Get revision notes on CBSE Class 12 Chemistry: Chapter 11 - Alcohols, Phenols and Ethers. These notes are based on the latest CBSE syllabus and very helpful for revision purpose.
The main topics covered in this part are:
o Physical properties
o Chemical properties
• Test for distinguishing 1o, 2o and 3o alcohols
• Commercially Important Alcohols
o Methanol: preparation, properties and uses
o Ethanol: preparation, properties and uses
• Denaturation of alcohol
The key notes of the chapter are as follows:
An alcohol is any organic compound in which the hydroxyl functional group (–OH) is bound to a saturated carbon atom. For example: methyl alcohol, CH3OH; ethyl alcohol C2H5OH, etc.
Classification of alcohols
(i) On the basis of number of carbons attached to the α-carbon, i.e., the carbon to which the OH group is attached, alcohols are classified as:
Primary (1o) alcohol:
This type of alcohol has only one carbon atom attached to the α-carbon atom.
Secondary (2o) alcohol:
Here α-carbon has two other carbon atoms attached to it.
Tertiary (3o) alcohol:
In such alcohol three other carbon atoms are attached to the α-carbon atom.
(ii) On the basis of number of hydroxyl groups (−OH) present, alcohols can be divided into the following categories:
They contain only one OH group. They have a general formula CnH2n+2O.
For example: CH3OH, CH3CH2OH, CH3CH2CH2OH, etc.
Such alcohol contains two OH groups.
This type of alcohol contains three OH groups.
these are the alcohols that contain more than three OH groups.
Nomenclature of alcohols
IUPAC naming system:
(i) Select the longest possible chain that contains the α-carbon
(ii) Number the carbon atoms in the chain in such a way that α-carbon gets the minimum number
(iii) Then identify the hydrocarbon group and name it as per the IUPAC naming system.
(iv) Now replace the ‘e’ in the end of the parent hydrocarbon name by ‘ol’.
Common naming system:
o The common name of alcohol is alkyl alcohol.
o Depending upon the upon the structure of the alkyl group, prefix n, iso, sec, tert or neo is added to the common name.
‘n’ is added when α-carbon and other carbon atoms make a straight chain.
‘Iso’ is added when the ─OH group is attached to
‘sec’ is added when the α-carbon is attached to two other carbon atoms.
‘tert’is added when the α-carbon is attached to three other carbon atoms.
‘Neo’ is used when quaternary carbon is present.
Preparation of alcohols
General methods of preparation of alcohols are described below:
(a) From alkenes:
(i) By acid catalysed hydration of alkenes in accordance with Markownikoffs rule.
(ii) By hydroboration-oxidation of alkenes in accordance with anti-Markownikoffs rule.
(b) From carbonyl compounds:
(i) By catalytic hydrogenation of aldehydes and ketones.
Note : Aldehydes yield primary alcohols whereas ketones give secondary alcohols.
(ii) By catalytic reduction of carboxylic acids and esters.
(c) Form Grignard reagent, RMgX:
It gives R group as a nucleophile to attack at the partrially positive carbon atom of carbonyl (–C=O) group.
Physical properties of alcohols
• Lower alcohols are liquid at room temperature while higher ones are solid.
• The boiling point of alcohols is higher than haloalkanes with the same number of carbon atoms. This is due to the fact that alcohols are associated via intermolecular hydrogen bonding. For the same alcohol, as the branching increases, boiling point decreases due to decrease in surface area.
• Lower alcohols are completely miscible in water due to their ability to form hydrogen bonds with water. Solubility in water decreases with increase in molecular mass due to a decrease in the extent of intermolecular hydrogen bonding.
• Alcohols are weaker acids than water due to the lower polarity of O – H bond in alcohols. The acidic strength of alcohols decreases as the number of electron donating groups increase at carbon.
Chemical properties of alcohols
(a) Reactions of alcohols involving cleavage of O – H bond
• Reaction with metals:
• Reaction with carboxylic acid (Esterification):
• Reaction with Grignard reagent:
(b) Reactions involving cleavage of C – O bond
For the reaction involving cleavage of C – O bond, the reactivity order is
• Formation of halides:
(c) Oxidation of alcohols
Depending upon the structure of the alcohol and the type of oxidising agent used, oxidation of alcohol gives different products.
(d) Dehydrogenation of alcohols
Dehydration of alcohols takes place in acidic medium. Intra-molecular dehydration leads to the formation of alkene while inter molecular dehydration forms ether.
Order of ease of dehydration is: 3° > 2° > 1°
(e) Dehydration of alcohols
Dehydration of alcohols with conc. H2SO4 at different temperatures yields different products.
The ease of dehydration of three alcohols increases in the order
1o ROH < 2o ROH < 3o ROH
Test for distinguishing 1o, 2o and 3o alcohols
Lucas reagent which s a mixture of conc. HCI and anhyd. ZnCl2 is used to distinguish and alcohols.
• 3o alcohol gives cloudiness or turbidity with Lucas reagent immediately.
• 2o alcohol gives cloudiness or turbidity after 5 – 10 min.
• 1o alcohol does not give cloudiness or turbidity at room temperature.
Commercially Important Alcohols
Two commercially important alcohols are methanol and ethanol.
1. Methanol, CH3OH
Methanol is mainly produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO – Cr2O3 catalyst.
• It is a colourless liquid and highly poisonous.
• It is completely soluble in water.
• It is used as a solvent for paints, varnishes, and for making formaldehyde.
2. Ethanol, C2H5OH
Ethanol is mainly obtained commercially by fermentation of sugars. The sugar sugarcane or fruits such as grapes is converted to glucose and fructose, in the presence of an enzyme, invertase. Glucose and fructose on fermentation in the presence of another enzyme, zymase, yield ethanol.
• Ethanol is a colourless liquid.
• The boiling point of ethanol is higher than methanol.
• It is used as s solvent in paint industry.
• It is also used in the preparation of a number of carbon compounds.
Denaturation of alcohol
The commercial alcohol is made unfit for drinking by mixing in it some copper sulphate and pyridine. This is known as denaturation of alcohol.