organic chemo lab 1- (copy)

Identification of Carboxylic Acids

• Presenter: Dr. Noor Al-Nabi Muhammad

Overview of Carboxylic Acids

• Carboxylic acids are organic compounds characterized by the presence of a carboxyl group.

  • Structure: RCOOH (alkyl group) or ArCOOH (aryl group).

  • Example: If R is hydrogen, the compound is formic acid.

• Types of Carboxylic Acids:

  • Mono carboxylated, multi carboxylated.

  • Substituted carboxylic acids (e.g., with hydroxyl groups).

  • Aromatic carboxylic acids.

Structure of Carboxylic Acids

• Common Structures:

  • Formic acid: HCOOH

  • Lactic acid: C3H6O3

  • Oxalic acid: C2H2O4

  • Tartaric acid: C4H6O6

  • Acetic acid: C2H4O2

  • Succinic acid: C4H6O4

  • Benzoic acid: C7H6O2

  • Salicylic acid: C7H6O3

  • Citric acid: C6H8O7

Physical Properties

• States at Room Temperature:

  • Formic acid, acetic acid, lactic acid are liquids; others are solids.

• Solubility:

  • Low molecular weight carboxylic acids are water-soluble (Classes S1 or S2).

  • Water-insoluble acids dissolve in sodium hydroxide or sodium bicarbonate, classified under Class A1.

  • Reaction with sodium bicarbonate releases carbon dioxide gas, indicating the presence of carboxylic acid.

• Boiling Points: High due to hydrogen bonding between molecules.

• Flame Colors: Aromatic carboxylic acids produce a yellow smoky flame; aliphatic acids burn with a blue flame without smoke.

Chemical Properties

• The acids' properties are mainly due to the proton of the carboxyl group.

• Mono carboxylic acids are generally weak acids; formic acid is the strongest.

• Influence of Alkyl Groups: An electron-releasing alkyl group weakens acidity; electron-withdrawing groups (like halogens) enhance acidity.

Reactions of Carboxylic Acids

• Related to:

  • Proton involvement in salt formation reactions.

  • Hydroxyl group removal for derivatives: esters, amides, acid chlorides.

  • Substitution at the alpha position (aliphatic) or meta position (aromatic).

Ferric Chloride Test

• Preparation: Neutralize acid solution using dilute ammonia and bromothymol blue indicator to achieve a basic solution.

• Procedure:

  • Heat gently to eliminate ammonia; test with ferric chloride.

  • Observe the color of solutions or precipitates for identification:

    • Formic/Acetic Acid: Red solution.

    • Succinic/Benzoic Acid: Light brown precipitate.

    • Salicylic: Violet solution.

    • Oxalic/Tartaric/Citric/Lactic: No special change.

Importance of Neutral pH

• Basic Solutions: Excess ammonia causes false results with ferric hydroxide precipitate.

• Acidic Solutions: Little ammonia leads to colorless complexes, resulting in false negatives.

Differentiating Light Brown Precipitates

• To distinguish between succinic acid (soluble) and benzoic acid (insoluble):

  • Add dilute sulfuric acid to light brown precipitate to liberate carboxylic acids.

Special Tests for Formic Acid

• Reducing Mercuric Chloride: Formic acid reduces mercuric chloride to mercurous chloride (white precipitate), further reducing it to elemental mercury (gray precipitate) when heated with excess acid.

  • Procedure: Add mercuric chloride to formic acid and heat for precipitate formation.

Additional Tests for Formic Acid**

• Tollen's Test: Refer to identification procedures for aldehydes and ketones.

• Potassium Permanganate Reaction: Formic acid decolorizes violet KMnO4 in alkaline medium, showing characteristic brown manganese dioxide precipitate.

Special Tests for Acetic Acid

• Ester Formation: Acetic acid undergoes this reaction:

  • Mix acetic acid with ethanol and concentrated sulfuric acid, heat, then add sodium bicarbonate to detect characteristic fruity odor of ethyl acetate indicating successful ester formation.

Tests for Succinic Acid (Fluorescence Test)

• Mix succinic acid with resorcinol and concentrated sulphuric acid, heat to obtain red color with green fluorescence. Dilute with water if unclear.

Tests for Tartaric Acid

• Reaction with Concentrated Sulfuric Acid: Heavy charring occurs, releasing CO, CO2, and SO2 gases.

• Calcium Chloride Reaction: White precipitate of calcium tartrate is formed when added to the neutral solution. It dissolves in dilute HCl but not acetic acid.

• Fenton's Reagent Reaction: Tartaric acid reacts, forming a violet color due to oxidation-reduction reaction with hydrogen peroxide and iron.

Tests for Oxalic Acid

• Potassium Permanganate Reaction: Oxalic acid reacts in acidic medium to decolorize violet solution. No reaction occurs in alkaline medium.

• Concentrated Sulfuric Acid Reaction: Similar gases are liberated as with tartaric acid; minor darkening occurs.

Tests for Lactic Acid

• Iodoform Test: Detection method analogous to alcohols due to the presence of a free methyl group.

• Concentrated Sulfuric Acid Reaction: Similar gaseous products as tartaric acid but with minimal charring.

Tests for Citric Acid**

• Concentrated Sulfuric Acid Reaction: Follows tartaric acid's procedure with gas evolution but no charring.

Tests for Salicylic Acid (Ester Formation)

• Salicylic acid can be detected through ester formation (e.g., methyl salicylate).

• Procedure: Similar to acetic acid esterification, but using methanol instead of ethanol (caution: methanol is toxic).