We are performing a reduction of 3-nitroacetophenone with sodium borohydride

a) We are performing a reduction of 3-nitroacetophenone with sodium borohydride, write the reaction mechanism(where you show the movement of the electrons)

b) When butanoic acid is treated with NaBH4, 1-butanol is NOT formed, but you can observe that something is happening. Which reaction can it be?

The Correct Answer and Explanation is :

a) Reduction of 3-nitroacetophenone with Sodium Borohydride

The reduction mechanism involves the transfer of a hydride ion ((H^-)) from sodium borohydride ((NaBH_4)) to the carbonyl group of 3-nitroacetophenone, reducing the ketone to a secondary alcohol. The presence of the nitro group does not interfere with the reduction of the ketone under these mild conditions.

Steps of the Mechanism:

1. Hydride Attack on the Carbonyl Carbon:

• A hydride ion ((H^-)) from (NaBH_4) attacks the electrophilic carbonyl carbon in 3-nitroacetophenone.
• The electrons in the carbon-oxygen (\pi)-bond are pushed onto the oxygen atom, forming an alkoxide intermediate. Movement of electrons:
  [
  \text{H:}^- + \text{R-C(=O)-R’} \rightarrow \text{R-CHO(-)-R’}
  ]

1. Protonation of the Alkoxide:

• The alkoxide ion is protonated by water or an alcohol solvent, leading to the formation of the secondary alcohol, 3-nitro-1-phenylethanol. Final Product:
  [
  \text{R-CHO(-)-R’} + \text{H}_2\text{O} \rightarrow \text{R-CH(OH)-R’} + \text{OH}^-
  ]

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b) Reaction of Butanoic Acid with Sodium Borohydride

When butanoic acid ((CH_3CH_2CH_2COOH)) is treated with (NaBH_4), no reduction to 1-butanol occurs because (NaBH_4) is not strong enough to reduce carboxylic acids. Instead, a different reaction takes place: the acid-base reaction between the carboxylic acid and (NaBH_4).

Explanation:

1. Reaction Details:

• Carboxylic acids are acidic and react with (NaBH_4), which is a mild base. This leads to the formation of the sodium salt of the acid and hydrogen gas ((H_2)).
  [
  CH_3CH_2CH_2COOH + NaBH_4 \rightarrow CH_3CH_2CH_2COONa + H_2
  ]

1. Why Reduction Does Not Occur:

• Sodium borohydride is selective and can reduce ketones and aldehydes but lacks the reducing power needed to break the stable (-COOH) bond in carboxylic acids. Stronger reducing agents like lithium aluminum hydride ((LiAlH_4)) are required to reduce carboxylic acids to alcohols.

1. Observable Evidence:

• The reaction generates hydrogen gas, which is visible as effervescence. This confirms that a reaction is occurring, even though the carboxylic acid is not reduced to 1-butanol.

Summary:

The reaction between butanoic acid and (NaBH_4) is an acid-base reaction, resulting in the formation of sodium butanoate and hydrogen gas, not a reduction to alcohol. This highlights the selective nature of (NaBH_4).