Class 12 Alcohols Notes

Unit – 10 : ALCOHOLS

Introduction

Hydroxy derivatives of aliphatic hydrocarbons are called aliphatic alcohols.

Aliphatic alcohols are represented by R – OH.

Examples:

Classification of Alcohols:

On the basis of number of hydroxy (−OH) group present in alcohol, they are classified into following types;

1. Monohydric (Monohydroxy) alcohols:

Alcohols containing only one – OH group per molecule.

Monohydric alcohols are further classified into primary, secondary and tertiary alcohols according to the type of carbon in which the – OH group is attached.

a) Primary (1^o) alcohols:

Alcohols in which the – OH group is attached to a primary carbon atom.

2. Dihydric (Dihydroxy) alcohols:

Alcohols containing two – OH group per molecule.

3. Trihydric (Trihydroxy) alcohols:

Alcohols containing three – OH group per molecule.

4. Polyhydric (Polyhydroxy) alcohols:

Alcohols containing more than three – OH group per molecule.

NOTE: It may be noted that more than one – OH group cannot be present on the same carbon atom. In such case, compound will be highly unstable and will easily loses water molecule.

If two – OH groups are present on the same carbon atom, the molecule loses water molecule and converted into carbonyl compound (aldehyde or ketone)

If three – OH groups are present on the same carbon atom, the molecule loses water molecule and converted into carboxylic acid.

Nomenclature:

IUPAC Name: Alk + an + ol Common Name: Alk + yl + alcohol

S.N.	Formula	IUPAC Name	Common Name
1.	CH₃– OH	Methanol	Methyl alcohol
2.	CH₃– CH₂– OH	Ethanol	Ethyl alcohol
3. 4.	CH₃– CH₂– CH₂– OH CH₃– CH – OH \| CH3	Propanol Propan -2-ol	n-propyl alcohol iso-propyl alcohol
5. 6. 7. 8.	CH₃– CH₂– CH₂– CH₂– OH CH₃– CH – CH₂– OH \| CH3 CH₃– CH₂– CH – OH \| CH3 CH3 \| CH₃– C – OH \| CH3	Butanol 2-methyl propanol Butan-2-ol 2-methylpropan-2-ol	n-butyl alcohol iso-butyl alcohol sec-butyl alcohol tert-butyl alcohol
9.	CH₂– OH \| CH₂– OH	Ethane-1,2-diol	Glycol
10.	CH₂– OH \| CH – OH \| CH₂– OH	Propane-1,2,3-triol	Glycerol

Isomerism in alcohols:

Alcohols exhibit following three types of structural isomerism.

1. Chain isomerism:

Isomers differ in the length of the carbon chain. Alcohols containing four or more carbon atoms exhibitchain isomerism.

Chain isomers of C₄H₁₀O are,

2. Position isomerism:

Isomers differ in the position of – OH group in the carbon chain. Alcohols containing three or more carbon atoms exhibit position isomerism.

Position isomers of C₃H₈O are,

3. Functional isomerism:

Isomers differ in the functional group. Alcohols containing two or more carbon atoms exhibit functional isomer.

Functional isomers of C₂H₆O are,

Distinction of Primary, Secondary and Tertiary alcohols by Victor Meyer Method: Primary, secondary and tertiary alcohols can be distinguished by Victor Meyer’s method. This method involves the following steps,

Step 1: The given alcohols are treated with red phosphorous and iodine (Red P₄+ I₂), which converts alcohols into respective iodoalkanes.

Step 2: The iodoalkanes are treated with silver nitrite (AgNO2), which converts iodoalkanes into respective nitroalkanes. Step 3: The nitroalkanes are treated with nitrous acid, HNO2 (NaNO2 + dil.HCl) which forms respective compounds. Step 4: Finally, aqueous NaOH or KOH is added to the resulting solution then different color are formed.

If the color is blood red, the given alcohol is primary alcohol.

If the color is blue, the given alcohol is secondary alcohol.

If it is colorless, the given alcohol is tertiary alcohol.

Reactions involved in Victor Meyer’s method.

Where R = CH₃– or CH₃CH₂– etc..

General Methods of Preparation of Monohydric Alcohols: 1. From Primary amines:

2. From haloalkanes:

3. By hydrolysis of esters:

a) Hydrolysis in acidic medium:

4. From Grignard reagents:

Grignard reagent (RMgX) forms addition product with aldehyde or ketone. The addition compound on acidic hydrolysis gives corresponding alcohol.

Industrial Method of Preparation of Alcohol:

1. By Oxo process:

Alkene reacts with water gas (CO + H2) in presence of dicobalt octacarbonyl, [Co(CO)₄]₂as a catalyst under high temperature and pressure to give aldehyde (having one carbon atom greater than alkene), which on catalytic hydrogenation gives primary alcohol.

i.e.

2. By Hydroboration-Oxidation of alkenes:

Alkene reacts with diborane to give trialkyl boron, which on oxidation with alkaline hydrogen peroxide gives alcohol.

3. By Fermentation:

The process of decomposition of complex organic compounds into simpler molecules in presence of enzyme secreted by microorganism like yeast is called fermentation.

Fermentation process is widely used to manufacture ethyl alcohol from carbohydrates. Preparation of ethyl alcohol from sugar (Sucrose)

Preparation of ethyl alcohol from starch:

Some Terminology

Absolute alcohol: 100% pure ethyl alcohol

Rectified spirit: 95% ethyl alcohol

Denatured alcohol (Methylated spirit): Ethyl alcohol containing poisonous substance likes methyl alcohol, acetone, pyridine etc. which is unfit for drinking is called denatured alcohol.

Wood spirit: Methyl alcohol is called wood spirit. It is poisonous and colorless.

Power alcohol: Mixture of 80% petrol & 20% absolute alcohol is called power alcohol. It is used as fuel. Alcoholic Beverage: Liquors use for drinking purposes and contain ethyl alcohol as a principal intoxicating agent are called alcoholic beverages. These are two types; Undistilled and distilled.

Physical Properties of Monohydric alcohols

a) Physical Character

State: Lower members up to C₁₁are liquid and higher member above C₁₁are solid

Color: Colorless

Odor: Lower members up to C₁₁have characteristic alcoholic smell and higher member above C₁₁are odorless. 1.Solubility:

Lower members of alcohols are soluble in water due to the formation of intermolecular H-bond with water molecule.

Intermolecular H-bond between alcohol and water molecules

Higher alcohols i.e. alcohols with larger alkyl groups are less or insoluble due to increase non-polar water repelling nature of alkyl groups.

2.Boiling point:

Alcohols have higher boiling point than alkane, haloalkane & isomeric ether because alcohol can form intermolecularH bonds with each other whereas alkane, haloalkane and ether cannot.

Intermolecular H- bond between alcohol moelcules

In isomeric alcohol, boiling point decreases with increasing branching. Branching decreases the surface area, which decreases Van der Waal’s force of attraction.

Boiling point decrease Within homologous series, boiling point increases with increase in molecular mass.

Chemical properties of Monohydric alcohols:

Alcohols are quite reactive than haloalkane because the alcohols has two reactive (polar) covalent bonds, the C–O bond and the O–H bond.

The electronegativity of oxygen is substantially greater than that of carbon and hydrogen. Consequently, the covalent bonds of this functional group are polarized so that oxygen is electron rich (nucleophilic) and both carbon and hydrogen are electron deficient (electrophilic).

The most reactive site in an alcohol molecule is the hydroxyl group, despite the fact that the O–H bond strength is significantly greater than that of the C–C, C–H and C–O bonds.

1) Reaction with halogen acids (HX) (Basic nature of alcohol)

i) Reaction with HI:

HI reacts with 1^o, 2^oand 3^oalcohols without any catalyst.

ii) Reaction with HBr:

HBr reacts with 1° alcohols in presence of Conc.H₂SO₄as catalyst but it reacts with 2° and 3° alcohols without any catalyst.

iii) Reaction with HCl:

HCl reacts with 1°and 2° alcohols in presence of Conc.H₂SO₄or anhydrous ZnCl₂as catalyst but it reacts with 3° alcohols without any catalyst.

2) Reaction with Phosphorous halides:

Note: PBr₃and PI₃are unstable so they are prepared in situ by the reaction of Br₂or I₂with red phosphorous.

3) Reaction with Thionyl chloride (SOCl₂):

4) Oxidation of alcohol:

Alcohols are oxidized by acidified K₂Cr₂O₇or Na₂Cr₂O₇or acidified / alkaline KMnO₄solution i.e. (K₂Cr₂O₇/ H⁺, Na₂Cr₂O₇/ H⁺or KMnO₄/ H⁺, KMnO₄/ OH⁻).

i) Primary alcohols are easily oxidized first into aldehyde & finally into carboxylic acid.

ii) Secondary alcohol is easily oxidized into ketones. But, ketone does not oxidized under normal condition. However, under drastic condition (i.e. high temperature and high pressure) ketones are oxidized into carboxylic acid (having one carbon less than ketone).

iii) Tertiary alcohol doesnot undergo oxidation under normal condition. However, under drastic condition tertiary alcohol first dehydrate into alkene which further oxidized into ketone and finally into carboxylic acid.

5) Esterification reaction:

When alcohol is heated with carboxylic acid in presence of little Conc. H₂SO₄as a catalyst, gives ester. This reaction is called Esterification reaction.

➢ Esters have fruity smell, easily detectable so this reaction can be used to test the presence of alcohol and carboxylic acid. (Esterification test)

6) Reaction with active metals (Acidic Nature of alcohol)

Active metals: Alkali metals (more electropositive metals) Li, Na, K

Reactivity of active metals Li < Na < K

CH₃CH₂OH + Na CH₃CH₂ONa + 1/2 H2

Ethanol Sodium ethoxide

CH₃CH₂OH + K CH₃CH₂OK + 1/2 H2

Ethanol Potassium ethoxide

CH₃CH₂OH + Li CH₃CH₂OLi + 1/2 H2

Ethanol Lithium ethoxide

Alcohols are weaker acid because of the presence of polar O─H bond. This polarity in O─H bond arises due to electronegativity difference between the oxygen and hydrogen which allows the separation of H as H+ ion. Since,H+ ion is the measure of acidity of compound as P^H= -log [H⁺].

Comparison of acidic strength of alcohol and water:

Both alcohol and water contain polar O – H group and ionizes in aqueous solution to give H⁺ion. H – O – H ↔ HO ^–+ H⁺

R – O – H ↔ RO ^–+ H⁺

Alcohols are weaker acid than water because of the presence of electron releasing alkyl group (R), which shows +I effect that increase the electron density in oxygen atom. As a result polarity of O─H bond decreases. Thus, O – H bond of alcohol becomes stronger & release of H as H⁺becomes difficult.

Comparison of acidic strength of primary, secondary and tertiary alcohols:

As the number of electron releasing alkyl groups (+I groups) increases from primary to tertiary, acidity decreases.

For example: 1^oalcohol > 2^oalcohol > 3^oalcohol

Propan-1-ol > Propan-2-ol > 2-methylpropan-2-ol

Litmus test: Alcohol turns blue litmus faintly red.