Alkanols – SS2 Chemistry Lesson Note (Third Term)

Alkanols are organic compounds whose functional group is the hydroxyl (-OH) group. They are named by replacing the “e” in the alkane name with “ol”, as in methanol (CH₃OH) and ethanol (CH₃CH₂OH). They are also grouped as primary, secondary or tertiary, and as monohydric, dihydric or polyhydric.

Behavioural objectives

By the end of the lesson, students should be able to:

• name simple alkanols and state their functional group;
• classify alkanols as primary, secondary or tertiary, and as monohydric, dihydric or polyhydric;
• describe the laboratory and commercial preparation of ethanol, including fermentation of starchy food;
• state the physical properties of ethanol;
• describe the chemical reactions of ethanol and write their equations;
• list the uses of ethanol.

What are alkanols?

The functional group in alkanols is the hydroxyl (-OH) group.

Nomenclature of alkanols

The names of alkanols are obtained by substituting “e” in alkanes with “ol”.

Example: methanol, CH₃OH; ethanol, CH₃CH₂OH.

How are alkanols classified?

Alkanols are classified by the number of alkyl groups directly linked to the carbon atom carrying the hydroxyl group.

1. Primary alkanols: These have only one alkyl group attached to the carbon atom that carries the hydroxyl group.
2. Secondary alkanols: These have two alkyl groups directly linked to the carbon atom carrying the hydroxyl group.
3. Tertiary alkanols: These have three alkyl groups attached to the carbon atom holding the hydroxyl group.

What are the types of alkanols?

The type of alkanol is determined by the number of hydroxyl groups (-OH) present in the molecule.

1. Monohydric alkanols: These have only one hydroxyl group (-OH) in the molecule. Examples: C₂H₅OH, C₃H₇OH.
2. Dihydric alkanols: These have two hydroxyl groups per molecule.
3. Polyhydric alkanols: These have three or more hydroxyl groups per molecule.

How is ethanol prepared?

Laboratory preparation

1. Hydrolysing ethyl esters with hot alkali.
2. Reducing ethanal with nascent hydrogen.

Commercial preparation

1. From ethene

Ethene is obtained by the cracking of petroleum. It is then absorbed in 95% H₂SO₄ at 80°C and 30 atm to form ethyl hydrogen tetraoxosulphate (VI).

C₂H₄ + H₂SO₄ → C₂H₅HSO₄

The ethyl hydrogen tetraoxosulphate (VI) is hydrolysed by boiling water to produce ethanol.

C₂H₅HSO₄ + H₂O → C₂H₅OH + H₂SO₄

The ethanol is distilled off, leaving the acid behind, which can be used again.

2. By fermentation

Ethanol is prepared industrially from raw materials containing starch or sugar by the process of fermentation. Fermentation is an enzymatic process which involves the decomposition of large organic molecules into simple molecules by micro-organisms. The common micro-organism used is yeast.

How is ethanol produced from starchy food?

Ethanol can be prepared from starchy food like rice, potatoes and maize. The following steps are involved:

1. Crush and pressure cook the starchy materials.
2. Extract the starch granules by mixing them with water.
3. Allow the starch granules to settle and decant.
4. Treat the starch granules with malt (partially germinated barley, which contains the enzyme diastase) at 50°C for one hour.
5. The starch is then converted to maltose.

   2(C₆H₁₀O₅)_n + nH₂O → nC₁₂H₂₂O₁₁

6. Add yeast at room temperature for some time (at least one day). Yeast contains two enzymes, maltase and zymase. Maltase converts maltose to two glucose units, while zymase converts the glucose to ethanol and carbon (IV) oxide.

   C₁₂H₂₂O₁₁ + H₂O → (maltase) 2C₆H₁₂O₆

   C₆H₁₂O₆ → (zymase) 2C₂H₅OH + 2CO₂

What are the properties of ethanol?

Physical properties

1. Ethanol is a colourless volatile liquid.
2. It is soluble in water.
3. It has a boiling point of 78°C.
4. It has no action on litmus paper.

Chemical properties

1. Combustion

The lower members of alkanols burn with clean flames in plenty of air.

2CH₃OH + 3O₂ → 2CO₂ + 4H₂O

2. Oxidation

The products of oxidation depend on the structure of the alkanol.

• Primary alkanols are oxidised first to alkanal and then to alkanoic acid in the presence of an oxidising agent, e.g. KMnO₄.

  CH₃CH₂OH → [O] CH₃CHO → [O] CH₃COOH

• Secondary alkanols are oxidised to alkanone.
• Tertiary alkanols are not oxidised because there is no carbon-hydrogen bond to be broken for the oxidation to take place.

Note: When the oxidising agent is acidified, purple KMnO₄ changes to colourless and orange K₂Cr₂O₇ turns green.

3. Esterification

This is the reversible reaction between an alkanol and an alkanoic acid to produce a sweet-smelling compound known as an ester. The reaction is catalysed by concentrated H₂SO₄.

Example:

CH₃CH₂OH + CH₃COOH ⇌ (conc. H₂SO₄) CH₃COOCH₂CH₃ + H₂O

4. Dehydration

Alkanols are dehydrated to alkenes in the presence of concentrated H₂SO₄.

CH₃CH₂OH + H₂SO₄ → CH₃CH₂HSO₄ + H₂O

CH₃CH₂HSO₄ → (170°C) C₂H₄ + H₂SO₄

5. Reaction with sodium and potassium

Sodium and potassium react vigorously with alkanols to liberate hydrogen gas and form the corresponding organic salt of sodium and potassium.

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

6. Reactions with the chlorides of phosphorus

Ethanol reacts vigorously with PCl₅ in the cold to produce fumes of HCl and chloroethane vapour.

C₂H₅OH + PCl₅ → C₂H₅Cl + POCl₃ + HCl

PCl₃ gives a similar reaction, but less vigorous.

3C₂H₅OH + PCl₃ → 3C₂H₅Cl + H₃PO₃

What are the uses of ethanol?

1. It is used as an organic solvent.
2. It is the main constituent of the methylated spirit used to clean wounds and dissolve paint.
3. It is used as a petrol additive for use as fuel in vehicles.
4. It is used to manufacture other chemicals such as ethanal and ethanoic acid.
5. It is used as an ingredient in making alcoholic drinks, e.g. beers, wines and spirits.
6. It is used as an antifreeze in automobile radiators because of its low freezing point (-117°C).

Evaluation

1. Name the functional group in alkanols and state how alkanols are named from alkanes.
2. Distinguish between primary, secondary and tertiary alkanols.
3. State the products of oxidation of primary, secondary and tertiary alkanols.
4. Describe how ethanol is produced from starchy food.
5. List four uses of ethanol.

Assignment

Write the equation for each of the following reactions of ethanol and state the conditions needed: (a) esterification with ethanoic acid, (b) dehydration with concentrated H₂SO₄, (c) reaction with sodium, (d) reaction with PCl₅.