Introduction To Organic Chemistry : 12.5 Organic Reaction

TYPE OF CLEAVAGE :
2 type of bond cleavage:-

• Hemolytic cleavage
• Heterolytic cleavage
  
  

Hemolytic cleavage
The breaking of a single (two-electron) bond in which one electron remains on each of the atoms. Also known as free-radical reaction; homolysis.

Heterolytic cleavage
The breaking of a single (two-electron) chemical bond in which both electrons remain on one of the atoms. Also known as heterolysis.

TYPES OF REACTANT:



Electrophiles

In organic chemistry, an electrophilic addition reaction is an addition reaction where, in a chemical compound, a π bond is broken and two new σ bonds are formed. The substrate of an electrophilic addition reaction must have a double bond or triple bond.[1]


 
The driving force for this reaction is the formation of an electrophile X⁺ that forms a covalent bond with an electron-rich unsaturated C=C bond. The positive charge on X is transferred to the carbon-carbon bond, forming a carbocation.


 
Electrophilic addition mechanism


 
In step 2 of an electrophilic addition, the positively charged intermediate combines with (Y) that is electron-rich and usually an anion to form the second covalent bond.


 

 
Step 2 is the same nucleophilic attack process found in an SN1 reaction. The exact nature of the electrophile and the nature of the positively charged intermediate are not always clear and depend on reactants and reaction conditions.


 
In all asymmetric addition reactions to carbon, regioselectivity is important and often determined by Markovnikov's rule. Organoborane compounds give anti-Markovnikov additions. Electrophilic attack to an aromatic system results in electrophilic aromatic substitution rather than an addition reaction.



Nucleophiles

In organic chemistry, an electrophilic addition reaction is an addition reaction where, in a chemical compound, a π bond is broken and two new σ bonds are formed. The substrate of an electrophilic addition reaction must have a double bond or triple bond.[1]


 
The driving force for this reaction is the formation of an electrophile X⁺ that forms a covalent bond with an electron-rich unsaturated C=C bond. The positive charge on X is transferred to the carbon-carbon bond, forming a carbocation.


 
Electrophilic addition mechanism


 
In step 2 of an electrophilic addition, the positively charged intermediate combines with (Y) that is electron-rich and usually an anion to form the second covalent bond.


 

 
Step 2 is the same nucleophilic attack process found in an SN1 reaction. The exact nature of the electrophile and the nature of the positively charged intermediate are not always clear and depend on reactants and reaction conditions.


 
In all asymmetric addition reactions to carbon, regioselectivity is important and often determined by Markovnikov's rule. Organoborane compounds give anti-Markovnikov additions. Electrophilic attack to an aromatic system results in electrophilic aromatic substitution rather than an addition reaction.

 

Free Radicals

Free radical addition is an addition reaction in organic chemistry involving free radicals [1]. The addition may occur between a radical and a non-radical, or between two radicals.

The basic steps with examples of the free radical addition (also known as radical chain mechanism) are:

* Initiation by a radical initiator: A radical is created from a non-radical precursor.

* Chain propagation: A radical reacts with a non-radical to produce a new radical species

* Chain termination: Two radicals react with each other to create a non-radical species

Free radical reactions depend on a reagent having a (relatively) weak bond, allowing it to homolyse to form radicals (often with heat or light). Reagents without such a weak bond would likely proceed via a different mechanism. An example of an addition reaction involving aryl radicals is the Meerwein arylation.

To illustrate, consider the alkoxy radical-catalyzed, anti-Markovnikov reaction of hydrogen bromide to an alkene. In this reaction, a catalytic amount of organic peroxide is needed to abstract the acidic proton from HBr and generate the bromine radical, however a full molar equivalent of alkene and acid is required for completion.


 

 
TYPES OF REACTION:



Elimination

An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism [2]. Either the unsaturation of the molecule increases (as in most organic elimination reactions) or the valence of an atom in the molecule decreases by two, a process known as reductive elimination.


 
An important class of elimination reactions are those involving alkyl halides, or alkanes in general, with good leaving groups, reacting with a Lewis base to form an alkene in the reverse of an addition reaction. When the substrate is asymmetric, regioselectivity is determined by Zaitsev's rule. The one and two-step mechanisms are named and known as E2 reaction and E1 reaction, respectively.



Substitution

In a substitution reaction, a functional group in a particular chemical compound is replaced by another group [1] [2]. In organic chemistry, the electrophilic and nucleophilic substitution reactions are of prime importance. Organic substitution reactions are classified in several main organic reaction types depending on whether the reagent that brings about the substitution is considered an electrophile or a nucleophile, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical or whether the substrate is aliphatic or aromatic. Detailed understanding of a reaction type helps to predict the product outcome in a reaction. It also is helpful for optimizing a reaction with regard to variables such as temperature and choice of solvent.

A good example of a substitution reaction is the photochemical chlorination of methane forming methyl chloride.Substitution reaction : chlorination of methane



Addition

An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one [1][2].


 
Addition reactions are limited to chemical compounds that have multiply-bonded atoms, such as molecules with carbon-carbon double bonds, i.e., alkenes, or with triple bonds, i.e., alkynes. Also included are molecules containing carbon - hetero double bonds like those with carbonyl (C=O) groups or those with imine (C=N) groups.


 
There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition. One non-polar addition reaction exists as well called free radical addition.

Addition reactions general overview. Top to bottom: electrophic addition to alkene, nucleophilic addition of nucleophile to carbonyl and free radical addition of halide to alkene


 
An addition reaction is the opposite of an elimination reaction. For instance the hydration reaction of an alkene and the dehydration of an alcohol are addition-elimination pairs. Addition reactions are also encountered in polymerizations and called addition polymerization.


 
Rearrangement

rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule [1] . Often a substituent moves from one atom to another atom in the same molecule. In the example below the substituent R moves from carbon atom 1 to carbon atom 2:

General scheme rearrangement


 
Intermolecular rearrangements also take place.


 
A rearrangement is not well represented by simple and discrete electron transfers (represented by curly arrows in organic chemistry texts). The actual mechanism of alkyl groups moving, as in Wagner-Meerwein rearrangement, probably involves transfer of the moving alkyl group fluidly along a bond, not ionic bond-breaking and forming. In pericyclic reactions, explanation by orbital interactions give a better picture than simple discrete electron transfers. It is, nevertheless, possible to draw the curved arrows for a sequence of discrete electron transfers that give the same result as a rearrangement reaction, although these are not necessarily realistic.


 
Three key rearrangement reactions are 1,2-rearrangements, pericyclic reactions and olefin metathesis.



1,2-rearrangements

Main article: 1,2-rearrangement

A 1,2-rearrangement is an organic reaction where a substituent moves from one atom to another atom in a chemical compound. In a 1,2 shift the movement involves two adjacent atoms but moves over larger distances are possible. Examples are the Wagner-Meerwein rearrangement: and the Beckmann rearrangement: