Mr Cole Chemistry

Oxidation and Reduction of Carbonyls Questions

Oxidation & Reduction of Carbonyls

Carbonyls Worksheet

1. Identifying Reactions

Consider the following molecules: CH₃COCH₃, CH₃CH₂CHO and (CH₃)₂CHCOCH₃.
a) Draw the skeletal formula for all three molecules and name them.
Skeletal formulas of Propanone, Propanal, and 3-methylbutanone

Left: Propanone

Middle: Propanal

Right: 3-methylbutanone

b) State the name(s) of the molecule or molecules above which can be oxidised to form a carboxylic acid.

Propanal (The middle one).

Aldehydes can be oxidised to carboxylic acids, but ketones cannot (without breaking carbon chains).

c) State the name(s) of the molecule or molecules above which can be reduced to form a primary alcohol.

Propanal.

Reduction of an aldehyde produces a primary alcohol.

d) State the name(s) of the molecule or molecules above which can be reduced to form a secondary alcohol.

Propanone and 3-methylbutanone.

Reduction of a ketone produces a secondary alcohol.

2. Oxidation of Butanal

Butanal can be oxidised to form butanoic acid.
a) Write an equation for the oxidation of butanal. Use [O] to represent the oxidising agent.

C₃H₇CHO + [O] → C₃H₇COOH

b) State the oxidation number of the end carbon in butanal and in butanoic acid to show that it is has been oxidised.

Butanal: +1 (Carbon bonds to: H (-1), O (-2), C (0). Net = +1)

Butanoic Acid: +3 (Carbon bonds to: O (-2), OH (-1), C (0). Net = +3)

The oxidation number increases by 2, showing oxidation.

c) State the reagents and conditions required for the oxidation of butanal.

Potassium dichromate (K₂Cr₂O₇) and concentrated sulfuric acid (H₂SO₄).

Heat under reflux.

d) State the colour change for the reaction.

Orange to Green.

e) Tollens’ reagent can be used to test for the presence of any butanal left over. Write the half equation for the oxidation of butanal in alkaline conditions.

C₃H₇CHO + 3OH⁻ → C₃H₇COO⁻ + 2H₂O + 2e⁻

f) Write the half equation for the reduction of Tollens’ reagent.

Ag(NH₃)₂⁺ + e⁻ → Ag + 2NH₃

g) Write the equation for the reaction between Tollens’ reagent and butanal.

C₃H₇CHO + 3OH⁻ + 2Ag(NH₃)₂⁺ → C₃H₇COO⁻ + 2Ag + 4NH₃ + 2H₂O

3. Reduction of Butanal

Butanal can be reduced to form an alcohol.
a) State the name of the alcohol that forms from the reduction of butanal.

Butan-1-ol

b) State the reagents and conditions for the reduction of butanal.

Sodium Borohydride (NaBH₄) dissolved in aqueous ethanol (or water).

c) Outline the mechanism for the reduction of butanal.

Nucleophilic Addition

Mechanism using skeletal formula

OR

Mechanism using displayed formula
d) Explain why butanal can be attacked by a nucleophile.

The C=O bond is polar due to oxygen being more electronegative than carbon. This makes the carbonyl carbon electron deficient (partially positive / δ+), attracting electron pairs from nucleophiles.

e) Explain why a hydride ion is a good nucleophile.

NaBH₄ provides hydride (H⁻) ions. The bond between Boron and Hydrogen is weak and polar. The hydride ion has a lone pair of electrons and a negative charge, making it a strong electron pair donor (nucleophile).

f) Explain why the pi bond is broken in the first step.

The pi (π) bond contains electrons that are in higher energy orbitals and are further from the nuclei than sigma electrons. Therefore, the pi bond is weaker and requires less energy to break than the sigma bond.

g) Explain why an acid is sometimes added following the reaction.

The initial reaction produces an alkoxide intermediate (R-CH₂O⁻). This ion must be protonated (accept a H⁺) to form the final alcohol group (-OH). While water can provide the protons, adding a dilute acid provides abundant H⁺ ions to ensure the reaction completes quickly.

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