What hydride reagent is commonly used to reduce ketones or aldehydes to alcohols?

Lithium aluminum hydride (LiAlH4) is a well-known and effective hydride reagent used primarily for the reduction of ketones and aldehydes to their corresponding alcohols. The process involves the transfer of a hydride ion (H-) from the metal hydride to the carbonyl carbon atom of the ketone or aldehyde. This reaction occurs under anhydrous conditions, as LiAlH4 is highly reactive with water and moisture.

When LiAlH4 is introduced to a ketone or aldehyde, it generates an alkoxide intermediate, which, upon protonation usually, via an acid work-up (such as adding water or an acid), becomes an alcohol. The ability of LiAlH4 to deliver a strong hydride species makes it a versatile and powerful reducing agent in organic synthesis.

Other reagents listed, like hydrogen peroxide, potassium permanganate, and chromium trioxide, are typically used for oxidation processes or have more specific roles that do not involve direct reduction of ketones or aldehydes to alcohols. Therefore, they are not suitable choices for this reduction reaction.