Alcohols

Alcohols Absolute

Absolute Alcohol, C2H5OH, (i.e. 100% Ethanol) is pure anhydrous ethanol. Absolute alcohol has a boiling point of 78.3 degC. It is a clear colourless liquid with a pleasant smell. It is completely miscible with water and organic solvents and is very hydroscopic.

Absolute Alcohol is obtained from 95% Alcohol by using a ternary azeotrope (i.e. by distillation using a three component Azeotrope).

A mixture of 7.5% Water (boiling point 100 degC), 18.5% Ethanol (boiling point 78.3 degC), and 74% Benzene (boiling point 80 degC), forms a ternary azeotrope (boiling point 64.9 degC), which is a minimum-boiling mixture.
Benzene and Ethanol form a binary azeotrope (boiling point 68.2 degC).
Thus, when a mixture of 95% ethanol and benzene is distilled, the above ternary azeotrope distills first, followed by the binary azeotrope, and the final fraction (b.p. 78.30C) is absolute alcohol.

Alcohols Aliphatic

The aliphatic alcohols are a series of homologous series organic compounds containing one or more hydroxyl groups [-OH] attached to an Alkyl Radical.

The aliphatic alcohols can be regarded as derivatives of alkanes in which one or more hydrogen atoms have been replaced by hydroxyl groups [-OH]. The general formula of saturated aliphatic alcohols is CnH2n+1OH, where n=1,2,3, etc. The saturated carbon chain is often designated by the symbol R, so that ROH can represent any alcohol in the homologous series.

Methanol and ethanol are the first two members of the series. Compounds of this type with one hydroxyl group per molecule are known as monohydric alcohols.

Alcohols dihydric

Ethylene glycol, CH2OHCH2OH, is the most important dihydric alcohol and approximately 75% of that produced is used as an anti-freeze agent.

Alcohols Preparation

A number of general methods for the preparation of aliphatic alcohols are described as follows.

Preparation of Alcohols by Hydration of an Alkene

The hydrolysis of Alkenes using sulphuric acid (i.e. the direct addition of water). The mechanism of the hydration reaction involves the addition of the Sulphuric Acid across the double bond of the alkene, followed by the displacement of the bisulphate ion by hydroxide ion to form the alcohol. This process can be used to prepare those alcohols whose formation is consistent with the application of the Markownikoff Rule for the addition of water across the double bond of the alkene. Because of the Markownikoff Rule, it is possible

to prepare isopropyl alcohol but not n-propyl alcohol, from propene,
to prepare sec-butyl alcohol, but not n-butyl alcohol from 1-butene, and
to prepare tert-butyl alcohol but not isobutyl alcohol, from 2-butene.


CH2=CH2  +  H2SO4    ==>     CH3CH2OSO3H + H20    ==>      CH3CH2OH
Ethylene                   Ethyl Hydrogen Sulphate          Ethanol
                                                           (Primary 
                                                            Alcohol)

CH3CH=CH2  +  H2SO4    ==>     CH3CHCH3 H20       ==>     CH3CHCH3
                                  OSO3H                       OH 
Propylene 	             Propyl Hydrogen Sulphate	   Isopropanol
            
CH3CH2CH=CH2 +  H2SO4  ==> CH3CH2CHCH3 + H2O  ==>    CH3CH2CHCH3 + H2O
                                 OSO3H                     OH 

Butylene   	            Butyl Hydrogen Sulphate      sec-butyl alcohol
                                                       (Secondary Alcohol)

The only primary alcohol which may be obtained in this way is ethyl alcohol, and this method is not suitable for the preparation of methanol.

Preparation of Alcohols by reduction of Aldehydes

The reduction of aldehydes , to primary alcohols, ketones to secondary alcohols by passing the vapour and molecular hydrogen over a heated nickel catalyst.


	CH3CH2 CHO + 2H         ==>       CH3CH2CH2OH     
	Propionaldehyde                    Propanol        

	CH3COCH3 + 2H           ==>       CH3CHCH3        
                                             OH        
	Acetone                          Isopropanol

Preparation of Alcohols from Amines

The action of Nitrous Acid, HNO2, on Primary Amines, RNH2.


         C2H5NH2  +  HON=O    ==>     C2H5OH  +  N2   +  H2O

Alcohols Primary, Properties Chemical

A primary alcohol, RCH2OH, can be oxidised to the corresponding aldehyde, RCHO, and then further oxidised to the corresponding carboxylic acid, RCOOH, depending on the reaction conditions.

A primary alcohol can undergo catalytic dehydrogenation to the corresponding aldehyde, RCHO.

Primary alcohols, RCH2CH2OH, can be dehydrated to Alkenes, RCH=CH2, or
to ethers, RCH2CH2OCH2CH2R,
depending on the reaction conditions.
Similarly, secondary alcohols, RR'CHCHOH, can be dehydrated to alkenes, RR'CH=CH2, or
to ethers, RR'CHCH2OCH2CHRR'.

Alcohols Properties Chemical

The chemical properties of any given aliphatic alcohol depends on the nature of the alkyl group in the molecule and on the properties of the hydroxyl group.

Alcohols react with organic acids to form Esters. The reaction proceeds slowly but the rate of esterification is increased by the presence of hydrogen ions, which act as a catalyst in the reaction. Sulphuric Acid in addition to acting as a source of hydrogen ions also helps to increase the yield of ester by absorbing the water as it is formed in the reaction.

Alcohols are very weak acids, intermediate in strength between acetylene and water. They undergo substitution with strongly electropositive metals such as sodium.

Alcohols react with phosphorus pentachloride, when the hydroxyl group is replaced by a chlorine atom. Thus, when the hydroxyl group is replaced by a chlorine atom from phosphorus pentachloride fumes of hydrogen chloride are evolved and this is used in testing for the presence of the hydroxyl group in a compound.

Alcohols Properties Physical

No gaseous alcohols are known. The lower members of the homologous series of aliphatic alcohols (containing C1 to C10) are clear colourless liquids at room temperature. They have varying solubility in water, the higher alcohols being less soluble. The alcohols higher than C12 are solids and are insoluble in water.

Methanol, ethanol and propanol are miscible with water. The alcohols are miscible in all proportions with most organic liquids. As we pass up the series, the specific gravity increases.

The boiling points of the straight chain alcohols increase as the number of carbon atoms in the molecule increase. For a given molecular weight, there is a decrease in the boiling point when branching of carbon atoms occurs. Thus, the primary alcohols boil at a higher temperature than the secondary alcohols of the same molecular weight, and similarly, secondary alcohols have higher boiling points than the tertiary alcohols. The boiling points are much higher than is to be expected from their molecular weights. For example, the boiling point of ethanol, 78 degC, can be explained by the attraction of ethanol molecules by means of hydrogen bonds to form extended groups of molecules.

Hydrogen bonds can arise in ethanol because the area around the oxygen atom is relatively rich in electrons and can attract hydroxyl hydrogen from a neighboring ethanol molecule. These intermolecular bonds are considered to be intermediate in strength between weak Van der Waals' forces and the strong forces between ions. The extra energy required to break the hydrogen bonds leads to an increase in boiling point .

The alcohols react with sodium and potassium with the evolution of hydrogen.


                    2 C2H5OH   +   2Na    ==>   H2   +   2C2H5ONa

Alcohols Trihydric

Glycerol, CH2OH.CHOH.CH2OH, is the most important trihydric alcohol and it is an important industrial chemical having many uses in war and peace.

Glycerol is used

in the production of explosives,
as a moistening agent for tobacco,
as a softening agent for cellophane films,
in cosmetics,
in food products,
as a commercial solvent, and
in the manufacture of plastics known as alkyd resins, which are used in paints.

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