Absolute Alcohol is obtained from 95% Alcohol by using a ternary azeotrope (i.e. by distillation using a three component Azeotrope).
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The aliphatic alcohols can be regarded as derivatives of alkanes in which one or more hydrogen atoms have been replaced by hydroxyl groups [-OH].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.
Ethylene glycol, CH2OHCH2OH, is the most important dihydric alcohol and approximately 75% of that produced is used as an anti-freeze agent.
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.
CH3CH2 CHO + 2H ==> CH3CH2CH2OH Propionaldehyde Propanol CH3COCH3 + 2H ==> CH3CHCH3 OH Acetone Isopropanol
C2H5NH2 + HON=O ==> C2H5OH + N2 + H2O
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'.
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.
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
Glycerol is used