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 ↓


Elimination reactions


Elimination reactions are a fundamental class of organic reactions in which two substituents are removed from a molecule, resulting in the formation of a new product. These reactions are not only important in the synthesis of alkenes and alkynes but also play a vital role in biochemical pathways and industrial chemical processes. Elimination reactions occur primarily via two main mechanisms: E1 and E2, which will be explored in depth in the following sections.

Basics of elimination reactions

In elimination reactions, a molecule loses two groups or substituents, usually on adjacent carbon atoms. This process results in the formation of a double or triple bond.

        Starting materials → intermediates → products

Generally, elimination reactions can be summarized as follows:

        C – C – X – Y → C = C + X – Y

Where X and Y are the two departing groups, and a double bond is formed between the carbon atoms from which these groups are departing.

E2 mechanism (bimolecular elimination)

The E2 mechanism is a single-step process that involves the simultaneous breaking of bonds and the formation of double bonds. The rate of the reaction depends on both the substrate (the molecule undergoing elimination) and the base (which abstracts the proton).

  • Characteristics: Second order reaction, bimolecular, coordinated mechanism.
  • Rate law: rate = k[substrate][base]

Mechanism of E2 reaction

E2 reactions occur when a strong base abstracts a proton from the β carbon atom and moves it to the leaving group. This action causes the leaving group to dissociate, and a π bond is formed between the α and β carbon atoms. The general requirement is the antiperiplanar arrangement of hydrogen and the leaving group.

           HH
           ,
        H--C---C--Br + base → H--C=C--H + H-Br + base 
           ,
           rr'

The above reaction shows a simple E2 mechanism where a strong base absorbs β-hydrogen, thereby losing the residual group (Br) and forming a double bond.

Energy profile diagram for E2

Transition state Reactants Products

E1 mechanism (monomolecular elimination)

The E1 mechanism is a two-step process, where the reaction rate depends only on the concentration of the substrate. In the E1 mechanism, the leaving group leaves first, forming a carbocation intermediate, followed by deprotonation to form a double bond.

  • Characteristics: First order reaction, unimolecular, stepwise mechanism, intermediates involved.
  • Rate law: rate = k[substrate]

Mechanism of E1 reaction

The E1 mechanism involves two main steps:

  1. Formation of a carbocation intermediate by departure of the abandoning group.
  2. Deprotonation by base to form double bond.
        R2C-CH3-X → R2C=CH2 + X-

For example, in the elimination of a haloalkane, the halide ion is removed, forming a carbocation. A base can then remove the proton from the carbocation, resulting in an alkene.

        Step 1: R-CH2-CH2-X → R-CH2-CH2+ + X- (Carbocation formation)
        Step 2: R-CH2-CH2+ + base → R-CH=CH2 + base-H (elimination)

Energy profile diagram for E1

Transition state 1 Carbocations Reactants Products

Factors affecting elimination reactions

There are many factors that affect the rate and outcome of elimination reactions, including:

  • Nature of the substrate: The structure of the organic compound undergoing elimination (e.g., primary, secondary, or tertiary) affects both the rate and mechanism of the reaction.
  • Type of base: Strong bases promote E2 mechanism, while weak bases may promote E1 mechanism, especially for tertiary substrates.
  • Ability of the leaving group: Good leaving groups (such as halides) facilitate both E1 and E2 mechanisms because they can be easily dissociated from the substrate.
  • Stereochemistry: The spatial arrangement of atoms in a molecule can affect the antiperiplanar requirement for E2 and carbocation stability in E1 reactions.
  • Solvent effect: Polar protic solvents stabilize carbocations and therefore promote E1 reactions. Aprotic solvents may favor the E2 mechanism.

Comparison between E1 and E2 reactions

Speciality E1 E2
Number of steps Two One
Rate laws Rate = k[substrate] Rate = k[substrate][base]
Formation of intermediate? Yes, carbocation No
Substrate preference Tertiary > Secondary > Primary Tertiary > Secondary > Primary
Base strength A weak base is enough Strong base required

Examples of elimination reactions

Understanding the elimination mechanism becomes concrete when explained with examples:

Dehydrohalogenation of haloalkanes

The classic example of an elimination reaction is the dehydrohalogenation of haloalkanes:

        CH3-CH2-Br + NaOH → CH2=CH2 + NaBr + H2O

This reaction uses sodium hydroxide (a strong base) to remove a hydrogen halide (Br) from an alkene (ethyl bromide), forming ethylene (an alkene).

Dehydration of alcohol

Another common elimination reaction is the dehydration of alcohols:

        CH3-CH2-OH → CH2=CH2 + H2O

Here, ethanol is dehydrated in the presence of acid to form ethylene.

Complete mechanism for hydroxide-induced E2 elimination

        R-CH2-CH2-X + NaOH → R-CH=CH2 + NaX + H2O

For example, 2-bromo-2-methylpropane undergoes E2 elimination with sodium hydroxide to afford isobutylene:

        (CH3)3C-Br + NaOH → (CH3)2C=CH2 + NaBr + H2O

Considerations for substituted substrates

Elimination reactions can be greatly affected by the substitution pattern of the substrate. For example, the Saytzeff rule predicts that more substituted alkenes are preferentially formed due to greater alkene stability.

        R1CH-CH2-X + base → R1C=CH2 + baseHX

The less hindered base will attack the more sterically accessible β-hydrogen, giving the more substituted (ortho) alkene.

Industrial applications of elimination reactions

Elimination reactions are integral to the pharmaceutical manufacturing and polymer industries.

  • The production of ethylene, used as a precursor to polyethylene, contributes extensively to the plastics industry.
  • Synthesis of isobutylene, which is used as a precursor in the production of isoctane, a compound that increases the octane level of gasoline.

Conclusion

Elimination reactions play a vital role in a vast range of organic chemistry, converting simple structures into many complex molecules with diverse functionalities. Understanding the mechanistic details of E1 and E2 helps to design efficient synthesis routes, optimize industrial chemical processes, and expand insights into understanding biochemical pathways.


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Elimination reactions

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