CN 2. LiAIH4 3. H2O NH2 Primary Amines Can Be Prepared From Nitriles By Reduction With LiAlH4. The Two-Step Sequence Involves SN2 Displacement Of Halide Ion By Cyanide Ion Followed By Reduction, And Yields A Primary Amine With One More Carbon Than Was Present In The Alkyl Halide. Draw Curved Arrows To Show The Movement Of Electrons In This Step Of The
The Correct Answer and Explanation is:
Correct Arrow-Pushing Mechanism (Reduction of Nitrile to Amine using LiAlH₄):
In the image provided, the mechanism step shown is the reduction of a nitrile (-C≡N) by lithium aluminum hydride (LiAlH₄). The curved arrows should show the following steps:
Correct Arrow Movements:
- Nucleophilic attack by hydride (H⁻) from the AlH₄⁻ anion:
- The hydride attacks the electrophilic carbon of the nitrile group (C≡N), breaking one π bond of the triple bond.
- The electrons from the π bond move to the nitrogen, forming an imine anion intermediate.
- Second hydride transfer:
- Another hydride from LiAlH₄ attacks the carbon again, reducing the imine to an amine anion.
- Coordination to AlH₃:
- The nitrogen donates its lone pair to aluminum, forming a coordinate bond with AlH₃ (as shown in the rightmost product).
- Final hydrolysis (with H₂O) step not shown here, but it would liberate the free primary amine (–NH₂) by breaking the Al–N bonds.
Explanation:
The transformation shown is a classic method to synthesize primary amines using a three-step sequence involving cyanide substitution followed by reduction. The substrate is a benzyl bromide, which undergoes SN2 substitution with cyanide ion (CN⁻) to form a nitrile intermediate. This nitrile is then reduced by lithium aluminum hydride (LiAlH₄).
LiAlH₄ is a strong reducing agent that donates hydride ions (H⁻). In the reduction step, the hydride nucleophilically attacks the electrophilic carbon of the nitrile (C≡N), breaking one of the carbon-nitrogen π bonds and producing an imine anion intermediate. A second hydride attack further reduces the imine to an amine anion, which is stabilized by coordination to AlH₃ through the lone pair on nitrogen.
The final step (not shown) involves aqueous work-up (addition of water), which hydrolyzes the Al–N bonds to yield the free primary amine product.
This process is extremely useful in synthetic organic chemistry because it adds a carbon to the original alkyl halide chain via the cyanide ion and then converts the nitrile into a primary amine. This method is particularly valuable for extending carbon chains while introducing nitrogen functionality.
The curved arrow mechanism illustrates the flow of electrons, helping to rationalize the bonding changes that take place during the reaction. Understanding these mechanisms is key to mastering organic synthesis and reaction design.
