[33seven]

Hello everyone, this is my first post on these forums. I am a student in an Organic Chemistry I course and am struggling a little, so I need some help. We just took our first test which I got an 84.2% on, but I would like to improve that so that my test scores are at least A's and hopefully some 100%'s. I have a bunch of questions which i will be posting from my 1st test which i need some guidance on, so here goes;

Which of the following covalent bonds would be considered most polar?
a) C-H  b)C-F  c)C-O  d)C-Na  e)C-N

I chose C-F, but it was marked wrong. Could someone explain what is the correct answer and why?

[macman104]

Well, if not C-F, which is your next guess?  Is there another C-__ pair that has very different electronegativities?

[33seven]

Well, since F is the most electronegative, then that pretty much eliminates O and N, I know C-H is not polar or barely polar, so the only wild card would be C-Na. If its not C-F then the only other possible answer is C-Na. If thats the answer though, then I think this was a BS question to put on our test b/c we dont have a chart of electronegativity values and our teacher specifically told us in class to not pay attention to those values anyways b/c they are not entirely representative of what really happens when two different atoms bond.

[Jorriss]

It's not BS.

Carbon is a non-metal.

Fluorine is a non-metal.

Sodium is a metal.


It's tricky and you gave it a fair shot.

[orgopete]

@33seven
I understand your dismay. However, it is difficult for me to be critical of a class I did not attend. It seems as though you did believe electronegativities are important to understanding bond polarity and this question would thus be answerable on that basis. Therefore, no table and wrong answer does seem BS. If your professor did not believe electronegativities are that important, then it seems reasonable that C-Na might be a polar bond. He might have even said that C-Li is a polar bond.

Two points, one is that polarity is a tricky term in a quantum world. If we compare HF and HI, the charge of the proton is +1, fluoride nucleus is +9, iodide nucleus +53, fluoride electrons -10, and iodide electrons -54. It seems paradoxical to think that HF is more polar than HI or that electrons (of a bond) can be more or less polar in different molecules. I think when we use the term, we use it in a reactivity sense. When we say a bond is polar, we are trying to explain why a bond is reactive. In that sense, a C-Na bond is more polar, meaning it is more reactive.

The second point is that I think electronegativity is simply wrong. This is derived from an early model of bonding suggested by Pauling in which the bond energy is the sum of a covalent and ionic component. He stated that an ionic component can only increase a bond strength (opposites attract). The discovery of metal hydrides with weak bonds should have disproved the two component concept. Inexplicably, Pauling changed the formulation (to match the data), but did not change the concept. Therefore we have contradictory ideas like, carbon is a better electron withdrawing group than hydrogen and a better electron donating group or that a C-F bond has a high degree of ionic content, but is resistant to forming ions while a C-I bond has a low ionic content and readily forms ions. Consequently, your professor might have been saying, "Electronegativity is not all that important."