Reaction mechanism in organic chemistry

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 Reaction mechanism:

  • An organic reaction mechanism is a complete, step-by-step account of how a reaction of organic compounds takes place. 
  • The description of an organic reaction mechanism typically includes designation of the overall reaction type (viz. substitution, addition, elimination, reduction, oxidation, or rearrangement), the presence of any reactive intermediates, the nature of the reagent that initiates the reaction, the presence of any catalysis, and its stereochemistry.
  • A common method for showing the progress of a reaction is a potential energy diagram, which plots the free energy of the system as a function of the completion of the reaction. The highest point along the reaction pathway called the transition state. The transition state indicates how easily the reaction can occur.
  • One of the other ways to illustrate the steps involved in a reaction mechanism is a kinetic scheme, which shows all the individual reaction steps and their rate constants.
  • The energy difference between the reactants and transition state is called the activation energy and represents the amount of energy that must be provided for the reactants to proceeds to products.

How Organic Reaction Occurs Mechanism?

  • A reaction mechanism describes the movement of electrons by using curved arrows to show bonds that are breaking and forming.
  • Homolysis occurs when a bond breaks with each atom keeping one electron.
  • Heterolysis occurs when a bond breaks and both electrons remain with one of the atoms.
  • Some reactions occur in more than one step with a reactive intermediate formed briefly on the way to the new product. 
  • Reactive intermediates can be changed species such as carbocations and carbanions or uncharged species such as radicals,
  • In organic chemistry Lewis acids are more often referred to as electrophiles, having an affinity for an elecron pair.
  • In organic chemistry Lewis bases are more often referred to as nucleophiles, having an electro pair that is available to bond to an electrophile.
  • Ionic reactions involve charged species. 

Reaction intermediates:

Reaction intermediates are chemical species, often unstable and short-lived( however sometimes can be isolated), which are not reactants and products of the overall chemical reaction, but are temporary products and/or reactants in the mechanisms reaction steps. Reaction intermediates are often free radicals.
The kinetics (relative rates of the reaction steps and the rate equation for the overall reaction) are explained in terms of the energy needed for the conversion of the reactant to the proposed transition states. 

Steps in mechanisms:

  • We classify the types of steps in a sequence 
  • A step involves either the formation or breaking of a covalent bond
  • Steps can occur individually or in combination with other steps 
  • When several steps occur at the same time they are said to be concerted 

Why is a reaction mechanism important?

The study of detailed processes of reaction mechanisms is important for many reasons, including the help it gives in understanding and controlling chemical reactions. The ability to draw such analogies frequently makes it possible to predict the course of untried reactions.

Types of organic reaction:

The organic reaction can be organized into several basic types. Some reactions fit into more than one category. For e.g, some substitution reactions follow an addition-elimination pathway. This overview isn't intended to include every single organic reaction. Rather, it is intended to cover the basic reactions.

Types of reactions:

  • Addition reactions 
Source: Internet

  • substitution reactions
Source: Internet

  • elimination reactions
Source: Internet

  • Rearrangement reactions
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  • Organic redox reactions
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