11.10 The E1 and E1cB Reactions
2 min read•may 7, 2024
Elimination reactions are crucial in organic chemistry, forming alkenes by removing adjacent atoms. E1 and E1cB mechanisms differ in their intermediates and conditions, but both result in double bond formation.
Understanding these pathways helps predict product formation and stereochemistry. Factors like substrate structure, solvent, and base strength influence which mechanism dominates, impacting reaction outcomes and product distributions.
E1 Reaction Mechanism and Characteristics
Mechanism of E1 reactions
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- eliminates a and an adjacent proton forming an (an example of an )
- Mechanism occurs in two steps:
- Rate-determining step heterolytically cleaves the bond to the creating a (tertiary carbocations form faster than secondary, which form faster than primary)
- Subsequent fast step deprotonates the carbocation by a (water, ethanol) forming the alkene product
- E1 shares similarities with SN1 reaction:
- Both form a carbocation intermediate in the rate-determining step
- Both are unimolecular reactions with rate depending only on substrate concentration
- Both are favored by substrates forming stable carbocations (tertiary > secondary > primary)
- Typical conditions use polar protic solvents (ethanol, water), heat, and a weak base (water, ethanol)
E1 vs E2 stereochemistry
- E1 reactions lack strict geometric requirements as the carbocation intermediate can rotate before deprotonation
- E2 reactions require between the leaving group and proton for concerted elimination
- E1 reactions often yield a mixture of alkene due to the carbocation being deprotonated at different positions
- E2 reactions typically yield a single alkene product determined by the antiperiplanar requirement and forming the more stable, more substituted alkene
- can apply in certain E2 reactions, favoring the less substituted alkene product
E1cB Reaction Mechanism and Conditions
E1cB reaction mechanism
- E1cB (Elimination Unimolecular conjugate Base) reaction forms a
- Mechanism occurs in two steps:
- Rate-determining step deprotonates the substrate by a strong base (, ) forming a carbanion intermediate
- Subsequent fast step eliminates the leaving group from the carbanion forming the alkene product
- Conditions favoring E1cB over E1 and E2:
- Strong base (alkoxides, amides)
- (DMSO, DMF)
- Substrates with acidic protons (-dicarbonyl compounds, -cyano compounds)
- Absence of good leaving groups (alcohols, ethers)
Additional Considerations
Kinetics and Reaction Pathways
- E1 and E1cB reactions follow first-order , with the rate-determining step depending only on the concentration of the substrate
- Some elimination reactions can proceed through an pathway, where the initial elimination product undergoes subsequent addition reactions
- reactions are a specific type of elimination reaction where water is removed from a molecule, often resulting in alkene formation
Key Terms to Review (27)
Alkene Product: An alkene product is the organic compound formed as the result of an elimination reaction, specifically the E1 and E1cB reactions, which involve the removal of a small molecule from a larger molecule to create a carbon-carbon double bond. These alkene products are important intermediates in many organic synthesis pathways.
Alkoxides: Alkoxides are negatively charged species formed when an alkoxide group, consisting of an alkyl group bonded to an oxygen atom, replaces a hydrogen atom on a molecule. They are important intermediates in various organic reactions, particularly in the context of the SN2 reaction and the E1 and E1cB elimination reactions.
Amides: Amides are a class of organic compounds that contain a carbonyl group (C=O) bonded to a nitrogen atom. They are derived from carboxylic acids and can be considered the result of replacing the hydroxyl group (-OH) of a carboxylic acid with an amino group (-NH2). Amides are important functional groups in many organic molecules, including proteins, and play a crucial role in various chemical reactions and processes.
Antiperiplanar Geometry: Antiperiplanar geometry refers to the spatial arrangement of atoms or groups in a molecule where they are positioned in opposite or anti positions relative to each other. This geometric configuration is particularly important in the context of elimination reactions and the conformational analysis of cyclic compounds.
Aprotic Solvent: An aprotic solvent is a type of organic solvent that does not contain an active hydrogen atom and does not participate in hydrogen bonding. These solvents are commonly used in various organic chemistry reactions, including the E1 and E1cB reactions, the reduction of carbonyl compounds, the Wittig reaction, and the Claisen condensation reaction.
Carbanion Intermediate: A carbanion intermediate is a negatively charged carbon species that serves as a key reactive intermediate in various organic reactions, particularly in the context of elimination reactions like the E1 and E1cB mechanisms, as well as in certain biological elimination processes.
Carbocation Intermediate: A carbocation intermediate is a positively charged carbon atom that acts as a reactive species in various organic chemistry reactions. These intermediates are formed during the course of a reaction and play a crucial role in determining the outcome and mechanism of the transformation.
Dehydration: Dehydration is a chemical process in which water is removed from a compound, typically resulting in the formation of a new compound with fewer hydrogen and oxygen atoms. This term is particularly relevant in the context of various organic reactions and transformations, where dehydration plays a crucial role in the preparation and interconversion of different functional groups.
E1 reaction: An E1 reaction is a type of elimination reaction in organic chemistry where a substrate, typically an alkyl halide, undergoes deprotonation to form an alkene. This process occurs in two steps, involving the formation of a carbocation intermediate followed by the loss of a proton.
E1 Reaction: The E1 reaction, or unimolecular elimination reaction, is a type of organic chemistry reaction in which a leaving group is removed from a substrate, resulting in the formation of an alkene. This process occurs in a stepwise manner, involving the formation of a carbocation intermediate.
E1cB reaction: The E1cB reaction is a type of organic chemical reaction where a substrate, usually an alkyl halide, undergoes unimolecular elimination via a carbanion intermediate to form an alkene. It involves the removal of a hydrogen atom (deprotonation) adjacent to the carbon bearing the leaving group, before the leaving group itself departs.
E1cB Reaction: The E1cB (Elimination, Unimolecular, Conjugate Base) reaction is a type of elimination reaction where the first step involves the removal of a proton from a carbon adjacent to a carbonyl group, followed by the loss of a leaving group to form a new carbon-carbon double bond. This reaction is characterized by the formation of a planar carbanion intermediate that is stabilized by the adjacent carbonyl group.
Elimination Reaction: An elimination reaction is a type of organic reaction in which two atoms or groups are removed from a molecule, typically resulting in the formation of a carbon-carbon double bond or a carbon-carbon triple bond. This process is an important step in the synthesis of alkenes and alkynes, as well as in various other organic transformations.
Elimination-Addition: Elimination-addition is a reaction mechanism in organic chemistry where a molecule first undergoes an elimination reaction, followed by the addition of a new functional group or atom. This process is particularly relevant in the context of the E1 and E1cB reactions, as well as the formation of benzyne intermediates.
Heterolytic Cleavage: Heterolytic cleavage is a type of bond breaking process where a covalent bond is cleaved in an unsymmetrical manner, resulting in the formation of two charged species - a positively charged species (carbocation) and a negatively charged species (anion). This process is in contrast to homolytic cleavage, where the bond is broken equally to produce two neutral radical species.
Hofmann's Rule: Hofmann's rule, also known as the Hofmann elimination, is a principle that predicts the major product in an elimination reaction involving a quaternary ammonium salt. It states that the elimination reaction will favor the formation of the alkene with the least substituted (most terminal) double bond.
Kinetics: Kinetics is the study of the rates and mechanisms of chemical reactions. It examines how quickly reactions occur and the factors that influence the speed of a reaction, such as temperature, pressure, and the presence of catalysts.
Leaving group: A leaving group in organic chemistry is an atom or group that detaches from the parent molecule during a nucleophilic substitution (SN2) reaction, forming a lone pair or negative ion. The ease with which a leaving group departs affects the rate and success of the reaction.
Leaving Group: A leaving group is a functional group or atom that is displaced or removed from a molecule during a chemical reaction. It is a key component in many organic reactions, particularly substitution and elimination reactions, as it facilitates the formation of a new bond or the creation of a new product.
Polar aprotic solvents: Polar aprotic solvents are solvents that have a net dipole moment but do not possess hydrogen atoms capable of forming hydrogen bonds. They are used in organic reactions to dissolve electrolytes and facilitate reactions without participating in them.
Polar Protic Solvent: A polar protic solvent is a solvent that is both polar and able to donate a proton (H+) to a solute. This type of solvent can stabilize charged intermediates and transition states, making it an important consideration in certain organic reactions like the E1 and E1cB mechanisms.
Regioisomers: Regioisomers are a type of structural isomers that differ in the position of a functional group or substituent within the molecule. These positional isomers have the same molecular formula but the atoms are arranged differently, leading to distinct chemical and physical properties.
Unimolecular Reaction: A unimolecular reaction is a type of chemical reaction where a single reactant molecule undergoes transformation to form one or more product molecules. This term is particularly relevant in the context of the E1 and E1cB reactions, which are examples of unimolecular elimination reactions.
Weak Base: A weak base is a chemical species that has the ability to accept a proton (H+) in an aqueous solution, forming a conjugate acid-base pair. Weak bases are less likely to completely ionize compared to strong bases, and their degree of ionization is dependent on the pH of the solution.
Zaitsev's Rule: Zaitsev's rule is a principle in organic chemistry that predicts the major product of an elimination reaction. It states that the major alkene product will be the one with the most substituted (most stable) double bond.
β-cyano compounds: β-cyano compounds are organic molecules that contain a nitrile (C≡N) functional group located at the beta (β) position, or two carbons away, from another functional group. These compounds are important intermediates in various organic reactions, particularly in the context of E1 and E1cB elimination reactions.
β-dicarbonyl compounds: β-dicarbonyl compounds are organic molecules that contain two carbonyl groups (C=O) separated by a single carbon atom. They are an important class of compounds in organic chemistry with unique reactivity and applications.