Question #44499

1 Answer
Ernest Z.
Nov 18, 2015

Alcohols are not oxidized by Benedict's solution because it is not a strong enough oxidizing agent.

Explanation:

The oxidizing agent is a citrate-stabilized complex of #"Cu"^(2+)#.

It is reduced to a precipitate of copper(I) oxide, #"Cu"_2"O"# when it acts as an oxidizing agent.

The standard reduction potentials in acid are:

#"acetate + 2H"^+ + 2e^- → "acetaldehyde; -0.6 V"#

#"acetaldehyde + 2H"^+ + 2e^- → "ethanol; -0.2 V"#

The potentials would be different in base, but they show that it is easier to oxidize an aldehyde to a carboxylic acid than it is to oxidize an alcohol to an aldehyde.

The standard reduction potential for

#"2Cu"^(2+)("stabilized") + 2e^(-) → "Cu"_2"O"#

in basic solution must lie between the values for aldehydes and alcohols.

That would explain why Benedict's solution can oxidize aldehydes but not alcohols.

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