Reaction Mechanism. Part 1 of 8

Basic Concepts of Organic Chemistry Reaction Mechanism

Whena chemical reaction takes place between two or more chemical species,new products are formed. This change is represented by chemicalequation. Most of the organic reactions involve conversion of onefunctional group into another by attack of a reagent. The reactantsnormally consist of two species.

(i) Substrate: Such organic compounds which are attacked by reagent are known as substrate or reactants.

(ii) Reagent: A simple inorganic or organic compounds which is used to create a desired transformation in the compound is known as reagent.

(I) A substrate and a chemical reagent from a highly energetic species, called activated complex, before it changes into the product.

(II) In certain cases, a relative energetically more stable species than the activated complex may also be formed. It is called reaction intermediate. Thus a chemical reaction, in general, may follow either of the following two paths.

Thedetail step by step description of a chemical reaction is calledmechanism of a reaction. If the reaction mechanism involves thebreaking and making of bonds simultaneously without the formation ofany intermediate, it is called concerted mechanism. Such reactions arecalled elementary reactions. On the other hand, if the reactionmechanism involves the formation of intermediates before the formationof products, it is called nonconcerted mechanism. These reactions arecomplex reactions.

It is convenient to classify the numerous reactions of the various classes of organic compounds into following four types.

1.Substitution or displacement reactions

2.Addition reactions

3.Elimination reactions

4.Rearrangementsa

1. Substitution Reaction:The replacement of an atom or group from a molecule by different atomor group is known as substitution or displacement reaction. These areof three types (i) Nucleophilic (ii) Electrophilic (iii) free radical

2. Addition Reaction:Reaction in which atoms or group of atoms are added to a molecule areknown as addition reactions. In other words, they involves thecombination of two molecules to give a single molecule

Thistype of reaction occurs only when there is a centre of unsaturation inthe molecule which is generally due to the presence of a multiple bondbetween two atoms, e.g.

Pi(p) bond is responsible for addition reactions. For each p bond of themolecule two sigma bonds are formed and the hybridization state ofcarbon changes from sp to sp² and sp² to sp³.

There are three types of addition reaction:

(i)Free radical addition reaction

(ii) Electrophilic addition reaction

(iii) Nucleophilic addition reaction

3. Elimination Reaction:These reactions are essentially the reversal of additional reactionsand involve loss of atoms or group of atoms from a molecule to form amultiple linkage. Most commonly, Loss of atoms or group occur fromadjacent carbon atoms to yield an olefin, e.g.

In elimination reaction sigma bond is converted to pi bond i.e. the state of hybridization of carbon change from sp³ to sp² and sp² to sp.

(i) b-Elimination Reactions:In this reaction, the losses of two atoms or groups occur from theadjacent atom of the substrate molecule. For example, acid-catalyseddehydration of alcohols and Base -Catalysed dehydrohalogenation ofalkyl halides:

Acid-Catalysed dehydration of alcohols:

Base-Catalysed dehydrohalogenation of alkyl halides:

(ii) a-Elimination Reactions:In these reactions, the loss of two atoms or groups occurs from thesame atom of the substrate molecule. For example, Base-catalyseddehydrohalogenation of chloroform to form dichlorocarbene:

Dichlorocarbene is the reactive intermediate involved in Carbylamine reaction and Reimer-Tiemann reaction.

Note –

-Removalof s proton and halide ion from the adjacent carbons is calleddehydrohalogenation and that a proton and hydroxide ion is calleddehydration.

- The carbonto which the leaving group (X, OH etc.) is attached is called thea-carbon. An adjacent carbon is a b-carbon because the eliminationreaction is initiated by removing a proton from a b -carbon, an E2reaction is also a b -elimination reaction or 1,2-elimination reaction.

5. Rearrangements:Rearrangement reactions involve either the migration of a functionalgroup to another position in the molecule containing double bond, e.g.

Orreshuffling of the sequence of atoms forming the basic carbon skeltonof the molecule to form a product with new structure, e.g.

The rearrangements vary in their types depending on the atom or group migrating in the molecule.