FSc Notes Part 2 Chemistry Important Chemicals Alcohol ROH
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
CnH2n+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.
- Monohydric Alcohols
- Dihydric Alcohols
- 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.
- Primary Alcohols
- Secondary Alcohols
- 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 + H2O —-> C2H5OH
2. From Grignard’s Reagent
The following chemical reaction show the preparation of alcohol from Grignard’s Reagent.
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.
Uses
- Ethanol is used as a solvent. It dissolves a large variety of organic substances such as gums, resins, tincture and varnishes.
- It is being extensively used in the form of different beverages.
- It is used as raw material for the preparation of other organic solvents such as CHCl3, ether and esters.
- It is used as fuel substitute and for low temperature thermometer.
- 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
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