Alkanols (Alcohols)

Alkanols are a homologous series of organic compounds with the general molecular formula \( C_nH_{2n+1}OH \) or \( ROH \). They contain the hydroxyl (\( -OH \)) functional group.

Nomenclature

Alkanol names are derived by replacing the “e” in the name of the corresponding alkane with “ol”.

Classification of Alkanols

Alkanols are classified based on the number of alkyl groups attached to the carbon atom bearing the hydroxyl group.

  1. Primary Alkanols (1°): One alkyl group is attached to the carbon atom carrying the hydroxyl group.
  2. Secondary Alkanols (2°): Two alkyl groups are attached to the carbon atom bearing the hydroxyl group.
  3. Tertiary Alkanols (3°): Three alkyl groups are attached to the carbon atom bearing the hydroxyl group.
Alcohol Diagram Credit: Libretexts

Types of Alkanols

The type of alkanol depends on the number of hydroxyl groups in the molecule:

  1. Monohydric Alkanols: Contain one hydroxyl group. e.g., \( C_2H_5OH \), \( C_3H_7OH \)
  2. Dihydric Alkanols: Contain two hydroxyl groups.
  3. Polyhydric Alkanols: Contain three or more hydroxyl groups.

Ethanol

Laboratory Preparation

  1. Hydrolysis of ethyl esters using hot alkali.
  2. Reduction of ethanol with nascent hydrogen.

Commercial Preparation

1. From Ethene

Ethene from petroleum cracking reacts with concentrated sulfuric acid at 80°C and 30 atm:

\( C_2H_4 + H_2SO_4 \rightarrow C_2H_5HSO_4 \)

The product is hydrolyzed to form ethanol:

\( C_2H_5HSO_4 + H_2O \rightarrow C_2H_5OH + H_2SO_4 \)

Ethanol is distilled, and sulfuric acid can be reused.

2. By Fermentation

Ethanol can be produced industrially from starch- or sugar-rich raw materials via fermentation using yeast.

Production from Starchy Foods

  1. Crush and cook the starchy material (e.g., maize, rice).
  2. Extract starch with water and allow to settle.
  3. Treat starch with malt (contains the enzyme diastase) at 50°C.
  4. Starch is converted to maltose:

    \( 2(C_6H_{10}O_5)_n + nH_2O \rightarrow nC_{12}H_{22}O_{11} \)

  5. Add yeast, which contains:
    • Maltase: Converts maltose to glucose

      \( C_{12}H_{22}O_{11} + H_2O \xrightarrow{maltase} 2C_6H_{12}O_6 \)

    • Zymase: Converts glucose to ethanol and carbon dioxide

      \( C_6H_{12}O_6 \xrightarrow{zymase} 2C_2H_5OH + 2CO_2 \)

Physical Properties of Ethanol

  1. Colourless and volatile liquid.
  2. Miscible with water.
  3. Boiling point of 78°C.
  4. Neutral to litmus paper.

Chemical Properties of Ethanol

1. Combustion

Ethanol burns in air with a clean flame:

\( 2CH_3OH + 3O_2 \rightarrow 2CO_2 + 4H_2O \)

2. Oxidation of Alkanols

The oxidation products of alkanols depend on their classification:

\( CH_3CH_2OH \xrightarrow{[O]} CH_3CHO \xrightarrow{[O]} CH_3COOH \)

Note: Colour changes of oxidizing agents:

3. Esterification

Esterification is a reversible reaction between an alkanol and an alkanoic acid to form a sweet-smelling ester. This reaction is catalyzed by concentrated sulfuric acid (\( H_2SO_4 \)).

\( CH_3CH_2OH + CH_3COOH \xrightarrow{H^+} CH_3COOCH_2CH_3 + H_2O \)

4. Dehydration of Alkanols

Alkanols can be dehydrated to form alkenes in the presence of concentrated \( H_2SO_4 \).

\( CH_3CH_2OH + H_2SO_4 \rightarrow CH_3CH_2HSO_4 + H_2O \)
\( CH_3CH_2HSO_4 \xrightarrow{170^\circ C} C_2H_4 + H_2SO_4 \)

5. Reaction with Sodium and Potassium

Sodium and potassium react vigorously with alkanols to release hydrogen gas and form the corresponding organic salts.

\( 2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2 \)

6. Reaction with Phosphorus Halides

Uses of Ethanol

  1. Used as an organic solvent.
  2. Main component of methylated spirit, used for cleaning wounds and dissolving paint.
  3. Added to petrol as a fuel additive for vehicles.
  4. Used in the manufacture of chemicals such as ethanoic acid and esters.
  5. Used as an ingredient in alcoholic beverages like beer, wine, and spirits.
  6. Used as antifreeze in car radiators due to its low freezing point (\( -117^\circ C \)).