ORGANIC CHEMISTRY FOR TEXTILE2. Bonding in organic compounds

L3: Secondary bonding

PRIMARY AND SECONDARY BONDING

A secondary bond is that where no true sharing or transfer of electron does occur while primary bond is formed by sharing or transfer of electron.

Primary bonding forces are short range strong forces, but the secondary bonding forces are long range weak forces.

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SECONDARY BONDING

The following types of secondary bonding occurs in organic compounds: Dipole forces Induction forces van der Waal’s forces Hydrogen bonding

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DIPOLE FORCES When two atoms having

difference in electronegativity are bonded together, the resulting bond is not neutral but bears partial negative and positive charges or polarity. Such polar bonds are said to have dipole. The greater the difference of electronegativity the greater the intensity of charges and greater the dipole.

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EFFECT OF DIPOLE FORCES Since similar charges repel

and opposite charges attract each other, the dipole forces effect the orientation of molecules in any material.

The orientation of molecules strongly effect physical properties of materials. For e.g. the melting and boiling point will rise.

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EFFECT OF DIPOLE FORCES Due to highly polar

nature PVC is very hard material while polyethylene is soft and flexible.

The melting point of polyethylene is 110 C while that of PVC is above 200 C.

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EFFECT OF DIPOLE FORCES This effect is very

prominent in polymers where these forces effect the strength of polyester (PET).

The dipole forces bind the polymer chains together and enhance the mechanical properties in polyvinyl chloride (PVC).

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INDUCTION FORCES When a molecule having a dipole comes near a

non polar molecule, the later experiences slight charge separation due to interaction with polar molecule. The forces acting due to such induced charge are called induction forces

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INDUCTION FORCES

Induction forces are weak and temporary forces and they die as and when the permanent dipole moves away.

however these forces do effect the properties of polymers to some extent.

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VAN DER WAAL’S FORCES In any material all the molecule

attract one another since the nucleus of each molecule exert slight attraction on electrons of the other molecules, so that the molecule come nearer to one another. However, the repulsion between the electrons of different molecules becomes prominent if they come too close to one other and thus keeps them at a certain distance. The forces acting to keep the molecules together are called van der Waal’s forces.

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EFFECT OF VAN DER WAAL’S FORCES

Due to van der Waal’s forces the free movement of molecules in a substance is effected. Thus stronger the forces the more restricted the movement.

If there were no van der Waal’s forces all molecule of any matter would move away from one another and everything would exist only in gaseous state.

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EFFECT ON MELTING, BOILING AND SOLUBILITY The melting points, boiling points and

physical state of materials is effected by Van der Waal’s forces.

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CHANGE OF STATE

When a material changes from one state to the other state it absorbs or releases energy, hence the kinetic energy of it’s molecules changes.

When changing from solid to liquid state the kinetic energy of molecules increases and they tend to move away from one another to a greater distance. The van der Waal’s forces thus oppose this change in kinetic energy, therefore, greater the intermolecular forces greater will be the energy required to change the state and higher will be it’s melting point.

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HYDROGEN BONDING When a hydrogen bonded to a

molecule comes in vicinity of another atom having a lone pair, like oxygen, on another molecule, it forms secondary bond with it. For example the hydrogen of a water molecule forms secondary bond with oxygen of the other water molecule.

The hydrogen bond is a not a primary bond but near to it in strength and strongest of all secondary bonds.

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HYDROGEN BONDINGEFFECT ON PHYSICAL PROPERTIES Hydrogen bonding

exerts a strong effect on physical properties of materials like boiling and melting point.

Boiling point of methanol is 64.5 ˚C, that of ethyl alcohol is 78 ˚C, while that of water is 100 ˚C.

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HYDROGEN BONDING In polyamides like nylon, wool, silk, inter-chain

hydrogen bonding enhances the mechanical properties.

The extraordinary strength of nylon is due to the heavy inter-chain hydrogen bonding.

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