Common Types of Organic Reactions- Elimination and Rearrangement

Organic reactions and chemical reactions that involve organic compounds. A few critical reactions are listed: Substitution reaction, Addition reaction, elimination reaction, and rearrangement reaction.

Elimination reaction:

An elimination reaction is a type of reaction in which a single atom or a group of atoms is removed by acid, bases, or metals and in few reactions even by heating at high temperature.

Types of elimination reactions:

E1 reaction: It involves unimolecular elimination

• It is a two-step process: Ionization and deprotonation.
• Ionization: The C—H bond breaks to form a carbocation intermediate
• Deprotonation: The H+ ion is removed to form the neutral molecule.
• E1 often takes place in 3° alkyl halides and some 2° alkyl halides.
•  Rate of reaction is based on concentration.
• The formation of cation is the rate-determining step.
• Takes place in acidic and high-temperature conditions.
• These reactions compete with S1N reactions as both have carbocation as intermediates.
• This type of elimination is seen in highly substituted alkyl halides.

E2 reaction: This is a bimolecular reaction.

•  Single-step process.
• Observed in primary alkyl halides and some secondary alkyl halides.
• It depends on both the alkyl halide and the base, therefore a second-order reaction.
• It takes place under strong bases such as NaOH and KCl.
• It involves pi bond formation.

E1cB reaction: stands for “Elimination unimolecular conjugate base.”

• It takes place in the presence of a base and the hydrogen atom to be removed is comparatively acidic.
• The intermediate form is a carbon anion.
• The hydrogen must be present on the beta-carbon, and the functional group on the alpha-carbon must be a poor leaving group.
• It takes place in two steps. 
• And stabilization is through the resonance of the electrons.

Saytzeff and Hoffman rule;

• Saytzeff’s rule states that the product with more substitutes is the primary product in an elimination reaction and is more stable.
• Hoffman’s rule states that in an elimination reaction, the product with fewer substitutes is the primary product and is more stable.

Pyrolytic eliminations 

• These reactions don’t require additional reagent or solvent to proceed and are carried out in the gaseous phase and can be performed in inert solvents. 
• All other elimination is different from this one because those require a base or acid to proceed further in the reaction.
• Pyrolytic Ei (Elimination internal) is another name for the elimination reactions. 
• The pyrolytic decomposition of carboxylic esters, xanthates, amine oxides, sulfoxides, and selenoxides under high temperature form a C—C double bond.

These reactions occur in order of a cyclic transition state and therefore proceed with syn stereochemistry so that the H – atom and the leaving group depart from the same side of the double-bonded carbon.

Rearrangement reaction:

It is a type of reaction in which a rearrangement of atoms or a group of atoms(functional group) gives rise to required organic molecules.

Pinacol-pinacolone rearrangement: It’s a rearrangement occurring in the conversion of 1,2-diol to form a carbonyl compound.

1. Hoffman’s rearrangement: The conversion of primary amide to a primary amine by forming cyanide as an intermediate.

1. Beckmann rearrangement: Conversion of oxime to amide in the presence of conc.H2SO4 and heat.

1. The Baeyer-Villiger Rearrangement: Conversion of ketone to an ester by adding peroxide to add an oxygen atom next to the carbonyl group. 

1. Favorskii Reaction : The rearrangement of cyclopropanes often formed as intermediates in reaction such as the base-catalyzed reaction of alpha – halo ketones to form carboxylic acids and their derivatives.

1. Fries rearrangement: Formation of acyl phenols by alcohols in the presence of bronsted acid.

1.  Wagner-Meerwein rearrangement: Rearrangement of the carbon skeleton of the reactant due to formation of more carbocation. 

1. Stevens rearrangement: The reaction involves converting quaternary sulfonium salts and ammonium salts into sulfides and amides, respectively.

Addition Reaction :

An addition Reaction is a simple reaction in which a single atom or a group of atoms add up together to form a larger molecule.

1. Electrophilic reaction: Reaction that involves Electrophile X+.

An addition reaction in which a carbon compound has a double or triple pi bond is cleaved to form two separate sigma bonds.

The electrophile attaches to the unsaturated c=c.

It takes place in 2 steps. 

• Firstly forms the carbocation by cleaving the bond.
• Then the reception C-X and C-Y bonds are formed.

1. Nucleophilic reaction: This type of reaction involves Nucleophile X-.

The double and triple bond is broken down into single sigma bonds.

The C=O and C=N bonds are polar; therefore, in these types of bonds, the carbon carries a positive charge because the nucleophile is attracted towards it. 

1. Nucleophilic conjugate addition: This type of reaction takes place when there is no polar bond found in alkenes. 

In these cases, a substitute like cyclohexane is added to form the polarity at the beta site, where the nucleophile can attack and form the alkane molecule.

These reactions are named as 1,4- nucleophilic addition.

Conclusion:

Organic reactions play the most vital role in our daily life. These reactions are used at the industrial level and in laboratories to achieve the required organic compounds. Elimination, addition, and rearrangement reactions are fundamental to understanding the other chemical reactions in organic compounds.