Erros Comuns com Grignard

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CHAPTER 13 445 
 
 
 
Common Mistakes to Avoid 
 
We have seen that epoxides will react with a wide variety of nucleophiles (causing ring-opening 
reactions), either under acidic conditions or under basic conditions. If the attacking nucleophile is itself a 
strong base, such as a Grignard reagent, then acidic conditions cannot be used. That is, the following 
transformation is not possible to achieve, so avoid trying to do something like this: 
 
 
 
This doesn’t work because Grignard reagents are not only strong nucleophiles, but they are also strong 
bases. And strong bases are incompatible with acidic conditions. If a Grignard reagent is subjected to a 
source of acid (even a relatively weak acid, such as H2O), the Grignard reagent is irreversibly protonated to 
give an alkane, for example: 
 
(MeMgBr)
H
O
H
H C
H
H
MgBr H C
H
H
H + HOMgBr
Methane 
 
In summary, never use a Grignard reagent in the presence of an acid. The same rule applies to the use of 
LiAlH4 (lithium aluminum hydride), which is both a strong nucleophile and a strong base. Therefore, much 
like a Grignard reagent, LiAlH4 also cannot be used to open an epoxide under acidic conditions: 
 
 
 
Once again, this doesn’t work because LiAlH4 is incompatible with acidic conditions. Avoid making this 
mistake. 
 
 
 
 
 
 
 
 
 
 
 
 
 
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