Alcohols - NEB Class 12 Chemistry 2080

Unit 10: Alcohols - Class 12 Chemistry

Class 12 Chemistry Chapter 10 Alcohols NEB Notes 2080. Class 12 Chemistry Unit 10 Alcohols Complete note, Exercise, Important Questions.

Introduction

When hydrogen atom of any alkane is replaced by –OH functional group, alcohol is formed. Alcohol can generally be represented by (R-OH) where R- is any alkyl group

 

Nomenclature

 

Common Name	IUPAC Name	Molecular Formula
Methyl alcohol	Methanol	CH3OH
Ethyl alcohol	Ethanol	C2H5OH
Propyl alcohol	Propanol	C3H7OH
Butyl alcohol	Butanol	C4H9OH
Pentyl alcohol	Pentanol	C5H11OH
Hexyl alcohol	Hexanol	C6H13OH
Heptyl alcohol	Heptanol	C7H15OH
Octyl alcohol	Octanol	C8H17OH

Classification of Alcohols (Monohydric alcohol)

Alcohols are classified as: 

 1. Primary alcohol (1O Alcohols) 

 2. Secondary alcohols (2O Alcohols) 

 3. Tertiary alcohols (3O Alcohols)

1. Primary alcohols (1° Alcohols)

   • If the -OH group is attached to a primary carbon (carbon attached to only one other carbon), the alcohol is called a primary alcohol.
   • Example: Methanol (CH3OH) is a primary alcohol because the -OH group is attached to the primary carbon of methane (CH4).

2. Secondary alcohols (2° Alcohols)

   • If the -OH group is attached to a secondary carbon (carbon attached to two other carbons), the alcohol is called a secondary alcohol.
   • Example: Isopropanol (CH3-CH(OH)-CH3) is a secondary alcohol because the -OH group is attached to the secondary carbon of propane (CH3-CH2-CH3).

3. Tertiary alcohols (3° Alcohols)

• If the -OH group is attached to a tertiary carbon (carbon attached to three other carbons), the alcohol is called a tertiary alcohol.
• Example: Tert-butyl alcohol [(CH3)3-C-OH] is a tertiary alcohol because the -OH group is attached to the tertiary carbon of tert-butane [(CH3)3-C-H].

Alcohols can be classified based on the number of -OH (hydroxyl) groups they contain:

(i) Monohydric alcohols: These alcohols have one -OH group in their molecular structure.

(ii) Dihydric alcohols: These alcohols have two -OH groups in their molecular structure.

(iii) Trihydric alcohols: These alcohols have three -OH groups in their molecular structure.

(iv) Polyhydric alcohols: These alcohols have more than two or three -OH groups in their molecular structure.

Isomerism in alcohols

Compounds having the same molecular formula but different structural formula and chemical properties are called isomers (Structural isomers) and the phenomenon is known as isomerism. Alcohols exhibit only following three types of structural isomerism.

Isomerism in alcohols can be classified into three types:

1. Chain isomerism

Alcohols having same molecular formula but differ only in the length of the carbon chain and properties are called chain isomers and phenomenon is known as chain isomerism.

2. Positional isomerism

Alcohol having the same molecular formula, same carbon chain length but differ only in the position of the –OH group in the carbon chain are called positional isomers and phenomenon is known as positional isomerism.

3. Functional isomerism

Alcohols are isomeric with ether. So alcohols can be functional isomers to each other.

General methods for the preparation of Monohydric alcohols

Alcohols can generally be prepared by the following methods

1.By hydrolysis of alkyl halides

When the alkyl halides are hydrolyzed in the presence of aqueous NaOH or KOH, alcohols are prepared.

2. By acid catalyzed hydration

Alkenes react with water in the presence of acid as catalyst to form alcohols. In case of unsymmetrical alkenes, the addition reaction takes place in accordance with Markovnikov’s rule.

3. From Grignard reagents

Alkyl magnesium chloride is known as Grignard reagent.

R-MgX 

Grignard reagents 

Grignard Reagents Preparation

1. Formaldehyde or methanal react with any Grignard reagent, always give primary (1° )alcohols

2. Aldehydes other than methanal of formaldehyde react with any Grignard reagent, always give secondary (2° ) alcohols.

3. Any ketones when react with any Grignard reagent , always give tertiary (3° ) alcohols.

4. From primary amines

When primary amines are treated with nitrous acid. Alcohols are formed. Nitrous acid is formed by the reaction of NaNO2 and HCl. In this reaction nitrogen gas is also liberated.

5. From Hydrolysis of Ester

Alcohols can also be prepared by the hydrolysis of ester in the presence of aq. Acidic or basic medium.

Industrial preparation of Alcohol

1. Fermentation of carbohydrates (sugar)

Fermentation is the slow decomposition of complex or higher organic compound into simpler compounds by the action of enzymes. The carbohydrates used for the fermentation are sucrose, glucose ,fructose, molasses and sugar containing fruits and starchy materials like wheat, rice, maize, barley, potato etc.

Fermentation is the old traditional method for the commercial manufacture of ethyl alcohol.

Enzyme used is the unicellular plant material which contains enzymes like invertase, diastase, maltase, zymase etc.

i. From fermentation of sugar Molasses is the big source of sucrose, glucose, fructose. Etc. Molasses is the dark brown colored mother liquor obtained after the crystallization of cane sugar in the sugar industries.

Sucrose obtained from molasses when hydrolyzed in the presence of enzyme ‘invertase’ give glucose or fructose.

Glucose or fructose then comes in contact with enzyme ‘zymase’ convert into ethyl alcohol along with the evolution of CO2 gas.

The fermented liquor from the above process is called ‘wash’. This wash contains 12-15% ethyl alcohol which can be obtained in pure form by distillation process.

ii. From fermentation of Starch

Starchy raw material used for the fermentation process are rice, wheat, maize, barley, potato etc. The raw materials are first thoroughly cocked or boiled with water to release starch which is called ‘Mesh’.

The mesh is then mixed with yeast (Enzyme) and kept for about 7-10 days or more.

This fermented liquor is called ‘wash’ this wash contains 12-15 % ethyl alcohol is obtained impure form by distillation.

Favorable condition for fermentation:

• Yeast, a type of single-celled fungus, provides the enzymes needed for fermentation.

• Little amount of ammonium sulphate or ammonium phosphate is added as nutrient of yeast.

• If the yeast cells become too cold, fermentation happens very slowly, or may not happen at all.

• If the yeast cells become too hot, their enzymes become denatured and fermentation stops.

• sugars dissolved in water, and mixed with yeast

• an air lock to allow carbon dioxide out, while stopping air getting in warm temperature, 25-35°C 

• The yeast dies when the ethanol concentration reaches about 15 % 

• If air is present, the oxygen causes the ethanol to oxidize to ethanoic acid, so the drink tastes of vinegar.

2. Oxo process

This is also the industrial process for manufacturing of alcohol above ethanol. In this process alkenes are treated with (CO+H2 )the presence of cobalt carbonyl catalyst (octacarbonyl dicobalt) to get aldehyde. This aldehyde on reduction in the presence of Ni/H2 or Pt/H2 catalyst Catalytic hydrogenation) gives alcohol.

3. Hydroboration-oxidation of ethene

Alkenes react with diborane (B2H6 ) or (BH3 )2 undergo hydroboration to give alkyl borane which on oxidation in the presence of H2O2 gives alcohol.

Properties of alcohols (Monohydric alcohols)

1. Physical properties

State:

Lower alcohols are colorless liquid with characteristic smell and burning taste while higher alcohols are colorless waxy solids. 

Solubility:

Lower alcohols are soluble in water due to presence of intermolecular hydrogen bonding. Solubility decreases with increase in the carbon chain or molecular masses. This is due to the difference in the sizes of the alcohol and water molecules.

Boiling Point:

The boiling points of the alcohols are much higher than those of other hydrocarbons having comparable molecular weight. It is because of intermolecular hydrogen bonding formation.

• Boiling points of alcohols decreases with increase of branching.
  
  
  

Chemical Properties of alcohols (Monohydric alcohols)

(1) Reaction with metals (Acidic nature of alcohols)

The reaction between highly electropositive metals like Na, K, Li etc. Involves in the presence of ether, breaking of the bond between -O-H takes place and metal alkoxide and H2 gas released showing the acidic nature of alcohols.

(2) Esterification:

Alcohols react with carboxylic acid in the presence of few drops of conc. H2SO4 to give esters. This reaction is called esterification reaction. Conc. H2SO4 acts as dehydrating agent.

This test is laboratory test for ethyl alcohol.

The order of acidic strength of alcohols is given as:

Primary alcohol  〉Secondary alcohol  〉 Tertiary alcohols

(3) Reaction with halogen acid (Basic nature of alcohol)

In this reaction –OH group is replaced by halogen atom.

The order of basic strength of alcohols is given as:

Primary alcohol 〈 Secondary alcohol〈 Tertiary alcohols

(4) Reaction with Phosphorus halide (PX3 ,PX5 )

Alcohols are easily converted to alkyl halide when react with phosphorus halides.

(5) Reaction with PX3

Alcohols also react with phosphorus trihalide to give alkyl halide.

(6) Reaction with Thionyl chloride (SOCl2 )

Alcohols react with thionyl chloride to give alkyl halides.

(7) Reaction with H2SO4:

Ethyl alcohol reacts with conc. H2SO4 to give different products at different temperatures.

 (i) At 100℃: Ethyl alcohol reacts with conc. H2SO4 at 100℃ to give ethyl hydrogen sulphate.

(ii) At 140℃: Ethyl alcohol reacts with conc. H2SO4  at 140℃ to give ethyl diethyl ether (Ethoxy ethane).

(iii) At 170℃: Ethyl alcohol reacts with conc. H2SO4 at 170℃ to give ethene.

(8) Oxidation of 1° ,2° and 3° alcohols by oxidizing agents:

(i) 1° Alcohol:

Alcohols can easily be oxidized into aldehydes and ketones in the presence of any oxidizing agents like acidic or alkaline K2Cr2O7 , KMNO4 etc.

(ii) 2° Alcohol:

2° alcohols are oxidized into ketone with the same no of carbon atoms. The ketones are further oxidized into carboxylic acid with one carbon less than original ketones.

(iii) 3° Alcohol:

3° alcohols are not oxidized in ordinary condition because in 3° alcohols carbon containing –OH has no hydrogen atom.

(9) Reduction of alcohols (Catalytic dehydrogenation & dehydration)

When alcohol vapors are passed through the red hot copper tube at 3000 C, Different class of alcohols give different products.

(i) 1° Alcohol:

1° or primary alcohols are dehydrogenated into aldehydes.

(ii) 2° Alcohol:

2° or secondary alcohols are dehydrogenated into ketones.

(iii) 3° Alcohol:

3° or tertiary alcohols are dehydrated into alkenes in the presence of Cu catalyst at 3000C.

Distinction between 1°, 2° and 3° alcohols by Victor Meyer’s Method

There are several methods for distinguishing 1°, 2° and 3° alcohols but most important method is Victor Meyer’s Method. 

• (i) Oxidation method

• (ii) Catalytic dehydrogenation

• (iii) Victor Meyer’s Method

•  (iv) Lucas Test

Victor Meyer’s Method:

Victor Meyer’s Method is most important and widely used method for distinguishing 1°, 2° and 3° alcohols. In this method the given alcohol is first treated with phosphorus and iodine solution (P+I2 ) to give iodo-alkane which is then treated with silver nitrite(AgNO2 ) to give nitro alkane. The nitro alkane is then treated with nitrous acid (HNO2 ) and the resulting solution is finally made alkaline by NaOH or KOH.

Following results are obtained.

• (i) Primary alcohol gives red colour

• (ii) Secondary alcohol gives blue colour

• (iii) Tertiary alcohol gives no any colour

Table for distinguishing 1°, 2° and 3° alcohols is given as

Table for distinguishing 1°, 2° and 3° alcohols

Test/Reagent	Primary (1°) Alcohol	Secondary (2°) Alcohol	Tertiary (3°) Alcohol
P+I2	Iodoalkane	Iodoalkane	Iodoalkane
AgNO2	Nitroalkane	Nitroalkane	Nitroalkane
HNO2 (NaNO2 + HCl) (Pseudonitrole Iodoalkane)	Nitrolic acid	Nitrolic acid	No Reaction
KOH	Red colour	Blue colour	No colour

Victor Meyer’s Method - Table for distinguishing 1°, 2° and 3° alcohols

Victor Meyer’s Method Example - Table for distinguishing 1°, 2° and 3° alcohols

Lucas Test

In This test the unknown alcohol is treated with the Lucas reagent (HCl + ZnCl2 ) . This is the reaction of alcohol with HCl in the presence of dilute HCl. The time taken for the reaction to occur is important to know the class of alcohols.

The occurrence of reaction can be observed by the appearance of white turbidity or cloudiness.

• (i) For 1° alcohol, reaction occurs only after heating. 

•  (ii) For 2° alcohol, reaction occurs within five minutes. 

•  (iii) For 3° alcohol, reaction occurs immediately.

Table:

Class of Alcohol	Reaction with Lucas Reagent	Observation
Primary (1°) Alcohol	R-CH2-OH + ZnCl2 → R-CH2-Cl + HCl (Chloroalkane)	Reaction occurs only after heating
Secondary (2°) Alcohol	R2CH-OH + ZnCl2 + HCl → R2CH-Cl + HCl + HCl (Chloroalkane)	Reaction occurs within five minutes
Tertiary (3°) Alcohol	R3C-OH + ZnCl2 + 2 HCl → R3C-Cl + ZnCl2 + 2 H2O (Chloroalkane)	Reaction occurs immediately

Lucas Test Table:

Test for ethyl alcohol:

• (i) Esterification Test (Already Studied)

• (ii) Iodoform test

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