orgo problem- don't understand

[dukejen04]

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so, this is probably really easy, but it's been almost five years since i've had chem, so try and hold back on any severe patronizing.

a practice question asked what the formal charge would be on a the central atom of POCl(3)

So, I figured there were two possibilities- 1) P is double bonded to O giving a formal charge of zero on both P and O (since I know P can have more than an octet, like in PCl5) 2) P is only single bonded to O, giving a formal charge of +1 on P and -1 on O. I figured the double bond would make more sense.

I was wrong- P is only single bonded to O. But why isn't it a double bond?

 

[Csv321]

okay..I may be wrong...which does happen but I believe it has to do with stability. A negative formal charge on the most electronegative atom is the most stable...

 

[ASDIC]

The formal charge is 0 because P is doubly bonded to O and singly to the 3 chlorines.

If P is singly bonded to O...it wont have the +1 charge. The +1 charge will only arise if P is bonded to 6 atoms or has 6 bonds.

 

[dukejen04]

ASDIC- i think you're wrong.
No, the formal charge on Phosphorous is definitely +1. That was the answer and that was what I got if I single bonded it to Oxygen.

P= 5(valence)- 1/2 bonding(4 bonds=8electrons)-lone pairs (none)
= 5-4= +1

Oxygen= 6(valence)-1/2 bonding(1 bond=2 electrons)-lone pairs(6 electrons)
= 6-1-6= -1

************************
Double bond O

P= (5)-1/2(10)-0= 0 with overfilled octet which is fine for P
O= (6)-1/2(4)-4= 0 with perfect octed cuz it is in the first two periods

I still don't get it!!! can anyone confirm what Csv said? i thought that rule only applied when there HAD to be a formal charge. but if there doesn't have to be a formal charge, i don't think having separation of charge would be better.

 

[imsotired]

I think (I may be wrong) that P likes to be single bonded to O. The elements that are capable of having double and triple bonds are O, N, C, and S. P is ONLY double bonded to O in nucleotide phosphates (ATP, ADP, GTP, etc). I believe that it's supposedly rare to have any other elements with double or triple bonds.

 

[BCmedgirl]

POCl3 has the P double bonded to O. The name of the compound is phosphoryl chloride, and as a chemist whose worked with this compound, I can fairly confidently say that the compound will be doubly bonded with a formal charge = to 0 on all.

 

[imsotired]

POCl3 has the P double bonded to O. The name of the compound is phosphoryl chloride, and as a chemist whose worked with this compound, I can fairly confidently say that the compound will be doubly bonded with a formal charge = to 0 on all.

she's right.....look up the structure at www.chemfinder.com

 

[bgreet]

Yea oxygen is double bonded to p with the chlorines singly bonded to p. Molecules are always going to attempt the most stable conformation, or that with a minimal formal charge.

 

[eltrebor]

BCmedgirl is right. Even if P was singly bonded to O, the O e- would be attracted to the positive charge on the P and consequently double bond with it anyways.

 

[coodoo]

The reason it's probably single bonded in the answer is it gives phosphorus an octet configuration which is very stable

 

[dukejen04]

so- i'm not wrong to have guessed that it might be double bonded- even though that's the wrong answer.
if it shows up again, safer to say single bond? i hate it when things aren't clear cut. damn.

 

[RayhanS1282]

If you draw it this way than the formal charges on all of 'em is 0.
Cl
I
P-O-Cl
I
Cl

* The I's are actually bonds to P.

 

[dukejen04]

yeah, but you should put the least electronegative ion in the middle- and that would be phosporus- correct?

besides- the answer to the question i +1, not 0

 

[coodoo]

so- i'm not wrong to have guessed that it might be double bonded- even though that's the wrong answer.
if it shows up again, safer to say single bond? i hate it when things aren't clear cut. damn.

yeah...octet rule dominates everything.

Just remember:

octet with a formal charge of +1 or -1 > d-orbital occupied w/no charge

 

[JohnHolmes]

yeah, but you should put the least electronegative ion in the middle- and that would be phosporus- correct?

besides- the answer to the question i +1, not 0

It is 0. Kaplan is wrong with that, email them.

P is an exception to the octet rule. Think of the phosphate group ... PO4 .... see?

Kaplan/other private testing companies are wrong sometimes. Their error checking is good, but not fool proof. If it was written POCl3 and the answer is +1, then the test writers were violating all sorts of conventions on how to write molecular formulas.

JH