CHAP 4: The Study of Chemical Reactions

Free Radical Chain Reaction (Free Radical Halogenation Mechanism)

The free radical halogenation of alkanes is a fundamental organic reaction involving the substitution of hydrogen atoms by halogen atoms via a radical mechanism. The process occurs in three main steps: initiation, propagation, and termination.

• Initiation: Formation of free radicals, typically by homolytic cleavage of a halogen molecule (e.g., Cl2 or Br2) under heat or light.

• Propagation: Radicals react with alkanes to form new radicals and products, continuing the chain reaction.

• Termination: Two radicals combine to form a stable molecule, ending the chain process.

• Example: Chlorination of methane:

  • Initiation:

  • Propagation:

  • Propagation:

  • Termination:

Free Energy

Free energy is a thermodynamic quantity that predicts the spontaneity of a chemical reaction. The change in Gibbs free energy () determines whether a reaction is favorable.

• Equation:

• Spontaneity: If , the reaction is spontaneous.

Enthalpy and Entropy Definitions

Enthalpy () is the heat change at constant pressure, while entropy () measures the disorder or randomness of a system.

• Enthalpy: is negative for exothermic reactions.

• Entropy: increases as disorder increases.

Transition State, Activation Energy, Reaction Kinetics

The transition state is the highest energy point along the reaction pathway. Activation energy () is the minimum energy required for a reaction to occur. Reaction kinetics studies the rate at which reactions proceed.

• Activation Energy Equation:

• Transition State: Denoted by a double dagger (), e.g.,

Radical Inhibitors

Radical inhibitors are substances that slow or stop radical chain reactions by reacting with free radicals to form stable, non-radical products.

• Example: Oxygen () acts as a radical inhibitor in many organic reactions.

Reactive Intermediate

Reactive intermediates are short-lived species formed during chemical reactions, such as free radicals, carbocations, and carbanions.

• Example: Methyl radical () in halogenation reactions.

CHAP 5: Stereochemistry

Chirality: Conditions, Mesocompound (Definition)

Chirality refers to the property of a molecule that is not superimposable on its mirror image. A mesocompound is an achiral compound despite having chiral centers, due to an internal plane of symmetry.

• Chiral Center: Typically a carbon atom bonded to four different groups.

• Mesocompound: Contains multiple chiral centers but is achiral overall.

• Example: Tartaric acid is a classic mesocompound.

R and S Nomenclature

The R/S system assigns absolute configuration to chiral centers using the Cahn-Ingold-Prelog priority rules.

• Steps:

  1. Assign priorities to substituents based on atomic number.

  2. Orient the molecule so the lowest priority group is away from you.

  3. If the sequence 1-2-3 is clockwise, the configuration is R; if counterclockwise, it is S.

• Example: 2-butanol has an R or S configuration depending on the arrangement of its groups.

Plane-Polarized Light (Equation, Rotation)

Chiral compounds rotate plane-polarized light, a property measured as optical rotation.

• Equation:

• Where: is specific rotation, is observed rotation, is path length (dm), is concentration (g/mL).

• Example: (+)-glucose rotates light to the right (dextrorotatory).

CHAP 6: Halides; Nucleophilic Substitution

Alkyl Halides (Nomenclature, Uses, Structure)

Alkyl halides are organic compounds containing a halogen atom bonded to an alkyl group. They are named by identifying the parent alkane and replacing the hydrogen with the halogen name.

• Nomenclature: E.g., chloromethane, bromoethane.

• Uses: Solvents, intermediates in synthesis, pharmaceuticals.

• Structure: General formula , where is alkyl, is halogen.

Free Radical Halogenation

Alkyl halides can be synthesized via free radical halogenation, as described in Chapter 4.

• Example: Formation of bromomethane from methane and bromine.

Unimolecular Nucleophilic Substitution (SN1) and Bimolecular Nucleophilic Substitution (SN2)

Nucleophilic substitution reactions involve the replacement of a leaving group by a nucleophile. There are two main mechanisms: SN1 (unimolecular) and SN2 (bimolecular).

• SN1 Mechanism:

  • Two-step process: formation of carbocation intermediate, then nucleophile attack.

  • Rate depends only on concentration of substrate:

  • Common for tertiary alkyl halides.

• SN2 Mechanism:

  • One-step process: nucleophile attacks substrate as leaving group departs.

  • Rate depends on both substrate and nucleophile:

  • Common for primary alkyl halides.

• Comparison Table: