Organic Reactions Types and Mechanism

ORGANIC REACTIONS TYPES AND MECHANISM

Types of Organic Reactions and their Examples

Organic reactions involve the transformation of organic compounds through various chemical changes, often to attain a more stable state. There are eight primary types of organic reactions, each characterized by the nature of the transformation that occurs.

They are further divided into subtypes.

Types of organic reactions

1. Substitution reactions

Substitution reactions involve the replacement of one atom or group in a molecule with another atom or group. These are common in saturated hydrocarbons and alkyl halides.

Types of substitution reactions

Substitution reactions are divided into three types based on types of reactants (substrate):

• Nucleophilic substitution reactions
• Electrophilic substitution reactions
• Free radical substitution reactions

Nucleophilic substitution reactions

In nucleophilic substitution reactions, a nucleophile (an electron-rich species) attacks an electron-deficient carbon atom in a substrate, leading to the replacement of a leaving group.

Types of Nucleophilic Substitution Reactions:

SN1 Reactions

• Mechanism: Unimolecular nucleophilic substitution involves a two-step mechanism. The first step is the formation of a carbocation intermediate after the leaving group departs. In the second step, the nucleophile attacks the carbocation.
• Rate Law: Rate = k[substrate]
• Example: Hydrolysis of tert-butyl chloride.
• Key Feature: The rate-determining step is the formation of the carbocation.

SN2 Reactions

• Mechanism: Bimolecular nucleophilic substitution involves a single concerted step where the nucleophile attacks the substrate from the opposite side of the leaving group, leading to the inversion of configuration.
• Rate Law: Rate = k[substrate][nucleophile]
• Example: Reaction of methyl bromide with hydroxide ion to form methanol.
• Key Feature: The nucleophile attacks the carbon center simultaneously as the leaving group departs.

SNI Reactions

• Mechanism: Substitution nucleophilic internal involves a nucleophile attacking a substrate in a concerted process without forming a carbocation.
• Example: Reaction of 1,2-dibromoethane with a strong base like potassium tert-butoxide to form an alkene.
• Key Feature: Common in systems where the nucleophile is very strong.`

SNAr Reactions

• Mechanism: Nucleophilic aromatic substitution involves the nucleophile attacking an electron-deficient aromatic ring, often facilitated by a leaving group.
• Example: Reaction of chlorobenzene with sodium hydroxide to form phenol (via the Benzyne mechanism).
• Key Feature: Typically occurs in aromatic systems with electron-withdrawing groups.

Electrophilic Substitution Reactions

In electrophilic substitution reactions, an electrophile (an electron-deficient species) replaces a leaving group in a substrate. These reactions are common in both aliphatic and aromatic systems, though they behave differently based on the nature of the substrate.

Aliphatic Electrophilic Substitution Reactions

Mechanism:

• Electrophiles attack aliphatic substrates, often resulting in the formation of products where the electrophile replaces a hydrogen atom or another substituent.
• The mechanism typically involves the formation of a carbocation intermediate, which is then attacked by the electrophile.

Examples:

Halogenation of Alkanes:

• Reaction: Methane reacts with chlorine in the presence of UV light to form chloromethane and hydrochloric acid.

• Mechanism:

1. Initiation: Formation of chlorine radicals.
2. Propagation: Chlorine radicals abstract a hydrogen atom from methane, forming methyl radicals, which then react with chlorine molecules to form chloromethane and regenerate chlorine radicals.
3. Termination: Combination of radicals to form stable products.

Nitration of Alkanes:

• Reaction: Alkanes react with nitric acid in the presence of sulfuric acid to form alkyl nitrates.

• Mechanism:

1. Formation of Nitronium Ion: Nitric acid and sulfuric acid produce the nitronium ion (NO₂⁺), the active electrophile.
2. Attack on Alkanes: The nitronium ion attacks the alkanes, resulting in the formation of alkyl nitrates.

Aromatic Electrophilic Substitution Reactions

Mechanism:

• Electrophiles attack the electron-rich aromatic ring, replacing a hydrogen atom. These reactions typically involve the formation of a carbocation intermediate (sigma complex or arenium ion) followed by the loss of a proton.
• Aromatic rings are stabilized by resonance, which affects the reactivity and orientation of electrophilic substitution.

Types of Aromatic Electrophilic Substitution Reactions:

1. Nitration:
   • Reaction: Benzene reacts with nitric acid in the presence of sulfuric acid to form nitrobenzene.
   • Mechanism:
     1. Generation of Nitronium Ion: Nitric acid and sulfuric acid produce the nitronium ion (NO₂⁺).
     2. Formation of Sigma Complex: The nitronium ion attacks the benzene ring, forming a sigma complex.
     3. Restoration of Aromaticity: Loss of a proton restores the aromaticity of the ring.

2. Halogenation:
   • Reaction: Benzene reacts with halogens (chlorine or bromine) in the presence of a Lewis acid catalyst (such as FeCl₃ or FeBr₃) to form halobenzene.
   • Mechanism:
     1. Formation of Electrophile: The Lewis acid catalyst reacts with the halogen to form a halogenium ion (e.g., Br⁺).
     2. Formation of Sigma Complex: The halogenium ion attacks the benzene ring, forming a sigma complex.
     3. Restoration of Aromaticity: Loss of a proton restores the aromaticity of the ring.

3. Friedel-Crafts Alkylation:
   • Reaction: Benzene reacts with an alkyl halide in the presence of a Lewis acid catalyst to form an alkylbenzene.
   • Mechanism:
     1. Formation of Carbocation: The Lewis acid catalyst facilitates the formation of a carbocation from the alkyl halide.
     2. Formation of Sigma Complex: The carbocation attacks the benzene ring, forming a sigma complex.
     3. Restoration of Aromaticity: Loss of a proton restores the aromaticity of the ring.

4. Friedel-Crafts Acylation:
   • Reaction: Benzene reacts with an acyl chloride in the presence of a Lewis acid catalyst to form an aryl ketone.
   • Mechanism:
     1. Formation of Acylium Ion: The Lewis acid catalyst facilitates the formation of an acylium ion (R-C=O⁺).
     2. Formation of Sigma Complex: The acylium ion attacks the benzene ring, forming a sigma complex.
     3. Restoration of Aromaticity: Loss of a proton restores the aromaticity of the ring.

General Considerations for Aromatic Electrophilic Substitution:

• Electronic Effects: The presence of substituents on the aromatic ring can influence the reactivity and orientation of the electrophilic substitution. Electron-donating groups (like -OH or -NH₂) generally activate the ring and direct electrophiles to the ortho and para positions, while electron-withdrawing groups (like -NO₂ or -COOH) deactivate the ring and direct electrophiles to the meta position.
• Regioselectivity: The position at which the electrophile substitutes on the aromatic ring is influenced by the existing substituents and their electronic effects.

Free Radical Substitution Reactions

Free radical substitution reactions involve the replacement of a substituent in a molecule with a free radical. These reactions are often termed chain reactions because they proceed through a series of steps involving the generation and consumption of free radicals. They are common in organic chemistry, particularly in the halogenation of alkanes.

Steps in Free Radical Substitution Reactions

1. Initiation:
   • Description: Free radicals are generated from non-radical precursors. This step usually involves the breaking of a bond to form radicals, often initiated by heat or light.
   • Example: In the halogenation of alkanes, halogen molecules (X₂) are dissociated into two halogen radicals (X•) by UV light or heat.
   • Reaction: X₂(UVlight)→2X•
2. Chain Propagation:
   • Description: The free radicals generated in the initiation step react with the substrate, creating new radicals and leading to the formation of the product. This step consists of a series of reactions where each step creates a radical that continues the chain reaction.
   • Example: In the chlorination of methane:
     • Step 1: A halogen radical abstracts a hydrogen atom from the alkane, forming a new radical.

CH₄+X•→CH₃X+H•

• Step 2: The newly formed radical reacts with another halogen molecule, generating a new halogen radical and continuing the chain.

X₂+H•→HX+X•

3. Chain Termination:
   • Description: The chain reaction terminates when two free radicals combine to form a stable product, thereby removing the free radicals from the system.
   • Example:
     • Combination of Two Radicals: X•+Y•→XY
     • Combination of Two Same Radicals: X•+X•→X₂

2. Addition Reactions

Addition reactions involve the combination of two or more molecules to form a single product, typically involving unsaturated compounds with double or triple bonds. These reactions result in the addition of new atoms or groups to the original molecules.

Types of Addition Reactions

Electrophilic Addition Reactions

Description: In electrophilic addition reactions, an electrophile (electron-deficient species) attacks the electron-rich π-bond of an unsaturated compound (such as alkenes or alkynes). This results in the formation of a carbocation intermediate, which then reacts with a nucleophile to form the final product.

Example: Addition of HBr to ethene.

CH₂=CH₂+HBr→CH₃CH₂Br

Mechanism:

Electrophile Attack: HBr (electrophile) adds to the double bond of ethene, forming a carbocation intermediate.

Nucleophile Attack: The bromide ion (Br⁻) attacks the carbocation to form the final product, bromoethane.

Nucleophilic Addition Reactions

Description: In nucleophilic addition reactions, a nucleophile (electron-rich species) attacks the carbonyl group (C=O) of aldehydes or ketones. The π-bond of the carbonyl group is broken, and the nucleophile adds to the carbon atom, forming a new chemical bond.

Example: Addition of sodium borohydride (NaBH₄) to acetone.

CH₃COCH₃+NaBH₄→CH₃CH(OH)CH₃

Mechanism:

Nucleophile Attack: NaBH₄ (nucleophile) adds to the carbonyl carbon of acetone, forming an alkoxide intermediate.

Protonation: The alkoxide intermediate is protonated to form the final alcohol product, isopropanol.

Free Radical Addition Reactions

Description: In free radical addition reactions, free radicals add to the double or triple bonds of unsaturated compounds. These reactions typically involve a chain mechanism similar to free radical substitution reactions.

Example: Addition of chlorine to ethene.

CH₂=CH₂+Cl₂→CH₂Cl−CH₂Cl

Mechanism:

Initiation: Formation of chlorine radicals (Cl•) by the dissociation of Cl₂.

Propagation: Chlorine radicals add to the ethene double bond, forming chlorinated intermediates.

Termination: Formation of the final product, 1,2-dichloroethane, by combining radicals.

Cyclic Addition Reactions

Description: Cyclic addition reactions involve the formation of a cyclic structure through the addition of reactants to a double or triple bond. These reactions often involve the formation of ring structures and can be part of more complex mechanisms such as cycloadditions.

Example: Diels-Alder reaction.

C₄H₆+C₂H₂Cl₂→C₆H₈Cl₂ ​

Mechanism:

Diene and Dienophile Reaction: A diene (such as 1,3-butadiene) reacts with a dienophile (such as ethylene) to form a cyclohexene ring structure.

Formation of Cyclic Product: The reaction results in a six-membered ring with new bonds formed between the reactants.

Characteristics of Addition Reactions

• Formation of Single Product: Addition reactions typically yield a single product with no by-products.
• Involves π-Bond Cleavage: The reaction involves the breaking of π-bonds in double or triple bonds.
• Can Be Reversible: Some addition reactions are reversible and can reach equilibrium.

3. Elimination reactions

Elimination reactions are opposite to the addition reactions. Sigma bondsare converted into the pi bond.This happens when two or more atoms or groups of atoms are removed to form a double or triple bond.

Types of elimination reactions

• 𝜶 Elimination reactions
• 𝜷 Elimination reactions
• 𝜸 Elimination reactions
• Conjugate Elimination reactions
• Pyrolytic Elimination reactions
• Extrusion reactions

𝜶 Elimination reactions

In 𝜶 elimination reactions both the groups are removed from the same atom. This results in the formation of carbene in the case of carbon and nitrene in the case of nitrogen.

𝜷 Elimination reactions

In 𝜷 elimination reactions, two groups are removed from adjacent carbon atoms. One is from𝜶 carbon and the second is from 𝞫 carbon. This is the most common type of elimination reaction.

𝜷 elimination reactions can proceed in a single step (E2) as well as in two steps (E1).

Types of 𝞫 Elimination reactions

𝞫 elimination reactions are subdivided into three types

• E1elimination reactions (unimolecular)
• E2elimination reactions (bimolecular)
• E1cBelimination reactions

𝜸 Elimination reactions

𝜸 elimination reactions make a three-membered ring structure. Here one atom is removed from 𝜶 carbon and the other is removed from 𝜸 carbon. Three-member rings are very unstable due to angle strain.

Extrusion reactions

Extrusion reactions involve the removal of a fragment from within a chain or a ring.

Conjugate Elimination reactions

They involve the removal of atoms from the conjugate system. The conjugate system has alternate single and double bonds.

Pyrolytic elimination reactions

As the name pyrolytic eliminations show that these elimination take place due to heating. There are several compounds that undergo elimination on heating when no other reagent is present. They are often in the gas phase. Such reactions are called pyrolytic elimination reactions.

4. Rearrangement reactions

The reactions in which an atom or group of atoms moves from one atom to another atom within a molecule are called rearrangement reactions. They result in the formation of structural isomers of original molecules. Connectivities of groups changes within the molecule but most of the migrations are from one atom to an adjacent atom and are called 1,2 shifts. However, there can be some migrations over a long distance.

Types of rearrangement reactions

• Nucleophilic rearrangement reactions
• Electrophilic rearrangement reactions
• Free radical rearrangement reactions

Nucleophilic rearrangement reactions

Nucleophilic rearrangement reactions are the most common rearrangement reactions and are also named as anionotropic rearrangements. In these rearrangements, the migrating group is a nucleophile. It will move with its electron pair.

Electrophilic rearrangement reactions

Nucleophilic rearrangement reactions are also named as cationotropic rearrangements or prototropic rearrangements. In these rearrangements, the migrating group is an electrophile. It will move without its electron pair.

Free radical rearrangement reactions

Free radical rearrangement reactions are not common. In these rearrangements, the migrating group is a free radical. It will move with just one electron.

5. Condensation reactions

Condensation reactions are the type of organic reaction that involves the combination of two molecules to form a single product by the loss of small molecules. In most cases, a water molecule is removed during condensation reactions. In the case of water molecules, these reactions are also called dehydration reactions.

There are certain other small molecules that can be removed during condensation reactions, i.e ammonia, ethanol, acetic acid, etc. Well known condensation reactions are:

• Aldol condensation
• Claisen condensation
• Deckman condensation
• Knoevenagel condensation

6. Pericyclic reactions

These are the type of organic reactions that requires light or heat to proceed and have a concerted mechanism. The bond breaking and bond, formation takes place simultaneously.

These reactions have a single transition state with cyclic geometry and are also known as nonionic reactions as no intermediate is formed during the chemical reaction. They are highly stereospecific.

Types of pericyclic reactions

Pericyclic reactions are classified into 7 different types:

• Cycloaddition reactions
• Electrocyclic reactions
• Sigmatropic reactions
• Group transfer reactions
• Ene reactions
• Cheletropic reactions
• Dyotropic reactions

7. Polymerization reactions

These are the organic reactions that involve the formation of larger molecules called a polymer. Polymers consist of a large number of small repeating units called monomers. These repeating units combine chemically to produce a polymer.

Polymerization reactions are also called as chain reactions. They proceed in three steps:

• Initiation
• Chain propagation
• Chain termination

Types of polymerization reactions

These are classified into two types

• Addition polymerization
• Condensation polymerization

8. Oxidation-reduction reactions

Oxidation-reduction reactions are also called redox reactions. They involve the loss or gain of electrons as a result of which the oxidation state of species changes. Oxidation and reduction take place simultaneously. One of the molecules gets oxidized while the other gets reduced at the same time.