Tests for Alcohols, Phenols and Ethers – Identification and Classification

Tests for Alcohols, Phenols and Ethers: Many compounds in the universe are made up of hydrocarbons, which means carbon and hydrogen are present in all the substances. But we should focus on the fact that all compounds or substances in the universe have different characteristics. Thus, apart from hydrogen and carbon, there are some other elements too present in different compounds. These groups of elements present in compounds that completely change their functionality are known as functional groups. When the hydrogen atom of a hydrocarbon is replaced by a hydroxyl group, then a functional group ‘alcohol’ is generated. There are three different types of compounds in this category: alcohol, phenol, and ether. 

What are Alcohols?

Alcohols are organic compounds having the functional group by \(-\text {OH}\) (hydroxyl) group. In the nomenclature of the alcohol, the suffix ‘-ol’ is added to the parent chain of the alkane name like ‘alkanol’. The general formula of homologous series of alcohol is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}} + 1}}{\rm{OH}}\), where \(\text {n}=1,2,3\) and so on. Some of the common alcohols are methanol, ethanol, butanol, etc., out of the ethanol \(\left( {{{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}}} \right)\) is most widely used in alcoholic drinks, as a preservative for biological specimens, as fuel (gasoline), and as a solvent for drugs and paints.

Test for Alcohol

There are various laboratory tests for alcohol such as sodium metal test, ester test, ceric ammonium nitrate test, Iodoform test, Acetyl Chloride Test. Primary, secondary and tertiary alcohols are distinguished by the Lucas test and Victor Meyer’s test. Let’s discuss some of them in detail:

1. Sodium Metal Test

Sodium is an active metal with which alcohols react to form a colourless solution of sodium alkoxide (example; ethoxide ‘\({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}}{{\rm{H}}_{\rm{2}}}{\rm{ONa}}\)’ ) along with the formation of a brisk effervescence due to the liberation of hydrogen gas. For example, in the chemical equation given below, methanol reacts with sodium to form sodium methoxide and release hydrogen gas.

\({\rm{2C}}{{\rm{H}}_3}{\rm{OH}} + 2{\rm{Na}} \to 2{\rm{C}}{{\rm{H}}_3}{{\rm{O}}^ – }{\rm{N}}{{\rm{a}}^ + } + {{\rm{H}}_2}\left( {\rm{g}} \right)\)

2. Ester Test

When alcohols react with carboxylic acids, a fruity-smelling compound called ester is formed. This reaction between alcohol and carboxylic acid catalysed by an acid such as concentrated sulphuric acid is known as esterification.

The chemical reaction between ethanol and ethanoic acid to form ethyl acetate and water is given below:

3. Lucas Test

Primary, secondary and tertiary alcohols react with Lucas reagent made up of zinc chloride and concentrated hydrochloric acid at different rates forming a cloud-like appearance on reacting with alcohols. We can observe that tertiary alcohols react immediately with Lucas reagent while secondary alcohols react slowly to form a cloud-like appearance after approximately \(5\) to \(10\) minutes. But there is no chemical reaction of Lucas reagent with primary alcohols. Thus, it can be summarised as cloudiness appearing immediately in Tertiary alcohols. Cloudiness appears within five to ten minutes in Secondary alcohols, and cloudiness does not appear at all in primary alcohols.

What are Phenols?

Phenol is an aromatic organic compound with having a chemical formula \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{OH}}\). It is also known as carbolic acid. It is a volatile and white crystalline solid. Phenol molecule consists of a phenyl group \(\left( { – {{\rm{C}}_6}{{\rm{H}}_5}} \right)\) bonded to a hydroxyl group \((−\text {OH})\). Phenols are mildly acidic.

Test for Phenol

Few laboratory tests of phenol are discussed below:

1. Litmus test

Phenol dissociates in water to give phenoxide ions and hydronium ions. Thus, it changes blue litmus paper to red, inferring that phenol is weakly acidic in nature. This reaction is represented as follows:

2. Ferric chloride test

A dark-coloured complex (blue to violet) is formed when phenol reacts with neutral ferric chloride solution.

What are Ethers?

Ether is a type of organic compound in which an oxygen atom connects two alkyl or aryl groups on each side. Ethers are represented by their general formula: \(\text {R}-\text {O}-\text {R}\)’ where \(\text {R}\) and \(\text {R}’\) represent two alkyl groups or aryl groups. Based on the type of alkyl or aryl groups attached to the oxygen atom, ethers are divided into two types:

1. Symmetrical Ether – Ethers in which both the alkyl or aryl groups attached to the oxygen atom are similar are known as symmetrical ethers. Like, dimethyl ether \(\left( {{\rm{C}}{{\rm{H}}_3} – {\rm{O}} – {\rm{C}}{{\rm{H}}_3}} \right),\) Diisopropyl Ether \(\left( {{{\left( {{\rm{C}}{{\rm{H}}_3}} \right)}_2}{\rm{CH}} – {\rm{O}} – {\rm{CH}}{{\left( {{\rm{C}}{{\rm{H}}_3}} \right)}_2}} \right)\) etc., are some examples of this type of ethers.

Asymmetrical Ether – Ethers in which different alkyl or aryl groups are attached to the oxygen atom are known as asymmetrical ethers. For example, ethyl methyl ether \(\left( {{\rm{C}}{{\rm{H}}_3} – {\rm{O}} – {\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_3}} \right).\)

Test for Ether

Ethers can be distinguished from alcohol and phenol by performing a few laboratory tests as follows:

1. Ferric chloride Test

If we take phenol and ether in two different test tubes and put a few drops of neutral ferric chloride solution in both, one test-tube solution will turn violet, and another will show no change at all. Thus, we can infer that the one test tube where colour change has occurred contains phenol and another one where no change occurs is ether.

\({\rm{FeC}}{{\rm{l}}_3} + {\rm{C}}{{\rm{H}}_3}{\rm{OC}}{{\rm{H}}_3} \to {\rm{No}}\,{\rm{Reaction}}\)

2. Halogenation of Ethers

Ether reacts with halogens like chlorine and bromine in the dark to form halogenated ethers that have anaesthetic properties. Thus, the formation of these anaesthetic products can help to identify ethers.

3. Acidic Hydrolysis

Ether undergoes acidic hydrolysis to form alcohol, and this can be tested easily in laboratories. Firstly, on hydrolysing ethoxy ethane under acidic conditions, we get ethanol, and by putting a few drops of acetic acid to it, we can get a fruity smelling compound characteristic of ester ethyl acetate. Thus, this two steps test can be used to identify that the starting compound was an ether, i.e., ethoxy ethane.

Step 1: 

Step 2:

Summary

Alcohols, phenols and ethers are organic compounds with hydrocarbons and oxygen. Replacement of a hydrogen atom of an aliphatic hydrocarbon compound by \( – {\rm{OH}}\) (hydroxyl) group leads to the formation of alcohol whereas, when the hydrogen of an aromatic hydrocarbon compound is replaced by \( – {\rm{OH}}\) (hydroxyl) group, phenol is formed. Ethers are formed when a hydrogen atom is replaced by an alkoxy group \(\left( {{\rm{R}} – {\rm{O}}} \right).\) Thus, there is a wide range of differences between alcohol, phenol, and ether. In a chemical laboratory, we can perform different tests to distinguish between these three compounds. 

Alcohols and phenols are classified based on the number of hydroxyl groups and hybridisation of the carbon atom to which hydroxyl groups are attached. At the same time, the ethers are classified on the basis of alkyl or aryl groups attached to the oxygen atom. Alcohols have higher boiling than ethers of comparable molecular masses due to stronger hydrogen bonding. All alcohols, phenols and ethers are soluble in water because of their ability to form intermolecular hydrogen bonding with water. Alcohols and phenols are acidic in nature, but ethers are not. Certain tests, such as litmus tests, ferric chloride tests, Lucas test, esterification, etc., can be used to distinguish between alcohol, phenol, and ether.

FAQs on Tests for Alcohols, Phenols and Ethers

Q.1. What happens when you add phenol to a ferric chloride solution?
Ans: When phenol is added to ferric chloride solution, it reacts with neutral ferric chloride solution; dark-coloured ‘violet’ iron-phenol complex is formed. Thus, confirming the presence of phenol.

Q.2. Which will easily release \({{\rm{H}}^ + }\) out of ethanol or phenol?
Ans: Phenol will release \({{\rm{H}}^ + }\) ions more easily because this leads to the formation of phenoxide ion that is very stable due to resonance, whereas ethoxide ion is not resonance stabilised. Thus, phenols exhibit a slightly more acidic character.

Q.3. What is the ether made of?
Ans: Ether is a special class of organic compounds in which an oxygen atom is connected to two similar or different alkyl or aryl groups. Their general formula is \({\rm{R}} – {\rm{O}} – {\rm{R}}’,\) where \({\rm{R}}\) and \({\rm{R}}’\) denote two alkyl groups or aryl groups in some cases.

Q.4. Is diethyl ether acidic or basic?
Ans: The oxygen atom in a diethyl ether molecule contains two lone pairs of electrons available for coordination. Thus, diethyl ether is a strong lewis base that reacts with a wide range of lewis acids and gives neutral products.

Q.5. What happens when ethers react with halogen in the dark?
Ans: Ether reacts with halogens like chlorine and bromine in the dark to form halogenated ethers with anaesthetic properties. Thus, the formation of these anaesthetic products can help to identify ethers.