Test for Alcohols: Alcohols are organic compounds with the functional group \({\rm{–OH,}}\) called the hydroxyl group. Ethanol is the most common alcohol widely used in alcoholic drinks as a preservative for biological specimens, fuel (gasoline), and solvent for drugs and paints.

While naming alcohols, the suffix \( – {\rm{ol}}\) is added to the parent chain of the alkane name. The position of the \( – {\rm{OH}}\) functional group is indicated in the name. The numbering of the parent chain is done at the end closest to where the \( – {\rm{OH}}\) is located.

The general formula of alcohol homologous series is \({{\rm{C}}_{\rm{n}}}{{\rm{H}}_{2{\rm{n}} + 1}}{\rm{OH}},\) where \({\rm{n}} = 1,2,3…..\)

Alcohol, Formula, Structure

The homologous series of alcohols consists of methanol, ethanol, propanol, and butanol with chemical formulas of \({\rm{C}}{{\rm{H}}_3}{\rm{OH}},\,{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}},\,{\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}\) and \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{OH}},\) respectively, where each successive compound differs from the previous one by a \( – {\rm{C}}{{\rm{H}}_2}\) group.

Study Structure of Alcohol Here

Based on the number of hydroxyl groups attached to the parent chain, alcohols are classified as monohydric (contain only one \( – {\rm{OH}}\) group), dihydric (contain two \( – {\rm{OH}}\) groups) and trihydric (contains three \( – {\rm{OH}}\) groups).

When the \( – {\rm{OH}}\) group is attached to the primary, secondary and tertiary carbon atoms, alcohols are further classified as primary \(\left( {1^\circ } \right),\) secondary \(\left( {2^\circ } \right),\) and tertiary \(\left( {1^\circ } \right).\)

Tests to Distinguish Alcohols from Other Functional Groups

To distinguish alcohols from other functional groups, the following tests are performed-

1. Sodium Metal Test

Alcohols react with active metals like sodium with a brisk effervescence to liberate hydrogen gas. A colourless solution of sodium ethoxide, \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{ONa}}\) is left behind.

The absence of phenyl or carboxyl groups favours this reaction.

2. Phosphorus Pentachloride Test

Alcohol reacts with \({\rm{PC}}{{\rm{l}}_5}\) with the evolution of \({\rm{HCl}}\) gas. Hydrogen chloride reacts with ammonium hydroxide to form white fumes of ammonium chloride.

\({\rm{R}} – {\rm{OH}} + {\rm{PC}}{{\rm{l}}_5} \to {\rm{POC}}{{\rm{l}}_3} + {\rm{HCl}}\)

\({\rm{N}}{{\rm{H}}_4}{\rm{OH}} + {\rm{HCl}} \to {\rm{N}}{{\rm{H}}_4}{\rm{Cl}} + {{\rm{H}}_2}{\rm{O}}\)

3. Ester Test

A fruity-smelling compound called esters is formed when alcohols react with carboxylic acids. The reaction between alcohol and carboxylic acid is known as esterification and is a slow and reversible reaction catalysed by an acid such as concentrated sulphuric acid.

The chemical reaction is given below.

4. Ceric Ammonium Nitrate Test

Alcohols react with ceric ammonium nitrate \({\left( {{\rm{N}}{{\rm{H}}_4}} \right)_2}{\rm{Ce}}{\left( {{\rm{N}}{{\rm{O}}_3}} \right)_6}\) to form a pink or red coloured precipitate of alkoxy cerium (IV) complex and ammonium nitrate. The red colour disappears if an excess of ceric ammonium nitrate solution is added to the alcoholic sample

The red colour also disappears after resting the reaction mixture.

The chemical reaction is given below.

5. Acetyl Chloride Test

Alcohols react with acid halides to form esters with the liberation of hydrogen chloride gas. The hydrogen chloride gas reacts with ammonium hydroxide to give white fumes of ammonium chloride.

The chemical equation is given below.

6. Iodoform Test

Secondary alcohols, ketones and acetaldehyde show a positive iodoform test. A yellow precipitate of iodoform is formed when the given compound is heated with sodium hydroxide solution and iodine.

The chemical reactions are given below

Primary, Secondary and Tertiary Alcohols

1. Lucas Test

Primary, secondary and tertiary alcohols react with Lucas reagent (zinc chloride and concentrated hydrochloric acid) at different rates. This reagent forms a cloud-like appearance on reacting with alcohols. Tertiary alcohols react immediately; secondary alcohols react slowly to form a cloud-like appearance after \(5\) to \(10\) minutes. However, there is no reaction with primary alcohols. Alcohols are soluble in Lucas reagent, so a clear solution is obtained, while alkyl chlorides being insoluble results in cloudiness in the solution.

The chemical reactions are given below.

Cloudiness appears immediately \( \to \) Tertiary alcohols
Cloudiness appears within five to ten minutes \( \to \) Secondary alcohols
Cloudiness does not appear  \( \to \) Primary alcohols

2. Victor Meyer Test

Victor Meyer’s test is an important test to distinguish between primary, secondary and tertiary alcohols. It is an organic compound that has a functional group \({\rm{RC}}{\left( {{\rm{NO}}} \right)_2} = {\rm{NOH}}\) and appears as red colour.

The reagents used in the Victor Meyers test are \({\rm{Hl}},\,{\rm{Ag}}{\left( {{\rm{NO}}} \right)_2}\) and \({\rm{KOH}}/{\rm{NaOH}}{\rm{.}}\)

In the test, three different colours are observed for different alcohols. These are listed below.

Blood Red Colour	Primary Alcohol
Blue Colour	Secondary Alcohol
No Colour Change	Tertiary Alcohol

When primary alcohol reacts with the reagents (\({\rm{Hl}},\,{\rm{Ag}}{\left( {{\rm{NO}}} \right)_2}\) and \({\rm{KOH}}/{\rm{NaOH}}\)) the final product is nitrolic acid. However, with tertiary alcohols, there would be no reaction. Victor Meyer’s test is also known as the Red, Blue and White test.

Tests for Phenolic group

Phenols are compounds that contain a hydroxyl group attached to an aromatic ring. The simplest phenol is \({{\rm{C}}_6}{{\rm{H}}_5}{\rm{OH}}.\) Other examples of phenols are \({\rm{o}}\)-cresol, \({\rm{m}}\)-cresol, \({\rm{p}}\)-cresol, quinol, catechol, resorcinol etc.

The following tests can detect the phenolic group:

1. Litmus Test

Phenol is weakly acidic and gives a red colour with blue litmus paper. Phenol dissociates in water to form phenoxide ions and water. This is represented as follows:

2. Ferric Chloride Test

Phenols react with neutral ferric chloride solution to give a coloured complex. For example, phenol gives a complex of violet colour as follows:

Observations:

1. Resorcinol, \({\rm{o}} – ,\,{\rm{m}} – ,\,{\rm{p}} – \)cresol, give violet or blue colouration.
2. \({\rm{\beta }}\)-Naphthol gives a green colouration.
3. \({\rm{\alpha }}\)-Naphthol gives pink colouration.
4. Formic acid and acetic acid give deep red colouration.

3. Liebermann’s Test

Phenol in concentrated sulphuric acid reacts with sodium nitrite to form a deep blue or green coloured complex. The blue or green colour changes to red or brown colour on being treated with water. The red colour is due to the formation of indophenol, which changes to blue or green by adding strong alkali. The blue or green colour is due to the formation of indophenol anion.

Note:
This test is not given by Nitrophenols and \({\rm{p}}\)-substituted phenols. Among the dihydroxy phenol, only resorcinol gives a positive test.

4. Phthalein Dye Test

Phenols in the presence of concentrated \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}\) condenses with phthalic anhydride to form phenolphthalein. It gives a dark pink colour with NaOH solution. This is called the phthalein dye test.

\({\rm{o}}\)-cresol	Red
\({\rm{m}}\)-cresol	Bluish Purple
\({\rm{p}}\)-cresol	No Colour
Catechol	Usually, blue takes a longer time to appear
Resorcinol	Green Fluorescent Colour of Fluorescein

Tests for Alchol Groups

The Tests for the alcohol group (Distinguishing test from other functional groups) are tabulated below:

Test	Procedure	Observation
Sodium metal Test	Reagent: \({\rm{5\% }}\) sodium hydroxide solution is prepared by dissolving \({\rm{5}}\,{\rm{g}}\) of solid sodium hydroxide in about \({\rm{50}}\,{\rm{ml}}{\rm{.}}\) It is then diluted to \({\rm{100}}\,{\rm{ml}}{\rm{.}}\) The organic compound to be tested is taken in a dry test tube. To it, \({\rm{1}}\,{\rm{gm}}\) of anhydrous calcium sulfate is added and shaken well. This is done to remove excess water. To the above solution, a small piece of sodium metal is added.	A brisk effervescence indicates the presence of an alcoholic group
Ester Test	\({\rm{1}}\,{\rm{ml}}\) of the organic liquid is taken in a clean, dry test tube. To the above solution, \({\rm{1}}\,{\rm{ml}}\) of glacial acetic acid and \(2-3\) drops of conc. Sulfuric acid is added. The mixture is heated in a water bath for \(10\) minutes. The hot mixture is poured into a beaker containing cold water.	A fruity smell confirms the presence of an alcoholic group.
Ceric Ammonium Nitrate Test	Reagent: The reagent is prepared by dissolving \({\rm{20}}\,{\rm{g}}\) of ceric ammonium nitrate orange crystals in \({\rm{200}}\,{\rm{ml}}\) of warm dilute nitric acid \(\left( {2\,{\rm{N}}} \right).\) \(1\,{\rm{ml}}\) of the given compound is taken in a dry test tube. A few drops of ceric ammonium nitrate reagent is added to the above solution and shaken.	A wine red coloured precipitate shows the presence of an alcoholic group
Acetyl Chloride Test	\(2\,{\rm{ml}}\) of given organic compound is taken in a clean test tube. To the above solution, \(1\,{\rm{gm}}\) of anhydrous calcium sulfate is added and filtered. To the filtrate, \(3\) to \(4\) drops of acetyl chloride solution is added and shaken well. A glass rod dipped in ammonium hydroxide solution is brought near the mouth of the test tube.	The formation of white fumes indicates the presence of alcohol
Iodoform Test	Reagent: \(2\,{\rm{g}}\) of solid potassium iodide is dissolved in about \(40\,{\rm{ml}}\) water. To it, \(1\,{\rm{g}}\) solid iodine is added and shaken. The solution is diluted to \(100\,{\rm{ml}}.\) \(1\,{\rm{ml}}\) of given organic compound is taken in a clean, dry test tube. \(1\,{\rm{ml}}\) of \(1\% \) iodine solution is added to it. A dilute solution of sodium hydroxide solution is added dropwise to the above solution until the brown colour of iodine is discharged. The mixture is heated gently in a water bath.	The formation of yellow precipitate shows the presence of alcohol

Tests for distinguishing primary, secondary and tertiary alcohols

Test	Procedure	Observation
Lucas Test	Dissolve \(135\,{\rm{g}}\) of anhydrous zinc chloride in \(100\,{\rm{ml}}\) of concentrated hydrochloric acid. Shake well and cool. \(1\,{\rm{ml}}\) of the organic compound is taken in a test tube, and to it, \(10\,{\rm{ml}}\) of Lucas reagent is added and shaken well.	Cloudiness appears immediately \( \to \) Tertiary alcohols Cloudiness appears within five to ten minutes \( \to \) Secondary alcohols Cloudiness does not appear  \( \to \) Primary alcohols
Victor Meyer’s Test	To the given organic compound, concentrated hydrogen iodide or red phosphorus and iodine are added. To the above solution, silver nitrite is added to produce nitroalkane. Nitroalkane is then treated with nitrous acid \(\left( {{\rm{NaN}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}} \right)\) and is made alkaline with potassium hydroxide.	Primary alcohol – Blue red Secondary alcohol – red Tertiary alcohol – No change

FAQs on Test for Alcohols

Q.1. Which alcohol gives red colour in Victor Meyer’s test?
Ans: Primary alcohols or \(1^\circ \) alcohol gives red colour in Victor Meyer’s test.

Q.2. Which alcohol gives Lucas test immediately?
Ans: Primary alcohols or \(1^\circ \) alcohol will not show turbidity immediately. However, tertiary alcohols or \(3^\circ \) alcohols show turbidity immediately, and they give a positive Lucas test.

Q.3. What is acidic dehydration of alcohol?
Ans: Alcohol reacts with protic acids and loses a molecule of water to form alkenes. As the reaction involves the removal of the water molecule, these are generally known as dehydration of alcohols. It is an example of an elimination reaction. The rate of the reaction differs for the primary, secondary and tertiary alcohols.

Q.4. What are the qualitative tests for alcohol?
Ans: The qualitative tests for alcohols are –
(i) Chromic Acid Test (or Jones Oxidation)
(ii) Lucas Test using \({\rm{ZnC}}{{\rm{l}}_2}\) and \({\rm{HCl}}\)
(iii) Iodoform Test.

Q.5. Which alcohol gives a positive iodoform test?
Ans: Ethanol is the only primary alcohol to give the triiodomethane (iodoform) reaction. However, secondary alcohols with a methyl group attached to the carbon with the \( – {\rm{OH}}\) group respond positively to this reaction.

Study Preparation of Alcohols Here

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