Chapter 2_organic Reaction Types

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CHAPTER 2

ORGANIC REACTION TYPES



CO1

= Have the ability to explain the concept

of hybridization in describing bonding

between atoms in organic molecules.

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• Why and how chemical reactions take place

– What kind of reaction occur

– How reaction occur

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Types of organic reaction

• Addition reaction

• Elimination reaction

•

Substitution reaction• Rearrangement reaction

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Addition reaction

• Two reactants add together to form singleproduct with no atoms left over

• Characteristic of compounds containing

double and triple bond• Less energy to break p than s bond (app.

15kJ/mole weaker)

•Electrophilic addition reaction (Br2 + =)

• Nucleophilic addition reaction (C=O + HCN)

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Rearrangement reaction

• Occur when a single reactant undergoes a

reorganization of bonds and atoms to yield

isomeric product

• Atoms/groups shift from one position to

another within the substrate molecule itself

giving a product with a new structure

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Mechanism

• Reaction mechanism: overall description ofhow a reaction occurs

• Describes in detail exactly what takes place at

each stage of chemical transformation-whichbonds are broken/formed and in what order.Also account for all reactants used and allproducts formed

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• Two ways in which a covalent two-electronbond can break/form

– Symmetrically/homolytic ( ) radical reaction

– Unsymmetrically/heterolytic ( ) polar reaction

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Radical reaction

• Not as common as polar reactions

• Radicals react to complete electron octet of valence shell

– A radical can break a bond in another molecule and

abstract a partner with an electron, giving substitution inthe original molecule

– A radical can add to an alkene to give a new radical,

causing an addition reaction

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Radical reaction•

Initiated by free radicals• Example: methane chlorination

• Initiation: irradiation with UV light begins thereaction by breaking the relatively weak Cl-Clbond to form chlorine radical

• Propagation: Chlorine radical collides withmethane, abstract hydrogen to give HCl and

methyl radical (*CH3). Which reacts with Cl2

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Radical reaction

• Termination: two radicals collide and combine

to form stable product

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Polar reactions•

An electrophile, an electron-poor species, (neutral or+ve) combines with a nucleophile, an electron-rich

species (neutral or –ve)

• The combination is indicate with a curved arrow

from nucleophile to electrophile

• Example: addition of HBr to Ethylene

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Polar reactions

• Using curved arrows

– Rule 1: electrons move from a nucleophilicsource(-ve or neutral) to an electrophilic sink (+ve

or neutral) – Rule 2: octet rule must be followed

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Describing a Reaction: Intermediates

• If a reaction occurs in morethan one step, it must involvespecies that are neither thereactant nor the final product

• These are called reaction

intermediates or simply“intermediates”

• Each step has its own freeenergy of activation

• The complete diagram for thereaction shows the free energy

changes associated with anintermediate

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Chapter 2_organic Reaction Types