Ochem Question

[Ibraiz]

The question asking which resonance structure is most stable. I picked C thinking that positive charge is on less electronegative that is Carbon and negative charge is on more electronegative which is Nitrogen. Now, for the octet rule for positively charged Carbon, I was thinking that this is a Carbocation so that is okay. But the explanation states that choice B only meets the octet rule, which makes sense but what I was thinking could be a possibility as well right? So which one is most stable then?



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[wantVCUdental]

Which of the following statements is true about the compounds pictured attached?

A. They are diastereomers with different melting points.
B. They are diastereomers with identical boiling points.
C. They are enantiomers with different melting points.
D. They are enantiomers which rotate plane-polarized light in different directions.
E. They are identical meso structures.



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The explanation in this question states that they are superimposable images, which I agree. But I was thinking this is a disastereomers because the left compound is S,S and right compound is S,R so doesn't that make these two compounds diastereomers? Or is it because they mirror images that automatically disqualifies these compounds to be diastereomers?

The compound on the right is R,R

 

[Ibraiz]

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[chromosome21]

The question asking which resonance structure is most stable. I picked C thinking that positive charge is on less electronegative that is Carbon and negative charge is on more electronegative which is Nitrogen. Now, for the octet rule for positively charged Carbon, I was thinking that this is a Carbocation so that is okay. But the explanation states that choice B only meets the octet rule, which makes sense but what I was thinking could be a possibility as well right? So which one is most stable then?



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Choices A and D are absolutely wrong. So they're out. B and C are resonance structures (http://en.wikipedia.org/wiki/Diazo), but the question asks which molecule is the MOST stable. This is a diazo compound. If you see carefully, choice B has potential to "free" the N2 gas, leaving the carbene behind. Choice C cannot do so. If you see diazo compound reactions with electrophiles, the end result or the product would be the nitrogen gas (N2 ---> :N=N🙂, and a new compound (carbene + electrophile). This is actually a smart way to make a carbene.

To answer your question, you are right too. But which one is the most stable can be reasoned out based on the given explanation.

 

[ScratchMan]

The compound on the right is R,R

Both left and right ones are S,S configuration.

 

[ScratchMan]

The question asking which resonance structure is most stable. I picked C thinking that positive charge is on less electronegative that is Carbon and negative charge is on more electronegative which is Nitrogen. Now, for the octet rule for positively charged Carbon, I was thinking that this is a Carbocation so that is okay. But the explanation states that choice B only meets the octet rule, which makes sense but what I was thinking could be a possibility as well right? So which one is most stable then?



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I used following rule to figure out which one is more stable on given resonance structures.

1) More bonds/ Fewer charges.
2) Full octect structure
3) Negative charge on electronegative atom.

c, d are out since those have two charges.
a is out since it is not octect structure.

b could be most stable one... 🙂

 

[Ibraiz]

Based on the 3 criteria you provided, choice C meets all the criteria as negative charge is on the most electronegative atom which is N and there is double bond between Carbon and Nitrogen. Only thing it doesn't meet is octet rule which I thought it would be a carbocation. For choice B it meets the octet rule but the negative charge is not on the most electronegative atom which is N rather its on less electronegative atom which in this case is C. Is that mean we over look the most electronegative atom and go with the structure that meets all the octet rule?

 

[ScratchMan]

Based on the 3 criteria you provided, choice C meets all the criteria as negative charge is on the most electronegative atom which is N and there is double bond between Carbon and Nitrogen. Only thing it doesn't meet is octet rule which I thought it would be a carbocation. For choice B it meets the octet rule but the negative charge is not on the most electronegative atom which is N rather its on less electronegative atom which in this case is C. Is that mean we over look the most electronegative atom and go with the structure that meets all the octet rule?[/QUOTE
As I know, rule number should be same as priority order so octet rule is higher priorty than negative charge on more electronegative atoms. I normaly follow that rule order to find most stable resonance structure given same molecular structure. If u find an example which is not applicable on that rule, plz let me know. I would like to make sure what I know just in case. 😀

 

[ScratchMan]

Based on the 3 criteria you provided, choice C meets all the criteria as negative charge is on the most electronegative atom which is N and there is double bond between Carbon and Nitrogen. Only thing it doesn't meet is octet rule which I thought it would be a carbocation. For choice B it meets the octet rule but the negative charge is not on the most electronegative atom which is N rather its on less electronegative atom which in this case is C. Is that mean we over look the most electronegative atom and go with the structure that meets all the octet rule?

And also, C has fewer bonds than B. More bonds more stable. ^^

 

[Ibraiz]

Ok I think I am getting hold of it now. Like you mentioned in your post, follow the rules, so now I will be looking out for

More bonds / fewer charges
Octet rule
Negative charge on more electronegative atom

Thanks scratchman, I notice you responds to the posts promptly, taking time out of your schedule to help others out. I really appreciate that.