11.7 Elimination Reactions: Zaitsev’s Rule
3 min read•may 7, 2024
are key in forming alkenes. helps predict the major product, stating that the most stable alkene (usually the most substituted) predominates. This rule guides chemists in understanding product formation and reaction outcomes.
Different elimination mechanisms (E2, E1, E1cB) affect how alkenes form from alkyl halides. Factors like base strength, structure, and reaction conditions influence which mechanism occurs. Understanding these helps predict and control alkene synthesis in organic chemistry.
Elimination Reactions and Zaitsev's Rule
Zaitsev's rule for alkene products
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Top images from around the web for Zaitsev's rule for alkene products
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- States most stable alkene product predominates in elimination reaction
- Most substituted alkene typically most stable due to increased and dispersal of electron density (ethene vs. 2-methylpropene)
- With multiple possible alkenes, major product has greatest number of alkyl substituents on (2-pentene vs. 1-pentene)
- Predict major product using Zaitsev's rule by:
- Identifying all possible alkene products
- Determining degree of substitution for each alkene (mono-, di-, tri-, or tetrasubstituted)
- Alkene with highest degree of substitution is major product
- Exceptions to Zaitsev's rule occur when steric hindrance prevents formation of most substituted alkene or reaction conditions favor over (bulky base)
Comparison of elimination mechanisms
- E2 mechanism ()
- Concerted mechanism with simultaneous breaking of C-H and C-X bonds in single
- Rate-determining step is formation of
- Requires strong base (hydroxide, ethoxide)
- orientation of C-H and C-X bonds necessary for
- E1 mechanism ()
- Stepwise mechanism with C-X bond breaking first to form intermediate in two transition states
- Rate-determining step is formation of carbocation intermediate
- Requires weak base or solvent acting as base (water, ethanol)
- No specific orientation required between C-H and C-X bonds
- E1cB mechanism (elimination unimolecular conjugate base)
- Stepwise mechanism with C-H bond breaking first to form intermediate in two transition states
- Rate-determining step is formation of carbanion intermediate
- Requires very strong base like or (sodium amide, potassium t-butoxide)
- Carbanion intermediate allows bond rotation leading to mixture of stereoisomers
Alkyl halide precursors for alkenes
- Identify potential precursors by:
- Analyzing structure of given alkene product
- Considering possible locations for halogen and hydrogen atoms on adjacent carbons
- Halogen and hydrogen must be anti-periplanar for E2 reactions
- Halogen and hydrogen can be in any orientation for E1 and E1cB reactions
- Determining degree of substitution for each potential precursor (substrate)
- Less substituted alkyl halides react faster in E2 reactions (ethyl bromide vs. t-butyl bromide)
- More substituted alkyl halides react faster in E1 reactions (t-butyl chloride vs. ethyl chloride)
- Considering ability of halogen ()
- Multiple alkyl halide precursors may lead to same alkene product depending on reaction conditions and mechanism (1-bromo-2-methylbutane and 2-bromo-2-methylbutane both yield 2-methyl-2-butene)
Reaction control and product formation
- :
- Favors formation of the most stable product (typically the more substituted alkene)
- Influenced by reaction conditions such as higher temperatures and longer reaction times
- Often results in products that follow Zaitsev's rule
- :
- Favors formation of the product that forms fastest (may not be the most stable)
- Influenced by reaction conditions such as lower temperatures and shorter reaction times
- Can lead to products that do not follow Zaitsev's rule
- plays a crucial role in determining product distribution
- E2 reactions often follow a concerted mechanism, leading to a single transition state
- E1 and E1cB reactions involve stepwise mechanisms with multiple transition states
- The in the product is formed through the elimination of adjacent atoms in the substrate
Key Terms to Review (38)
Alkoxide: An alkoxide is a functional group consisting of an alkyl group (R-) bonded to an oxygen atom (O-). Alkoxides are important intermediates in many organic chemistry reactions, including Grignard reactions, elimination reactions, and carbonyl condensation reactions.
Alkyl halide: An alkyl halide is an organic compound in which one or more hydrogen atoms in an alkane (saturated hydrocarbon) have been replaced by a halogen atom (fluorine, chlorine, bromine, or iodine). This substitution results in a molecule with distinct chemical and physical properties compared to its alkane precursor.
Alkyl Halide: An alkyl halide is a type of organic compound that consists of an alkyl group (a hydrocarbon chain) bonded to a halogen atom (fluorine, chlorine, bromine, or iodine). These compounds are important intermediates in many organic reactions, including polar reactions, elimination reactions, and substitution reactions.
Amide: An amide is a functional group consisting of a carbonyl group (C=O) linked to a nitrogen atom (N). Amides are important in organic chemistry and play a crucial role in various topics, including functional groups, elimination reactions, alcohol reduction, nitrile chemistry, amide chemistry, and the spectroscopy of carboxylic acid derivatives.
Anti stereochemistry: Anti stereochemistry describes the spatial arrangement in a chemical reaction where two substituents are positioned on opposite sides of a double bond or ring structure after the reaction. It is particularly relevant in the halogenation of alkenes, resulting in products where the added atoms are located across from each other.
Anti-periplanar: The anti-periplanar arrangement refers to the spatial orientation of atoms or groups in an organic molecule where they are positioned on opposite sides of a planar carbon-carbon double bond or a carbon-carbon single bond. This specific arrangement is crucial in understanding certain types of organic reactions, particularly elimination reactions.
Beta Hydrogen: Beta hydrogen refers to the hydrogen atom that is located on the carbon atom that is two positions away from the reactive site in an elimination reaction. This beta hydrogen plays a crucial role in the mechanism and stereochemistry of elimination reactions, such as the E2 reaction and Zaitsev's rule.
Bimolecular Elimination: Bimolecular elimination, also known as the E2 reaction, is a type of elimination reaction in organic chemistry where two molecules (the substrate and a base) participate simultaneously in the rate-determining step to remove a leaving group and a hydrogen atom, resulting in the formation of an alkene product.
Carbanion: A carbanion is a negatively charged species that contains a carbon atom with three bonds and a lone pair of electrons, giving it a formal negative charge. This species is crucial in various organic reactions, as it acts as a strong nucleophile and can participate in forming new bonds by attacking electrophiles.
Carbocation: A carbocation is a positively charged carbon atom that is part of an organic molecule. These reactive intermediates play a crucial role in various organic reactions, including electrophilic additions, nucleophilic substitutions, and elimination reactions.
Double bond: A double bond in organic chemistry is a chemical bond between two atoms involving four bonding electrons instead of the usual two. It results in stronger attraction and shorter distance between the bonded atoms compared to a single bond.
Double Bond: A double bond is a covalent chemical bond that forms between two atoms, with the sharing of four valence electrons. This type of bond is commonly found in organic compounds, particularly in alkenes, and is a key structural feature that influences the properties and reactivity of these molecules.
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.
E2 reaction: An E2 reaction is a bimolecular elimination reaction where a hydrogen atom is removed from a carbon adjacent to the one bonded to the leaving group, resulting in the formation of an alkene. This process involves a single concerted step, where both the base removing the hydrogen and the leaving group departure occur simultaneously.
E2 Reaction: The E2 reaction is an elimination reaction in organic chemistry where a base removes a hydrogen atom and a leaving group from adjacent carbon atoms, resulting in the formation of an alkene. This reaction is characterized by the simultaneous removal of the hydrogen and the leaving group, proceeding through a concerted mechanism.
Elimination reactions: Elimination reactions are a type of organic reaction where two atoms or groups are removed from a molecule, resulting in the formation of a double bond. These reactions often involve the loss of small molecules like water or hydrogen halides from larger organic molecules.
Elimination Reactions: Elimination reactions are a class of organic reactions where two atoms or groups are removed from a molecule, typically resulting in the formation of a new carbon-carbon double bond. These reactions are an important aspect of organic chemistry, as they allow for the conversion of various functional groups and the synthesis of alkenes and other unsaturated compounds.
Hyperconjugation: Hyperconjugation is a type of conjugation in organic chemistry where the sigma bonds of alkyl groups (such as methyl or ethyl) interact with adjacent pi bonds, leading to increased stability of the molecule. This stabilizing effect is particularly important in understanding the stability of carbocations and the orientation of electrophilic additions.
Kinetic control: Kinetic control in organic chemistry refers to reaction conditions under which the product distribution is determined by the rate at which products are formed, favoring the formation of products that are formed fastest. These conditions often lead to products that are not necessarily the most stable but are reached more quickly due to lower activation energies.
Kinetic Control: Kinetic control refers to the principle that the initial product formed in a reaction is determined by the reaction pathway that has the lowest activation energy, regardless of the thermodynamic stability of the final products. It describes how the kinetics of a reaction, rather than just the thermodynamics, can dictate the outcome of a transformation.
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.
Reaction mechanism: A reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs. It outlines the specific way in which reactants convert to products, including the formation and breaking of bonds.
Reaction Mechanism: A reaction mechanism is the step-by-step sequence of elementary reactions by which overall chemical change occurs. It describes the detailed pathway that a reaction follows, including the formation and rearrangement of chemical bonds, the generation of intermediates, and the movement of electrons. Understanding reaction mechanisms is crucial for predicting the products of a reaction, explaining reactivity trends, and designing new synthetic pathways.
Regioselectivity: Regioselectivity refers to the preference of a chemical reaction to occur at a specific site or region of a molecule, leading to the formation of one regioisomeric product over another. This concept is particularly important in the context of electrophilic addition reactions of alkenes, electrophilic aromatic substitution, and other organic transformations.
Stereochemistry: Stereochemistry is the study of the three-dimensional arrangement of atoms in molecules and how this arrangement affects the chemical and physical properties of the substance. It examines the spatial orientation of atoms and their relationship to one another, which is crucial in understanding many organic chemistry concepts.
Substrate: In the context of organic chemistry, a substrate is the molecule or compound that undergoes a chemical reaction, typically catalyzed by an enzyme or a reagent in a laboratory setting. Substrates serve as the starting material for various types of reactions, including biological reactions and laboratory reactions.
T-Butoxide: The t-butoxide ion (CH3)3CO⁻ is a strong nucleophilic alkoxide base that is commonly used in organic chemistry reactions. It is derived from the tert-butyl alcohol and is a key participant in elimination reactions following Zaitsev's rule.
Thermodynamic control: In organic chemistry, thermodynamic control describes conditions under which the products of a reaction are determined by the relative stability of the products rather than the rates at which they are formed. This often results in the formation of the most stable product over time, even if it is not the most rapidly produced.
Thermodynamic Control: Thermodynamic control refers to the principle that the most stable and thermodynamically favored product will be the predominant product of a reaction, regardless of the kinetic pathway. It is a concept that governs the outcome of various organic chemistry reactions, including those related to energy diagrams, elimination reactions, electrophilic additions to conjugated dienes, and the dehydration of aldol products.
Transition state: In organic chemistry, the transition state is a high-energy, temporary condition where reactants are transformed into products during a chemical reaction. It represents the point of maximum energy on the energy diagram before the formation of products.
Transition State: The transition state is a key concept in organic chemistry that describes the highest-energy intermediate along the reaction pathway. It represents the point where the reactants are being converted into products, with the system at its most unstable and energetically unfavorable configuration.
Unimolecular Elimination: Unimolecular elimination, also known as E1 elimination, is a type of elimination reaction in organic chemistry where a single reactant molecule undergoes a reaction to form two or more products. This process is particularly relevant in the context of Zaitsev's rule, which describes the preferred formation of the more substituted alkene in elimination reactions.
Zaitsev’s rule: Zaitsev's rule predicts the outcome of elimination reactions, stating that the more substituted alkene product is preferred over less substituted alternatives. It helps in determining the major product when a reactant can potentially lead to two or more different alkenes.
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.