PHD
  1. Inorganic chemistry  1.1. Coordination chemistry  1.1.1. Ligand field theory  1.1.2. Crystal field theory  1.1.3. Molecular Orbital Theory for Coordination Compounds  1.1.4. Stability of Coordination Compounds  1.1.5. Electronic spectra of coordination compounds  1.1.6. Isomerism in Coordination Compounds  1.1.7. Kinetics and mechanism of coordination reactions  1.2. Organometallic Chemistry  1.2.1. Metal Carbonyls  1.2.2. Metal alkyl and aryl  1.2.3. Fischer and Schrock carbonaceans  1.2.4. Oxidative Addition and Reductive Elimination  1.2.5. Metal Hydrides  1.2.6. Catalysis by organometallic compounds  1.2.7. Metallocenes and Sandwich Complexes  1.2.8. CH activation  1.3. Solid state chemistry  1.3.1. Crystal structures and lattices  1.3.2. Band theory and electrical properties  1.3.3. Defects in the crystal  1.3.4. Synthesis of solid state materials  1.3.5. Magnetic and optical properties of solids  1.3.6. Superconductivity  1.3.7. Solid state ionics  1.4. Bio-inorganic chemistry  1.4.1. Metalloproteins and enzymes  1.4.2. Metal ion transport and storage  1.4.3. The role of metals in medicine  1.4.4. Bioinspired catalysis  1.4.5. Metal–DNA interactions  1.4.6. Metal Complexes in Medical Science  1.5. Lanthanides and Actinides  1.5.1. Electronic configuration and oxidation states in lanthanides and actinides  1.5.2. Spectral and Magnetic Properties in Lanthanides and Actinides  1.5.3. Separation and Extraction of Lanthanides and Actinides  1.5.4. Coordination chemistry of f-block elements  1.6. Main Group Chemistry  1.6.1. Chemistry of boron compounds  1.6.2. Chemistry of Silicon and Germanium Compounds  1.6.3. Chemistry of Phosphorus and Sulfur Compounds  1.6.4. Organosilicon chemistry  1.6.5. Noble Gas Chemistry
  2. Organic chemistry  2.1. Reaction mechanism  2.1.1. Nucleophilic substitution reactions  2.1.2. Electrophilic addition and substitution reactions  2.1.3. Elimination reactions  2.1.4. Rearrangement reactions  2.1.5. Radical reactions  2.1.6. Pericyclic Reactions  2.1.7. Photochemical reactions  2.2. Stereoscopic  2.2.1. Chirality and optical activity  2.2.2. Structural analysis  2.2.3. Stereoelectronic effects  2.2.4. Asymmetric synthesis  2.2.5. Dynamic Stereochemistry  2.3. Organometallic Chemistry in Organic Synthesis  2.3.1. Organolithium and organomagnesium reagents  2.3.2. Transition metal catalyzed coupling reactions  2.3.3. Palladium-catalyzed reactions  2.3.4. Olefin Metathesis  2.3.5. Gold and silver catalysis  2.4. Natural products chemistry  2.4.1. Alkaloids and terpenoids  2.4.2. Steroids and Flavonoids  2.4.3. Total synthesis of natural products  2.4.4. Biosynthesis of natural products  2.5. Supramolecular Chemistry  2.5.1. Host-Guest Chemistry  2.5.2. Self-assembly and molecular recognition  2.5.3. Supramolecular Catalysis  2.5.4. Molecular Machines
  3. Physical Chemistry  3.1. Thermodynamics  3.1.1. Laws of Thermodynamics  3.1.2. Chemical Potential and Equilibrium  3.1.3. Statistical thermodynamics  3.1.4. Non-equilibrium thermodynamics  3.2. Quantum Chemistry  3.2.1. Schrödinger equation and its applications  3.2.2. Molecular orbital theory  3.2.3. Density functional theory  3.2.4. Post-Hartree–Fock methods  3.3. Chemical kinetics  3.3.1. Reaction rate theory  3.3.2. Mechanism and rate laws  3.3.3. Catalysis and enzyme kinetics  3.3.4. Reaction dynamics  3.4. Spectroscopy and molecular structure  3.4.1. Infrared Spectroscopy  3.4.2. UV-visible spectroscopy  3.4.3. Nuclear Magnetic Resonance Spectroscopy  3.4.4. Mass Spectrometry  3.4.5. Raman Spectroscopy  3.5. Surface and Colloid Chemistry  3.5.1. Absorption and Catalysis  3.5.2. Surfactants and micelles  3.5.3. Colloidal Stability  3.5.4. Nanomaterials and Interfaces
  4. Analytical chemistry  4.1. Chromatography  4.1.1. Gas Chromatography  4.1.2. High Performance Liquid Chromatography  4.1.3. Thin Layer Chromatography  4.1.4. Ion Chromatography  4.2. Electroanalytical techniques  4.2.1. Voltammetry and Polarography  4.2.2. Conductometry and potentiometry  4.2.3. Colometry  4.3. Spectroscopic Methods  4.3.1. Atomic absorption spectroscopy  4.3.2. Fluorescence Spectroscopy  4.3.3. X-ray diffraction  4.3.4. Electron paramagnetic resonance spectroscopy  4.4. Chemometrics  4.4.1. Data processing and statistical methods  4.4.2. Multidisciplinary analysis  4.4.3. Machine Learning in Analytical Chemistry
  5. Theoretical and Computational Chemistry  5.1. Molecular dynamics simulation  5.2. Quantum chemistry methods  5.3. Computational drug design  5.4. Machine Learning in Chemistry  5.5. Ab initio molecular dynamics
  6. Biophysics and Medicinal Chemistry  6.1. Drug discovery and design  6.2. Enzyme kinetics and inhibition  6.3. Chemical biology approach  6.4. Protein-ligand interactions  6.5. Bioorthogonal chemistry
  7. Materials chemistry  7.1. Polymer chemistry  7.1.1. Polymerization mechanism  7.1.2. Functional Polymers  7.1.3. Biodegradable Polymer  7.1.4. Conducting and semiconducting polymers  7.2. Nano Chemistry  7.2.1. Nanoparticles and nanostructures  7.2.2. Carbon-based nanomaterials  7.2.3. Quantum dots  7.3. Energy and Environmental Chemistry  7.3.1. Battery and Fuel Cell Chemistry  7.3.2. Green chemistry and sustainable materials  7.3.3. Photocatalysis

PHD → Organic chemistry → Reaction mechanism ↓


Elimination reactions


Introduction to elimination reactions

In organic chemistry, reaction mechanisms provide a route for the transformation of organic molecules. Among these transformations, elimination reactions are of vital importance. Elimination reactions involve the removal of atoms or groups from a molecule, resulting in the formation of unsaturated structures such as alkanes or alkynes. Common types of elimination reactions are represented as E1 and E2, where "E" stands for elimination.

General concept of elimination reactions

Elimination reactions generally involve the removal of two substituents from a molecule. Usually, this reacts to form a double bond. The simplest example can be taken from the dehydrohalogenation of haloalkanes, where one halogen atom and one hydrogen atom are eliminated to form an alkene. The general form of an elimination reaction can be represented by the following equation:

R–C–C–X + Base ⟶ R–C=C + Base–H + X–,
HH

In this general reaction, X is a leaving group, and Base is an electron pair donor that removes a proton. The result is the formation of a new bond between the two carbon atoms.

E1 and E2 reaction mechanisms

E1 elimination

E1 reaction is a two-step process. It usually occurs under neutral or acidic conditions, often with secondary or tertiary haloalkanes. The mechanism involves:

  1. Formation of carbocation: The molecule loses its base group and forms a carbocation.
  2. Deprotonation: A base removes a proton from the beta-carbon atom adjacent to the carbocation, resulting in the formation of an alkene.
Step 1: R3C–X ⟶ R3C+ +
Step 2: R3C+ + Base ⟶ R2C=CR2 + Base–H

E1 reactions are unimolecular and follow first-order kinetics depending on the substrate concentration. They are favored by weak bases and highly substituted carbocations because of the increased stability.

Haloalkanes alkene Base essence H +

E2 elimination

E2 mechanism occurs in a single coordinated step and is characterized by a strong base. Experimentally, this mechanism proceeds through the following concurrent events:

  1. Base-induced deprotonation: Base deprotonates hydrogen from the beta-carbon.
  2. Leaving group together: As the proton is removed, the leaving group also leaves.
RCH2CH2X + base ⟶ RCH=CH2 + base–H + X–

E2 eliminations are bimolecular and show second-order kinetics, which depend on both substrate and base concentrations. The reaction occurs faster with strong bases and is less sensitive to steric hindrance than E1 reactions.

Haloalkanes alkene Base and leaving group leave together

Stereochemistry of elimination reactions

The stereochemistry of elimination reactions, especially in E2 mechanisms, is heavily influenced by the orientation of the atoms and bonds. In E2 reactions, there are generally two primary orientations: anti-periplanar and syn-periplanar.

Anti-periplanar orientation

In E2 reactions, the anti-periplanar conformation is usually preferred. This refers to a state where the hydrogen and the leaving group are on opposite sides of the molecule, leading to a more stable transition state, as shown:

H BR H C C results in alkene formation

Syn-periplanar orientation

The syn-periplanar configuration is less common because it usually faces steric hindrance. In it, the hydrogen and the leaving group are on the same side:

H BR Static hindrance

Zaitsev's law

An important rule in elimination reactions, especially when determining product distribution, is Zaitsev's rule. It indicates that in an elimination reaction, the more substituted alkene is usually the major product. This preference arises because more substituted alkenes are more stable due to hyperconjugation and electron donating effects.

Examples of elimination reactions

Dehydration of alcohol

Elimination of alcohols can be carried out to form alkenes, usually via the removal of water (dehydration). Concentrated sulfuric acid often serves as a catalyst:

RCH2CH2OH ⟶ RCH=CH2 + H2O

This reaction usually follows E1 mechanism in secondary and tertiary alcohols due to carbocation formation, while primary alcohols may follow E2 pathway.

Dehydrohalogenation of alkyl halides

As mentioned earlier, alkyl halides can form alkenes by undergoing dehydrohalogenation. This process is often carried out in the presence of a strong base such as potassium tert-butoxide, which particularly favors E2 reactions. Here's an example:

CH3CH2Br + KOH ⟶ CH2=CH2 + KBr + H2O

E1cB elimination reaction

Another less common elimination reaction type is E1cB mechanism, which means elimination of the unimolecular conjugate base. This reaction occurs via a carbanion intermediate.

RCH2CHOHCH2-X ⟶ RCH=CH–CHO +

In this mechanism, the residue group is removed after deprotonation and formation of the carbanion, making it a stepwise process distinct from other removal pathways.

Conclusion

Understanding elimination reactions is fundamental in organic chemistry because these reactions play a key role in the synthesis of compounds, particularly in forming alkenes and alkynes. The mechanistic routes - E1, E2, and E1cB - offer variety and complexity, allowing chemists to control product outcomes. Factors such as the nature of the substrate, the strength and orientation of the base, and the effect of Zaitsev's rule explain the depth of elimination reactions. Mastery of these concepts underlies successful organic synthesis and a comprehensive understanding of molecular transformations.


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Elimination reactions

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