Lecture Notes: Formation of Alcohols Using Hydride Reducing Agents

Introduction to Hydride Reducing Agents

Key Concept: Hydride reducing agents transfer hydrides (H⁻) to reduce carbonyl compounds (aldehydes and ketones) to alcohols.
Hydride (H⁻): Hydrogen atom with two electrons, giving it a negative formal charge.

Structure: Boron atom bonded to four hydrogens, boron has a -1 formal charge, and a sodium cation (Na⁺) is present.
Function: Transfers a hydride to the carbonyl group.
Mechanism:
- Carbon in carbonyl is partially positive, attracts electrons.
- Hydride transfer to carbonyl carbon, forming an alkoxide ion (oxygen with a -1 charge).
- Protonation of alkoxide to form alcohol can be done using a proton source like water, methanol, or ethanol.
Example: Reduction of ketone to secondary alcohol (carbon bonded to OH is bonded to two other carbons).

Structure: Aluminum bonded to four hydrogens, aluminum has a -1 formal charge, and a lithium cation (Li⁺) is present.
Mechanism:
- Similar hydride transfer from LiAlH₄ to carbonyl.
- Formation of alkoxide followed by protonation to form alcohol.
- Example: Reduction of aldehyde to primary alcohol (carbon bonded to OH is bonded to one other carbon).
Important Note: LiAlH₄ reacts violently with water, forming hydrogen gas (H₂).

Electronegativity:
- Boron (≈2) is more electronegative than Aluminum (≈1.5).
- Al-H bond in LiAlH₄ is more reactive than B-H bond in NaBH₄.
Functional Group Reactivity:
- NaBH₄: Reduces aldehydes and ketones but not esters and carboxylic acids.
- LiAlH₄: Reduces a wider range, including esters and carboxylic acids.

NaBH₄ Reduction: Selectively reduces aldehyde in a compound, leaving ester group untouched.
LiAlH₄ Reduction: Reduces both aldehyde and ester groups in a compound.

Summary: Hydride reducing agents are crucial in synthesizing alcohols from carbonyl compounds.
Further Study: More on the mechanism of ester reduction by LiAlH₄ in subsequent lectures.