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Reaction Mechanisms

The Walden inversion, an umbrella turned inside–out in a gale The Walden inversion, an umbrella turned inside–out in a gale In a bimolecular nucleophilic substitution reaction, if the substrate is optically active, we observe an inversion in the configuration of the substituted product. It was first observed by chemist Paul Walden in 1896. This Walden inversion of chirality can be visualized by imagining an umbrella turned inside‐out in a gale. Since an optically active molecule can form two enantiomers around a chiral center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to the other.

Learning Objectives

After completing the topic, the student will be able to:

  • Identify the characteristics and significance of reaction mechanism.
  • Discuss the cleavage of a covalent bond and its products.
  • List the components of a reaction, substrate, leaving group, reagent and intermediates.
  • Define the electronic displacements such as inductive effect, mesomeric effect, hyper conjugation etc. in organic compounds.
  • Describe effects of electronic displacements on acidity, basicity and reactivity of substrates.
  • Classify and describe major type organic reactions, substitutions (SN1 and SN2 ) and eliminations (E1 and E2) reactions and factors effecting them.
  • Compare SN1, E1, E2 and SN2 reactions.
Chemical reactions Chemical reactions A chemical reaction is a process that leads to the transformation of one set of chemical substances to another.
Reaction Mechanism

To the uninitiated, a chemical reaction must seem like an act of magic. A chemist puts one or two reagents into a flask, warms them for a period of time, and then takes from the flask one or more completely different compounds. It is, until we understand the details of the reaction, like a magician who puts apples and oranges in a hat, shakes it, and then pulls out rabbits and parrots.

Now we'll try to understand how this chemical magic takes place. We need to be able to explain how the products of the reaction are formed. This explanation will take the form of a mechanism for the reaction–a description of the events that take place on a molecular level as reactants become products.

Often the reaction takes place in more than one step. We will know what chemical species, called intermediates, intervene between each step along the way.

Examples of reaction pathway Examples of reaction pathway
  • Hydrogenation of alkenes to alkanes
  • Hydrolysis of alkyl halides to alcohols in presence of a base
Reaction flow chart

In any reaction a substrate is attacked by a reagent forming a transition state or intermediate which is converted in to product. It is the common process takes place in any chemical reaction. Before understanding the reaction mechanism a chemist should,

  1. Aware of the properties of various substrates and attacking reagents.
  2. Isolate and characterize the intermediate or transition state.

Reaction pathway:

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