Erros Comuns em Mecanismos

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280 CHAPTER 9 
 
Common Mistakes to Avoid 
When drawing a mechanism for the acid-catalyzed tautomerization of an enol, the first step is protonation. 
Students sometimes get confused about where to place the proton during this first step. Indeed, there are 
two possible locations where protonation could occur (a lone pair or the  bond): 
 
 
 
Don’t forget that it is the  bond that is protonated during tautomerization of the enol. It is a common 
student error to protonate the OH group, like this: 
 
 
 
If you make this mistake, you might then be tempted to make another critical mistake - formation of a vinyl 
carbocation, which is likely too unstable to form (and does not get us any closer to obtaining the ketone). 
 
 
 
Whenever possible, avoid the formation of high-energy intermediates that are unlikely to form. This is a 
general rule that should be followed whenever you are drawing a mechanism (exceptions are rare). The 
correct first step for acid-catalyzed tautomerization of an enol is protonation of the  bond to generate a 
resonance-stabilized cation: 
 
 
There is one other (unrelated) common error that should be avoided. When designing a synthesis in which 
acetylene is alkylated twice, make sure to alkylate each side separately, even if both alkyl groups are the 
same: 
 
 
It is a common student error to show the reagents just one time, assuming that alkylation will occur on both 
sides of acetylene. If the reagents for alkylation are only shown once, then alkylation will only occur once: 
 
 
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