Undergraduate
  1. General chemistry  1.1. Introduction to Chemistry  1.2. Atomic Structure  1.2.1. Subatomic particles  1.2.2. Atomic Model  1.2.3. Quantum numbers  1.2.4. Electron Configuration  1.2.5. Periodic trends  1.2.6. Isotopes and their applications  1.3. Chemical bond  1.3.1. Ionic bond  1.3.2. Covalent bonds  1.3.3. Metal Bond  1.3.4. Hybridization  1.3.5. Molecular geometry and VSEPR theory  1.3.6. Bond polarity and dipole moment  1.4. Stoichiometry  1.4.1. Mole concept  1.4.2. Empirical and molecular formula  1.4.3. Limiting reactant and excess reactant  1.4.4. Percent Yield and Purity  1.4.5. Dilution calculation  1.5. States of matter  1.5.1. Gas Laws  1.5.2. Matter and Intermolecular Forces  1.5.3. Solid and Crystal Structures  1.5.4. Phase diagram  1.5.5. Plasma and supercritical fluid  1.6. Chemical reactions  1.6.1. Types of Chemical Reactions  1.6.2. Balancing Chemical Equations  1.6.3. Thermochemical reactions  1.6.4. Reaction energy profiles  1.7. Solutions and Mixtures  1.7.1. Concentration units  1.7.2. Syndrome properties  1.7.3. Solubility and Solubility Rules  1.7.4. Henry's Law and Raoult's Law  1.7.5. Partition coefficient  1.8. Chemical equilibrium  1.8.1. Law of mass action  1.8.2. Le Chatelier's principle  1.8.3. Reaction quotient  1.8.4. To use this online calculator for Equilibrium Constant, enter Equilibrium Constant (Eq.) and hit the calculate button. Here is how the Equilibrium Constant calculation can be explained with given input values -> 0.05 = 0.05/0.  1.8.5. Acid-base balance  1.9. Acids and bases  1.9.1. Arrhenius, Brønsted–Lowry, and Lewis definitions  1.9.2. pH and pOH  1.9.3. Acid-base titration  1.9.4. Buffer Solution  1.9.5. Acid-base indicator  1.9.6. Polyprotic Acids and Bases  1.10. Electrochemistry  1.10.1. redox reactions  1.10.2. Galvanic and Electrolytic Cells  1.10.3. Nernst equation  1.10.4. Electrolysis  1.10.5. Faraday's laws of electrolysis  1.11. Thermodynamics  1.11.1. Laws of Thermodynamics  1.11.2. Enthalpy, entropy and Gibbs free energy  1.11.3. Spontaneity of reactions  1.11.4. Thermodynamic cycles (Hess's law, Carnot cycle)  1.12. Kinetics  1.12.1. Rate law and reaction order  1.12.2. Activation Energy and Catalyst  1.12.3. Collision theory  1.12.4. Reaction mechanism  1.12.5. Transition state theory
  2. Organic chemistry  2.1. Structure and relationships  2.1.1. Hybridisation in Carbon Compounds  2.1.2. Resonance and aromatization  2.1.3. Molecular orbitals  2.1.4. Inductive and mesomeric effects  2.2. Hydrocarbons  2.2.1. Hydrocarbons  2.2.2. Alkene  2.2.3. Alkynes  2.2.4. Aromatic compounds  2.2.5. Cycloalkanes and Conformation  2.3. Functional Group  2.3.1. Alcohols and phenols  2.3.2. Ethers and epoxides  2.3.3. Aldehyde and ketone  2.3.4. Carboxylic acids and derivatives  2.3.5. Amin  2.3.6. Esters and amides  2.3.7. Thiols and sulfides  2.4. Stereoscopic  2.4.1. Isomerism in Stereochemistry  2.4.2. Chirality and optical activity  2.4.3. Structural analysis  2.4.4. Diastereomers and Enantiomers  2.5.1. Substitution reactions  2.5.2. Elimination reactions  2.5.3. Addition reactions  2.5.4. Radical reactions  2.5.5. Rearrangement reactions  2.5.6. Pericyclic Reactions  2.6. Spectroscopy and structural analysis  2.6.1. Infrared Spectroscopy  2.6.2. Nuclear magnetic resonance spectroscopy  2.6.3. Mass Spectrometry  2.6.4. UV-visible spectroscopy  2.6.5. X-ray crystallography  2.7. Polymer chemistry  2.7.1. Addition and Condensation Polymers  2.7.2. Polymerization mechanism  2.7.3. Biodegradable Polymer  2.7.4. Conducting and smart polymers
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Ligands and coordination compounds  3.1.2. Crystal field theory  3.1.3. Molecular Orbital Theory in Coordination Compounds  3.1.4. Jahn–Taylor distortion  3.2. Main Group Chemistry  3.2.1. Alkali and alkaline earth metals  3.2.2. Halogens and Noble Gases  3.2.3. Transition metals and their complexes  3.2.4. Lanthanides and Actinides  3.3. Solid state chemistry  3.3.1. Crystal lattices and unit cells  3.3.2. Defects in solids  3.3.3. Electrical and magnetic properties  3.3.4. Superconductivity  3.4. Bio-inorganic chemistry  3.4.1. Metal ions in biology  3.4.2. Enzymatic reactions with metals  3.4.3. Metal poisoning and detoxification  3.4.4. Metalloproteins and Metalloenzymes
  4. Physical chemistry  4.1. Quantum Chemistry  4.1.1. Wave–particle duality  4.1.2. Schrödinger Equation  4.1.3. Quantum Numbers and Orbitals  4.1.4. Atomic and molecular spectroscopy  4.2. Statistical mechanics  4.2.1. Maxwell–Boltzmann distribution  4.2.2. Partitioning functions  4.2.3. Bose–Einstein and Fermi–Dirac statistics  4.3. Chemical thermodynamics  4.3.1. Gibbs Free Energy  4.3.2. Phase transition  4.3.3. Thermodynamic efficiency  4.4. Surface chemistry  4.4.1. Absorption and Catalysis  4.4.2. Colloids and Emulsions
  5. Analytical Chemistry  5.1. Classical methods  5.1.1. Gravimetric Analysis  5.1.2. titrimeter  5.2. Instrumental methods  5.2.1. Chromatography  5.2.2. Spectroscopy  5.2.3. Electroanalytical methods  5.2.4. X-ray diffraction
  6. Industrial Chemistry  6.1. Petrochemistry  6.2. Chemical engineering principles  6.3. Green Chemistry  6.4. Pharmaceuticals and Drug Synthesis
  7. Biochemistry  7.1. Carbohydrates  7.2. Proteins and enzymes  7.3. Lipids and membranes  7.4. Nucleic acids  7.5. Metabolism and Bioenergetics  7.6. Enzyme kinetics and mechanism

Undergraduate → Organic chemistry ↓


Elimination reactions


An elimination reaction is a type of organic reaction in which two atoms or groups are removed from a molecule, forming a new multiple bond or ring system. This process usually results in the formation of an alkene or alkyne from an alkyl halide or alcohol. Elimination reactions are fundamental to organic chemistry and are classified into different types, such as E1, E2, and E1cB mechanisms. In this document, we will explore these mechanisms, their underlying principles, and provide visual and textual examples to enhance understanding.

Types of elimination reactions

Elimination reactions may be mainly classified into the following types based on their mechanism:

  • E1 (monomolecular elimination)
  • E2 (bimolecular elimination)
  • E1cB (elimination unimolecular conjugate base)

E1 mechanism

The E1 mechanism is a two-step process in which elimination occurs via the formation of a carbocation intermediate. The steps are as follows:

  1. Formation of a carbocation by the departure of a retiring group.
  2. Formation of a double bond by removal of a proton (H+), usually with the help of a base.

Specific characteristics:

  • Occurs with substrates that can form stable carbocations (e.g., tertiary haloalkanes).
  • Is favored over polar protic solvents that can stabilize the carbocation.
  • The rate of the reaction depends on the concentration of the substrate.

Example of E1 reaction:

Consider the dehydration of tert-butyl alcohol to form isobutylene:

        (CH₃)₃C-OH → (CH₃)₂C=CH₂ + H₂O
(CH₃)₃C-OH (CH₃)₂C=CH₂ + H₂O

E2 mechanism

The E2 mechanism is a single-step, coordinated reaction in which the proton is removed, and the leaving group is also removed simultaneously. This mechanism is characterized by:

  • This occurs with strong bases that can quickly donate or absorb protons.
  • This usually involves primary or secondary haloalkanes.
  • The reaction rate depends on both the substrate and the base.

Specific characteristics:

  • A good leaving group (halide like Cl-, Br-, I-) is required.
  • Takes up space in anti-periplanar geometry for optimal orbital overlap.

Example of E2 reaction:

Consider the dehydrohalogenation of 2-bromo-2-methylpropane using a strong base such as potassium tert-butoxide:

        (CH₃)₃CBr + KOtBu → (CH₃)₂C=CH₂ + KBr + tBuOH
(CH₃)₃CBr + KOtBu (CH₃)₂C=CH₂ + KBr + tBuOH

E1cB mechanism

The E1cB mechanism involves a carbanion intermediate. It is a two-step process where:

  1. A base absorbs a proton to form a carbanion.
  2. The carbenium residue then expels the group to form an alkene.

Specific characteristics:

  • This occurs when the group to be left is a bad one (not a halide), such as a hydroxide.
  • Often seen in compounds containing electron-withdrawing groups, such as carbonyl.
  • The reaction rate is affected by the stability of the carbanion.

Example of an E1cB reaction:

Consider the base-induced elimination of β-hydroxy carbonyl compounds:

        R-CH(OH)-CH₂-COR' → R-CH=CH-COR' + H₂O
R-CH(OH)-CH₂-COR' R-CH=CH-COR' + H₂O

Factors affecting elimination reactions

Several factors can affect the order and outcome of elimination reactions:

1. Substrate structure

The tendency to undergo E1 versus E2 reactions depends largely on the structure of the substrate:

  • Tertiary substrates favor the E1 mechanism due to easier carbocation formation.
  • Primary substrates favor the E2 mechanism because the carbocation would be destabilized.
  • Static hindrance favors elimination over substitution.

2. Base power

Elimination reactions are strongly affected by the strength and nature of the base:

  • Strong bases promote the E2 mechanism, accelerating the one-step coordinated process.
  • Weak bases may favor E1 as they will favor carbocation formation.

3. Leaving the group

Good leaving groups facilitate both E1 and E2 mechanisms because they leave easily, allowing the transition state to be reached:

  • Halides such as bromide and chloride are excellent residual groups.
  • Poor leaving groups slow down the reaction and can change the mechanism unpredictably.

4. Solvent effect

The type of solvent can stabilize intermediates or transition states, and favor one mechanism over another:

  • Polar protic solvents favor E1 by stabilizing the carbocation.
  • Aprotic solvents prefer E2 because they do not stabilize the intermediate, but instead help make the base soluble.

Applications of elimination reactions

Elimination reactions play an important role in organic synthesis, helping chemists create unsaturated molecules that serve as key intermediates and final products:

  • Synthesis of alkenes and alkynes, which are important in building more complex molecular structures.
  • Facilitating reactions in pharmaceutical, polymer and materials science applications.
  • Understanding these reactions helps to design selective routes for building complex molecules.

Concluding remarks

Elimination reactions are versatile, forming the backbone of organic reaction mechanisms. By changing factors such as substrate structure, base strength, or reaction conditions, chemists can manipulate these reactions for desired results. Whether creating industrially important polymers or enabling complex drug synthesis, understanding elimination reactions allows chemists to conduct these transformations with precision and creativity.


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Elimination reactions

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