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Phenols react with PCl5 ?

Do phenols react with PCl5 ? Are the products the same as with an alcohol+PCl5 ?
Reply 1
Avatar for illusionz
illusionz
20
Original post by Ari Ben Canaan
Do phenols react with PCl5 ? Are the products the same as with an alcohol+PCl5 ?


No they don't. The mechanism for the process requires Chloride acting as a nucleophile attacking R-OH+-PCl4, which works fine for an sp3 hybridised R but will not work for an sp2 hybridised carbon in a benzene ring as the angle of attack for SN2 is in the centre of the ring.

The alcohol reaction is effectively the same reaction as this:



Whereas for an aromatic ring (copied from wikipedia).

Aryl halides cannot undergo SN2 reaction.The C–Br bond is in the plane of the ring as the carbon atom is trigonal. To attack from the back, the nucleophile would have to appear inside the benzene ring and invert the carbon atom in an absurd way. This reaction is not possible.



SN1 reaction is possible but very unfavourable. It would involve the unaided loss of the leaving group and the formation of an aryl cation.

(edited 11 years ago)
Sn2.png2.2KB
Original post by illusionz
A random yahoo answers says not but I checked on reaxys and found 0 results so it's looking like they won't.


Hmmm.... I thought so but a textbook I have alludes to such a reaction. It states that the reaction would take longer than with an alcohol.

I have another question.

When carrying out the hydrolysis of an amide we usually reflux the amide with DILUTE NaOH or HCl.

However, when we hydrolyse a protein we use CONCENTRATED NaOH/HCl.

Why this difference in reagent conditions when we are essentially dealing with breakage of the same bond type ?
Reply 3
Avatar for illusionz
illusionz
20
Original post by Ari Ben Canaan
Hmmm.... I thought so but a textbook I have alludes to such a reaction. It states that the reaction would take longer than with an alcohol.

I have another question.

When carrying out the hydrolysis of an amide we usually reflux the amide with DILUTE NaOH or HCl.

However, when we hydrolyse a protein we use CONCENTRATED NaOH/HCl.

Why this difference in reagent conditions when we are essentially dealing with breakage of the same bond type ?


Edited my post with a mechanistic view of why it wouldn't work. You wouldn't get Sn1 either from a thermodynamic point of view as the loss of aromaticity is very unfavourable.

As for hydrolysis of proteins and amides, there are two reasons.

Firstly, a protein has a vast number of peptide bonds so you'll need quite a lot of nucleophile to cleave the bond.

Secondly, proteins are all folded up and only the surface would be available for attack at neutral (or close to neutral pH). Use of very high/very low pH denatures the protein, exposing all the peptide bonds for cleavage, which will make the reaction a lot, lot faster.
Reply 4
Avatar for cpchem
cpchem
14
Original post by illusionz
Edited my post with a mechanistic view of why it wouldn't work. You wouldn't get Sn1 either from a thermodynamic point of view as the loss of aromaticity is very unfavourable.

As for hydrolysis of proteins and amides, there are two reasons.

Firstly, a protein has a vast number of peptide bonds so you'll need quite a lot of nucleophile to cleave the bond.

Secondly, proteins are all folded up and only the surface would be available for attack at neutral (or close to neutral pH). Use of very high/very low pH denatures the protein, exposing all the peptide bonds for cleavage, which will make the reaction a lot, lot faster.


Formation of an aryl cation does not involve loss of aromaticity. The pi system is orthogonal.
Nope :no:

I did this today in class :mmm:

My teacher said that phenols are the only ones with a OH group that doesn't react with PCL5 to give white steamy/misty fumes of HCL
Reply 6
Avatar for illusionz
illusionz
20
Original post by cpchem
Formation of an aryl cation does not involve loss of aromaticity. The pi system is orthogonal.


Ah so it is. Silly me.
Reply 7
Avatar for Tweed
Tweed
post for future use*
No, your Teacher taught wrong .Actually when phenol reacts with phosphorous pentachloride it gives triphenyl phosphate. (ph)3P=o

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