Organic Chemistry Chapter 6

Organic chemical reactions are primarily classified into four types:
- Addition Reactions: Two reactants combine to form a single product without leftover atoms (e.g., alkene + HBr = alkyl bromide).
- Elimination Reactions: A single reactant splits into two products, often releasing a small molecule (e.g., alcohol yields water + alkene).
- Substitution Reactions: Two reactants exchange parts to create two new products (e.g., ester + water = carboxylic acid + alcohol).
- Rearrangement Reactions: A single reactant reorganizes bonds and atoms to yield isomers (e.g., dihydroxyacetone phosphate to glyceraldehyde 3-phosphate).

A reaction mechanism describes the detailed process of how a reaction occurs, including bond-breaking and bond-making steps.
Bond-breaking can be:
- Heterolytic (Unsymmetrical): Bond breaks with both electrons going to one fragment.
- Homolytic (Symmetrical): Bond breaks with one electron retained by each fragment.
Indicated by arrows:
- Full-headed curved arrow for heterolytic.
- Half-headed (fishhook) arrow for homolytic.

Polar Reactions: Involve unsymmetrical bond-breaking and bond-making; common in organic chemistry.
Radical Reactions: Involve symmetrical bond-breaking and bond-making; typically less common.
Pericyclic Reactions: Discussed later, less common type.

Polar bonds result from differences in electronegativity, leading to partial charges. For example,
- Oxygen and nitrogen are more electronegative than carbon, affecting charge distribution.
In polar reactions:
- Nucleophiles: Electron-rich species that donate electrons.
- Electrophiles: Electron-poor species that accept electrons.

The equilibrium constant (K_{eq}) expresses the position of equilibrium in a reaction:
- Large K_{eq} > 1 favors products.
- Small K_{eq} < 1 favors reactants.
Gibbs Free Energy Change (ΔG): Determines reaction favorability:
- ΔG < 0: Exergonic (favorable).
- ΔG > 0: Endergonic (unfavorable).
ΔG relates to K{eq} by: $ext{ΔG}^o = -RT ext{ln}(K</em>{eq})$

Enthalpy change (ΔH) and entropy change (ΔS):
- ΔH: Measure of total bond energy change; negative indicates exothermic reaction.
- ΔS: Measure of molecular randomness change; positive indicates more randomness.
Example: Reaction of ethylene and HBr is exothermic with ΔH < 0 and ΔS < 0 due to decreased randomness.

Energy diagrams illustrate energy changes during reactions, including activation energy.
- Activation Energy (ΔG^‡): Energy needed to reach the transition state.
- Lower activation energies lead to faster reactions.
Carboxylic intermediates and transition states affect overall energy changes during the reaction pathway.

Biological reactions occur in aqueous environments and are catalyzed by complex enzymes, while laboratory reactions may not require complex setups.
Specificity is high in biological reactions, with enzymes typically catalyzing specific reactions with particular substrates.