Mr Cole Chemistry

Forming and Testing for Carbonyls Questions

Forming and Testing for Carbonyls

Carbonyls Worksheet

1. Preparation of Propanone

Propanone can be formed from an alcohol.
a) Write the equation for the formation of propanone from an alcohol. Use [O] for the oxidising agent.

CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O

b) State the reagents and conditions for the formation of propanone.

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

Conditions: Heat under reflux.

c) Write the half equation for the oxidation of the alcohol.

CH3CH(OH)CH3 → CH3COCH3 + 2H+ + 2e–

d) Write the half equation for the reduction of the oxidising agent.

Cr2O72- + 14H+ + 6e– → 2Cr3+ + 7H2O

e) Combine the half equations to form the overall equation.

Cr2O72- + 8H+ + 3CH3CH(OH)CH3 → 2Cr3+ + 7H2O + 3CH3COCH3

f) State the oxidation number on the carbon in the alcohol and in propanone.

Carbon in Propan-2-ol: 0

Carbon in Propanone: +2

2. Distinguishing Aldehydes & Ketones

A chemist has a chemical. It is either butanal or butanone, but they aren’t sure which it is.
a) Draw butanal and butanone.
Structures of Butanal and Butanone

b) They conduct a series of tests to determine whether it is butanal or butanone. Copy and complete the table below for the expected results for each chemical.

Test Colour change with butanal Colour change with butanone
Tollens’ Reagent
Fehling’s solution
Acidified dichromate
Bromine water
Ethanoic acid and conc sulfuric acid
Test Colour change with butanal Colour change with butanone
Tollens’ Reagent Silver mirror forms No change
Fehling’s solution Brick red precipitate No change
Acidified dichromate Orange → Green Stays orange
Bromine water Remains orange Remains orange
Ethanoic acid and conc sulfuric acid No change No change

3. Tollens’ Reagent (Redox)

Tollen’s reagent can be used to distinguish between carbonyls.
a) Explain why Tollens’ reagent will only react with some carbonyl compounds.

Tollens’ reagent is a mild oxidising agent. It will only react with carbonyls that can be easily oxidised (Aldehydes). Ketones cannot be oxidised further without breaking carbon-carbon bonds, so they do not react.

b) Write the half equation for the oxidation when ethanal reacts with Tollens’ reagent.

Note: Reaction occurs in alkaline conditions. Oxygen is provided by OH–.

CH3CHO + 3OH– → CH3COO– + 2H2O + 2e–

c) Tollens’ reagent is Ag(NH₃)₂⁺. State the oxidation state of the silver ion.

+1

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

Ag(NH3)2+ + e– → Ag + 2NH3

e) Combine the half equations to form the overall equation for the reaction between ethanal and Tollens’ reagent.

CH3CHO + 3OH– + 2Ag(NH3)2+ → CH3COO– + 2Ag + 4NH3 + 2H2O

4. Acidified Dichromate

Acidified dichromate (K₂Cr₂O₇/H⁺) can also be used to distinguish between carbonyls.
a) Explain why K₂Cr₂O₇/H⁺ can be used to distinguish propanal from propanone.

Acidified dichromate is an oxidising agent. Propanal (aldehyde) can be oxidised to propanoic acid, causing a colour change. Propanone (ketone) cannot be oxidised, so there is no reaction.

b) Explain why K₂Cr₂O₇/H⁺ cannot be used to distinguish between 3-hydroxylpropanal and hydroxypropanone.

Both 3-hydroxypropanal and hydroxypropanone contain a primary alcohol group (-CH2OH). This primary alcohol group in both molecules can be oxidised by acidified dichromate. Therefore, both compounds would turn the solution from orange to green, meaning they cannot be distinguished by this test alone.

c) State why the colour changes during the reaction between propanal and acidified potassium dichromate.

The colour change is due to the reduction of Chromium. Chromium in the dichromate ion is in the +6 oxidation state (Orange), which is reduced to the +3 oxidation state (Green) during the reaction.

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