Graduate
  1. Physical Chemistry  1.1. Thermodynamics  1.1.1. Laws of Thermodynamics  1.1.2. Enthalpy and heat capacity  1.1.3. Entropy and free energy  1.1.4. Phase Equilibrium  1.1.5. Statistical thermodynamics  1.1.6. Thermodynamic cycle  1.1.7. Fugacity and activity  1.2. Quantum Chemistry  1.2.1. Principles of quantum mechanics  1.2.2. Schrödinger Equation  1.2.3. Particle in a box  1.2.4. Operators and eigenvalues  1.2.5. Atomic orbitals  1.2.6. Molecular orbital theory  1.2.7. Perturbation theory  1.2.8. Valence bond theory  1.2.9. Hybridization and chemical bonding  1.3. Chemical kinetics  1.3.1. Rate laws and reaction mechanisms  1.3.2. Collision theory  1.3.3. Transition state theory  1.3.4. Enzyme Kinetics  1.3.5. Chain Reactions and Polymerization  1.3.6. Photochemical reactions  1.4. Statistical mechanics  1.4.1. Partitioning functions  1.4.2. Molecular distribution functions  1.4.3. Boltzmann Distribution  1.4.4. Bose–Einstein and Fermi–Dirac statistics  1.5. Spectroscopy  1.5.1. Rotational Spectroscopy  1.5.2. Vibrational Spectroscopy  1.5.3. Electronic Spectroscopy  1.5.4. Nuclear magnetic resonance spectroscopy  1.5.5. Mass Spectrometry  1.5.6. Raman Spectroscopy  1.5.7. Electron paramagnetic resonance spectroscopy  1.6. Surface and colloidal chemistry  1.6.1. Absorption isotherm  1.6.2. Surface tension and wetting  1.6.3. Colloidal Stability  1.6.4. Catalysis  1.6.5. Emulsions and micelles  1.7. Electrochemistry  1.7.1. Nernst equation  1.7.2. Electrochemical cells  1.7.3. Conductivity and mobility  1.7.4. War  1.7.5. Fuel cells  1.7.6. Electrolysis
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic reactions  2.2. Spectroscopy and structural determination  2.2.1. UV-Vis Spectroscopy  2.2.2. IR Spectroscopy  2.2.3. NMR Spectroscopy  2.2.4. Mass Spectrometry  2.2.5. X-ray crystallography  2.3. Stereoscopic  2.3.1. Chirality and optical activity  2.3.2. Structural analysis  2.3.3. Geometrical isomerism  2.3.4. Dynamic Stereochemistry  2.4. Organometallic Chemistry  2.4.1. Organolithium and organomagnesium reagents  2.4.2. Palladium-catalyzed cross-coupling reactions  2.4.3. Transition Metal Complexes  2.4.4. Metal–carbon bond  2.5. Polymer chemistry  2.5.1. Polymerization mechanism  2.5.2. Polymer Characteristics  2.5.3. Biodegradable Polymer  2.5.4. Conducting polymer  2.5.5. Supramolecular polymers  2.6. Medicinal chemistry  2.6.1. Drug design and development  2.6.2. Pharmacokinetics and pharmacodynamics  2.6.3. Structure-activity relationships  2.6.4. Molecular docking and drug screening
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Crystal field theory  3.1.2. Ligand field theory  3.1.3. Spectrochemical Series  3.1.4. Chelation and stability  3.2. Organometallic Chemistry  3.2.1. Metal Carbonyls  3.2.2. Catalysis by organometallic complexes  3.2.3. Metallocenes  3.3. Bio-inorganic chemistry  3.3.1. Metalloproteins and enzymes  3.3.2. The role of metals in biological systems  3.3.3. Metal ion transport and storage  3.4. Solid state chemistry  3.4.1. Crystal Structures  3.4.2. Band theory of solids  3.4.3. Superconductors  3.4.4. Defects in the crystal  3.5. Lanthanides and Actinides  3.5.1. Electronic configuration in lanthanides and actinides  3.5.2. Coordination chemistry of the lanthanides  3.5.3. Magnetic Properties of Lanthanides
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.2. Spectroscopic Techniques  4.2.1. Atomic absorption spectroscopy  4.2.2. X-ray diffraction  4.2.3. Inductively coupled plasma spectroscopy  4.3. Electroanalytical methods  4.3.1. Potentiometry  4.3.2. Voltammetry  4.3.3. Colometry  4.4. Mass Spectrometry  4.4.1. Ionization Technique  4.4.2. Fragmentation Pattern  4.5. Chemometrics  4.5.1. Multidisciplinary analysis  4.5.2. Machine Learning in Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.1.1. Force fields and energy minimization  5.1.2. Monte Carlo simulation in molecular dynamics simulations  5.2. Quantum chemical methods  5.2.1. Hartree–Fock theory  5.2.2. Density functional theory  5.2.3. Semi-empirical methods  5.3. Computational drug design  5.3.1. Molecular Docking  5.3.2. QSAR Modeling  5.3.3. Virtual Screening
  6. Biochemistry  6.1. Enzyme Kinetics  6.1.1. Michaelis-Menten kinetics  6.1.2. Inhibition mechanism  6.2. Metabolism and Bioenergetics  6.2.1. Glycolysis  6.2.2. Citric acid cycle  6.2.3. Electron transport chain  6.3. molecular Biology  6.3.1. DNA replication and repair  6.3.2. Protein synthesis  6.3.3. Gene Regulation  6.4. Structural Biochemistry  6.4.1. Protein Folding  6.4.2. Membrane Biophysics
  7. Environmental Chemistry  7.1. Atmospheric Chemistry  7.1.1. Greenhouse gases  7.1.2. Ozone depletion  7.1.3. Air pollutants and smoke  7.2. Water Chemistry  7.2.1. pH and water hardness  7.2.2. Wastewater treatment  7.2.3. Aquatic Chemistry  7.3. Soil Chemistry  7.3.1. Heavy metal contamination  7.3.2. Soil pH and Buffering  7.3.3. Nutrient cycling  7.4. Toxicology and Chemical Safety  7.4.1. Toxicokinetics  7.4.2. Risk Assessment in Toxicology and Chemical Safety in Environmental Chemistry  7.4.3. Effects of pollutants on the environment

Graduate → Organic chemistry → Reaction mechanism ↓


Nucleophilic substitution reactions


Nucleophilic substitution reactions are an essential class of reactions in organic chemistry. In these reactions a nucleophile is substituted for a leaving group on a saturated carbon atom. Nucleophilic substitution is widely used in the synthesis of various organic compounds. Understanding the underlying principles, mechanisms, and applications of these reactions is important to master organic chemistry.

Types of nucleophilic substitution reactions

There are two primary mechanisms by which nucleophilic substitution reactions proceed: SN1 and SN2.

SN2 mechanism

The SN2 mechanism is short for "bimolecular nucleophilic substitution." This process occurs in a single coordinated step, where the nucleophile attacks the electrophilic carbon, and the leaving group leaves simultaneously. Since the rate of the reaction depends on both the nucleophile and the substrate, this process is called bimolecular.

A typical representation of an S2 reaction is shown below, where Nu: represents the nucleophile and LG represents the leaving group:

R-LG + NEW: → R-NEW + LG

The active stereochemistry of the substrate is important in the SN2 mechanism, and the reaction usually proceeds with a backside attack, leading to inversion of configuration at the carbon center.

Visual example

Consider the following SVG of the SN2 reaction mechanism involving methyl chloride and hydroxide ion.

CH3 Cl yes- CH3 OH

SN1 mechanism

The SN1 mechanism means "unimolecular nucleophilic substitution." It is characterized by a two-step process. First, the leaving group dissociates, forming a carbocation intermediate. Then, the nucleophile attacks the carbocation, completing the substitution.

The rate of the reaction depends only on the concentration of the substrate, making it a monomolecular reaction. The general reaction scheme for the SN1 reaction can be illustrated as follows:

R-lg → R+ + lg R+ + nu: → R-nu

Due to the formation of a carbocation intermediate, the SN1 reaction is prone to racemization when the carbon center is chiral, as the nucleophile can attack from either side.

Visual example

Consider the following scenario of an SN1 reaction involving tert-butyl chloride and water.

(CH3)3 CCl C+ (CH3)3 COH

Factors affecting nucleophilic substitution reactions

Substrate

The nature of the substrate plays an important role in determining whether the SN1 or SN2 pathway is preferred. Primary substrates often prefer the SN2 mechanism, while tertiary substrates usually undergo SN1 reactions.

Leaving the group

A good leaving group is important for nucleophilic substitution reactions. Generally, leaving groups that can stabilize the negative charge after isolation are considered good. Common leaving groups include halides such as Cl-, Br-, and I-.

Nucleophile

The strength and concentration of the nucleophile can also affect the reaction pathway. Strong nucleophiles favor the SN2 mechanism, while weak nucleophiles often lead to SN1 reactions.

Solvent

The type of solvent can dramatically affect which substitution mechanism prevails. Polar protic solvents stabilize the carbocation, facilitating SN1 reactions, while polar aprotic solvents increase the nucleophilic strength, favoring SN2 mechanisms.

Comparative summary of SN1 and SN2 mechanism

Features SN1 SN2
Molecularity Unimolecular Bimolecular
Step Two-stage One step
Rate determining factors Substrate concentrations Substrate and nucleophile concentrations
Reaction rate K[r-lg] k[r-lg][new]
Stereoscopic Racemization Inversion of configuration
Substrate preference Tertiary Primary

This lesson provides a comprehensive and accessible overview of nucleophilic substitution reactions, helping you understand their mechanisms and applications in organic chemistry.


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Nucleophilic substitution reactions

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