ROH

Definition

“Organic compounds that contain monovalent functional group OH are called alcohols.”

General Formula

Alcohols are denoted by ROH, where R may be any alkyl group. The general formula of aliphatic alcohol is :

C_nH_2n+1 – OH

Where n may be any natural number.

Examples

CH₃ – OH                               Methyl Alcohol (Methanol)

C₂H₅ – OH                                   Ethyl Alcohol (Ethanol)

Classification Of Alcohols

On the basis of number of groups, alcohols have been classified into the following:

1.      Monohydric Alcohols

2.      Dihydric alcohols 

3.      Polyhydric Alcohols

1. Monohydric Alcohols (Hydrins)

Those aliphatic compounds that contain only one hydroxyl group (OH) are known as Monohydric Alcohols. They are also known as Hydrins.

Types of Hydrins

On the basis of carbon atom, monohydric alcohols have been further classified into the following:

a)      Primary Alcohols

b)      Secondary Alcohols

c)      Tertiary Alcohols.

a)Primary Alcohols

When one hydrogen atom of methyl group is replaced by any alkyl group, then the carbon atom of the substituted methyl is called Primary carbon atom.

H-CH₂-            ®       R-CH₂-

Those monohydric alcohols in which OH group is directly bonded with primary carbon atom are called Primary Alcohols.

H-CH₂-OH      ®       R-CH₂-OH

b)Secondary Alcohols

When two hydrogen atoms of methyl group are replaced by alkyl groups, then the carbon atom of the substituted methyl is called Secondary carbon atom. These alkyl groups may be different or similar.

H-CH₂-            ®       R­₂-CH-

Those monohydric alcohols in which OH group is directly bonded with Secondary carbon atom are called Secondary Alcohols.

H-CH₂-OH      ®       R­₂-CH-OH

c)Tertiary Alcohols

When three hydrogen atoms of methyl group are replaced by alkyl groups, then the carbon atom of the substituted methyl is called Tertiary carbon atom. These alkyl groups may be different or similar.

H-CH₂-            ®       R­₃C-

Those monohydric alcohols in which OH group is directly bonded with Tertiary carbon atom are called Tertiary Alcohols.

H-CH₂-OH      ®       R­₃C-OH

2. Dihydric Alcohols

Those aliphatic compounds that contain two hydroxyl group (OH) are known as dihydric alcohols. They are also known as Glycol.

OH-CH₂-CH₂-OH

3. Polyhydric Alcohols

Those aliphatic compounds that contain three or more hydroxyl group (OH) are known as Polyhydric Alcohols. They are also known as Glycerol.

OH-CH₂-CHOH-CH₂-OH

Preparation

Alcohols can be prepared by the following methods:

1. From Alkene

In presence of catalyst, dilute H₂SO₄, when ethene reacts with water, then ethyl alcohol or ethanol is formed.

C₂H₄    +          H₂0      ®       C₂H₅OH

2. From Grignard’s Reagent

The following chemical reactions show the preparation of alcohol from Grignards Reagent.

a)             Reaction with Formaldehyde

In presence of dry ether and halogen acid when grignard’s reagent reacts with form aldehyde, then primary alcohol is obtained.

R^- – Mg⁺ – X + H⁺CHO^- ® RCH₂OH + MgX₂

1° Alcohol

CH₃^- – Mg⁺ – Cl + H⁺CHO^- ® CH₃CH₂OH + MgCl₂

Ethanol

b)             Reaction with Aldehyde

In presence of dry ether and halogen acid when grignard’s reagent reacts with aldehyde then secondary alcohol is formed.

R^- – Mg⁺ – X + R⁺CHO^- ® R₂CHOH + MgX₂

2° Alcohol

CH₃^- – Mg⁺ – Cl + CH₃⁺CHO^- ® (CH₃)₂CHOH + MgCl₂

                                                                           2° Propanol

c)              Reaction with Ketone

In presence of dry ether and halogen acid, when Grignard’s reagent reacts with Ketone then tertiary alcohol is formed.

R^- – Mg⁺ – X + R⁺CRO^- ® R₃COH + MgX₂

3° Alcohol

CH₃^- – Mg⁺ – Cl + CH₃⁺C(CH₃)O^- ® (CH₃)₃COH + MgCl₂

3. From Fermentation

On large scale, ethyl alcohol is produced by fermentation. Fermentation means gentle bubbling or boiling. In presence of microorganism enzymes, one compound is converted into other. Carbondioxide gas is evolved in form of bubbles, therefore the process is called fermentation.

Ethyl alcohol may be obtained by the following two processesses:

a)Fermentaion of Starch

The main source of starch is rice and potato. For the fermentation of starch, grains are cooked to form a pulp. Than, malt is added to the pulp. Then, malt is added to the pulp, which hydrolysis starch to sugar maltose. The enzyme present in malt is diastase.

Starch                                    Sugar Maltose

The reaction is then carried out in presence of yeast, which contain two enzymes maltose and zymase. With the help of maltase, Sugar Maltose is converted into glucose.

Sugar Maltose                                  Glucose

Finally with the help of zymase, glucose is converted into ethyl alcohol with the liberation of Carbondioxide.

Glucose                    Ethyl Alcohol

Alcohol is then separated by distillation. The boiling point of ethyl alcohol is 78.5°C.

b)Fermentation of Molasis

The thick dark brown coloured syrup left after the isolation of crystalline sugar cane is called Molasis. It fermentation is carried out in presence of yeast, which contains two enzymes, sucrose, molasis converts into glucose and fructose.

Glucose     Fructose

Glucose and fructose further react with zymase to form ethyl alcohol and carbondioxide.

Glucose                    Ethyl Alcohol

Alcohol is then separated by distillation. The boiling point of ethyl alcohol is 78.5°C.

Chemical Properties

1. Dehydration

Removal of water molecules from a compound is called dehydration. In presence of catalyst, concentrated H₂SO₄, when ethyl alcohol is heated at about 170°C, then dehydration takes place. As a result, ethene is formed.

C₂H₅OH          ®       C₂H₄    +          H₂O

2. Reaction with Grignard’s Reagent

When Grignard’s Reagent reacts with alcohol, then alkane is formed.

R^- – Mg⁺ – X + R⁺OH^- ® RH + RO – Mg – X

Alkane

H₃C^- – Mg⁺ – I + C₂H₅⁺OH^-® CH₄ + C₂H₅O – Mg – I

Methane

3. Reaction with HalogenAcid

When alcohol reacts with halogen acid then alkyl halide is formed.

R⁺OH^- +          H⁺X^-     ®       RX       +          H₂O

 Alkyle halide

C₂H₅OH                      +          HCl      ®       C₂H₅Cl            +          H₂O

Ethyl Choride

4. Reaction with Thionyl Chloride

In presence of solvent Pyridine, when alcohol reacts with thionyl chloride (SOCl₂) then alkyl chloride is formed.

Alkyl Chloride

Ethyl Chloride

5. Reaction of Alcohol with Phosphorus Penta Chloride (PCl₅)

When alcohol reacts with phosphorus penta chloride the alkyl chloride is formed.

ROH + PCl₅ ® RCl + HCl + POCl₃

Alkyl              Phosphorus

Chloride             OxyTrichoride

C₂H₅OH + PCl₅ ® C₂H₅Cl + HCl + POCl₃

Ethyle Chloride

6. Reaction of Alcohol with Phosphorus Tri Bromide (PBr₃)

When alcohol reacts with Phosphorus tri bromide, then alkyl bromide is formed.

3ROH + PBr₃ ® 3RBr + H₃PO₃

Alkyl Bromide

C₂H₅OH + PBr₃ ® C₂H₅Br + H₃PO₃

Ethyl Bromide

7. Reaction with Sodium Metal

When ethyl alcohol reacts with sodium then hydrogen atom of OH group is replaced by sodium atom to form sodium ethoxide. This reaction shows the acidic behaviour of alcohol.

ROH    +          Na       ®       RONa +          1/2H2

C₂H₅OH          +          Na       ®       C₂H₅ONa        +          1/2H₂

Sodium Ethoxide

8. Esterification

Formation of ester is called Esterification. In presence of catalyst H₂SO₄, when ethyl alcohol reacts with acetic acid, then ester is formed.

9. Formation of Ether

When ethyl alcohol reacts with concentrated H₂SO₄ at low temperature, then ether is formed in two steps.

10.      Oxidation

In presence of oxidizing agents such as KMnO₄ and H₂SO₄ when ethyl alcohol is oxidized, then acetaldehyde is formed.

C₂H₅OH          +          [O]       ®       CH₃COH         +          H₂0

11.      Dehydrogenation

Removal of hydrogen from a compound is called dehydrogenation. In presence of Ni – Cu couple, when vapours of ethyl alcohol are heated at 180°C then dehydrogenation takes place. As a result, acetaldehyde is formed.

C₂H₅OH          ®       CH₃COH         +          H₂

Uses

1.      Ethanol is used as a solvent. It dissolves a large variety of organic substances such as gums, resins, tincture and varnishes.

2.      It is being extensively used in the form of different beverages.

3.      It is used as raw material for the preparation of other organic solvents such as CHCl₃, ether and esters.

4.      It is used as fuel substitute and for low temperature thermometer.

5.      Ethanol is used as inert solvent for certain organic reactions and re-crystallization of many organic compounds.

Classification On The Basis Of Composition

On the basis of composition, alcohols have been classified into following types.

1. Absolute Alcohol

Organic compounds that contain 99.99% pure ethyl alcohol are known as absolute alcohols.

2. Rectified Spirit

Organic compounds that contain 92% - 95% ethyl alcohol are known as rectified spirits.

3. Denatured Alcohol

Organic compounds that contain 85% ethyl alcohol, 11% water and 4% methyl alcohol are known as denatured alcohol. They are also known as methylated spirits.