Why Does the Central Carbon Have a Positive Charge?

[alingy1]

I was looking at this problem. It says I should write appropriate resonance structures. But, even with resonance, I do not see how a carbon cannot fill its octet. Why does the carbon in the middle have a positive charge. Is that not unstable?

 

[Ygggdrasil]

The structure you have drawn is known as a carbocation. Although these structures are indeed very unstable, they are often transient intermediates in many reactions, and understanding their resonance structures can aid in understanding how these intermediates react in various organic reaction mechanisms (e.g. in unimolecular nucleophilic substitution and unimolecular elimination reactions.)

 

[Yanick]

See: Allylic Carbocation.

In your case it is "doubly allylic" so you should have several resonance structures available. Can you draw them now?

 

[alingy1]

I could find three! If you could confirm me the number that would be great! If they are wrong, I am fully ready to draw them out fully.

 

[Yanick]

Well, you could technically draw many more resonance structure than simply three. It doesn't mean that all of them will be contributors to the true nature of the actual molecule though. I guess for the typical O-chem question where there it is implied that the most stable resonance structures should be drawn, then the answer is the first structure you posted and two more. So three total. You can post a picture for us to double check that you are doing it properly.