? If the answer is yes, is it anything to do with the $\ce{-COOH}$ group?





> 
> a phenol will not react (or react very slowly) with $\ce{PCl5}$
> 
> 
> 


That's not true. Phenol and $\ce{PCl5}$ do react, producing mixture of compounds of formula $\ce{PCl\_n(C6H5O)\_{5-n}}$ (with $n > 1$ ). It is even possible to produce $\ce{(C6H5O)5P}$![1]


However, unlike in alcohols, in phenols:


* the carbon–oxygen bond is much stronger, having a bond order of more than one and being formed with $\mathrm{sp^2}$ hybridized orbitals
* the formation of the corresponding transition state for nucleophilic substitution would destroy the aromaticity of the benzene ring.


Thus, the formation of chlorobenzene does not proceed at a significant rate. 


It is still possible to replace the phenol group by a chloride atom in some cases. For example, it is possible to prepare picrylchloride (1,3,5-trinitrochlorobenzene) by reaction of pyridine picrate and $\ce{PCl5}$, since three nitro groups greatly increase the reactivity of the phenyl fragment in nucleophilic aromatic substitution reactions.[2] However, as a general rule, benzene derivatives are not active in nucleophilic substitution except if there are a lot of strong π-electron withdrawing groups in the ring. 


As for 4-hydroxybenzoic acid, my guess is that it should react as both phenol (as above) and benzoic acid (which [forms benzoyl chloride](http://classroom.synonym.com/chemical-properties-benzoic-acid-2721.html)).




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Addition
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While I was unable to find precise information on *para*-hydroxybenzoic acid, there is some info on *ortho*-hydroxybenzoic (salicylic acid). $\ce{PCl5}$ replaces the $\ce{OH}$ fragment of the carboxylic acid group with chlorine. The phosphorus oxochloride then reacts with the phenolic group, producing an $\ce{O-POCl2}$ fragment. The [information](https://books.google.ru/books?id=OeZ46Zgz2yQC&lpg=PA302&ots=gX5kjCqM0V&dq=salicylic%20acid%20PCl5&pg=PA302#v=onepage&q=salicylic%20acid%20PCl5&f=false), however, does not look absolutely trustworthy. Still, assuming it is correct and extrapolating it onto *para*-hydroxybenzoic acid, I'd say that the product should be $\ce{Cl-C(O)-C6H4-O-P(O)Cl2}$.




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References
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[1]: Ramirez, F.; Bigler, A. J.; Smith, C. P. Pentaphenoxyphosphorane. *J. Am. Chem. Soc.* **1968,** *90* (13), 3507–3511. [DOI: 10.1021/ja01015a038](https://doi.org/10.1021/ja01015a038).


[2]: Boyer, R.; Spencer, E. Y.; Wright, G. F. *Can. J. Chem.* **1946,** *24b* (5), 200–203. [DOI: 10.1139/cjr46b-025](https://doi.org/10.1139/cjr46b-025).


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