Question 1

How does lithium aluminum hydride reduce nitriles to amines?

Accepted Answer

Lithium aluminum hydride reduces nitriles through nucleophilic acyl substitution. The hydride ion attacks the nitrile carbon, forming an imine anion. A second hydride equivalent attacks this anion to generate a dianion. Finally, water protonates the dianion, yielding a primary amine as the product.

Question 2

Why does the nitrile reduction require two equivalents of lithium aluminum hydride?

Accepted Answer

Two equivalents of lithium aluminum hydride are needed because each hydride ion performs a distinct role. The first hydride attacks the nitrile carbon to form an imine anion intermediate. The second hydride then attacks this anion to generate the dianion, which is subsequently protonated to yield the primary amine.

Question 3

What is the difference between reducing nitriles with lithium aluminum hydride versus diisobutylaluminum hydride?

Accepted Answer

Lithium aluminum hydride, a strong reducing agent, converts nitriles to primary amines using two equivalents. Diisobutylaluminum hydride, a milder reducing agent, uses only one equivalent and stops at the aldehyde stage. Hydrolysis of the intermediate from diisobutylaluminum hydride yields the aldehyde product rather than the amine.

Question 4

What intermediate forms when hydride attacks the nitrile carbon?

Accepted Answer

When a hydride ion attacks the nitrile carbon as a nucleophile, an imine anion intermediate forms. This anion is then attacked by a second hydride equivalent to generate a dianion. The dianion is the key intermediate that, upon protonation with water, produces the primary amine product.

Question 5

How does the protonation step complete nitrile reduction to amines?

Accepted Answer

After the second hydride attack generates a dianion, water provides the proton needed to complete the reduction. This protonation converts the dianion into a primary amine, the final product. The protonation step is essential for releasing the amine from the reaction intermediate and regenerating the reducing agent.

Question 6

What role do hydride ions play in nitrile reduction?

Accepted Answer

Hydride ions act as nucleophiles and reducing agents in nitrile reduction. They attack the electrophilic nitrile carbon, initiating the reduction sequence. Lithium aluminum hydride serves as the source of these hydride ions, providing two equivalents to drive the reaction to completion and form the primary amine.

Question 7

Why is water necessary in the final step of nitrile reduction?

Accepted Answer

Water provides the proton required to protonate the dianion intermediate formed during nitrile reduction. This protonation step converts the negatively charged dianion into a neutral primary amine product. Without water protonation, the reaction cannot reach completion and release the amine product.

腈到胺:LiAlH44 还原

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