ALKANOLS

Alkanols is a homologous series with general molecular formula of C_nH_2n+1OH or ROH.

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

 

NOMENCLATURE

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

Example:

Methanol – CH₃OH, Ethanol – CH₃CH₂OH

 

CLASSIFICATION

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

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

H         H                                             H

 

 

H     C         C   OH                             H    C   OH

 

 

H         H                                             H

Ethanol                                                Methanol

1. Secondary alkanols (2⁰): They have two alkyl groups directly linked with the carbon atom carrying the hydroxyl group. Example

H         H         H

 

 

HC      C         C         H

 

 

H         OH      H

Propan – 2- ol

2. Tertiary alkanols (3⁰ ): The alkanols here have three alkyl groups attached to the carbon atom holding the hydroxyl group

CH₃

 

 

CH₃     C     CH₃

 

 

OH

2 – methylpropan -2 –ol

 

TYPES OF ALKANOLS

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 hydroxyl(–OH), present in its molecule.

Example: C₂H₅OH, C₃H₇OH.

2. Dihydric alkanols: This type has two of hydroxyl group per molecule.

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 hydroxyl groups per molecule.

Example

H

 

 

H         C         OH

 

 

H         C         OH

 

 

H         C         OH

 

 

H

 

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.

 

ETHANOL

LABORATORY PREPARATION

1. Hydrolyzing ethyl esters with hot alkali
2. Reducing ethanol with nascent hydrogen

 

COMMERCIAL PREPARATION

1. From ethene: Ethene is obtained by the cracking of petroleum. It is then absorbed in 95% H₂SO₄ at 80⁰C and 30 atm to form ethyl hydrogen tetraoxosulphate (VI)

C₂H₄  +  H₂SO₄                       C₂H₅HSO₄

 

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

C₂H₅HSO₄+ H₂O       C₂H₅OH + H₂SO₄.

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

2. Preparation 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 organicmolecules to simple molecule by micro-organism.The common micro-organism used is YEAST

 

PRODUCTION OF ETHANOL FROM STARCHY FOOD

Ethanol can be prepared from starchy food like 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 (partially germinated barley which contains the enzyme, DIASTASE) at 50⁰C for one hour.
• The starch is then converted to MALTOSE.

2(C₆H₁₀O₅)n + nH₂O              nC₁₂H₂₂O₁₁

• Then yeast is added 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  ^maltase2C₆H₁₂O₆

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         2CO₂+H₂O

2. Oxidation: The products of oxidation depend on the structure of the alkanol.

–           Primaryalkanols are oxidized to alkanal first and then to alkanoic acid in the presence of oxidizing agent e.g KMnO₄

CH₃CH₂OH    ^O          CH₃CHO        ^O          CH₃COOH

–           Secondary alkanols oxidize to alkanone. Example

CH₃                                         CH₃

 

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

propanone

H

Propon-2-ol

 

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

 

Note: The colour change of oxidizing agent if acidified is purple KMnO₄ change to colourless and range K₂Cr₂O₇ turns green.

 

3. Esterification:

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

CH₃CH₂OH  +CH3COOH        H⁺     CH₃COOCH₂CH₃  +  H₂O

4. Dehydration:

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

CH₃CH₂OH    +  H₂SO₄            CH₃CH₂HSO₄  +  H₂O

CH₃CH₂HSO₄₁₇₀^o_CC₂H₄  +  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.

2C₂H₃OH  +  Na                  2C₂H₃ONa  +  H₂

6. Reactions with the chlorides of phosphorus: Ethanol reacts vigorously with PCl₅ in the cold to produce fumes of HCl and chloroethanevapour.

C₂H₅OH + PCl₅C₂H₅Cl  +  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. It is 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).

GENERAL EVALUATION/REVISION

1. Describe how ethanol can be prepared from cane sugar.
2. Using balanced equations, state five chemical properties of ethanol.
3. Describe a test to identify an unknown solution to be ethanol.
4. What is the number of oxygen atoms in 32g of the gas? [N_A = 6.02 x 10²³]
5. 5.6dm3 of oxygen gas was evolved at the anode during the electrolysis of dilute copper (II) tetraoxosulphate (VI) using platinum electrodes. What mass of copper is deposited at the cathode during the process? [Cu = 64, Molar volume of a gas at s.t.p = 22.4dm³, 1F = 96500C]

 

READING ASSIGNMENT

New School Chemistry for Senior Secondary School by O.Y.Ababio (6^th edition) pages 539-544.

 

WEEKEND ASSIGNMENT

SECTION A:Write the correct option ONLY.

1. The functional groups of the alkanolis(a).double bond (b). carboxyl group    (c). hydroxyl group    (d). triple bond
2. Primaryalkanols are oxidized to carboxylic 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 alkenes
3. The solubility of alkanols in water is due to (a). the covalent nature(b).hydrogenbonding

(c).their low melting point  (d).their low melting point

4. When acidified KMnO₄ is used as oxidizing agent for alkanol, the colour change observed is(a). yellow to red (b). purple to colourless(c). orange to green (d).white to black
5. Which of the following enzymes converts glucose to ethanol?(a). maltose (b).zymase

(c).diastase (d).amylase

 

SECTION B

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

(b). Explain the structural different between secondary and tertiary alkanols giving one example each.

2          (a).What is fermentation?

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

 

See also

ALKYNES II

ALKENES I

ALKANES

INTRODUCTION TO ORGANIC CHEMISTRY

ACID/BASE REACTIONS