Nucleophilic Substitution & Elimination Reactions: Chapter 6 Terms

Posted on Aug 27, 2024 in Chemistry

Chapter 6: Nucleophilic Substitution & Elimination Reactions – Key Terms

Common Organic Halides

Alkyl Halide

An alkyl halide (also called haloalkane) is a compound where a halogen atom replaces a hydrogen atom of an alkane. Its general formula is RX (Sec 2.5, 4.3E, 6.1).

Vinylic Halide

A vinylic halide (also called alkenyl halide or haloalkene) is an organic halide where the halogen atom is attached to a carbon atom of a C=C double bond (Sec 6.1).

Aryl Halide

An aryl halide (also called haloarene) is an organic halide where the halogen atom is attached to an aromatic ring, such as a benzene ring (Sec 6.1).

Reaction Types and Mechanisms

Ionic Reaction

An ionic reaction involves ions as reactants, intermediates, or products. These ions are usually produced through the heterolysis of covalent bonds (Sec 3.1A, 6.2, 6.6, 6.10, 8.3, 10.1).

Substitution Reaction

A substitution reaction is a reaction where an atom or group is replaced by a different atom or group (Sec 3.1, 6.2, 10.3, 13.2).

Nucleophilic Substitution Reaction

A nucleophilic substitution reaction is initiated by a nucleophile that reacts with a substrate and replaces a leaving group. The leaving group departs with a lone pair of electrons (Sec 6.2, 6.5 – 6.14, Lab 14).

S_N2 Reaction

An S_N2 reaction (Substitution, Nucleophilic, Bimolecular) is a bimolecular nucleophilic substitution. It occurs in a single step where a nucleophile attacks the carbon bearing a leaving group from the backside. This attack causes an inversion of configuration at this carbon and displacement of the leaving group (Sec 6.5 – 6.8, 6.13, Lab 14).

S_N1 Reaction

An S_N1 reaction (Substitution, Nucleophilic, Unimolecular) is a multistep nucleophilic substitution. The leaving group departs in a unimolecular step before the attack of the nucleophile. The rate equation is first order in substrate, but zero order in nucleophile (Sec 6.9 – 6.13, Lab 14).

Elimination Reaction

An elimination reaction results in the loss of two groups from a substrate, forming a pi bond. The most common elimination is a β elimination (1,2 elimination), where the two groups are lost from adjacent atoms (Sec 3.1, 6.15 – 6.18, 7.5 – 7.7).

E2 Reaction

An E2 reaction is a bimolecular β elimination. In a single step, a base removes a proton while a leaving group departs from the substrate, forming a pi bond (Sec 6.15C, 6.16, 6.18A, 7.6).

E1 Reaction

An E1 reaction is a unimolecular β elimination. In a slow, rate-determining step, a leaving group departs from the substrate to form a carbocation. Then, in a fast step, the carbocation loses a proton to form a pi bond (Sec 6.15C, 6.17, 6.18B, 7.7).

Important Concepts and Terms

Nucleophile

A nucleophile (meaning “nucleus loving”) is an anion or molecule with a lone pair of electrons. It donates its electron pair to form a new covalent bond. Lewis bases (electron-pair donors) are nucleophiles (Sec 3.4A, 6.3, 6.13B, 8.1, Lab 14).

Substrate

The substrate is the reactant considered to be attacked in a chemical reaction (Sec 6.2).

Leaving Group

The leaving group is the substituent that departs with a lone pair of electrons from the substrate in a nucleophilic substitution reaction. Good leaving groups depart as stable, very weak bases (Sec 6.2, 6.4, 6.13E, 17.4, Lab 14).

Bimolecular Reaction

A bimolecular reaction’s rate-determining step involves two chemical species (Sec 6.5).

Unimolecular Reaction

A unimolecular reaction’s rate-determining step involves only one chemical species (Sec 6.9).

Inversion of Configuration

Inversion of configuration (also called Walden inversion) is a change of configuration (e.g., from R to S, or from cis to trans, or vice versa) that occurs when a nucleophile attacks a stereogenic carbon from its backside in an S_N2 reaction (Sec 6.6, 6.8).

Concerted Reaction

A concerted reaction involves bond breaking and bond forming occurring simultaneously (in concert) through a single transition state (Sec 6.6).

Transition State

The transition state (also called activated complex) is an unstable species with a very brief existence. It occurs at a state of maximum potential energy and is shown on a potential energy diagram at an energy maximum (Sec 6.6, 7.7B, Lab 14).

Carbocation

A carbocation (also called carbonium ion) is a chemical species where a carbon atom bears a formal positive charge. The C⁺ cation has three sp² hybrid orbitals, plus one empty p orbital, and a trigonal planar molecular geometry. It can act as a Lewis acid and a powerful electrophile (Sec 3.4, 6.11).

Other Important Terms

• Gibbs Free Energy Change (ΔG): A measure of the amount of free energy of a system (Sec 3.10, 6.7, 10.5A).
• Exergonic Reaction: A reaction that releases energy and has a negative ΔG (Sec 6.7).
• Endergonic Reaction: A reaction that absorbs energy and has a positive ΔG (Sec 6.7).
• Free Energy of Activation (ΔG^‡): The difference in Gibbs free energy (ΔG) of the transition state and the reactants (Sec 6.7).
• Activation Energy (E_act): The difference in enthalpy (ΔH) of the transition state and the reactants (Sec 10.5B).
• Rate-Determining Step: The slowest step in a multistep reaction, which determines the overall reaction rate (Sec 6.9).
• Intermediate: A transient species that exists between reactants and products (Sec 6.9, 6.10).
• Delocalization: The dispersal of electron density or electrical charge by resonance (Sec 1.8, 2.1D, 3.11A, 6.11B, 7.7B, 13.3 – 13.7, 14.6).
• Hyperconjugation: Electron delocalization via orbital overlap (Sec 4.8, 6.11B, 10.2B).
• Inductive Stabilization: Stabilization of a reactive intermediate by donation or withdrawal of electron density through sigma bonds (Sec 6.11B, 15.11).
• Racemization: A reaction that transforms an optically active compound into a racemic form (Sec 5.10A, 6.12A).
• Solvolysis: A nucleophilic substitution where the nucleophile is a solvent molecule (Sec 6.12B).
• Steric Effect: An effect on the relative rate of reaction caused by the spatial arrangement of atoms (Sec 6.13A).
• Steric Hindrance: Interference between two groups when they come close together (Sec 4.8, 6.13A).
• Hammond-Leffler Postulate: Describes the relationship between transition state structure and the stable species nearest to it in free energy (Sec 6.l3A, 15.11).
• Nucleophilicity: The strength of a nucleophile (Sec 6.13B).
• Basicity: The strength of a base (Sec 6.13B).
• Polarizability: The susceptibility of an electron cloud to distortion (Sec 2.13B, 6.13C).
• Protic Solvent: A solvent with acidic protons (Sec 3.12, 6.13C).
• Aprotic Solvent: A solvent without acidic protons (Sec 6.13C).
• Polar Aprotic Solvent: A solvent with a strong dipole moment but no acidic protons (Sec 6.13C).
• Solvent Effect: An effect on relative rates of reaction caused by the solvent (Sec 6.l3D).
• Dehydrohalogenation: An elimination reaction resulting in the loss of hydrogen halide (Sec 6.15A, 7.6).