Reaction Mechanism-I

In organic Chemistry, a reaction typically involves an organic molecule (referred to as the substrate) interacting with a suitable attacking reagent. This interaction leads to the formation of intermediate(s) and ultimately product(s).

The substrate is the reactant that contributes carbon to the new bond being formed. The other reactant involved is called the reagent. If both reactants contribute carbon atoms to the new bond, the choice of which one to call the substrate is arbitrary; generally, the substrate is the molecule that is the primary focus of the reaction.

Reaction mechanism is a detailed account of each step of the reaction including descriptions of electron movement, the energetics of bond breaking and forming, and the kinetics - how the rate at which reactants transform into products. 

1.0Attacking Reagents

Attacking reagents in organic chemistry are species that initiate reactions by forming new bonds with other molecules.

A molecule whose carbon is involved in new bond formation is called substrate and the other one is called reagent. When a carbon-carbon bond is formed, the choice of naming the reactants as substrate and reagent is arbitrary and depends on the molecule under observation.

                                   CH₂=CH₂    +   Br₂    →    CH₂Br - CH₂Br

Types of Attacking Reagent

In chemistry, particularly in organic reactions, the term attacking reagent refers to a species that attacks a substrate molecule or an intermediate, leading to the formation of a product. This attacking reagent can be a nucleophile, an electrophile, or a radical, depending on the nature of the reaction. Main types of Attacking reagents are:

1. Electrophiles

• Electrophiles are "electron-loving" species that seek electrons to complete or stabilize their valence shell. They are typically electron-poor and have a positive or partial positive charge.
• Electrophiles can act as Lewis acids.
• Examples of electrophiles- H^⊕, Br^⊕, Cl^⊕, SO₃, BF₃, FeCl₃

2. Nucleophiles

• Nucleophiles are "nucleus-loving" entities that are rich in electrons. They are attracted to positive or partially positive centers in other molecules, where they can donate or share their electrons.
• Nucleophile is always a lewis base.
• Examples of Nucleophiles- H^⊖,OH^⊖, O-R^⊖, R-NH₂, NH₃, H₂O.

3. Amphiphile

• Amphiphilic molecules, particularly those containing a multiple bond between carbon and a more electronegative atom, exhibit versatile chemical behavior that allows them to act both as electrophiles and nucleophiles. 
• Examples of Amphiphile-  >C = O  ,   CH₃―C☰N

2.0Reaction Intermediates

In organic Chemistry, understanding reaction mechanisms first, we need to understand reaction intermediates. Here is brief description of important intermediates:

1. Carbon Free Radical: A carbon free radical is an atom of carbon that possesses an unpaired electron, making it highly reactive. Free radicals are typically generated through homolytic bond cleavage and are involved in many types of reactions, including polymerizations and combustion processes.

2. Carbocation: A carbocation is a positively charged carbon ion, where the positive charge is due to a carbon atom having only six electrons in its valence shell. Carbocations are electrophilic and can be stabilized by alkyl groups or resonance. 

They are important intermediates in reactions like electrophilic aromatic substitution and alkene addition reactions.

3. Carbanion: A carbanion features a negatively charged carbon atom with three bonds and a lone pair of electrons. Carbanions are nucleophilic and can be stabilized by electron-withdrawing groups and by resonance.

They play a significant role in various organic reactions, including nucleophilic substitution and elimination reactions.

4. Carbene: Carbenes are neutral species with a carbon atom having two non-bonded electrons and only six valence electrons, making them highly reactive. 

Carbenes can insert into C-H bonds, add to double bonds, and rearrange to form more stable structures. They are used in cyclopropanation reactions and in the formation of ylides.

3.0Bond Clevage

A covalent bond can be cleaved in two primary ways, which are fundamentally different in their process and result. Let’s learn how a bond fission takes place-

Heterolytic Cleavage

• In heterolytic cleavage, the bond between two atoms breaks unevenly. One atom retains both electrons from the shared pair, resulting in the formation of an anion, while the other atom loses both electrons, forming a cation. This type of cleavage often occurs in polar bonds where one atom is significantly more electronegative than the other.

• The outcome is the formation of ions, which can be stabilized by solvents in solution. Heterolytic cleavage is common in reactions that occur in polar solvents, which help stabilize the charged species.

Homolytic Cleavage

• Homolytic cleavage involves the even splitting of a covalent bond, where each atom takes away one electron from the bonded pair. This results in the formation of two radicals, each bearing an unpaired electron.

• Radicals are highly reactive due to their unpaired electrons and can rapidly engage in further chemical reactions. Homolytic cleavage is often facilitated by high energy conditions such as heat or light (photochemical reactions).

4.0Types of Reactions and Reaction Mechanism

Each type of bond cleavage plays an important role in determining the path and products of chemical reactions, influencing how molecules interact and transform under various conditions.

Organic reactions are varied and complex, but they can be broadly categorized into four main types, each involving distinct changes in the molecular structure and bonding. Here's a brief description of each:

Substitution Reactions

• In nucleophilic substitution reactions, the nucleophile targets the electron-deficient part of the substrate molecule. This type of reaction occurs when a nucleophile displaces an existing nucleophile within a molecule. 
• Haloalkanes commonly serve as the substrates in these reactions. Here, the nucleophile reacts with a haloalkane, where the carbon atom bonded to the halogen carries a partial positive charge. During the reaction, the halogen atom, acting as the leaving group, is expelled as a halide ion. This process is specifically termed a nucleophilic substitution reaction because it is initiated by the nucleophile attacking the electron-deficient carbon.
• Nucleophilic Subtitution of Alkyl Halides (R-X) will be-

                                  R-X     +    Nu^–   ⟶     R-Nu     +  X^– 

Mechanisms:

Substitution reactions can proceed via two primary mechanisms:

• S_N1 (Unimolecular Nucleophilic Substitution): Involves the formation of an intermediate carbocation, followed by a nucleophilic attack. It is common in tertiary substrates where carbocations are stable.

Example:

• S_N2 (Bimolecular Nucleophilic Substitution): Involves a single step where the nucleophile attacks the substrate from the opposite side of the leaving group, leading to inversion of configuration. 

Characteristics:

• The nucleophile attacks the substrate from the side directly opposite the leaving group.
• During the transition state, the hybridization of the carbon where the substitution takes place shifts from sp³ to sp².
• The reaction is bimolecular and occurs as a one-step concerted process.      

Addition Reactions

Addition reactions involve two or more molecules combining to form a single product. This type of reaction is characteristic of alkenes and alkynes, where the pi bond is broken and new sigma bonds are formed.

Mechanisms:

These reactions can be:

• Electrophilic Addition: Typical of alkenes and alkynes, where the pi bond electrons are attracted to an electrophile, followed by nucleophilic attack.
• Nucleophilic Addition: Common in carbonyl chemistry, where a nucleophile adds to the carbon of the carbonyl group.

Elimination Reactions

• Elimination reactions result in the removal of two atoms or groups from a molecule, leading to the formation of a double or triple bond. These are the reverse of addition reactions.

Mechanisms:

• E₁ (Unimolecular Elimination): Involves the formation of a carbocation intermediate followed by the loss of a leaving group.’
• E₂ (Bimolecular Elimination): Occurs in a single concerted step where a base removes a hydrogen atom, leading to the formation of a double bond as the leaving group exits.

Rearrangement Reactions

• Rearrangement reactions involve a structural reordering of the molecular framework. Atoms or groups within a molecule move to different positions, leading to a new molecular structure.
• Characteristics: These reactions often involve the formation of more stable or more functional molecules. They can occur as part of other reaction processes (like during an S_N1 reaction) or can be initiated by heat or light.
• For example-  The Benzil-Benzilic Acid rearrangement is a well-known organic reaction in which a 1,2-diketone (benzil) undergoes a rearrangement to form an α-hydroxy-carboxylic acid (benzilic acid) under the influence of a base. This reaction is a type of 1,2-rearrangement, a fundamental transformation in organic synthesis.