Jacaranda 9.2 – Laboratory analysis: Tests for C=C, -OH and -COOH functional groups

Learning Intention

• To learn about the different laboratory tests used to detect key functional groups in organic compounds:

  • Carbon–carbon double bonds (C=C)

  • Hydroxyl groups (-OH)

  • Carboxyl groups (-COOH)

• Emphasis on how these tests inform the chemical and physical properties of substances, including polarity and intermolecular forces.

The Study Design

• Focus: qualitative tests for the presence of C=C, -OH, and -COOH functional groups.

• Applications and principles include:

  • Verifying components and purity of consumer products.

  • Melting point determination and distillation (simple and fractional).

  • Measurement of the degree of unsaturation using the iodine number.

  • Redox titrations for quantitative analysis (including calculations of excess and limiting reactants; back titrations excluded).

Introduction

• Targeted functional groups:

  • Carbon–carbon double bonds, C=C

  • Hydroxyl groups, R–OH

  • Carboxyl groups, R–COOH

• Relevance:

  • These groups determine polarity and intermolecular forces.

  • Detection helps predict chemical and physical properties of the substance.

Detection of carbon-to-carbon double bonds

The bromine water test

• Purpose: Qualitative determination of degree of unsaturation (presence of C=C).

• Mechanism: Bromine (Br2) reacts across C=C bonds, decolorizing the Br2 solution.

• Colour change: Br2 solution is orange-brown; it fades and can become colourless if Br2 is consumed by reaction with an unsaturated compound.

• Interpretation: Decolorization indicates the presence of C=C bonds (unsaturation).

• Quantitative hint: The test can be used to estimate the iodine number via stoichiometry (next section).

The potassium permanganate test (KMnO4)

• Reagent: Acidified KMnO4 (aq) tests for the presence of double or triple bonds.

• Reaction with unsaturated compounds:

  • Addition of acidified KMnO4 reduces purple ext{MnO}_4^- to colorless solution.

• Alternative (alkaline KMnO4): Can be reduced to a dark green solution with a brown precipitate of ext{MnO}_2 .

• Interpretation: Decolorization/green-to-brown changes indicate unsaturation (C=C or C≡C).

The Iodine number test

• Purpose: Quantitative determination of unsaturation by measuring how much iodine reacts with 100 g of substance.

• Concept: Iodine ext{I}_2 adds across C=C bonds via an addition reaction. Stoichiometry: 1 mole of ext{I}_2 reacts with 1 mole of C=C.

• Higher iodine number → greater degree of unsaturation (more C=C bonds).

• Applications: Determine whether fats/oats/oils are saturated, monounsaturated, or polyunsaturated.

Calculate the Iodine number

• Key idea: 100 g of a compound reacts completely with ext{I}_2 .

• How to determine the iodine number:

  • Measure mass of iodine that reacted, ext{m(I}_2) .

  • Iodine number, I.V. = (mass of ext{I}_2 that reacted in grams) × 100 / (mass of sample in grams).

• General reaction:
  ext{I}_2 + ext{C=C} \rightarrow ext{diiodinated product}

• Example workflow (qualitative data to a numeric value):

  • Let ext{m(sample)} be the sample mass used (often 100 g for the standard method).

  • Determine ext{m(I}_2) that has reacted from the mass gain or titration/detection method.

  • Compute ext{I.V.} = \frac{ ext{m(I}_2)}{ ext{m}_{\text{sample}}} \times 100

• Example interpretation steps (as shown in the transcript):

  • Iodine adds across all C=C sites in the sample; the more C=C bonds, the more ext{I}_2 is consumed, hence a higher I.V.

  • If a sample of fat is used, determine the mass change due to iodine addition and apply the I.V. formula to gauge saturation vs unsaturation.

Practice: Iodine number example structure (from transcript guidance)

• Step 1: Assume 100 g of a fat reacts completely with ext{I}_2 . Determine the mass of ext{I}_2 that reacted, ext{m(I}_2) .

• Step 2: Calculate ext{n(I}_2) = \text{m(I}_2) / \text{M(I}_2) , with \text{M(I}_2) = 253.8 \text{ g/mol} .

• Step 3: Because each C=C reacts with 1 mole of ext{I}_2 , set ext{n(C=C)} = ext{n(I}_2) .

• Step 4: Determine ext{n(fat)} = \text{m(sample)} / \text{M(fat)} using the given molar mass ext{M(fat)} .

• Step 5: Compute the total number of C=C bonds per fat molecule: Number of C=C per fat molecule = ext{n(C=C)} / ext{n(fat)} .

• Step 6: Interpret results: higher value indicates greater unsaturation per fat molecule.

Practice Problem 1 (Iodine number exercise)

• Task: A 100 g sample of a fat reacts with iodine, and the final mass after reaction is 200 g. ext{M(fat)} = 254.0 \text{ g/mol} .

• Questions:

  • a) Calculate the iodine number of the fat.

  • b) Calculate the amount of ext{I}_2 (in mol) that reacted with the fat.

  • c) Calculate the amount (in mol) of C=C present in the fat sample.

  • d) Calculate the amount (in mol) of fat in the 100 g sample.

  • e) Determine the number of C=C present (degree of unsaturation) in each fat molecule.

Detection of hydroxyl groups

The sodium metal test (hydrogen pop test)

• Purpose: Qualitative detection of hydroxyl (-OH) groups (often used for alcohols).

• Reaction: Alcohols react with active metals (e.g., Na) to form an ionic salt and hydrogen gas:
  2 \text{CH}3\text{OH (l)} + 2 \text{Na (s)} \rightarrow 2 \text{CH}3\text{ONa (l)} + \text{H}_2 \text{(g)}

• Notes:

  • Alcohols are typically liquids (not aqueous) in these tests.

  • Procedure: add a piece of Na metal to a sample in a test tube; observe hydrogen gas bubbles and a pop when a lit taper is introduced near the mouth.

  • Safety: Na is highly reactive; perform under proper supervision and with appropriate safety protocols.

The Lucas test

• Purpose: Qualitative differentiation of primary, secondary, and tertiary alcohols.

• Reagents: Concentrated HCl with ext{ZnCl}_2 catalyst (Lucas reagent).

• Observation: Presence of an alcohol generally yields a cloudy liquid; the rate and persistence help distinguish substitution class:

  • Primary alcohols: slower cloudy formation

  • Secondary alcohols: quicker cloudiness

  • Tertiary alcohols: immediate/rapid cloudiness

The oxidation test

• Purpose: Determine the class of alcohol after initial detection of -OH.

• Reagents: Acidified dichromate ( ext{Cr}2\text{O}7^{2-} ) or permanganate ( ext{MnO}4^- ) in acidic medium ( ext{H}2\text{SO}_4 ).

• Color changes (typical with ext{Cr}2\text{O}7^{2-} ):

  • Primary or secondary alcohols under reflux: ext{Cr}2\text{O}7^{2-} (orange) → Green ( ext{Cr}^{3+} )

  • Tertiary alcohols: ext{Cr}2\text{O}7^{2-} remains orange (no reaction)

• Color changes (typical with ext{MnO}_4^- ):

  • Primary or secondary alcohols: ext{MnO}_4^- (pink) → colorless ( ext{Mn}^{2+} )

  • Tertiary alcohols: ext{MnO}_4^- pink persists (no reaction)

• Note: Reflux is used to apply heat over an extended period for the reaction.

• Interpretation: Green color indicates oxidation of primary/secondary alcohols; persistence of orange or pink indicates tertiary alcohols or no reaction, respectively.

Esterification context for alcohol detection

• Alcohols can be identified via esterification (with a carboxylic acid) using concentrated sulfuric acid as a catalyst.

• Procedure: add an alcohol and a carboxylic acid to a tube with concentrated ext{H}2\text{SO}4 and heat.

• Observation: Fruity smell indicates ester formation, confirming the presence of an alcohol reacting with a carboxylic acid.

Esterification role in alcohol testing (summary)

• Esterification tests can help confirm alcohol presence by producing esters with carboxylic acids (fruit-like odors).

• This test is often discussed alongside the dehydration esterification reaction:
  \text{Alcohol} + \text{Carboxylic acid} \rightarrow \text{Ester} + \text{H}_2\text{O}

Detection of carboxyl groups

Carboxylic acids as weak acids

• Carboxylic acids ionize in aqueous solutions to release ext{H}^+ ions, lowering pH.

• Detection methods include:

  • pH measurement using indicators or pH probes.

  • Hydrogen carbonate limewater tests to confirm ext{CO}_2 evolution.

The hydrogen carbonate test (carbonate test)

• Procedure: add powdered ext{NaHCO}_3 (or ext{Na}_2\text{CO}_3 ) to a sample containing a carboxylic acid.

• Observation: production of carbon dioxide gas ( ext{CO}_2 ) bubbles.

• Confirmation: ext{CO}_2 will turn limewater ( ext{Ca(OH)}_2 ) cloudy due to the formation of calcium carbonate ( ext{CaCO}_3 ).

• Other relevant reactants: metal hydrogen carbonates and metal carbonates can also react to produce ext{CO}_2 .

The esterification test (carboxyl group detection context)

• Carboxylic acids can be detected by reacting with an alcohol in the presence of a concentrated sulfuric acid catalyst to produce an ester, which has a characteristic fruity smell.

• This test parallels the alcohol detection test, reinforcing the presence of -COOH groups when ester formation occurs.

Observations summary for carboxyl detection

• Acidic pH indicates presence of carboxyl group due to acidity of carboxylic acids.

• Hydrogen carbonate reaction with ext{CO}_2 confirms carboxyl groups via gas evolution and limewater cloudiness.

• Fruity smell from esterification supports the presence of a carboxyl-containing compound that reacts with an alcohol to form an ester.

Summary of tests and observations (as in the endpoint table)

• Bromine water test (C=C detection):

  • Unsaturated: ext{Br}_2 color fades to colorless; saturation: color persists as orange-brown.

• Sodium metal test (OH detection):

  • Hydrogen gas evolution indicates presence of an -OH group (alcohol) or carboxyl group (both can react with Na).

• Oxidation tests ( ext{Cr}2\text{O}7^{2-} or ext{MnO}_4^- ) for alcohols:

  • Primary/secondary alcohols: ext{Cr(VI)} reduction to ext{Cr(III)} (orange ext{Cr}2\text{O}7^{2-} → green ext{Cr}^{3+} ); ext{MnO}4^- reduction (pink ext{MnO}4^- → colorless ext{Mn}^{2+} ).

  • Tertiary alcohols: little to no change (orange persists for ext{Cr}2\text{O}7^{2-} ; ext{MnO}_4^- pink persists).

• Esterification test (presence of alcohol or carboxyl):

  • Fruity smell indicates ester formation, confirming alcohol and carboxyl group interaction.

• Carboxyl tests:

  • pH measurement shows acidic solution for carboxyl-containing compounds.

  • Hydrogen carbonate test produces ext{CO}_2 ; limewater turns cloudy.

  • Esterification can confirm carboxyl presence via ester formation.

• Observations across tests help identify:

  • C=C presence (bromine decolorization, ext{KMnO}_4 reaction)

  • -OH presence (sodium metal, Lucas, oxidation tests)

  • -COOH presence (acidic pH, bicarbonate/limewater tests, esterification outcomes)

Sample Problem 2 (interpretation exercise)

• Situation: Unknown organic compound is a hydrocarbon, alcohol, or carboxylic acid; tests performed:
  1) Bromine water: orange-brown colour disappeared (decolorized).
  2) pH probe: pH = 4.
  3) Sodium metal: hydrogen gas bubbles observed.

• THINK (interpretation):

  • a) Decolorization of bromine water indicates unsaturation (C=C) present.

  • b) Hydrogen gas with sodium metal can indicate hydroxyl-containing compounds (alcohols) but carboxylates may also respond similarly under some conditions.

  • c) pH 4 indicates acidity consistent with a carboxyl group (carboxylic acids are weak acids and lower the pH).

  • d) The molecule likely contains a carboxyl group and a C=C bond (unsaturation) and may also contain an -OH group (if alcohol present).

• WRITE (concise conclusions):

  • a) The unknown is unsaturated (due to ext{Br}_2 decolorization).

  • b) Tests consistent with hydroxyl group: sodium metal test ( ext{H}_2 gas).

  • c) Acidic pH (pH = 4) is consistent with a carboxyl group.

  • d) Functional groups present: C=C (unsaturation) and -COOH (carboxyl), with potential -OH contribution.

Practice Problem 2 (interpretation)

• Observations:
  1) Bromine water: orange-brown colour persisted (no decolorization).
  2) Acidified dichromate oxidation: orange colour persisted (no oxidation).
  3) Reaction with an alcohol in the presence of concentrated sulfuric acid: fruity smell was not detected.
  4) Reaction with a carboxylic acid: fruity smell produced.

• Questions and answers:

  • a) Saturated vs unsaturated: The orange-brown ext{Br}_2 colour persisted, so the substance is saturated.

  • b) Tests showing carboxyl group: test 4 (reaction with carboxylic acid) indicates carboxyl presence; acidic behavior could be inferred if pH were measured, but here the fruity smell confirms esterification context.

  • c) Test observation that could only be produced by an alcohol: the fruity smell from ester formation (test 3 would yield such a smell if an alcohol reacted with a carboxyl group; in this case, no fruity smell was detected, so alcohol presence is unlikely).

  • d) Functional groups present: carboxyl group (–COOH) is present (test 4), and the lack of unsaturation and lack of alcohol-reactivity in test 3 reduces likelihood of an alcohol; overall, the data point to a carboxylic acid without an alcohol or with limited reactive alcohol content in the setup.

Conclusion

• This section highlighted the main laboratory tests for detecting C=C, -OH, and -COOH functional groups, including qualitative and quantitative approaches.

• You should now be able to:

  • Identify the different tests used to detect C=C, -OH, and -COOH groups.

  • Understand how these tests are conducted in the laboratory.

  • Use qualitative test data to identify unknown organic compounds.

• Homework: Complete Jacaranda 9.2 questions and seek help if stuck.

Connections to foundational principles and real-world relevance

• These tests illustrate core organic qualitative analysis, linking molecular structure (functional groups) to observable properties (color changes, gas evolution, smells).

• Understanding degree of unsaturation informs properties such as melting/boiling behavior, reactivity, and nutritional aspects for fats/oils.

• Practical skills developed include acid–base concepts (pH changes), redox chemistry ( ext{Cr(VI)} and ext{MnO}_4^- tests), and reaction mechanisms (esterification).

Safety and ethics in the laboratory (practical implications)

• Handle reactive metals (e.g., sodium) with care; they react vigorously with water and air; use appropriate PPE and work in a suitable fume hood.

• Work with concentrated acids ( ext{H}2\text{SO}4 ) and oxidizing agents ( ext{Cr}2\text{O}7^{2-} , ext{MnO}_4^- ) under supervision and with proper waste disposal protocols.

• Properly label and dispose of chemical wastes, especially organic/inorganic oxidizers and halogen-containing reagents.

• Document observations accurately and cite uncertainties in qualitative assessments.

Key formulas and equations to remember

• Iodine addition across C=C (qualitative):
  \text{I}_2 + \text{C=C} \rightarrow \text{diiodinated product}

• Iodine number (I.V.) definition (per 100 g sample):
  \text{I.V.} = \frac{\text{m}(\text{I}2 \text{ reacted})}{\text{m}{\text{sample}}} \times 100

• Molar relation for \text{I}2 in the iodine test: \text{n}(\text{I}2) = \frac{\text{m}(\text{I}2)}{\text{M}(\text{I}2)} \text{ with } \text{M}(\text{I}_2) = 253.8 \text{ g/mol}

• Alcohol oxidation (general):

  • With \text{Cr}2\text{O}7^{2-} : orange \text{Cr}2\text{O}7^{2-} → green \text{Cr}^{3+} for oxidizable primary/secondary alcohols.

  • With \text{MnO}4^- : pink \text{MnO}4^- → colorless \text{Mn}^{2+} for oxidizable primary/secondary alcohols.

• Esterification (condensation):
  \text{Alcohol} + \text{Carboxylic acid} \rightarrow \text{Ester} + \text{H}_2\text{O}