FSc Notes Part 2 Chemistry Important Chemicals Polymerization, Cracking Or Pyrolysis, Detergent and Isomerism

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Polymerization

Definition

The phenomenon in which simple and small molecules are converted into large and complex molecules is known as Polymerization.

The simple and small molecules are known as monomer, where as large and complex molecules are called polymers.

Classification of Polymerization

Polymerization has been classified the following.

1. Addition Polymerization
2. Condensation Polymerization

1. Addition Polymerization

The process of Polymerization in which molecules are added into each other is called Addition Polymerization.

Examples

In presence of traces of oxygen when is heated at about 200ºC under 100 atm pressure, then Polymerization takes place. As a result, monomers add together to form polymer i.e. Polyethene or Polythene.

Similarly the Polymerization of Vinyl Chloride to Poly Vinyl Chloride (PVC) and Polymerization of Vinyl Acetate to Poly Vinyl Acetate (PVA) are the examples of Addition Polymerization.

2. Condensation Polymerization

The process of Polymerization in which removal of water molecule takes place with the formation of polymer is called Condensation Polymerization.

Example

Bakelite Plastic is the Polymer that is obtained by the polymerization of Phenol and formaldehyde. During this process removal of water molecule takes place.

Cracking Or Pyrolysis

Definition

The phenomenon in which large molecules of hydrocarbons are thermally decomposed into smaller molecules in the absence of air or oxygen is known as Cracking or Pyrolysis.

Examples of Cracking

Cracking can be explained with the help of following examples.

1. Methane

When methane is burnt in absence of air then cracking takes place, as a result carbon black and hydrogen are formed.

2. Gasoline

The most important fraction of petroleum is gasoline (petrol) which is used as motor fuel. The fractional distillation of petroleum yields only a small percentage of total petrol demand. The additional quantity of petrol is obtained by the cracking of diesel.

The large less volatile molecules of diesel that have high boiling point are heated in the absence of air using a catalyst. As a result, cracking takes place and smaller more volatile molecules of petrol having lower boiling point are obtained.

Thus cracking is applied to obtain additional amount of gasoline from crude oil. In cracking a number of other useful by products are also obtained.

Detergent

Definition

Detergents are the salts of alkyl sulphate or aryl sulphate that improves the cleaning action of solvents, particularly ionic solvents.

Explanation

Detergents are long chain molecules, which when dissolve in water dissociate into positive and negative ions, these ions react with the ions of dirt and grease to produce soluble compounds with are readily carried away by the water molecules.

General Formula

Detergents are sodium or potassium salts of alkyl sulphate or aryl sulphate.

R-OSO3-Na+ Sodium Alkyl Sulphate

Where R is an alkyl group containing 7 to 18 carbon atoms. Soaps are sodium or potassium salts of long chain fatty acids.

R – COO-Na+

Composition of Detergents

Detergents are composed of two main parts namely.

1. Hydrophobic Part
2. Hydrophilic Part

1. Hydrophobic Part

It is a water repelling part of detergent, which consist of long hydrocarbon chain. The hydrocarbon chain being non-polar attracts dirt particles, which are normally non polar, i.e. greasy or oily molecules. Therefore, this part of detergent removes dirt particles from thing being washed and water takes away the dirt particles with it.

2. Hydrophilic Part

Hydrophilic part of the detergent is water attracting part. It consists of small ionic groups such as sulphonate (SO3) and sulphate (-SO3-). The hydrophilic group being ionic gets attracted by the polar water molecules and in this way, this part of the detergent removes the ionic dirt particles.

Advantage of Detergents

Detergents can act in hard water, since the calsium and magnesium salts of detergents are soluble in water. Whereas, the corresponding salts of soap are insoluble in water. Thus detergents are better cleaning agents than soaps.

Disadvantages of Detergents

One disadvantage of detergent over soap is that hydrocarbon chain, unlike those of soaps, which are derived from food substances (fats and oil) cannot be broken down by bacteria and dispersads. Therefore, it causes water pollution.

Isomerism

Definition

The phenomenon in which different compounds have the same molecular formula but different structural formula or electronic configuration is known as Isomerism.

The different compounds are called Isomers.

Types of Isomerism

There are various types of isomerism, but the four important types are

1. Chain Isomerism
2. Position Isomerism
3. Functional group Isomerism
4. Metamerism

1 Chain Isomerism

The phenomenon in which different compounds have the same molecular formula but differ in length of carbon chain is called Chain Isomerism. The structures are known as Chain Isomers.

Examples

1. Methane, Ethane and Propane have no chain isomers because the cannot be rearranged with small carbon chain.

CH4 CH3-CH3 CH3-CH2-CH3

2. The carbon chain of butane (C4H10) is long enough. So, butane has two isomers.

CH3-CH2-CH2-CH3 (n-butane)

CH3-CHCH3-CH3 (iso-butane)

3. Pentane (C5H12) shows the following isomers.

CH3-CH2-CH2-CH2-CH3 (N-PENTANE)

CH3-CHCH3-CH2-CH3 (ISO-PENTANE)

In saturated hydrocarbons, the number of isomers increase with increase in number of carbon atoms. This is the reason why decane shows 75 isomers.

2. Position Isomerism

The phenomenon in which different compounds have same molecular formula but differ in position of functional group, double bond or triple bond in same length of carbon chain is called Position Isomerism. The structures are called Position Isomers.

Examples

1. Propyl alcohol shows two position isomers.

2. Butene C4H8 has two position isomers.

3. Functional Group Isomerism

The phenomenon in which different compounds have same molecular formula but differ in functional groups is called functional group Isomerism. The structures are known as functional group isomers.

4. Metamerism

The phenomenon in which different compounds have same molecular formula and functional group but different alkyl groups attached to the same multivalent atom is called metamerism.