Identifying Alcoholic Functional Groups in Organic Compounds

Identifying the Presence of Alcoholic Functional Groups in Organic Compounds

Theory:

Several tests can be performed to detect the presence of an alcoholic functional group in a given organic compound. These include:

  • Sodium Metal Test
  • Ester Test
  • Ceric Ammonium Nitrate Test
  • Acetyl Chloride Test
  • Iodoform Test

(a) Sodium Metal Test:

This test relies on the observation of brisk effervescence due to the liberation of hydrogen gas when an alcohol reacts with active metals like sodium.

Chemical Reaction:

2R-OH + 2Na → 2R-O-Na + H2

2CH3-OH + 2Na → 2CH3-O-Na + H2

Note: The alcohol being tested should be dry, as water also reacts with sodium. Handle sodium with care and destroy any unreacted sodium by adding excess alcohol. This test is favorable in the absence of phenyl or carboxyl groups. The evolution of hydrogen gas causing brisk effervescence indicates an alcoholic group.

(b) Ester Test:

Carboxylic acids react with alcohols to form esters with a fruity smell. This reaction, called esterification, is slow and catalyzed by concentrated sulfuric acid.

Chemical Reaction:

R-OH + R-COOH → R-COOR + H2O

CH3OH + CH3-COOH → CH3-COOCH3 + H2O

Note: A sweet smell indicates the presence of an alcoholic group.

(c) Ceric Ammonium Nitrate Test:

Alcohols react with ceric ammonium nitrate to form a pink or red precipitate due to the formation of a complex compound and ammonium nitrate.

Chemical Reaction:

(NH4)2 [Ce(NO3)6] + 3ROH → [Ce(NO3)4(ROH)3] + 2NH4NO3

(NH4)2 [Ce(NO3)6] + 3CH3OH → [Ce(NO3)4(CH3OH)3] + 2NH4NO3

Note: The appearance of a wine-red precipitate indicates the presence of an alcoholic group.

(d) Acetyl Chloride Test:

Alcohols react with acetyl chloride to form an ester and hydrogen chloride. The resulting hydrogen chloride reacts with ammonium hydroxide to form white fumes of ammonium chloride and water.

Chemical Equation:

R-OH + CH3-CO-Cl → CH3-COOR + HCl

HCl + NH4OH → NH4Cl + H2O

Note: The formation of white fumes indicates the presence of alcohol.

(e) Iodoform Test:

This test is specific to secondary alcohols, ketones, and acetaldehyde. The compound is heated with sodium hydroxide solution and iodine. The formation of a yellow precipitate of iodoform indicates the presence of alcohol.

Chemical Reactions:

CH3-CH(OH)-CH3 + I2 + 2NaOH → CH3-CO-CH3 + 2NaI + 2H2O

CH3-CO-CH3 + 3I2 + 4NaOH → CHI3 (Iodoform) + CH3COONa + 3NaI + 3H2O

Note: The formation of a yellow precipitate indicates the presence of alcohol, acetaldehyde, or methyl ketones.

Distinguishing Between Primary, Secondary, and Tertiary Alcohols: Lucas Test

The Lucas reagent, a mixture of zinc chloride and concentrated hydrochloric acid, reacts with primary, secondary, and tertiary alcohols at different rates, forming cloudiness. Tertiary alcohols react immediately, secondary alcohols react slowly (within 5-10 minutes), and primary alcohols show no reaction.

Chemical Reactions: (Refer to a chemistry textbook for detailed reactions)

Note:

  • Immediate cloudiness: Tertiary alcohols
  • Cloudiness within 5-10 minutes: Secondary alcohols
  • Cloudiness only on heating: Primary alcohols

Materials Required:

  • Sodium metal
  • Acetic acid
  • Ceric ammonium nitrate
  • Acetyl chloride
  • Ammonium hydroxide
  • Iodine
  • Sodium hydroxide
  • Calcium sulfate
  • Lucas reagent (zinc chloride and Conc. HCl)
  • Test tubes
  • Test tube holder
  • Filter paper

Apparatus Setup:

(Refer to a chemistry textbook for a visual setup of the Lucas Test)

Procedure:

Preparation of Reagents:

  • Ceric Ammonium Nitrate: Dissolve 20g of ceric ammonium nitrate in 200ml of warm dilute nitric acid.
  • Lucas Reagent: Dissolve 135g of anhydrous zinc chloride in 100ml of concentrated hydrochloric acid.
  • Iodine Solution: Dissolve 5g of potassium iodide in 40ml of water. Add 1g of solid iodine and dissolve completely.
(a) Sodium Metal Test:
  1. Place the organic compound in a dry test tube.
  2. Add 1g of anhydrous calcium sulfate and shake to remove excess water.
  3. Decant the solution into another clean test tube.
  4. Add a small piece of sodium metal.
  5. Observe for brisk effervescence (hydrogen gas evolution) indicating an alcoholic group.
(b) Ester Test:
  1. Take 1ml of the organic liquid in a clean, dry test tube.
  2. Add 1ml of glacial acetic acid and 2-3 drops of concentrated sulfuric acid.
  3. Heat the mixture in a water bath for 10 minutes.
  4. Pour the hot mixture into a beaker of cold water.
  5. Smell the water for a fruity odor, confirming the presence of an alcoholic group.
(c) Ceric Ammonium Nitrate Test:
  1. Take 1ml of the compound in a dry test tube.
  2. Add a few drops of ceric ammonium nitrate reagent and shake well.
  3. Observe for a red precipitate, confirming the presence of an alcoholic group.
(d) Acetyl Chloride Test:
  1. Take 2ml of the organic compound in a clean test tube.
  2. Add 1g of anhydrous calcium sulfate and shake well.
  3. Filter the solution. Add 3-4 drops of acetyl chloride to the filtrate and shake well.
  4. Hold a glass rod dipped in ammonium hydroxide solution near the mouth of the test tube.
  5. Observe for white fumes, confirming the presence of an alcoholic group.
(e) Iodoform Test:
  1. Take 1ml of the organic compound in a clean, dry test tube.
  2. Add 1ml of 1% iodine solution.
  3. Add dilute sodium hydroxide solution dropwise until the brown color of iodine disappears.
  4. Heat the mixture gently in a water bath.
  5. Observe for a yellow precipitate, indicating the presence of ethanol, acetaldehyde, or a methyl ketone.

Observations:

  • Sodium Metal Test: Brisk effervescence indicates an alcoholic group.
  • Ester Test: A sweet smell indicates the presence of an alcoholic group.
  • Ceric Ammonium Nitrate Test: A wine-red precipitate indicates the presence of an alcoholic group.
  • Acetyl Chloride Test: Formation of white fumes indicates the presence of alcohol.
  • Iodoform Test: Formation of a yellow precipitate indicates the presence of alcohol, acetaldehyde, or methyl ketones.