Chemistry 2008 JAMB Past Questions - Question 47: Reduction of alkanonoes with LiAIH,

Correct Answer

A

Explanation

The reduction of alkanones with LiAlH₄ (lithium aluminum hydride) is a classic reaction in organic chemistry that primarily leads to the formation of **primary alcohols**. Here's a breakdown of the key aspects:

**Reaction:**

R-CO-CH₃ + LiAlH₄ → R-CH₂OH + Al(OH)₃ + LiOH

**Mechanism:**

1. **Hydride transfer:** LiAlH₄ acts as a hydride donor. One of its hydrogen atoms, attached to aluminum, gets transferred to the carbonyl carbon of the alkanone.
2. **Intermediate formation:** This creates an alkoxide intermediate (R-CH₂O⁻AlH₃).
3. **Protonation:** The intermediate is then protonated, typically by water or another source of protons present in the reaction mixture. This step leads to the formation of the final product, a primary alcohol.

**Advantages of using LiAlH₄:**

* **Strong reducing agent:** Effectively reduces a wide range of carbonyl compounds, including alkanones.
* **Selective:** Primarily forms primary alcohols from alkanones.
* **High yields:** The reaction usually leads to good yields of the desired product.

**Things to consider:**

* **Exothermic reaction:** The reaction can be exothermic, requiring careful control of the temperature to avoid decomposition of the starting materials or products.
* **Flammable and pyrophoric:** LiAlH₄ is a flammable and pyrophoric compound, requiring proper handling and safety precautions.
* **Moisture sensitive:** Reacts readily with water, necessitating anhydrous conditions for the reaction.

**Alternative reducing agents:**

* Other hydride donors like NaBH₄ (sodium borohydride) or DIBAL-H (diisobutylaluminum hydride) can be used depending on the specific needs of the reaction.

**Overall, the reduction of alkanones with LiAlH₄ is a powerful and versatile reaction in organic synthesis for synthesizing primary alcohols.**