ALKANOLS

SOURCES OF ALKANOLS

–           From destructive distillation wood.

–           From starchy food and sugar

 

General molecular formular

Alkanol is a homologous series with general molecular formular of C_n H_2n+1OH or ROH.

Or (C_nH_2n+2O).

 

Nomenclature

The names of alkanols are obtained by substituting “e” in alkanes with “Ol” in alkanol e.g. methanol

(CH₃OH), ethanol (CH₃CH₂OH).

 

Classification

The alkanols are classified based on the number of alkyl groups directly linked to the carbon atom

holding the hydroxyl group.

1. Primary alkanols (1⁰): It has only one alkyl group attached to the carbon atom that carries the

hydroxyl group e.g.                                     H

R –       C     OH

H

OR

H          H                                              H

H      C          C   OH          or     H     C   OH

H          H                                              H

Ethanol                                                Methanol

1. Secondary alkanols (2⁰):- They have 2 alkyl groups directly linked with the carbon atom holding the hydroxyl group e.g.

 

 

H          H          H                                              H

H          C          C          C          H          or         R          C          R¹

H          OH       H                                              OH

Propan – 2-ol

III. Tertiary alkanols (3⁰ ):  the alkanols here have 3 alkyl groups attached to the carbon atom holding the hydroxyl group

CH₃                                           R¹

 

CH₃       C          CH₃         or       R          C          OH

 

OH                                           R¹¹

2 – methylpropan -2 –ol

The R¹, R¹¹ are alkyl groups which may be the same or not.

N.B:      Examine the structure of the alkanol, identify the carbon atom bonded to the –OH, count the number of alkyl groups that are directly attached to the carbon atom.  Then you can say whether it is 1⁰, 2⁰ or 3⁰.

 

EVALUATION

1. Name the functional group in the alkanol.
2. Give an example each by writing the structure and names of the classes of alkanols.

 

Types of Alkanol

The type of alkanols is determined by the number of the hydroxyl group –OH, present in the molecule.

 

1. MONOHYDRIC ALKANOLS: This type has only one –OH, present in its molecule

e.g. C₂H₅OH, C₃H₇OH e.t.c.

2. DIHYDRIC ALKANOLS: This type has two of –OH group per molecule e.g.

H                                              H

 

H          C          OH                   H          C          OH

 

H          C          OH                   H          C          H

 

H                                  H          C          OH

 

H

Ethan -1, 2- diol                             Propan 1, 3-diol

3. POLYHYDRIC ALKANOLS: This type has three or more.  OH groups per molecule e.g. glycerol.

 

H                                                                       H

 

H          C          OH                                           H          C          OH

 

H          C          OH                                           H          C          OH

 

H          C          OH                                           H          C          OH

 

H                                                          H          C          OH

 

H          C          OH

 

H          C         OH

 

H

PREPARATION   e.g. ethanol

 

1. Laboratory preparation

–           Hydrolyzing ethyl esters with hot alkali

–           Reducing ethanol with nascent hydrogen

 

2. Commercial preparation

–           From ethene

Ethene is obtained by the cracking of petroleum. It is then absorbed in 95% H₂SO4 at 80⁰C and

30 atoms to form C₂H₅HSO4

C₂H₄  +  H₂SO₄              C₂H₃HSO₄

The C₂H₃HSO₄ is bedded in water to produce ethanol; C₂H₃HSO4 + H₂O               C₂H₃OH + H₂SO₄.

 

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

-Preparation by fermentation ethanol is prepared industrially from raw materials containing

starch or sugar by the process of fermentation.

-The fermentation is an enzymatic process which involves the decomposition of large organic

molecules to simple molecule by micro-organism.

-The common micro-organism used is YEAST

 

EVALUATION

1. Name three (3) types of alkanol and give one example each.
2. State two methods by which ethanol can be prepared industrially.

PRODUCTION OF ETHANOL FROM STARCHY MATERIALS

(e.g. rice, potatoes, maize etc)

The following steps are involved;

• Crush and pressure cook the starchy materials.
• Extract the starch granules by mixing with water.
• Allow the starch granules to settle and decant
• Treat the starch granules with malt (a partially germinated barley which contains the enzyme, DIASTASE) at 50⁰C for I hour.
• The starch is then converted to MALTOSE. 2(C₆H₁₀O₅)n + nH₂O nC₁₂H₂₂O₁₁

Then add yeast at room temperature for some time (at least one day).  Yeast contains two enzymes,

namely 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

C₆ H₁₂ O₆    ^Zymase                    2C₂H₃OH   +   CO₂

Ethanol

EVALUATION

1. Describe fully, the production of ethanol from a named starchy material/food.
2. What type of chemical reaction is involved in fermentation of sugar?

 

PHYSICAL PROPERTIES

1. Ethanol is a colourless volatile liquid.
2. It is soluble in water.
3. It has boiling point of 78⁰C.
4. It has no action on litmus paper.

 

CHEMICAL PROPERTIES

1. Combustion: the lower members of alkanols burn with clean flames in plenty air

2CH₃OH  +  3O₂                2CO_{2            4} +H₂O

2. Oxidation

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

–           Primary alkanols are oxidized to alkanol first and then to alkanoic acid in the presence of oxidizing agent e.g KNnO₄

CH₃CH₂OH        ^O           CH₃CHO            ^O           CH₃COOH

–           Secondary alkanols oxidize to ketone e.g.

R                                                          R

 

R          C          OH       [O]                    R        C   = O  +  H₂O

 

H

 

CH₃                                           CH₃

 

CH₃       C          OH         [O ]               CH₃        C  = O  +   H₂O

 

 

H

Propon-2-ol                              propanone

 

–           Tertiary alkanols are not oxidized because there is no carbon-hydrogen bond to be broken for the oxidation to take place.

 

N.B:      Note the colour change of oxidizing agent if acidified.

–           Purple KMnO₄ changes to colourless.

–           Orange K₂Cr₂O₇ turns green.

 

3. Esterification

This is the reversible reaction between alkanol and alkanoic and to produce ester.  The reaction is catalysed by conc H₂SO₄ e.g. CH₃CH₂OH  +  CH₂COOCH₂CH₃  +  H₂O

 

4. Dehydration

Alkanols are dehydrated to alkenes in the presence of conc H₂SO₄

e.g. CH₃CH₂OH    + conc H₂SO₄ CH₃CH₂HSO₄  +  H₂O

CH₃CH₂HSO_{4170 C}           C₂H₄  +  H₂SO₄

 

5. Reaction with sodium and potassium

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

E.g.2C₂H₃OH  +  Na               2C₂H₃ONa  +  H₂

N.B:   If sodium ethoxide reacts with water, it hydrolyses to ethanol and alkaline solution of NaOH.

6. Reactions with the chlorides of phophorus: Ethanol reacts vigorously with PCl₅ in the cold

to produce time of HCl and chloroethanevapour  C₂H₅OH + PCl₅         C₂H₃Cl  +  POCl₃  + HCl

-PCl₃ gives a similar reaction, but less vigorous C₂H₅OH  +  PCL₃             3C₂H₅Cl   +  H₃PO₃.

 

USES OF ETHANOL

1. It is used as organic solvent
2. It is the main constituent of methylated spirit used to clean wounds and to dissolve paint.
3. It is used as petrol addictive for use as fuel in vehicles.
4. Used to manufacture other chemicals such as ethanol and ethanoic acid.
5. It is used as ingredient in making alcoholic drinks e.g. beers, wines and spirits.
6. It is used as anti-freeze in automobile radiator because of its low freezing point (-117⁰C).

 

TEST FOR ALKANOLS

–           It librates hydrogen gas on reacting with sodium methal.

–           It changes to alkanoic acid (primary alkanol) on exposure to air or reacting with oxidizing agent.

 

EVALUATION

1. State five (5) chemical properties of ethanol.

2a.       Identify two (2) physical properties of ethanol

1. State two (2) uses of alkannol.

 

WEEKEND ASSIGNMENT

1. The functional groups of the alkanol is
2. CnH_{2n + 1} OH     B.  carboxylic group    C.  hydroxyl group    D.  CnH_{2n+ 2}
3. Primary alkanols are oxidized to carbonylic acid, secondary alkanols are oxidized to alkanones while tertiary alkanols are A.  oxidized to alkanols    B.  oxidized to alkanones    C.  not oxidized    D.  oxidized to alkene
4. The soluble of alkanols in water is due to
5. the covalent nature    B.  hydrogen bonding    C.  their low melting point
6. their low melting point
7. When acidified KMnO4 is used as oxidizing agent for alkanol, the colour change observed is
8. yellow to red     B.  purple to colourless    C.  orange to green    D.  white to black
9. Which of the following enzymes converts glucose to ethanol?
10. maltose       B.  zymase        C.  diastase       D.  amylase

 

THEORY

1(a)      Write the structural formula of two named secondary alkanols.

(b)      Explain the structural difference between primary and tertiary alkanols.

2(a)      What is fermentation?

(b)      Describe the preparation of ethanol from table sugar.

 

See also

AROMATIC HYDROCARBONS

ALKYNES

UNSATURATED HYDROCARBONS

Saturated Hydrocarbons

SIMPLE MACHINE