[Twickel]

Hi
I am unable to figure out how many pi electrons are in this system. Please explain how the answer of. There are 2a+3 caron atoms which each contribute one electron each. Recognize that the pi systems extends over both nitrogens, there are two resonance forms. So the final answer is 2a+6 electrons.
( please see attached document for the image),

Your help is much appreciated

[fledarmus]

Which part of the answer don't you understand?

1) can you see that this is a conjugated system, which includes both nitrogens?

2) can you see two different resonance forms?

3) do you count 6 pi electrons outside the paranthesis?

4) do you count 2 pi electrons inside the parenthesis?

5) can you see that the total number of electrons inside the parenthesis is 2a?

6) can you see that the total number of electrons in the conjugated system is 2a + 6?

[Twickel]

I do not see the resonance forms. I also do not understand the nitrogen resonance leading to the pi electrons being spread over the entire system. One N has 2 pi electrons the other N has 1. Why?

I also cannot see the 6 pi electrons outside the parentheses.

[fledarmus]

Actually, one nitrogen has a pi bond shared with an adjacent carbon atom (so is sharing two pi electrons) and the other does not have a pi bond, but has a lone pair of electrons in a p orbital which can overlap with the double bond chain and conjugate with it.

The resonance structures are formed by moving the lone pair of electrons on the singly bonded nitrogen to form a double bond to the next carbon, and rearranging the other double bonds appropriately, driving the last pi bond pair up into a lone pair on the other nitrogen. See attached diagram. The six pi electrons outside the bracket are marked in purple on each structure.

[Twickel]

Thank you very much.

May I ask, how did you generate the compounds on your PC?

[Borek]

Not sure what software Fledarmus uses, but it is always worth to check free ChemSketch from ACDLabs (http://www.acdlabs.com/download/)

[Twickel]

Thanks very much.
I have anaother question related to that moecule.  What is the length over which the pi electrons are delocalised?
Looking at the structure there are 2a+4 bonds in the pi system, if we add a bond at the end of each side we get 2a+6.
I am really having trouble with this concept.
Where do 2a+44 bonds come from. I get the 2a, but the 4 then add 2 part got no idea.

Now,if I was to apply this to cycloehexatriene. Is the bond length just 3x1.40?

[fledarmus]

I'm using ChemBioDraw Ultra 12.0 for drawing software - the successor to ChemDraw. A really nice system, but I wouldn't even ask how much it costs. Fortunately my company provides it to me.

For the length, you need to think carefully about what you are adding. There is a difference between single and double bonds. For your compound, outside the parentheses you have one C=N double bond and one C-N single bond, one C=C double bond and one C-C single bond (well, actually two halves of a C-C single bond, which add up to one). Inside the parenthesis you have one C=C double bond and one C-C single bond (again, two half bonds). So what would your total length be?

The actual length of each bond would be an average between single and double bonds due to the resonance and you could reformulate the total length using an average bond length, but again, the bond lengths of C-N and C-C bonds, single or double, will be slightly different. The charge on the end of the C=N double bond will also make a slight difference.

[Twickel]

My notes have 2a+6.
L=2a+6
so, I only count the ones in the pi system.
So there are 2a bonds ( 1 from each). Then one C-N 2 C-C and one C-N bond from the other side. Why do we include the single bonds if we interested in the the pi system.
Is it because in any pi conjugate system aromatic or not, the elcetrons form pi are dolocalised over a ath which includes single bonds?

[fledarmus]

The form of your answer depends on the question. What are you supposed to be counting for the length of your conjugated system? 2a + 6 is the number of electrons in the conjugated system. The number of total bonds the system is conjugated over is 2a + 4. The number of total atoms the system is conjugated over is 2a + 5. In terms of types of bonds, the system is conjugated over (a + 2) single bonds and (a + 2) double bonds. In terms of the total calculated length of the system, it is conjugated over [(bond length C=N⁺) + (bond length C-N) + (a+1)(bond length C=C) + (a+1)(bond length C-C)]

[Twickel]

I have added the question and answer for you to see.

Btw under what section is this in my chem textbooks?
I use clayden organic and mcmurry chem

[fledarmus]

I only see 2a + 5 bonds the way they are calculating them. I don't know how they come up with 2a + 6. That is the number of total electrons involved, not the number of total bonds.

As for where to get the answers, this is usually found in the analytical chemistry textbooks covering UV-Visible spectrometry. These structures are usually strong absorbers in those wavelengths, and the spectra can tell you how long the conjugated system is. At one time it was a very important relationship used to guide dye molecule design.

Of course, the last time I was consistently using textbooks was a few dozen years ago. They may have changed somewhat since then.