is pcl5 and water nucleophilic or electrophilic substitution?

^^^ Thanks!

Reply 1

^^^ Thanks!

It’s definitely a nucleophilic reaction, but I am inclined to say it’s addition-elimination.

PCl5 in the solid state is an ionic substance with the structure [PCl4]^+ [PCl6]^-. The cationic part should be susceptible to hydrolysis by water, given the +ve charge predominantly centred on the phosphorus and the tendency of phosphorus to form strong bonds to oxygen. I’d expect the water to attack this to form an intermediate with the structure [PCl4(OH2)]^+, which rapidly decomposes to Cl3P=O, HCl and “H^+”. Meanwhile, I’d expect the “H+” to break up the anionic part, starting the cycle again.

Reply 2

anonymous56754

Original post by TypicalNerd

It’s definitely a nucleophilic reaction, but I am inclined to say it’s addition-elimination.
PCl5 in the solid state is an ionic substance with the structure [PCl4]^+ [PCl6]^-. The cationic part should be susceptible to hydrolysis by water, given the +ve charge predominantly centred on the phosphorus and the tendency of phosphorus to form strong bonds to oxygen. I’d expect the water to attack this to form an intermediate with the structure [PCl4(OH2)]^+, which rapidly decomposes to Cl3P=O, HCl and “H^+”. Meanwhile, I’d expect the “H+” to break up the anionic part, starting the cycle again.

i see, how do we judge if plc5 is a nucleophile or electrophile, is it based on the electronegativity?

Reply 3

TypicalNerd

Original post by anonymous56754

i see, how do we judge if plc5 is a nucleophile or electrophile, is it based on the electronegativity?

Well, think again about the definitions of the terms nucleophile and electrophile. You know a nucleophile is anything with a lone pair that is attracted to formally positive charged and than an electrophile is anything with a positive charge that is drawn to electron-density (i.e lone pairs).

In this example reaction, which species has the lone pair and which one has the positive charge?

Reply 4

TypicalNerd

Original post by TypicalNerd

Well, think again about the definitions of the terms nucleophile and electrophile. You know a nucleophile is anything with a lone pair that is attracted to formally positive charged and than an electrophile is anything with a positive charge that is drawn to electron-density (i.e lone pairs).
In this example reaction, which species has the lone pair and which one has the positive charge?

Also, electronegativity won’t help you a great deal if you assume PCl5 is purely covalent - if it was purely covalent and existed as discrete molecules, PCl5 would be nonpolar as all the dipoles would face opposing directions (trigonal bipyramidal shape).

Reply 5

anonymous56754

Original post by TypicalNerd

Well, think again about the definitions of the terms nucleophile and electrophile. You know a nucleophile is anything with a lone pair that is attracted to formally positive charged and than an electrophile is anything with a positive charge that is drawn to electron-density (i.e lone pairs).
In this example reaction, which species has the lone pair and which one has the positive charge?

the oxygen on a water molecule has a lone pair of electrons and is it P in PCL5 positive?

Reply 6

TypicalNerd

Original post by anonymous56754

the oxygen on a water molecule has a lone pair of electrons and is it P in PCL5 positive?

Yes.

Though technically speaking, if you assume the structure to be the simple covalent PCl5 rather than [PCl4]^+ [PCl6]^- (the true structure in the solid state), then no positive charge could exist on the phosphorus as PCl5 has all of its dipoles facing opposing directions, leading them all to cancel each other out.

Reply 7

anonymous56754

Original post by TypicalNerd

Yes.
Though technically speaking, if you assume the structure to be the simple covalent PCl5 rather than [PCl4]^+ [PCl6]^- (the true structure in the solid state), then no positive charge could exist on the phosphorus as PCl5 has all of its dipoles facing opposing directions, leading them all to cancel each other out.

ok thanks

Reply 8

Nitrotoluene

Original post by anonymous56754

^^^ Thanks!

I comment things as follows:
One good example of a strong electrophile is PCl5. The phosphorus in PCl5 has a positive charge and is surrounded by chlorine atoms, which are really good at pulling in electrons. This means phosphorus ends up with fewer electrons.

Because PCl5 is an electrophile, it tends to react with nucleophiles, which are atoms that can donate a pair of electrons. Water (H2O) is a nucleophile because it has lone pairs of electrons on the oxygen atom.

So, when water comes into contact with PCl5, it can share its electrons (O2) with it.
Overall reaction, IMHO ...of course😀:

PCl5+4 H2O ==> H3PO4+5 HCl

Here are my 2 cents!

(edited 1 month ago)

Reply 9

TypicalNerd

Original post by Nitrotoluene

I comment things as follows:
One good example of a strong electrophile is PCl5. The phosphorus in PCl5 has a positive charge and is surrounded by chlorine atoms, which are really good at pulling in electrons. This means phosphorus ends up with fewer electrons.
Because PCl5 is an electrophile, it tends to react with nucleophiles, which are atoms that can donate a pair of electrons. Water (H2O) is a nucleophile because it has lone pairs of electrons on the oxygen atom.
So, when water comes into contact with PCl5, it can share its electrons (O2) with it.
Overall reaction, IMHO ...of course😀:

PCl5+4 H2O ==> H3PO4+5 HCl

Here are my 2 cents!

Depends on how much water you use. In excess, yes, you get complete hydrolysis to phosphoric acid - however, O=PCl3 is the initial product and if the ratio of water to PCl5 used initially is 1:1, you get no further reaction.

This does make for an interesting point - are the subsequent reactions of O=PCl3 with water nucleophilic substitution or not? Arguably, it could be addition-elimination, followed by a mixture of addition and substitution, but I would imagine this is unlikely given the strength of the P=O bond (making it unfeasible for addition into it) and the vacant 3d orbitals of phosphorus would allow it to behave as a lewis acid and therefore undergo nucleophilic substitution.

Reply 10

anonymous56754

Original post by Nitrotoluene

I comment things as follows:
One good example of a strong electrophile is PCl5. The phosphorus in PCl5 has a positive charge and is surrounded by chlorine atoms, which are really good at pulling in electrons. This means phosphorus ends up with fewer electrons.
Because PCl5 is an electrophile, it tends to react with nucleophiles, which are atoms that can donate a pair of electrons. Water (H2O) is a nucleophile because it has lone pairs of electrons on the oxygen atom.
So, when water comes into contact with PCl5, it can share its electrons (O2) with it.
Overall reaction, IMHO ...of course😀:

PCl5+4 H2O ==> H3PO4+5 HCl

Here are my 2 cents!

So since one is a nucleotide and one is an electrophile, how do we know if it is a nucleophilic or electrophilic reaction? Thanks!

Reply 11

Nitrotoluene

Original post by anonymous56754

So since one is a nucleotide and one is an electrophile, how do we know if it is a nucleophilic or electrophilic reaction? Thanks!

During the reaction:

4H2O + PCl5 ==> H3PO4 + 5HCl

Beginning with the oxygen in water (the nucleophile), the process uses its lone pair to strike the phosphorus in PCl5 (the electrophile). This pushes out a chlorine atom, therefore bonding oxygen to phosphorus.
Water replaces chlorine atoms on phosphorus, therefore this is a nucleophilic substitution reaction.
Especially, water is flexible; it can be both a nucleophile and an electrophile.
As a nucleophile, oxygen's lone pairs can attack electron-poor centers, including phosphorus in PCl5.
As an electrophile, water's hydrogen atoms can be attacked by strong bases because of the polarized O-H bond, which makes hydrogen partly positive. This is when water is functioning as an acid and donating a proton.
In short, to identify if a reaction is nucleophilic or electrophilic, look for the species donating an electron pair, which in this reaction is water, the nucleophile.
Bye,
Sandro

Reply 12

Nitrotoluene

I would say that a nucleotide is unrelated to your first question concerning PCl5. I do not understand why you used the term nucleotide as "a nucleotide is the basic building block of nucleic acids (RNA and DNA)."

Reply 13

TypicalNerd

Original post by Nitrotoluene

During the reaction:

4H2O + PCl5 ==> H3PO4 + 5HCl

It seems you are modelling PCl5 as being discrete molecules rather than an ionic solid with the structure [PCl4]^+ [PCl6]^-. This is not a reliable way to approach the mechanism.

I’d argue it’s nucleophilic addition-elimination at the phosphorus in the [PCl4]^+ cation which first forms the O=PCl3 (via the PCl4(OH2)^+ intermediate, hence addition and then this decomposes to HO=PCl3^+ and HCl, hence elimination) and then this substrate undergoes several successive nucleophilic substitutions at phosphorus to give O=P(OH)3 (or H3PO4, phosphoric acid).