Alkanols

Back to: CHEMISTRY SS2

 

Welcome to Class !!

We are eager to have you join us !!

In today’s Chemistry class, We will be learning about Alkanols. We hope you enjoy the class!

 

 

CONTENT

• Types and Classes
• Industrial Production by Fermentation
• Properties and Uses

 

Alkanols is a homologous series with the 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.
2. Secondary alkanols (2⁰): They have two alkyl groups directly linked with the carbon atom carrying the hydroxyl group.
3. Tertiary alkanols (3⁰ ): The alkanols here have three alkyl groups attached to the carbon atom holding the hydroxyl group

 

 

 

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 hydroxyl group per molecule.

 

3. Polyhydric alkanols: This type has three or more hydroxyl groups per molecule.

 

 

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 30atm 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 organic molecules into simple molecules by micro-organisms. 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  ^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 the fermentation of sugar?

 

PHYSICAL PROPERTIES

1. Ethanol is a colourless volatile liquid.
2. It is soluble in water.
3. It has a 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 of air

2CH₃OH  +  3O₂                  2CO₂+H₂O

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

–  Primary alkanols 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 are oxidized to alkanone. Example

 

–  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 a 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

CH₃CH₂HSO      ₄₁₇₀^o_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.

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 chloroethane vapour.

C₂H₅OH + PCl₅            C₂H₅Cl  +  POCl₃  + HCl

PCl₃ gives a similar reaction, but less vigorous.

C₂H₅OH  +  PCl₃                  3C₂H₅Cl   +  H₃PO₃

 

USES OF ETHANOL

1. It is used as an 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 alkanols is  (a).double bond     (b). carboxyl group    (c). hydroxyl group    (d). triple bond
2. Primary alkanols 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).hydrogen bonding (c).their low melting point  (d).their low melting point

4. When acidified KMnO₄ is used as an 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 primary alkanols.

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

 

2        (a).What is fermentation?

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

 

 

 

We have come to the end of this class. We do hope you enjoyed the class?

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

We have come to the end of this term and hence SS2. It’s been a remarkable journey and we are glad that you have made it this far. For making it this far, we commend you for being resilient, you have taken charge of your education and future.

The Journey still continues though, we are moving on to SS3. we hope to meet you there. 

 

Get more class notes, videos, homework help, exam practice on Android [DOWNLOAD]

Get more class notes, videos, homework help, exam practice on iPhone [DOWNLOAD]