Possible resonance forms? Watch

kiiten
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What are the possible resonance forms of this compound?

I know how you move the double bond with curly arrows but im not sure where to put the +ve charge.

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MexicanKeith
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(Original post by kiiten)
What are the possible resonance forms of this compound?

I know how you move the double bond with curly arrows but im not sure where to put the +ve charge.

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The positive charge in this case will always be on a carbon that has only 3 bonds
Perhaps easier to see precisely what that means if you display the H atoms.
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kiiten
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(Original post by MexicanKeith)
The positive charge in this case will always be on a carbon that has only 3 bonds
Perhaps easier to see precisely what that means if you display the H atoms.
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these are the resonance forms i got (sorry about the arrows - i hate using chemdraw )

Im not sure what you mean because where theres a single bond i just assume the carbon has 4 bonds?
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MexicanKeith
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(Original post by kiiten)
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these are the resonance forms i got (sorry about the arrows - i hate using chemdraw )

Im not sure what you mean because where theres a single bond i just assume the carbon has 4 bonds?
I'm assuming that this is a Wheland intermediate (ie you have produced it by protonating a benzene ring)?

If this is the case then draw the structure with Hydrogen atoms and try drawing your resonance forms again. The ones you have written are not correct, if you draw them with the Hydrogens you will see that some of your resonance structures have 5 bonds on certain carbon atoms.
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kiiten
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(Original post by MexicanKeith)
I'm assuming that this is a Wheland intermediate (ie you have produced it by protonating a benzene ring)?

If this is the case then draw the structure with Hydrogen atoms and try drawing your resonance forms again. The ones you have written are not correct, if you draw them with the Hydrogens you will see that some of your resonance structures have 5 bonds on certain carbon atoms.
Im not sure what you mean by wheland?? (do you mean deprotonating?) - the first structure was given to me and i had to draw the possible resonance forms.

Ive redrawn the structures with H atoms but i still cant see where i add the charge Name:  Screenshot 2017-12-03 15.48.14.png
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MexicanKeith
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(Original post by kiiten)
Im not sure what you mean by wheland?? (do you mean deprotonating?) - the first structure was given to me and i had to draw the possible resonance forms.

Ive redrawn the structures with H atoms but i still cant see where i add the charge Name:  Screenshot 2017-12-03 15.48.14.png
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You have magically disappearing and reappearing hydrogen atoms!

Each carbon must have the same number of hydrogens in each resonance forms, otherwise they aren't in resonance, they are different structures!

start with the first structure, which is correct (ie it is a benzene ring with an added proton) the only bonds that move in resonance are pi bonds, so you can only change the double bonds.

Notice that, in your first structure, the + is on the carbon which only has 3 bonds! so when the double bonds move, the + will be on whichever carbon has 3 bonds afterwards.
Also bare in mind, one of the carbon has no pi bonds (the one with 2 hydrogens in your first structure) so this carbon cannot be involved in resonance, so the electrons are not delocalised round the whole ring.

So, try again but this time make sure no Hydrogen atoms appear/move/disappear and put the positive charge on the carbon atom with only 3 bonds!
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kiiten
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(Original post by MexicanKeith)
You have magically disappearing and reappearing hydrogen atoms!

Each carbon must have the same number of hydrogens in each resonance forms, otherwise they aren't in resonance, they are different structures!

start with the first structure, which is correct (ie it is a benzene ring with an added proton) the only bonds that move in resonance are pi bonds, so you can only change the double bonds.

Notice that, in your first structure, the + is on the carbon which only has 3 bonds! so when the double bonds move, the + will be on whichever carbon has 3 bonds afterwards.
Also bare in mind, one of the carbon has no pi bonds (the one with 2 hydrogens in your first structure) so this carbon cannot be involved in resonance, so the electrons are not delocalised round the whole ring.

So, try again but this time make sure no Hydrogen atoms appear/move/disappear and put the positive charge on the carbon atom with only 3 bonds!
Probably because i dont really understand resonance

Ahhh so the functional groups at each carbon stay the same for each structure? Ive tried it again and added +ve charges. Maybe ive done something wrong but the last 2 structures have 2 charges??

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thanks for putting up with me btw
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MexicanKeith
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(Original post by kiiten)
Probably because i dont really understand resonance

Ahhh so the functional groups at each carbon stay the same for each structure? Ive tried it again and added +ve charges. Maybe ive done something wrong but the last 2 structures have 2 charges??

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thanks for putting up with me btw
Getting closer. and the atoms are staying in the right place now. But take a look at the bonds round each carbon, anything with 5 bonds round a carbon can't exist
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kiiten
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(Original post by MexicanKeith)
Getting closer. and the atoms are staying in the right place now. But take a look at the bonds round each carbon, anything with 5 bonds round a carbon can't exist
Yes of course! .....But that means only these 2 structures exist? Ive written in my notes that the 3rd structure also exists but like you said it cant because it has 5 bonds Name:  Screenshot 2017-12-03 16.18.27.png
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MexicanKeith
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(Original post by kiiten)
Attachment 707952

Yes of course! .....But that means only these 2 structures exist? Ive written in my notes that the 3rd structure also exists but like you said it cant because it has 5 bonds
of the ones you have drawn, only those 2 exist. There is a third structure.

What happens if you take your first structure and only do one of the two curly arrows you have drawn?
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kiiten
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(Original post by MexicanKeith)
of the ones you have drawn, only those 2 exist. There is a third structure.

What happens if you take your first structure and only do one of the two curly arrows you have drawn?
Like this? - do i just draw the structure ive circled as it is?

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MexicanKeith
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(Original post by kiiten)
Like this? - do i just draw the structure ive circled as it is?

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Yep, so I would draw your first structure, one arrow to take you to the circled structure then one further arrow to take you to your final structure.

You will remember from A level that benzene is unreactive, because it is aromatic (ie the 6 electrons are delocalised all the way round the ring) and therefore very stable

When benzene reacts with an electrophile, the intermediate is the structure we have here. Notice that now the electrons are not delocalised all the way round the ring (because one of the carbon atoms is saturated), so this intermediate is a lot less stable than the starting benzene ring, this big energy difference means that there is an energy barrier to reaction, so benzene reacts slowly if at all.
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kiiten
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(Original post by MexicanKeith)
Yep, so I would draw your first structure, one arrow to take you to the circled structure then one further arrow to take you to your final structure.

You will remember from A level that benzene is unreactive, because it is aromatic (ie the 6 electrons are delocalised all the way round the ring) and therefore very stable

When benzene reacts with an electrophile, the intermediate is the structure we have here. Notice that now the electrons are not delocalised all the way round the ring (because one of the carbon atoms is saturated), so this intermediate is a lot less stable than the starting benzene ring, this big energy difference means that there is an energy barrier to reaction, so benzene reacts slowly if at all.
So draw the first structure twice? One with both arrows and one with one arrow??

Yeah i see
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MexicanKeith
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(Original post by kiiten)
So draw the first structure twice? One with both arrows and one with one arrow??

Yeah i see
yeah you could do that.

you could also draw one structure, one arrow to get to a second structure.
then one arrow on the second structure to get to the third structure.

Ie draw the two arrows in your original but do them one at a time.

So the positive charge goes from position 1 to position 3 to position 5
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kiiten
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(Original post by MexicanKeith)
yeah you could do that.

you could also draw one structure, one arrow to get to a second structure.
then one arrow on the second structure to get to the third structure.

Ie draw the two arrows in your original but do them one at a time.

So the positive charge goes from position 1 to position 3 to position 5
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MexicanKeith
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(Original post by kiiten)
like this?
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perfect
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kiiten
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(Original post by MexicanKeith)
perfect
Thank youu so much for helping me
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