Organic Reactions

Organic reactions are chemical processes involving organic compounds, characterized by the transformation of molecules primarily composed of carbon and hydrogen, along with other atoms like Oxygen, Nitrogen, Sulfur, and Halogens. These reactions play a central role in the synthesis and transformation of organic molecules in both nature and industrial applications.

1.0Types of Organic Reactions

Substitution Reactions

In substitution reactions, one atom or group in a molecule is replaced by another atom or group. These reactions can occur in two main mechanisms: nucleophilic substitution and electrophilic substitution.

1. Nucleophilic Substitution Reactions (S_N1 and S_N2): In nucleophilic substitution, a nucleophile (electron-rich species) attacks a carbon atom bonded to a leaving group (electron-deficient), replacing it.

S_N1 (Unimolecular Nucleophilic Substitution):

• A two-step process involving the formation of a carbocation intermediate.
• The rate of reaction depends only on the concentration of the substrate (hence unimolecular).
• Example: Hydrolysis of tert-butyl chloride in water:
• (CH₃)₃CCl + H₂O → (CH₃)₃COH + HCl   

S_N2 (Bimolecular Nucleophilic Substitution):

• A one-step reaction where the nucleophile attacks the substrate from the opposite side of the leaving group, displacing it in a single step.
• The reaction rate depends on the concentration of both the nucleophile and substrate (hence bimolecular).
• Example: Hydrolysis of methyl bromide with hydroxide ions:

CH₃Br + OH⁻ → CH₃OH + Br⁻ 

2. Electrophilic Substitution Reactions (Aromatic Compounds):

• Commonly seen in aromatic compounds like benzene, where an electrophile replaces a hydrogen atom on the benzene ring.
• Example: Nitration of benzene:

C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (using H₂SO₄ as a catalyst)

Addition Reactions

In addition reactions, atoms or groups are added to a molecule, usually across a multiple bond (double or triple bond), resulting in a more saturated product. These reactions are common in alkenes and alkynes.

1. Electrophilic Addition:

• Occurs when an electrophile is attracted to the electron-rich double or triple bond, forming a carbocation intermediate.
• Example: Hydrogenation of ethene:
• • CH₂=CH₂ + H₂ → CH₃CH₃ (using a metal catalyst like Pd or Pt) 

2. Nucleophilic Addition:

• In nucleophilic addition, a nucleophile attacks an electrophilic center, often seen in carbonyl compounds (aldehydes and ketones).
• Example: Addition of water to acetaldehyde:
• • CH₃CHO + H₂O → CH₃CH(OH)₂ (forms a hydrate) 

Elimination Reactions

Elimination reactions involve the removal of atoms or groups from a molecule, resulting in the formation of a double or triple bond. These reactions are the reverse of addition reactions.

1. E₁ (Unimolecular Elimination):

• A two-step reaction where a carbocation intermediate forms first, followed by the elimination of a proton to form a double bond.
• Example: E₁ elimination reaction, which involves two steps: the loss of a leaving group (Br⁻) followed by the removal of a β-hydrogen to form an alkene.

2. E₂ (Bimolecular Elimination):

• A one-step process where the base abstracts a proton as the leaving group departs, forming a double bond.
• Example: 

Rearrangement Reactions

In rearrangement reactions, the structure of a molecule changes by the migration of atoms or groups within the molecule, resulting in a constitutional isomer.

Beckmann Rearrangement:

• Conversion of an oxime to an amide:
• • R₂C=NOH → RCONHR'

Oxidation-Reduction Reactions (Redox Reactions)

In oxidation-reduction reactions, the oxidation state of atoms changes due to the transfer of electrons. Oxidation refers to the loss of electrons (increase in oxidation state), while reduction refers to the gain of electrons (decrease in oxidation state).

Oxidation of Alcohols:

• Primary alcohols oxidize to aldehydes, and further oxidation leads to carboxylic acids.
• Example: Oxidation of ethanol to acetic acid using potassium dichromate (K₂Cr₂O₇):
• • CH₃CH₂OH → CH₃CHO → CH₃COOH img 9

Reduction of Carbonyl Compounds:

• Reduction of aldehydes to primary alcohols and ketones to secondary alcohols.
• Example: Reduction of acetone with NaBH₄:
• • CH₃COCH₃ → CH₃CH(OH)CH₃

Condensation Reactions

In condensation reactions, two molecules combine, often losing a small molecule such as water, ammonia, or alcohol in the process.

Aldol Condensation:

• Involves the reaction of two aldehydes or ketones in the presence of a base to form a β-hydroxy aldehyde or ketone.
• Example: Aldol reaction between two molecules of acetaldehyde:
• • CH₃CHO + CH₃CHO → CH₃CH(OH)CH₂CHO (aldol product)