Carbocation Rearrangements

For each of the reactions on the left, predict the structure of the initially formed carbocation, and the structure of the most stable carbocation. Pay particular attention to the possibility of rearrangements.

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The initially formed secondary carbocation undergoes a 1,2-hydride shift to give the more stable tertiary carbocation, as shown below.































The initially formed secondary carbocation undergoes a 1,2-methyl shift to give the more stable tertiary carbocation, as shown below.  If the carbocation forms on carbon #1 of the alkene, a hydride- and a methyl-shift are necessary.

 
 





































The initially formed secondary carbocation undergoes a 1,2-hydride shift to give the more stable tertiary carbocation, as shown below. 

  










       


































The initially formed secondary carbocation undergoes a 1,2-methyl shift to give the more stable tertiary carbocation, as shown below.








































The secondary carbocation does not undergo methyl transfer to give the tertiary carbocation since the carbocation formed would be at a bridgehead position and carbocations must be planar.






































The reaction of an alkene with OsO4 proceeds through the formation of a osmium diester intermediate in which two of the osmium oxygens are bonded to the carbons of the alkene.  Work-up with bisulfite results in the formation of a glycol (a 1,2-diol) with cis stereochemistry.  The same products are obtained from reaction with alkaline KMnO4;  this latter reaction does not require bisulfite work-up and proceeds through the formation of a permanganate diester, as above.
 



































The initially formed secondary carbocation undergoes a 1,2-hydride shift to give the more stable tertiary carbocation.















































The initially formed secondary carbocation undergoes a 1,2-methyl shift to give the more stable tertiary carbocation.  If the carbocation forms on carbon #1 of the alkene, a hydride- and a methyl-shift are necessary.















































The initially formed secondary carbocation undergoes a 1,2-hydride shift to give the more stable tertiary carbocation. 















































The initially formed secondary carbocation undergoes a 1,2-methyl shift to give the more stable tertiary carbocation.












































The secondary carbocation does not undergo methyl transfer to give the tertiary carbocation since the carbocation formed would be at a bridgehead position and carbocations must be planar.















































The reaction of an alkene with OsO4 proceeds through the formation of a osmium diester intermediate in which two of the osmium oxygens are bonded to the carbons of the alkene.  Work-up with bisulfite results in the formation of a glycol (a 1,2-diol) with cis stereochemistry.  The same products are obtained from reaction with alkaline KMnO4;  this latter reaction does not require bisulfite work-up and proceeds through the formation of a permanganate diester, as above.