Organic Chemistry Laboratory: Carboxylic Acid Derivatives Experiment #7

Compiled by JUHLIAN SANTOS, RChE, RChT, MEngChE

Intended Learning Outcomes

Upon the completion of this experiment, students should be able to:

• 1. Compare the relative reactivity of carboxylic acid derivatives through systematic chemical testing.

• 2. Apply hydrolysis, alcoholysis, aminolysis, and hydroxamic acid test reactions to distinguish between different carboxylic acid derivatives.

• 3. Analyze reaction mechanisms and explain the structural factors that influence the reactivity order of carboxylic acid derivatives.

• 4. Interpret experimental results to identify unknown carboxylic acid derivatives based on their characteristic reaction patterns and products.

What Are Carboxylic Acid Derivatives?

General Structure

• Carboxylic acid derivatives share a common structural feature represented as:

  • $R–CO–L$

• Description: A carbonyl group (C=O) bonded to a leaving group (L).

• Importance: The identity of L determines the type of derivative and its reactivity.

Types of Carboxylic Acid Derivatives

1. Acid Halides

   • Structure: $R–CO–X$ where X = Cl, Br (halogen)

   • Reactivity: Highly reactive due to strong leaving group properties of halogens.

2. Anhydrides

   • Structure: $R–CO–O–CO–R'$

   • Description: Two acyl groups joined by an oxygen.

3. Esters

   • Structure: $R–CO–OR'$

   • Description: Bonded to an alkoxy group (derived from alcohols).

4. Amides

   • Structure: $R–CO–NH_2$

   • Description: Bonded to a nitrogen atom.

Nucleophilic Acyl Substitution

Mechanism of Reaction

• The carbonyl carbon carries a partial positive charge due to the electron-withdrawing effect of the leaving group (L).

• Reaction Process:

  1. A nucleophile (electron-rich species) attacks the electrophilic carbon (electron-deficient atom), forming a tetrahedral intermediate.

  2. The leaving group departs, resulting in a new acyl compound.

Reactivity Order

• Reactivity decreases in the following order:

  • Acid Halides > Anhydrides > Esters > Amides

• Key Concept: The better the leaving group, the more reactive the derivative.

  • Halides are excellent leaving groups.

  • In amides, nitrogen donates electrons into the carbonyl, reducing the electrophilicity of the carbon.

Key Reactions

1. Hydrolysis

   • Nucleophile: Water ($H₂O$)

   • Product: Carboxylic acid + leaving group

   • Detection: Litmus paper turns red (acidic product).

2. Alcoholysis

   • Nucleophile: Alcohol/Ethanol ($C₂H₅OH$)

   • Product: Ester

   • Detection: Fruity/pleasant odor; two layers may form.

3. Aminolysis

   • Nucleophile: Amine/Ammonia/Aniline ($C₆H₅NH₂$)

   • Product: Amide

   • Detection: A white precipitate forms.

4. Hydroxamic Acid Test

   • used to distinguish between carboxylic acid derivatives based on their reactivity patterns.

   • Nucleophile: Hydroxylamine ($NH₂OH$)

   • Product: Fe³⁺-hydroxamate complex

   • Detection: Burgundy/magenta or violet color.

Apparatus, Equipment, and Materials

Apparatus and Equipment

• 1. Beaker

• 2. Graduated Cylinder

• 3. Hot Plate

• 4. Parafilm

• 5. Pipettes

• 6. Test Tubes

Reagents and Materials

1. 1 M Potassium Hydroxide ($KOH$)

2. 10% Hydrochloric Acid ($HCl$)

3. 20% Sodium Hydroxide ($NaOH$)

4. 25% Sodium Hydroxide ($NaOH$)

5. 5% Ferric Chloride ($FeCl₃$)

6. Acetic Anhydride

7. Aniline

8. Benzamide

9. Ethanol

10. Ethyl Acetate

11. Hydroxylamine

12. Red Litmus Paper

13. Blue Litmus Paper

14. Saturated Sodium Bicarbonate

15. 2% Silver Nitrate

16. Acetyl Chloride

Experimental Procedures

Part A:

Hydrolysis of Carboxylic Acid Anhydride

Compound: Acetic Anhydride

1. Place 20 drops of water in a clean, dry test tube.

2. Add 5 drops of acetic anhydride. Gently shake the test tube and immediately observe the reaction. Note any temperature change.

3. Add 1 mL of saturated sodium bicarbonate. Observe for effervescence.

   • Result: Acetic anhydride reacts with water to produce acetic acid, giving a positive test with $NaHCO₃$ (gas bubbles from $CO₂$ release).

Hydrolysis of Ester

Compound: Ethyl Acetate

1. Place 20 drops of ethyl acetate in a test tube.

2. Add 25 drops of 25% sodium hydroxide. Note the formation of two distinct layers.

3. Test the solution with red and blue litmus papers. If the mixture is basic, acidify it dropwise with 10% HCl. Observe and note any odor released.

   • Description: This reaction is called saponification. $NaOH$ breaks the ester bond, producing sodium acetate(salt) and ethanol(alcohol). After acidification, the acetic acid released has a characteristic vinegar-like smell.

Hydrolysis of Amide

Compound: Benzamide

1. Place a small pinch (approximately 0.5 g) of benzamide in a test tube.

2. Add 5 mL of 10% sodium hydroxide and heat the mixture to boiling.

3. Hold a piece of moist red litmus paper near the mouth of the test tube. Note the color change and any odor.

   • Result: Benzamide undergoes basic hydrolysis to produce sodium benzoate and ammonia gas ($NH₃$). Ammonia turns moist red litmus paper blue and has a pungent odor.

Part B: Alcoholysis

Compound: Acetic Anhydride + Ethanol

1. Add a mixture of 0.5 mL ethanol and 1 mL water into a clean test tube.

2. Carefully add 5 drops of acetic anhydride.

3. Add 2 mL of 20% sodium hydroxide solution. Seal the test tube with parafilm and shake gently for about 3 minutes.

4. Observe and record any odor and the formation of two liquid layers.

   • Description: Ethanol acts as the nucleophile and reacts with acetic anhydride to form ethyl acetate (an ester). Ethyl acetate has a fruity, sweet odor. Two layers may appear since the ester(on top) is less dense and not fully miscible with the aqueous layer.

Part C: Aminolysis

Compound: Acetic Anhydride + Aniline

1. Place 0.5 mL of aniline into a clean test tube.

2. Add 0.5 mL of acetic anhydride to the test tube.

3. Cover the test tube with parafilm and shake gently.

4. Add 0.5 mL of cold water to the mixture.

5. Observe and note the color of any precipitate that forms.

   • Result: Aniline (a primary amine) reacts with acetic anhydride to form acetanilide, a white solid precipitate. This is a classic aminolysis reaction. The cold water helps the solid product crystallize out of solution.

Part D: Hydroxamic Acid Test

Compounds Tested: Acetic Anhydride, Ethyl Acetate, Benzamide

1. Place 2 drops of each compound in separate labeled test tubes.

2. Add 2 mL of alcoholic hydroxylamine hydrochloride and 1 mL of 1 M KOH to each.

3. Heat the mixture in a boiling water bath for 2 minutes. Allow it to cool to room temperature.

4. Add 1 mL of 5% ferric chloride ($FeCl₃$) solution and observe the resulting color change.

   • Mechanism: Hydroxylamine converts the acyl compound into a hydroxamic acid. The hydroxamic acid then chelates with $Fe^{3+}$ ions, producing a colored complex. The color and intensity depend on the derivative type.

Expected Results

1. Hydrolysis

   • Acetic Anhydride Reaction with Water: Exothermic; solution becomes warm.

   • Saturated NaHCO₃: Effervescence ($CO₂$ gas released).

2. Alcoholysis

   • Acetic Anhydride + Ethanol: Fruity/sweet odor; two liquid layers (ester layer on top).

3. Aminolysis

   • Acetic Anhydride + Aniline: White solid precipitate (acetanilide) forms upon addition of cold water.

4. Hydroxamic Acid Test

   • Acetic Anhydride: Burgundy/magenta color.

   • Ethyl Acetate: Burgundy/magenta color.

   • Benzamide: Red to violet color.

Interpreting Results

Positive Results

• Hydrolysis (NaHCO₃): Vigorous effervescence (bubbles)

• Ester Hydrolysis: Vinegar-like odor after acidification.

• Amide Hydrolysis: Moist red litmus turns blue.

• Alcoholysis: Fruity odor; two separate layers observed.

• Aminolysis: White solid precipitate formed.

• Hydroxamic Acid Test: Burgundy, magenta, or violet color observed.

Negative Results

• No gas evolution with NaHCO₃.

• No distinct odor released.

• No color change in litmus paper.

• No layer separation; no fruity smell.

• No solid forms after adding cold water.

• No color change after adding $FeCl₃$ (stays yellow/brown).

The Hydroxamic Acid Test

• The hydroxamic acid test serves as a key identification tool for carboxylic acid derivatives.

Process Overview

1. The derivative reacts with hydroxylamine ($NH₂OH$) to form a hydroxamic acid ($R–CO–NHOH$).

2. $KOH$ ensures the reaction medium is basic, promoting nucleophilic substitution.

3. Heating drives the reaction to completion.

4. $FeCl₃$ is added. The hydroxamic acid chelates with $Fe^{3+}$, forming a colored iron-hydroxamate complex.

Color Indicators

• Acid Halides, Anhydrides, and Esters: Give a burgundy/magenta color.

• Amides: Yield a red to violet color due to their lower reactivity.

Safety Considerations

General Precautions

• Always wear safety goggles and lab gloves throughout the experiment.

• Work in a well-ventilated area or use a fume hood, as several reagents release toxic or irritating fumes.

• Acetic Anhydride: Highly reactive with water and releases heat; add it slowly and carefully.

• Aniline: Toxic and can be absorbed through the skin; avoid direct contact.

Specific Hazards

• NaOH (20% and 25%): Highly corrosive; rinse any skin contact immediately with running water.

• HCl (10%): Corrosive and produces irritating fumes; handle with care.

• FeCl₃: Can stain and is mildly irritating; wash hands after handling.