14.27
Nitriles to Ketones: Grignard Reaction
Organomagnesium halides, commonly known as Grignard reagents, convert nitriles to ketones and proceed through a nucleophilic acyl substitution. Nitril…
Nitriles can be converted to ketones in the presence of organomagnesium halides, known as Grignard reagents.
The reaction proceeds via a nucleophilic acyl substitution, introducing a new carbon–carbon bond.
Notably, the alkyl–magnesium bond in the Grignard reagent is highly polar. As a result, the alkyl carbon develops a carbanionic character and acts as a nucleophile in the reaction.
The first step of the mechanism begins with a nucleophilic attack by the Grignard reagent at the nitrile carbon, forming an anion.
The second step involves protonation of the anion with an aqueous acid to form an imine as an intermediate.
Since the imine intermediate is not isolated, it enters the next step, where the imine is hydrolyzed under acidic conditions to yield ketone as the final product.
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Q1: How do Grignard reagents convert nitriles to ketones?
Grignard reagents, or organomagnesium halides, convert nitriles to ketones through nucleophilic acyl substitution. The alkyl carbon in the Grignard reagent develops carbanionic character and attacks the nitrile carbon, forming an anion. After protonation with aqueous acid and imine hydrolysis, the ketone product forms.
Q2: Why is the alkyl-magnesium bond in a Grignard reagent so reactive?
The alkyl-magnesium bond in a Grignard reagent is highly polar, causing the alkyl carbon to develop carbanionic character. This negative charge makes the alkyl carbon an effective nucleophile, enabling it to attack the electrophilic nitrile carbon and initiate the reaction mechanism.
Q3: What is the role of the imine intermediate in nitrile-to-ketone conversion?
The imine intermediate forms after the Grignard reagent attacks the nitrile carbon and the resulting anion is protonated with aqueous acid. Although not isolated, the imine immediately undergoes hydrolysis under acidic conditions to yield the final ketone product. This step is essential for product formation.
Q4: What new bond is formed when a Grignard reagent reacts with a nitrile?
A new carbon-carbon bond is formed when the Grignard reagent's alkyl carbon attacks the nitrile carbon. This bond formation is central to converting the nitrile functional group into a ketone, connecting the alkyl group from the Grignard reagent to the nitrile carbon.
Q5: What sequence of steps occurs after the Grignard reagent attacks the nitrile?
After the Grignard reagent attacks the nitrile carbon, forming an anion, aqueous acid protonates this anion to generate an imine intermediate. The imine then undergoes hydrolysis under acidic conditions, ultimately yielding the ketone as the final product of the reaction.
Q6: Why is aqueous acid necessary in the nitrile-to-ketone Grignard reaction?
Aqueous acid serves two critical functions: it protonates the anion formed after the Grignard attack to generate the imine intermediate, and it provides the acidic conditions needed to hydrolyze the imine into the ketone product. Without aqueous acid, the reaction cannot proceed to completion.
Q7: How does the polarity of the Grignard reagent enable nucleophilic attack?
The highly polar alkyl-magnesium bond in the Grignard reagent causes the alkyl carbon to develop significant carbanionic character. This negative charge makes the alkyl carbon nucleophilic, allowing it to attack the electrophilic carbon of the nitrile and initiate the reaction mechanism.