10 Alcohols

1. From Haloalkane (studied in substitution reactions of haloalkanes)
2. From Esters (By hydrolysis in acidic/alkaline medium and reduction)

a. Acidic hydrolysis

Esterification is the reaction of an alcohol with a carboxylic acid to form an ester and water. The reverse reaction is ester hydrolysis.

Being a reversible reaction, the equilibrium could be reached by starting from either direction. If the above reaction is started from the product side, it is called the esterification reaction/test.

Reaction of alcohols with carboxylic acid – Esterification (test of alcohol)

When a mixture of alcohol and carboxylic acid is heated in the presence of a calculated amount of conc. H₂SO₄ and an ester are formed. The ester has a characteristic fruity smell. Hence, this reaction is used as a test of both alcohol and carboxylic acid functional groups.

Concentrated sulphuric acid acts as a dehydrating agent, and intermolecular dehydration occurs between the alcohol and carboxylic acid functional groups. Equilibrium can be shifted to the right side by removing water as soon as it is formed.

b. Hydrolysis in basic medium

Basic hydrolysis of esters is called saponification; it produces an alcohol and the sodium salt of the carboxylic acid. When fatty esters are hydrolysed, the salts formed are soaps.

c. Reduction by Na/C₂H₅OH or LiAlH₄

From Primary amines (by reaction with nitrous acid)

Primary amines react with nitrous acid (prepared in situ by reacting sodium nitrite with hydrochloric acid) in ice-cold conditions, resulting in alcohols.

e.g.

1. Oxo process

Alkenes react with carbon monoxide and hydrogen in the presence of octocarbonyldicobalt or cobalt tetracarbonyl at high temperatures and pressure to produce aldehydes. Aldehydes can be changed into alcohols by hydrogenation.

2. Fermentation of sugar 🍇

Fermentation is a sophisticated biochemical process where complex organic compounds, primarily carbohydrates like sugars and starches, undergo decomposition into simpler molecules such as alcohols, organic acids, and CO₂. This transformation is mediated by biological catalysts known as enzymes, typically secreted by microorganisms like yeast or bacteria.

Crucially, this process occurs under anaerobic conditions — meaning it takes place in the absence of oxygen.

Beyond the distilleries, fermentation is a cornerstone of Nepalese traditional food science, utilized in the artisanal production of local beverages and preserved foods like pickles.

Primary Feedstocks for Alcohol Production

In industrial and laboratory settings, two main raw materials serve as the carbon source for fermentation:

1. Molasses (The Sugar Route)

Molasses is the concentrated, dark, viscous byproduct of the sugar refining process, remaining after the crystallization of sugarcane juice. It is rich in sucrose C₁₂H₂₂O₁₁, a disaccharide that serves as the starting point for ethanol synthesis.

The Biochemical Pathway

• Hydrolysis: The enzyme invertase (derived from yeast) catalyzes the hydrolysis of sucrose into its constituent monosaccharides: glucose and fructose.

Fermentation of Glucose and Fructose

The monosaccharides are then converted into ethanol and carbon dioxide by the enzyme zymase present in yeast.

Starch (The Complex Carbohydrate Route)

Starch is a polysaccharide abundant in grains, fruits, and vegetables. To produce ethanol, starch must undergo a systematic enzymatic breakdown:

• Saccharification: Diastase (found in germinating barley) first converts starch into the disaccharide maltose.
• Hydrolysis: Maltase then breaks down maltose into glucose.
• Fermentation: Finally, zymase converts the glucose into ethyl alcohol (ethanol).

Key Concepts for Revision

3. Hydroboration-oxidation of Ethene (Alkene) ⚡

Alkenes can be treated with Diborane (BH₃ or (BH₃)₂ or B₂H₆) followed by treatment with alkaline hydrogen peroxide to yield 1° alcohols.

• Pure (100%) and anhydrous Ethyl alcohol is called absolute alcohol.
• Rectified spirit is the 95% pure alcohol containing 5% water.
• Undrinkable Ethyl alcohol containing poisonous substances, especially Methyl alcohol, is called denatured alcohol or methylated spirit.
• Alcoholic beverages are liquors for drinking purposes. They contain Ethanol, water, colouring materials, and flavouring materials. They may be distilled, e.g., vodka, gin, whisky, brandy, etc., or undistilled, e.g., beer, wine, etc.
• Wood alcohol is methanol, generally made by distilling wood.
• Power alcohol — a blend of ethanol (about 20%) with petrol, used as an engine fuel.

1. State

The lower members of alcohols are usually colourless liquids. They have a characteristic alcoholic or spirit-like smell. The higher members are usually colourless solids, and their smell is less noticeable.

2. Solubility

Alcohols are more soluble in water than corresponding alkanes, haloalkanes, and ethers of similar molecular mass. This is because the alcohol molecule has a polar –OH group, and it forms hydrogen bonds with water molecules.

As the number of carbon atoms increases, the solubility of alcohols decreases. This is because the alkyl part of the molecule becomes larger, and the alkyl group does not dissolve in water.

3. Boiling and melting points

• Alcohols have higher boiling points and melting points than corresponding ethers and haloalkanes of similar molecular mass.
• This is due to their greater polarity and the formation of intermolecular hydrogen bonds between alcohol molecules.
• In a homologous series, the boiling point and melting point generally increase as molecular mass increases.
• This is because the van der Waals forces between molecules become stronger with increasing molecular mass.
• For isomeric alcohols, boiling point and melting point decrease as branching increases.
• This is because branching reduces the surface area of the molecule.
• Therefore, the order of boiling point and melting point is generally 1° > 2° > 3°.

[A] Reaction involving the cleavage of O–H bond

1. Action with active metals like Li, Na, K (acidic nature of alcohol)

When alcohols are treated with active metals like Na, K, Mg, etc., they liberate hydrogen with the formation of metal alkoxides. This shows the acidic nature of alcohol. E.g.,

However, alcohols are weak acids, weaker than water. They neither turn litmus red nor neutralise even strong alkali or aq. alkali solutions.

[B] Reaction involving cleavage of C–O bond (Nucleophilic Substitution rxⁿ)

1. Reaction with HX, PX₃, PCl₅, SOCl₂

(studied in preparations of haloalkanes)

[C] Other Reactions

1. Dehydration of alcohols 🔥

Alcohols can be dehydrated under different conditions with concentrated sulphuric acid, producing Ether or Alkene.

a. When an excess amount of ethyl alcohol is heated with conc. H₂SO₄ as a dehydrating agent at about 140 °C, ethyl alcohol undergoes intermolecular dehydration to give diethyl ether.

This reaction is supposed to occur in the following two steps.

b. When the calculated amount of ethyl alcohol is heated with an excess of conc. H₂SO₄ at about 170 °C, intramolecular dehydration results in Ethene.

Ethyl alcohol can undergo intermolecular dehydration to give diethyl ether by passing its vapour over heated alumina at a temperature less than 300 °C.

Ethyl alcohol can undergo intramolecular dehydration to give ethene by passing its vapours over heated alumina (Al₂O₃) at a temperature above 300 °C.

2. Oxidation of primary, secondary, and tertiary alcohols 🧪

1° alcohol: Primary alcohols can be oxidised into aldehydes; the reaction does not stop there, and the aldehydes are further oxidised into carboxylic acids. The number of carbon atoms in the main skeleton do not change. Different oxidising agents, such as alkaline or acidified potassium permanganate, sodium dichromate, and potassium dichromate, can be used.

2° alcohol: Secondary alcohols are oxidised into ketones with the same number of carbon atoms.

The ketones are not further oxidised easily.

3° alcohol: Tertiary alcohols are quite resistant to oxidation. But when oxidation is carried out under drastic acidic conditions with a strong oxidising agent like KMnO₄ and conc. H₂SO₄, tertiary alcohol first undergoes dehydration into alkenes. The alkene formed is then oxidised to a ketone, which is finally oxidised to a carboxylic acid with a lesser number of carbon atoms. e.g.

3. Catalytic dehydrogenation of alcohols 🚀

Primary and secondary alcohols decompose to give aldehydes and ketones, respectively, in the presence of Cu at 300 °C.

This process is also called dehydrogenation.

When a tertiary alcohol is passed over a red-hot copper tube, it is dehydrogenated to an alkene.

🧪 Laboratory test of Ethanol

1. Esterification test — (studied in chemical properties – esterification reaction)

2. Iodoform test (test for the group)

Alcohols having the CH₃CH(OH)– group respond to this test. When alcohols having the above-mentioned group are treated with I₂ and aq. Alkali (NaOH), iodoform is formed, which is a yellow crystalline precipitate with a characteristic hospital smell.