Alkanols is a homologous series with general molecular formula of CnH2n+1OH or ROH.
The functional group in alkanols is the hydroxyl (-OH) group.
The names of alkanols are obtained by substituting “e” in alkanes with “ol”.
Methanol – CH3OH, Ethanol – CH3CH2OH
The alkanols are classified based on the number of alkyl groups directly linked to the carbon atom carrying the hydroxyl group.
- 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
- 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
- Tertiary alkanols (30 ): The alkanols here have three alkyl groups attached to the carbon atom holding the hydroxyl group
CH3 C CH3
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.
- Monohydric alkanols: This type has only one hydroxyl(–OH), present in its molecule.
Example: C2H5OH, C3H7OH.
- Dihydric alkanols: This type has two of hydroxyl group per molecule.
H C OH H C OH
H C OH H C H
H H C OH
Ethan -1, 2- diol Propan- 1,3-diol
- Polyhydric alkanols: This type has three or more hydroxyl groups per molecule.
H C OH
H C OH
H C OH
- Name the functional group in the alkanol.
- Give an example each by writing the structure and names of the classes of alkanols.
- Hydrolyzing ethyl esters with hot alkali
- Reducing ethanol with nascent hydrogen
- 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.
- 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
- Describe fully, the production of ethanol from a named starchy material/food.
- What type of chemical reaction is involved in fermentation of sugar?
- Ethanol is a colourless volatile liquid.
- It is soluble in water.
- It has boiling point of 780C.
- It has no action on litmus paper.
- Combustion: The lower members of alkanols burn with clean flames in plenty air
2CH3OH + 3O2 2CO2+H2O
- 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 C OH [O ] CH3 C = O + H2O
– 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.
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
Alkanols are dehydrated to alkenes in the presence of concentrated H2SO4.
CH3CH2OH + H2SO4 CH3CH2HSO4 + H2O
CH3CH2HSO4170oCC2H4 + H2SO4
- 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
- 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
- It is used as organic solvent.
- It is the main constituent of methylated spirit used to clean wounds and to dissolve paint.
- It is used as petrol addictive for use as fuel in vehicles.
- It is used to manufacture other chemicals such as ethanol and ethanoic acid.
- It is used as ingredient in making alcoholic drinks e.g. beers, wines and spirits.
- It is used as anti-freeze in automobile radiator because of its low freezing point (-1170C).
- Describe how ethanol can be prepared from cane sugar.
- Using balanced equations, state five chemical properties of ethanol.
- Describe a test to identify an unknown solution to be ethanol.
- What is the number of oxygen atoms in 32g of the gas? [NA = 6.02 x 1023]
- 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]
New School Chemistry for Senior Secondary School by O.Y.Ababio (6th edition) pages 539-544.
SECTION A:Write the correct option ONLY.
- The functional groups of the alkanolis(a).double bond (b). carboxyl group (c). hydroxyl group (d). triple bond
- 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
- 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
- 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
- Which of the following enzymes converts glucose to ethanol?(a). maltose (b).zymase
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.
INTRODUCTION TO ORGANIC CHEMISTRY