Identification of functional groups by test-tube reactions

What are test tube reactions used to do?

Identify the functional group present in an organic compound

To test for alkenes is to test for un…?

Unsaturation

What is the function group for alkenes?

C=C

How do you test for alkenes?

By shaking the sample with bromine water

What is the positive result for testing for alkenes?

Sample turns from orange to colourless

What is the functional group for halogenolkanes?

R—X

• R = carbon chain

• X = halogen

What three reagents are used to test for halegenoalkanes?

Sodium hydroxide (NaOH), nitric acid (HNO₃) and silver nitrate (AgNO₃)

How do you test for halogenoalkanes?

• Add aqueous sodium hydroxide to the sample and heat gently.

• Acidify with nitric acid then add silver nitrate

What is the expected result when testing for halogenoalkanes?

Coloured precipitates of AgX will form

What are the three AgX precipitates which can form?

Silver precipitates made of chlorine, bromine and iodine:

• AgCl which forms a white precipitate

• AgBr which forms a cream precipitate

• AgI which forms a yellow precipitate

What is the functional group for alcohols?

R—OH

What reagent is used for testing for alcohols?

Acidified potassium dichromate (K₂Cr₂O₇)

How do you test for alcohols?

By adding acidified potassium dichromate solution to the sample

What is the positive result for testing for primary and secondary alcohols?

The sample turns from orange to green

What is the positive result for testing for tertiary alcohols?

There is no colour change as there is no reaction

What is the functional group for aldehydes?

R—CHO

What are two ways of testing for aldehydes?

By warming the sample with Tollen's reagent or Fehling's solution

What is the positive result for testing for aldehydes with Tollen's reagent?

Silver mirror forms

What is the positive result for testing for aldehydes with Fehling's solution?

The blue solution turns into a red precipitate

What is the functional group for carboxylic acids?

R—COOH

What reagent is used to test for carboxylic acids?

Sodium hydrogen carbonate (NaHCO₃)

How do you test for carboxylic acids?

By adding sodium hydrogen carbonate to the sample

What is the positive result for testing for carboxylic acids?

CO₂ gas is formed