Experiment 54

Objective

To identify the presence of the hydroxyl group (-OH) in an alcohol by observing the reaction with phosphorus pentachloride (PCl₅), which produces hydrochloric acid (HCl) that reacts with ammonia to form white smoke.

Introduction

Alcohols react with phosphorus pentachloride (PCl₅) to form alkyl halides, phosphoryl chloride (POCl₃), and hydrochloric acid (HCl). The HCl then reacts with ammonia gas to produce ammonium chloride, visible as white smoke. This reaction is a qualitative test for the hydroxyl (-OH) group.

Reaction Overview

R-OH + PCl5​ → R-Cl + POCl3 ​+ HCl NH3 + HCl → NH4Cl (white smoke)

Materials

Chemicals

·       Ethanol

·       Phosphorus pentachloride (PCl₅)

·       Ammonia gas (via ammonia stick)

Apparatus

·       Test tube

·       Test tube holder

·       Bunsen burner

·       Ammonia stick

Reagents

1-2 mL of ethanol and a small amount of PCl₅ in a test tube

Procedure

1.    Preparation of Test Solution

o   Place 1-2 mL of ethanol in a test tube and add a small amount of PCl₅.

2.    Heating

o   Heat the test tube gently for about one minute.

3.    Testing with Ammonia

o   Hold an ammonia stick at the mouth of the test tube.

4.    Observation

o   White smoke forms around the ammonia stick, indicating the presence of the hydroxyl (-OH) group.

Observation

The formation of white smoke (ammonium chloride) confirms the presence of the hydroxyl group in the alcohol.

Questions

1.    What happens when phosphorus pentachloride reacts with alcohol?

2.    How does the white smoke form during the -OH group test?