ALKANOL

 

Week:                                                                                                  Date:                                                    

Period:                                 Duration: 1 HR 20 MIN.                                                 Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                        Class: SS TWO

Topic:                                    ALKANOL

Sub topic:  Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials:  Ethanol

Entry behavior: The students have been taught benzene

Behavioural objective: At the end of the lesson the students should be able to:

i.                     explain alkanol  and its classes

ii.                   explain the laboratory preparation of alkanol

iii.                  state the materials used in ethanol production

iv.                 State the product of oxidation of alkanol

CONTENT

ALKANOLS

Alkanols are a group of compounds containing the hydroxyl (-OH) group. They form a homologous series with general formula CnH2n+1OH, where n is equal to or greater than 1. Alkanols are regarded as being derived from alkanes by substituting one or more hydrogen atoms with hydroxyl groups.  They have –OH functional group (hydroxyl group). All alcohols end with suffix -ol

First three members of the series (so that you’d have idea on the next)

        -     Methanol, CH3OH
        -     Ethanol, C2H5OH or CH3CH2OH
        -     Propanol, C3H7OH or CH3CH2CH2OH

For alkanol, the –OH is not of hydroxide ion, OH-, but is covalent bond between oxygen and hydrogen, O – H

CLASSIFICATION OF ALKANOLS BASED ON THE NUMBER OF ALKYL GROUP

The general formula is R-OH where R is alkyl group. There are three types of alkanols based on this classification.

1.       PRIMARY  ALKANOL: As only alkyl group attached to the carbon atom, that carries the hydroxyl group e.g [RCH2OH]

Ethanol

2.       SECONDARY ALKANOL: It has two alkyl groups  attached to the carbon atom that carries the hydroxyl group e.g

[R2CHOH] i.e  propan-2-ol.

3.       TERTIARY ALKANOL: It has three alkyl groups attached to the carbon atom that carries the hydroxyl group e.g [R3COH]

2- methyl propan-2-ol

Properties of OH- group as functional group

Alkanols e.g ethanol are more reactive than the corresponding alkanes. The increase in reactivity is due to the presence of the OH- group which is the functional group.

Ethanol

Ethanol has a molecular formula C2H5OH and the structure

It is a primary alkanol and a very important compound, because it has more uses than almost any other organic compound. It is commonly called alcohol.

LABORATORY PREPARATION OF ETHANOL

Ethanol can be prepared in the laboratory by

I.Hydration of ethene

CH2=CH2(g)   +  H2O(g)  à   CH3CH2OH(l)

A mixture of ethene  and steam with tetraoxosulphate (vi) acid as catalyst is heated to a temperature of 600oC and a pressure of 80 atm.

II.Reducing ethanal with nascent hydrogen in the presence of platinum catalyst.

CH3CHO(aq)     +   H2(g)      à    CH3CH2OH(l)

III.Hydrolysing ethyl ester with a hot alkali

CH3COOC2H5  +   KOH    à   C2H5OH  +  CH3COOK

IV.Hydrolysing iodoethane with an alkali

C2H5I  +  NaOH     à   C2H5OH    +    NaI

Industrial preparation of ethanol

Commercially, ethanol is prepared:

i.                     From ethene             ii. by fermentation


1. Reacting Ethene with Steam

Ethene and steam are passed over phosphoric acid H3PO4 (as a catalyst) under high temperature of 300oC and pressure of 65 atm or over tetraoxosulphate (vi) acid as catalyst at 600oC and 80 atm.

C2H4 (g) + H2O (g) ⇌ C2H5OH (aq)

Since this is reversible reaction, both ethene and water are produced aside from ethanol. 

The ethanol is separated by fractional distillation.

2. Fermenting glucose

Fermentation is breakdown of sugars into smaller molecules by microorganisms.

C6H12O6 (aq) --> 2C2H5OH (aq) + 2CO2 (g)

Temperature is kept constant at 37oC to prevent destruction of yeast at higher temperatures. 

Oxygen is removed by limewater and carbon dioxide is produced during fermentation. 

Alcohol is separated from solution by fractional distillation.

Materials used in ethanol production

In practice, glucose is not the starting material.

Potatoes, cassava and cereal: the starchy food is crushed and treated with steam to extract the starch granules. Malt is added at 55oC for one hour. ( malt is a partially germinated barley which contain enzyme diastase). This enzyme converts starch to maltose.

2(C6H10O5)n(s) +  nH2O    à    nC12H22O11(aq)

    [starch]                                   [maltose]

 Yeast is then add at room temperature to release maltase, which converts maltose to glucose

C12H22O11(aq)    +  H2O   à    C6H12O6(aq)

Enzyme zymase which is also present in yeast convert the glucose into ethanol and carbon (iv) oxide

C6H12O6(aq)   à   2C2H5OH(aq)   +   2CO2

PHYSICAL PROPERTIES OF ETHANOL

I.                    Is a colourless, volatile liquid with a characteristic taste and smell.

II.                  It is readily soluble in water in all proportions. This is because the hydroxyl groups in their molecules can form hydrogen bond with water.

III.                It has a boiling point of 780C

IV.                It is neutral to litmus.

Note: boiling point and solubility of alkanols are affected by the presence of hydrogen bonding.

CHEMICAL PROPERTIES OF ETHANOL

1) Combustion

Ethanols burn in air or oxygen with a pale-blue flame to produce carbon dioxide and water.

E.g. combustion of ethanol
C2H5OH (aq) + 3O2 (g) -->  2CO2 (g) + 3H2O (l)
Example:


2) Oxidation
a. Alcohol can be prepared in laboratory by warming alcohol with oxidizing agent (e.g. acidified potassium chromate(VI)). 

The product is carboxylic acid and water.


E.g. oxidation of ethanol produces water and ethanoic acid
C2H5OH (aq) + 2[O] {from oxidizing agent} ---> 2CH3COOH (g) + 3H2O (l)
b. Alcohol can be oxidized when left in air with bacterial enzymes as catalyst. 

The products are carboxylic acid and water.
E.g. ethanol produces water and ethanoic acid when left in air.
C2H5OH (aq) + O2 (g) ---> 2CH3COOH (aq) + 3H2O (l)

3) Dehydration

With excess concentrated tetraoxosulphate (vi) acid at 180oC , ethanol reacts to form ethyl hydrogen tetraoxosulphate (vi) which then decomposes to yield ethene.

C2H5OH   +  H2SO4       à    C2H5HSO4   +  H2O

C2H5HSO4     à     C2H4  +    H2SO4    

If on the other hand, the alkanol  is in excess at a lower temperature, it will react with the ethyl hydrogen tetraoxosulphate (vi) to yield ethoxyethane

C2H5HSO4     à     C2H5O C2H5  (ethoxyethane)   +    H2SO4    

4) Esterification

Alkanols react reversibly with carboxylic acids in the presence of mineral acid to form alkanoates (esther). The process is known as esterification.

Alkanol  + acid     à  ester   + water.  E.g  ethanol reacts  with ethanoic acid to give ethyl ethanoates

CH3COOHaq     +   C2H5OHaq   à    CH3COOC2H5( l)    +  H2O(l)

5) Reaction with sodium or potassium

Ethanol reacts with sodium at room temperature, to produce hydrogen and sodium ethoxide

C2H5OHaq     + Na  àC2H5ONa(aq)    +  H2(g)

USES OF ETHANOL

1.       Ethanol  is a useful solvent for the production of perfumes, varnishes, soap, detergent etc

2.       Used as fuel

3.       Used as  an anti-freeze in automobile radiators (because it has low freezing point of – 117oC)

4.       Used as antiseptic in surgical spirits for hardening skin in treatment of pimples and sterilization of biological specimen and food

PRESENTATION

Step I: The teacher explains alkanol and its classification.

Step II: The teacher explains the laboratory preparation of alkanol.

Step III: The teacher states the materials used in the ethanol production.

Step IV: The teacher explains  the physical and the chemical properties of alkanol

Step V: The students state some uses of ethanol.

EVALUATION

The teacher evaluates the lessons by asking these questions from the students

i.                     explain alkanol  and its classes

ii.                   explain the laboratory preparation of alkanol

iii.                  state the materials used in ethanol production

iv.                 State the product of oxidation of alkanol

 ASSIGNMENT

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