Describe the method to identify the alkene functional group in an organic compound.

Add bromine water dropwise to the organic compound.

In the presence of an alkene (C=C) group, orange bromine water is decolourised.

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Describe the method to identify a halogenoalkane using silver nitrate.

  • Add silver nitrate solution and ethanol to the halogenoalkane and warm in a water bath.
  • In the presence of a chloroalkane, a white precicipitate of silver chloride is formed.
  • In the presence of a bromoalkane, a cream precicipitate of silver bromide is formed.
  • In the presence of an iodoalkane, a yellow precicipitate of silver iodide is formed.

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Describe the method to identify the carbonyl functional group in an organic compound.

Add 2,4-dinitrophenylhydrazine (2,4-DNPH) to the organic compound.

In the presence of a carbonyl (C=O) group, an orange precipitate is produced.

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Describe and explain the method to identify a phenol functional group in an organic compound.

  1. Add solid sodium hydroxide to the organic compound - the formation of a colourless solution suggests the compound could be a phenol. 
  2. Next, add solid sodium carbonate - the absence of effervescence confirms the compound is a phenol. 


Phenols are weak acids but, unlike carboxylic acids, they are not acidic enough to react with sodium carbonate.

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Describe the method to identify a carboxylic acid functional group in an organic compound.

Add a metal carbonate such as sodium carbonate to the organic compound.

In the presence of the carboxylic acid (-COOH) group, carbon dioxide gas is evolved which turns limewater cloudy.

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Explain how you would use Tollens' reagent to distinguish between an aldehyde and a ketone.

Warm the compound with Tollens' reagent, [Ag(NH3)2]+(aq).

Aldehydes are oxidised to carboxylic acids, causing a silver mirror to form on the inside of the test tube due to the formation of Ag(s).

Ketones do not react so the solution remains colourless.

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Explain how you would use Fehling’s reagent to distinguish between an aldehyde and a ketone.

Warm the organic compound with Fehling's solution (containing blue Cu2+ ions).

Aldehydes are oxidised to carboxylic acids, causing a brick red precipitate to form due to the formation of Cu2O(s). Ketones do not react so the solution remains blue.

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What classes of alcohols (primary, secondary or tertiary) can be identified using acidified potassium dichromate?

Primary and secondary alcohols

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Describe the method to identify a primary or secondary alcohol using potassium dichromate.

Heat the alcohol in a solution of acidified potassium dichromate.

In the presence of a primary or secondary alcohol, the orange solution turns green. 

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Name three tests that can confirm the presence of an aldehyde.

Three tests that confirm the presence of an aldehyde are:

  1. Tollens' reagent.
  2. Fehling's solution.
  3. Acidified potassium dichromate.

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Describe and explain the test for the presence of a CH3CH(OH) group in an alcohol.

Add an alkaline solution of iodine to the sample. In the presence of the CH3CH(OH) group, a yellow precipitate of triiodomethane (CHI3) will form.

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