Qualitative Tests for Functional Groups

Introduction

• Qualitative tests are used to identify the presence or absence of specific functional groups in organic compounds. They do not provide information about the quantity of the functional group.
• The three functional groups covered in this section are:
  • Carbon-carbon double bonds (alkenes)
  • Hydroxyl groups (alcohols and phenols)
  • Carboxyl groups (carboxylic acids)

KEY TAKEAWAY: Qualitative tests provide a ‘yes’ or ‘no’ answer regarding the presence of a functional group, unlike quantitative tests which measure the amount.

1. Tests for Carbon-Carbon Double Bonds (Alkenes)

1.1. Bromine Water Test

• Reagent: Bromine water ($Br_2(aq)$), which is brown/orange in color.
• Procedure: Add bromine water to the unknown organic compound.
• Observation:
  • Positive result (alkene present): The brown/orange color of the bromine water disappears (decolorizes).
  • Negative result (alkene absent): The brown/orange color of the bromine water persists.

• Reaction: The bromine adds across the double bond in an addition reaction.

  $R-CH=CH-R’ + Br_2(aq) \rightarrow R-CHBr-CHBr-R’$

• Explanation: The reaction consumes the bromine, causing the color to disappear.

• Limitations: Other compounds (e.g., phenols) can also decolorize bromine water.

1.2. Baeyer’s Test (Potassium Permanganate Test)

• Reagent: Potassium permanganate ($KMnO_4(aq)$), which is purple in color.
• Procedure: Add potassium permanganate solution to the unknown organic compound.
• Observation:
  • Positive result (alkene present): The purple color of the potassium permanganate disappears, and a brown precipitate of manganese dioxide ($MnO_2(s)$) forms.
  • Negative result (alkene absent): The purple color of the potassium permanganate persists.

• Reaction: The alkene is oxidized by the potassium permanganate.

  $R-CH=CH-R’ + KMnO_4(aq) + H_2O \rightarrow R-CH(OH)-CH(OH)-R’ + MnO_2(s) + KOH$

• Explanation: The permanganate ion ($MnO_4^-$) is reduced to manganese dioxide ($MnO_2$), causing the color change.

• Notes: This test also works for other oxidizable functional groups, so it is not specific to alkenes.

EXAM TIP: When describing the bromine water test, explicitly state “the brown/orange colour of bromine water decolourises” for full marks.

2. Tests for Hydroxyl Groups (Alcohols and Phenols)

2.1. Reaction with Sodium Metal

• Reagent: Sodium metal ($Na(s)$).
• Procedure: Add a small piece of sodium metal to the unknown organic compound.
• Observation:
  • Positive result (alcohol or phenol present): Fizzing (evolution of hydrogen gas, $H_2(g)$) is observed.
  • Negative result (alcohol or phenol absent): No fizzing is observed.

• Reaction: The alcohol or phenol reacts with sodium metal to form an alkoxide or phenoxide and hydrogen gas.

  $2R-OH + 2Na(s) \rightarrow 2R-ONa + H_2(g)$ (Alcohol)

  $2Ar-OH + 2Na(s) \rightarrow 2Ar-ONa + H_2(g)$ (Phenol, where ‘Ar’ represents an aromatic ring)

• Explanation: The hydrogen gas produced causes the fizzing.

• Caution: This test can be dangerous due to the reactivity of sodium metal. Use small amounts and ensure proper safety precautions.
• Note: Water will also react with sodium, so the sample must be dry.

2.2. Reaction with Acidified Potassium Dichromate (Distinguishes between primary, secondary, and tertiary alcohols)

• Reagent: Acidified potassium dichromate ($K_2Cr_2O_7/H^+$, orange).
• Procedure: Add acidified potassium dichromate to the unknown alcohol and warm the mixture.
• Observation:
  • Primary Alcohol: Orange dichromate turns green. Aldehyde produced, which can be further oxidised to a carboxylic acid.
  • Secondary Alcohol: Orange dichromate turns green. Ketone produced.
  • Tertiary Alcohol: No color change (orange remains orange) as tertiary alcohols are not easily oxidized.
• Reaction: Oxidation of the alcohol by the dichromate ion.
  • Primary Alcohol: $RCH_2OH \xrightarrow{[O]} RCHO \xrightarrow{[O]} RCOOH$
  • Secondary Alcohol: $R_1CHOHR_2 \xrightarrow{[O]} R_1COR_2$
• Explanation: Dichromate ion ($Cr_2O_7^{2-}$) is reduced to $Cr^{3+}$ which is green.

COMMON MISTAKE: Saying “the solution turns green” without specifying what turns green (the dichromate).

3. Tests for Carboxyl Groups (Carboxylic Acids)

3.1. Reaction with Sodium Carbonate

• Reagent: Sodium carbonate ($Na_2CO_3(aq)$).
• Procedure: Add sodium carbonate solution to the unknown organic compound.
• Observation:
  • Positive result (carboxylic acid present): Fizzing (evolution of carbon dioxide gas, $CO_2(g)$) is observed.
  • Negative result (carboxylic acid absent): No fizzing is observed.

• Reaction: The carboxylic acid reacts with sodium carbonate to form a sodium carboxylate, water, and carbon dioxide.

  $2R-COOH + Na_2CO_3(aq) \rightarrow 2R-COONa(aq) + H_2O(l) + CO_2(g)$

• Explanation: The carbon dioxide gas produced causes the fizzing.

• Distinction: This test distinguishes carboxylic acids from phenols, as phenols are generally not acidic enough to react with sodium carbonate.

3.2. Litmus Paper Test

• Reagent: Blue litmus paper.
• Procedure: Dip blue litmus paper into a solution of the unknown organic compound.
• Observation:
  • Positive result (carboxylic acid present): Blue litmus paper turns red.
  • Negative result (carboxylic acid absent): Blue litmus paper remains blue.
• Explanation: Carboxylic acids are acidic and donate protons ($H^+$) to the litmus dye, causing the color change.

VCAA FOCUS: VCAA often requires students to design experimental procedures to identify unknown compounds using these qualitative tests. Be prepared to describe the reagents, procedure, observations, and expected results.

Summary Table

STUDY HINT: Create flashcards with each functional group, test, reagent, and expected observation. This will help you quickly recall the information during exams.

Additional Notes

• Safety Precautions: Always wear appropriate personal protective equipment (PPE) when performing chemical tests.
• Control Experiments: It is good practice to run control experiments with known compounds to ensure the reagents are working correctly and to have a basis for comparison.
• Interferences: Be aware of potential interferences from other functional groups or compounds that might give false positive results.

REMEMBER: BROMINE - Alkenes DECOLORIZE. SODIUM CARBONATE - Carboxylic Acids FIZZ.