Regioselectivity of Hydride Reduction

The carbon-carbon double bond of alkenes is not polar therefore, alkenes do not react with nucleophiles. This difference in reactivity accounts for the regioselectivity of sodium borohydride or lithium aluminum hydride reduction. Neither reagent reduces alkenes both reduce aldehydes and ketones. 

In the reduction of an aldehyde by sodium borohydride in ethanol, a hydride is transferred to the carbonyl carbon atom, and the carbonyl oxygen atom is protonated by ethanol. 

The ethoxyborohydride product in the above reaction has three remaining hydride ions available for further reduction reactions, and the ultimate boron product is tetraethoxyborohydride, (RO) B . Thus, one mole of NaBH reduces four moles of a carbonyl compound. A dilute solution of acid is used to destroy any excess reagent as part of the standard work-up procedure. 

When hthium aluminum hydride is used to reduce carbonyl compounds, an ether, such as diethyl ether, (CH CH ljO, is used as the solvent. The reduction of a carbonyl group by lithium aluminum hydride occurs by transfer of a hydride anion from A1H to the carbonyl carbon atom. The carbonyl oxygen atom forms an alkoxyaluminate salt. 

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