Qvault O-chem question

[FeralisExtremum]

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Took exam 5 today and came across this:

It seems to me that the answer should actually be the product below, due to the cation shifting to the more stable tertiary carbon in the intermediate of the second HCl addition:

Thoughts?

 

[DAT Destroyer]

Took exam 5 today and came across this:

It seems to me that the answer should actually be the product below, due to the cation shifting to the more stable tertiary carbon in the intermediate of the second HCl addition:

Thoughts?

The answer is correct.....A shift would have indeed made a stable carbocation.....BUT....it is not all that great. Why ? A carbocation is sp2.....and necessitates 120 degree geometry...this is not attained by placing the positive charge on the ring.....Without the shift....the carbocation is stabilized by resonance from the electron donation of the Chlorine atom. Usually tertiary carbocations win the race...but not this time !!

Hope this helps .

Dr. Romano

 

[FeralisExtremum]

Thank you! Should I always assume that a carbocation stabilized directly by a halogen will not go through a carbocation shift?

Also, somewhat unrelated question - let's say I had a carbocation and a halogen was not directly attached to it, but one carbon away. In that case the halogen would actually destabilize the carbocation via the inductive effect rather than stabilizing via resonance, right? Just want to make sure I've got this fully understood.

Some images I see on this are confusing like here:

I would expect the Fluorine to stabilize the cation and destabilize the anion for the reasons mentioned above...

 

[DAT Destroyer]

Thank you! Should I always assume that a carbocation stabilized directly by a halogen will not go through a carbocation shift?

Also, somewhat unrelated question - let's say I had a carbocation and a halogen was not directly attached to it, but one carbon away. In that case the halogen would actually destabilize the carbocation via the inductive effect rather than stabilizing via resonance, right? Just want to make sure I've got this fully understood.

Some images I see on this are confusing like here:

I would expect the Fluorine to stabilize the cation and destabilize the anion for the reasons mentioned above...

I never use words like " always " in organic chemistry.....but yes, usually resonance stabilization trumps all other factors.WHOA WHOA WHOA..... a halogen or electron withdrawing group one carbon away is the KISS OF DEATH !!!! This would destabilize the carbocation. I absolutely love your questions....I can feel your enthusiasm !!!

Fluorine is one of my favorite elements !!!! As a matter of fact, Fluorine chemistry is now EXPLODING !!!! Many new reagents have been developed that allows us to do Fluorine Chemistry safely ...such as Selectfluor. Fluorine.....as well as any electron withdrawing group....will stabilize an anion since the negative charge is delocalized. How about carbocations ? I have seen in Advanced Organic chemistry many studies on this. Yes....There is some destabilization from the inductive effect......yes......there is some resonance stabilization. Who wins ? It is close.....but again.....Resonance trumps out the inductive effect.....even against the mighty Fluorine.

Hope this helps.

Dr. Jim Romano

 

[tooth_hurty]

Interesting...I would've expected the same product you drew out.

By the way, how do you like Qvault's Ochem so far? I haven't purchased it yet but have read some threads on SDN of people not being too satisfied with it :/ so I'm not sure if it's worth it

 

[FeralisExtremum]

I never use words like " always " in organic chemistry.....but yes, usually resonance stabilization trumps all other factors.WHOA WHOA WHOA..... a halogen or electron withdrawing group one carbon away is the KISS OF DEATH !!!! This would destabilize the carbocation. I absolutely love your questions....I can feel your enthusiasm !!!

Fluorine is one of my favorite elements !!!! As a matter of fact, Fluorine chemistry is now EXPLODING !!!! Many new reagents have been developed that allows us to do Fluorine Chemistry safely ...such as Selectfluor. Fluorine.....as well as any electron withdrawing group....will stabilize an anion since the negative charge is delocalized. How about carbocations ? I have seen in Advanced Organic chemistry many studies on this. Yes....There is some destabilization from the inductive effect......yes......there is some resonance stabilization. Who wins ? It is close.....but again.....Resonance trumps out the inductive effect.....even against the mighty Fluorine.

Hope this helps.

Dr. Jim Romano

Thanks so much Dr. Romano! You've been a huge help with everything leading up to my DAT...I owe you and Destroyer big time!

Interesting...I would've expected the same product you drew out.

By the way, how do you like Qvault's Ochem so far? I haven't purchased it yet but have read some threads on SDN of people not being too satisfied with it :/ so I'm not sure if it's worth it

The full length tests haven't been too bad. They definitely don't feel as challenging as Destroyer or even Bootcamp, but I think they've made for good practice to at least keep my skills sharp. Maybe a bit too easy, although my test average on Bootcamp and Qvault have been roughly the same.

 

[Anthony Bogdan]

I have done the same..Bootcamp /Destroyer to qvault and I've noticed very similar things. Did you feel like qvault PAT was a little easier than Bootcamp? I wonder which is more accurate

 

[FeralisExtremum]

For me Qvault PAT feels easier than Bootcamp overall with the major exception of the TFE section which feels a lot harder. Some of the Qvault TFE questions are insanely challenging, I don't even bother.