Introduction to Organic Reactions

Overview

This chapter introduces the foundational concepts of organic reactions, including the classification of reaction types, the use of arrow-pushing to depict electron movement, and the distinction between radical and polar mechanisms. Understanding these principles is essential for predicting products, drawing mechanisms, and analyzing reactivity in organic chemistry.

Kinds of Organic Reactions

General Types of Reactions

• Addition: Two reactants combine to form a single product. Example: (e.g., alkene + HBr → alkyl bromide)

• Elimination: A single reactant splits into two products, often with the loss of a small molecule. Example: (e.g., alkyl halide + base → alkene + HX)

• Substitution: Parts of two reactants exchange to form new products. Example: (e.g., alkyl halide + OH⁻ → alcohol + halide)

• Rearrangement: The structure of a molecule changes without addition or loss of atoms. Example: (e.g., migration of a group within a molecule under heat or light)

Mechanisms – Arrow Pushing

Depicting Electron Movement

• Radical Reactions: Involve unpaired electrons; depicted with single-headed arrows (fishhook arrows).

• Polar (Ionic) Reactions: Involve electron pairs; depicted with double-headed arrows.

• Arrows always show the movement of electrons, not atoms.

Radical Reactions

Mechanism and Steps

Radical reactions are less common in organic chemistry but are important for certain transformations, such as halogenation of alkanes.

• Initiation: Formation of radicals, often by light or heat.

• Propagation: Radicals react with stable molecules to form new radicals.

• Termination: Two radicals combine to form a stable molecule.

Radical Halogenation

• Alkanes react with or under light to form alkyl halides.

• Substitution occurs preferentially at the most substituted carbon (especially with ).

• Drawbacks: Poor selectivity (especially for ), multiple substitutions possible, and use of unpleasant reagents.

Polar Reactions

Mechanism and Key Terms

Polar reactions dominate organic chemistry, involving the movement of electron pairs and the formation of charged intermediates.

• Nucleophile: Species that donates electrons (typically negative or neutral).

• Electrophile: Species that accepts electrons (typically positive or neutral).

• Arrows start from the nucleophile (electron source) and point to the electrophile (electron sink).

• Charges are conserved throughout the reaction.

Examples of Polar Mechanisms

• Substitution:

• Addition:

Drawing Reaction Mechanisms

Guidelines

• Use arrows to show electron movement only.

• Draw arrows from nucleophile to electrophile.

• Follow the octet rule; if an atom exceeds the octet, something must leave.

• Each mechanistic step is independent, with conservation of charge and electron flow.

• Consider the fate of electrons and the formation/breaking of bonds at each step.

Example Mechanism: Addition of HBr to an Alkene

• Step 1: Alkene reacts with HBr, forming a carbocation intermediate.

• Step 2: Bromide ion attacks the carbocation, yielding the alkyl bromide product.

Summary Table: Types of Organic Reactions

Key Takeaways

• Organic reactions are classified by the type of transformation: addition, elimination, substitution, rearrangement, and radical.

• Mechanisms are depicted using arrows to show electron movement; single-headed for radicals, double-headed for polar reactions.

• Understanding nucleophiles and electrophiles is crucial for predicting reaction outcomes.

• Drawing mechanisms requires attention to electron flow, charge conservation, and the octet rule.

Additional info: The notes reference Luis E. Miramontes and the synthesis of norethindrone, an important historical context for organic synthesis and pharmaceutical chemistry.