Ethanol

Ethanol, C2H5OH, (also called Ethyl Alcohol) is the second member of the aliphatic alcohol series. It is a clear colourless liquid with a pleasant smell. Except for alcoholic beverages, nearly all the ethanol used industrially is a mixture of 95% ethanol and 5% water, which is known simply as 95% alcohol. Although pure ethyl alcohol (known as absolute alcohol) is available, it is much more expensive and is used only when definitely required.

Preparation

Ethanol is prepared as 95% alcohol (i.e. a 95% solution of ethanol in water) by distillation of the solution which results from the fermentation of sugars.

Manufacture

There are two major industrial pathways to ethanol. Ethanol which is intended for industrial use is made by the first method, while ethanol intended for food use tends to be made by the second method.

Reaction of Ethene with Steam

Most of the ethanol used in industry is made, not by alcoholic fermentation, but by an addition reaction between ethene and steam.


	C2H4     +      H2O   ==>       C2H5OH  
	Ethene          Steam             Ethanol

Alcoholic Fermentation

A solution of sucrose, to which yeast is added, is heated. An enzyme, invertase, which is present in yeast is added and this acts as a catalyst to convert the sucrose into glucose and fructose.,


                               invertase       
C12H22O11  +  H2O               ==>     C6H12O6 + C6H12O6       
Sucrose                                              Glucose     Fructose

The glucose, C6H12O6, and fructose, C6H12O6, formed are then converted into ethanol and carbon dioxide by another enzyme, zymase, which is also present in yeast.
```
	        zymase  
C6H12O6              ==>     2C2H5OH + 2CO2  
		        Ethanol 
```
The fermentation process takes three days and is carried out at a temperature between 250C and 300C. The ethanol is then obtained by fractional distillation.

Absolute Ethanol

Whatever method of preparation is used, the ethanol is initially obtained in admixture with water. The ethanol is then extracted from this solution by fractional distillation.

Although the boiling point of ethanol, 78.3 degC, is significantly lower than the boiling point of water, 100 degC, these material cannot be separated completely by distillation. Instead, an azeotropic mixture (i.e. a mixture of 95% ethanol and 5% water) is obtained, and the boiling point of the azeotrope is 78.15oC. In a distillation, the most volatile material (i.e. the material that has the lowest boiling point) is the first material to distill from the distillation flask, and this material is the azeotrope of 95% ethanol which has the lowest boiling point. If an efficient fractionating column is used, there is obtained first 95% alcohol, then a small intermediate fraction of lower concentration, and then water. But no matter how efficient the fractionating column used, 95% alcohol cannot be further concentrated by distillation.

The separation of a mixture by fractional distillation occurs because the vapour has a different composition from the liquid from which it distils (i.e. the vapour is richer in the more volatile component). We cannot separate 95% alcohol into its components by distillation, because here the vapour has exactly the same composition as the liquid; towards distillation, then, 95% alcohol behaves exactly like a pure compound.

A liquid mixture that has the peculiar property of giving a vapour of the same composition is called an azeotrope (i.e. a constant-boiling mixture). Since it contains two components 95% alcohol is a binary azeotrope. Most azeotropes, like 95% alcohol, have boiling points lower than those of their components, and are known as minimum-boiling mixtures. Azeotropes having boiling points higher than those of their components are known as maximum-boiling mixtures.

Physical Properties

Ethanol is a colourless liquid with a pleasant smell. It is completely miscible with water and organic solvents and is very hydroscopic.


		Melting Point           -1150C  
		Boiling Point              780C 
		Specific Gravity         0.79.

Chemical Properties of Ethanol

Combustion of Ethanol

Ethanol burns with a pale blue, non luminous flame to form carbon dioxide and steam.


	C2H5OH + 3O2    ==>         2CO2 + 3H2O 
	Ethanol

Oxidation of Ethanol

Ethanol is oxidised

with acidified Potassium Dichromate, K2Cr2O7, or
with acidified Sodium Dichromate, Na2Cr2O7, or
with acidified potassium permanganate, KMnO4,

to form ethanal, (i.e. acetaldehyde).


		[O]     
	C2H5OH  ==>     CH3CHO + H2O    
	Ethanol              Ethanal

The ethanal is further oxidised to ethanoic acid (i.e. acetic acid) if the oxidising agent is in excess.


		[O]     
	CH3CHO  ==>     CH3COOH 
	Ethanal         Ethanoic Acid

The oxidising agent usually used for this reaction is a mixture of sodium dichromate or potassium dichromate and sulphuric acid which react together to provide oxygen atoms as follows.


              Na2Cr2O7    +    4 H2SO4 ==>     Na2SO4 + Cr2(SO4)3 + 4H2O + 3[O]

Dehydration of Ethanol

When ethanol is mixed with concentrated sulphuric acid with the acid in excess and heated to 170 degC, ethylene is formed. (One mole of ethanol loses one mole of water)


		   H2SO4   
                  C2H5OH        ==>             C2H4 + H2O      
		170 degC

When ethanol is mixed with concentrated sulphuric acid with the alcohol in excess and heated to 140 degC, diethyl ether distils over (two moles of ethanol loses one mole of water) .


                                     H2SO4   
	2 C2H5OH        ==>     C2H5OC2H5    +    H2O   
                                      140 deg

Reaction of Ethanol with Sodium

Sodium reacts with ethanol at room temp to liberate hydrogen. The hydrogen atom of the hydroxyl group is replaced by a sodium atom, forming sodium ethoxide.


                         C2H5OH    +    Na    ==> C2H5ONa    +    (H2(

Apart from this reaction, ethanol and the other alcohols show no acidic properties.

Dehydrogenation of Ethanol

Ethanol can also be oxidised to ethanal (i.e. acetaldehyde) by passing its vapour over copper heated to 300 degC. Two atoms of hydrogen are eliminated from each molecule to form hydrogen gas and hence this process is termed dehydrogenation.


                      C2H5OH          ==>             CH3CHO    +    H2       
                        Ethanol                              Ethanal

Esterification of Ethanol

Ethanol, C2H5OH, reacts with organic acids to form esters.


                                                         H(+)    
	C2H5OH   +      CH3COOH  ==>    CH3COOC2H5 + H2O        
	Ethanol                Ethanoic                Ethyl              Water        
			Acid                    Acetate

Halogenation or Substitution of Ethanol with PCl5

Ethanol reacts with phosphorus pentachloride at room temperature to form hydrogen chloride, ethyl chloride (i.e. chloroethane) and phosphoryl chloride.


	C2H5OH  +       PCl5     ==>     C2H5Cl +       POCl3   +       HCl  
	Ethanol         Phosphorus        Ethyl            Phosphorus     Hydrogen
	                    Pentachloride      Chloride       Pentachloride   Chloride

Halogenation or Substitution of Ethanol with HCl

Ethanol reacts with hydrogen chloride to form ethyl chloride (i.e. chloroethane) and water. A dehydrating agent (e.g. zinc chloride) is used as a catalyst.


			ZnCl2   
	C2H5OH  +       HCl     ==>     C2H5Cl + H2O    
	Ethanol                                    Ethyl   
                                                              Chloride

Uses of Ethanol

Ethanol is used

in the manufacture of alcoholic drinks, e.g. Vodka, etc.,
as a widely used solvent for paint, varnish and drugs,
in the manufacture of ethanal, (i.e. acetaldehyde), and ethanoic acid, (i.e. acetic acid),
as a fuel (e.g. in Gasahol),
as the fluid in thermometers, and
in preserving biological specimens.

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