The reduction of acetophenone (a ketone) using sodium borohydride (NaBH₄) is a classic example of nucleophilic addition to a carbonyl group. Here’s a step-by-step explanation of the mechanism:
Step 1: Formation of the Nucleophile
Sodium borohydride (NaBH₄) contains borohydride ions (BH₄⁻), which act as a source of hydride (H⁻). In the presence of acetophenone, one of the hydride ions is made available for the reaction.
Step 2: Nucleophilic Attack
Acetophenone has a carbonyl group (C=O) that is electrophilic due to the polarization of the double bond. The oxygen atom is more electronegative than carbon, making the carbon atom partially positively charged and susceptible to nucleophilic attack.
- The borohydride ion (BH₄⁻) donates a hydride ion (H⁻) to the electrophilic carbonyl carbon in acetophenone.
- This results in the formation of a tetrahedral alkoxide intermediate.
Step 3: Tetrahedral Intermediate
When the hydride ion attacks the carbonyl carbon, the double bond between carbon and oxygen breaks, leading to the formation of a tetrahedral carbon species:
- The carbon atom becomes a sp³ hybridized center with four substituents: one hydride from NaBH₄, one phenyl group from acetophenone, one methyl group (from the acetophenone), and the negatively charged alkoxide oxygen.
Step 4: Protonation
The alkoxide formed in the previous step is typically then protonated to give the final alcohol product. This can occur via:
- Addition of water (if the reaction is conducted in aqueous solution) or simply any protic solvent present.
- The oxygen atom in the alkoxide intermediate can abstract a proton (H⁺) from water, forming an alcohol.
Final Product
The product of this reaction is 1-phenylethanol (or α-phenylethanol) along with sodium borate as a by-product (if the NaBH₄ is consumed completely).
Summary
Overall, the reduction of acetophenone with sodium borohydride involves the nucleophilic attack of a hydride ion on the carbonyl carbon, formation of a tetrahedral alkoxide intermediate, and final protonation to yield an alcohol. This process is a key transformation in organic synthesis for converting ketones to alcohols.