How do I correctly move pi electrons towards a pi bond in resonance structures?

[donaldparida]

In my chemistry study material a set of rules to draw resonance structures are given. They are:

(1) Only electrons move. The nuclei of the atoms never move.

(2) The only electrons that can move are pi electrons (electrons in pi bonds) and lone-pair electrons.

(3) The total number of electrons in the molecule does not change, and neither does the number of paired and unpaired electrons.

The electrons can be moved in one of the following ways:

(a) Move pi electrons toward a positive charge or towards a pi bond.

(b) Move lone-pair electrons toward a pi bond.

(c) Move a single non-bonding electron toward a pi bond.

As an illustration of the second part of point (a)- move pi electrons towards a pi bond, this picture is given:

https://i.stack.imgur.com/D98F1.jpg

In the first and second examples pi electrons are moving towards sigma bond and not pi bonds. Right? Then how does this support the first rule that "Move pi electrons...towards a pi bond."? Where am i going wrong?

As an illustration of point (b) this image has been given:

https://i.stack.imgur.com/5a3jj.jpg

In the first example,

(i) Why are electrons moving towards oxygen from the pi bond?
(ii) Why are electrons moving from the lone pair above nitrogen towards the sigma bond? According to the rules electrons should move from lone pair towards pi bond?

In other examples also the same thing is happening. Where am i going wrong exactly?

Please help.

 

[mjc123]

I don't think the rules are very well expressed. When they say electrons move "toward" a pi bond, I think that means "in the direction of a pi bond", but not "onto a pi bond". The electrons move onto a sigma bond, but not any old sigma bond - it must be one with a pi bond on the other side, whose pi electrons can move somewhere else to allow the new pi bond to be formed. In other words, there must be a continuity of pi orbitals on adjacent atoms, which can be described as overlapping in different ways (in terms of conventional pi bonds). For example, in 1-butene, you couldn't move the pi electrons onto the C2-C3 sigma bond because C3 is saturated - it has no room to accept those electrons and form a C2-C3 pi bond.
Similarly, a lone pair can move onto a sigma bond with a pi bond on the other side, to give a new pi bond, and wherever the other pi bond electrons go. And of course you can reverse the process, and pi bond electrons can go to a lone pair.

 

[donaldparida]

@mjc123 , could you please explain this line in your post: "whose pi electrons can move somewhere else to allow the new pi bond to be formed"

 

[mjc123]

Well, for example, in 1-methoxypropene, when the O lone pair moves to form a C=O pi bond, the C=C pi electrons move to C2 to form a lone pair on a carbanion.

 

[donaldparida]

@mjc123 why does this happen?

 

[DrDu]

In my chemistry study material a set of rules to draw resonance structures are given. They are:

(1) Only electrons move. The nuclei of the atoms never move.

(2) The only electrons that can move are pi electrons (electrons in pi bonds) and lone-pair electrons.

(3) The total number of electrons in the molecule does not change, and neither does the number of paired and unpaired electrons.

The electrons can be moved in one of the following ways:

(a) Move pi electrons toward a positive charge or towards a pi bond.

(b) Move lone-pair electrons toward a pi bond.

(c) Move a single non-bonding electron toward a pi bond.

That's utter nonsense:
1) It is not a movement of electrons but only of lines on your paper which move. These movements do not correspond to the actual motion of the electrons.
2) Resonance is a term from valence bond theory. However, in valence bond theory, there is no distinction between sigma and pi electrons. So it makes no sense to say that only electrons in pi bonds move. Even then, there are many cases where resonance involves sigma bonds, e.g. in SF6.

 

[donaldparida]

@DrDu, but the electrons are delocalised (due to which they can move). Aren't they? If these rules are wrong then how does one draw the resonance structures of a compound?

 

[Borek]

Resonance structures are just an approximation (or simplification) of the real world. Yes, they happen to be useful when explaining some observed phenomena, but it doesn't mean they in any way reflect the reality.

Problem is, explaining the real thing in real terms requires digging much deeper into the QM, which is not always an option.

 

[jim mcnamara]

I think what @Borek is telling you: if you are not a Chemistry or Physics major, then the resonance theory is okay for you. Take it as more of a moral tale, use what it tells you because it is very useful, and move on.

BTW: is this homework? It sure looks like it to me.

 

[donaldparida]

@jim mcnamara, Nope this is not homework. I was just trying to figure out the steps to draw resonance structures of any compound, that is, the rules for "delocalising" the electrons.