16.4 Substituent Effects in Electrophilic Substitutions
3 min read•may 7, 2024
Substituents on rings can make or break reactions. boost reactivity and direct substitution to ortho and para positions, while do the opposite, slowing things down and favoring the meta position.
Understanding these effects is key to predicting reaction outcomes. The type of substituent influences the stability of the formed during the reaction, ultimately determining how fast and where on the ring the substitution occurs.
Substituent Effects on Electrophilic Aromatic Substitution
Substituent effects on aromatic substitution
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Organic chemistry 28: Aromaticity - electrophilic aromatic substitution View original
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Organic chemistry 28: Aromaticity - electrophilic aromatic substitution View original
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- Substituents on the benzene ring influence reactivity and orientation of (EAS) reactions
- Electron-donating groups (EDG) increase reactivity direct substitution to ortho and para positions (-OH, -OR, -NH2, -NHR, -NR2, -R, -CH3)
- Electron-withdrawing groups (EWG) decrease reactivity direct substitution to meta positions (-NO2, -CN, -CHO, -COR, -COOH, -COOR, -SO3H, -NH3+)
- Substituents affect ability to stabilize the cationic intermediate formed during rate-determining step of EAS
- EDG stabilize the cationic intermediate increasing reactivity
- EWG destabilize the cationic intermediate decreasing reactivity
- and inductive effects of substituent determine orientation
- EDG stabilize cationic intermediate more effectively at ortho and para positions through resonance
- EWG destabilize cationic intermediate less at meta positions due to reduced resonance interaction
Electron-donating vs electron-withdrawing substituents
- EDG stabilize the cationic intermediate formed during EAS
- Donate electron density to the ring increasing electron density at ortho and para positions
- Increased electron density stabilizes positive charge of cationic intermediate through resonance
- Stabilization of cationic intermediate lowers activation energy increasing reaction rate
- EWG destabilize the cationic intermediate formed during EAS
- Withdraw electron density from the ring decreasing electron density at ortho and para positions
- Decreased electron density destabilizes positive charge of cationic intermediate through reduced resonance interaction
- Destabilization of cationic intermediate increases activation energy decreasing reaction rate
- The relative of the aromatic ring is influenced by these
Product prediction for mono-substituted benzenes
- Mono-substituted benzenes with EDG yield ortho and para substituted major products
- EDG direct substitution to ortho and para positions due to increased stability of cationic intermediate through resonance
- (methylbenzene) undergoing chlorination yields ortho-chlorotoluene and para-chlorotoluene as major products
- Mono-substituted benzenes with EWG yield meta substituted major product
- EWG direct substitution to meta positions due to reduced destabilization of cationic intermediate
- undergoing bromination yields meta-bromonitrobenzene as major product
- Relative reactivity of mono-substituted benzenes compared to benzene predicted based on substituent
- EDG increase reactivity compared to benzene (toluene more reactive than benzene)
- EWG decrease reactivity compared to benzene (nitrobenzene less reactive than benzene)
Reaction Kinetics and Mechanism
- The rate-determining step in EAS reactions is typically the formation of the intermediate
- helps explain the energetics of the reaction, including the activation energy barrier
- The of the attacking species plays a crucial role in the reaction rate and overall feasibility of the substitution
Key Terms to Review (28)
Activating groups: Activating groups are substituents attached to an aromatic ring that increase its reactivity toward electrophilic aromatic substitution by donating electrons either through resonance or inductively. These groups enhance the electron density on the ring, making it more attractive to electrophiles.
Activating Groups: Activating groups are substituents on an aromatic ring that increase the reactivity of the ring towards electrophilic aromatic substitution reactions. These groups facilitate the addition of electrophiles to the aromatic system by stabilizing the intermediate carbocation formed during the reaction.
Alkyl group: An alkyl group is a type of hydrocarbon chain that branches off from the main molecular structure in organic compounds, typically derived by removing one hydrogen atom from an alkane, giving it the general formula CnH2n+1. These groups are not stable on their own but can form strong covalent bonds with other atoms or groups, influencing the compound's physical and chemical properties.
Alkyl Group: An alkyl group is a hydrocarbon substituent derived from an alkane by the removal of a single hydrogen atom. Alkyl groups are commonly found in various organic chemistry topics, including substituent effects in electrophilic substitutions, the properties of alcohols and phenols, and the preparation of aldehydes and ketones.
Arenium Ion: The arenium ion, also known as the arene cation, is a positively charged intermediate species that arises during electrophilic aromatic substitution reactions. It is a key concept in understanding the mechanisms of various aromatic substitution reactions, including bromination, other aromatic substitutions, and the Friedel-Crafts reactions.
Benzene: Benzene is a planar, aromatic hydrocarbon compound with the chemical formula C6H6. It is a key building block in organic chemistry and has a unique resonance structure that contributes to its stability and reactivity.
Cationic Intermediate: A cationic intermediate is a positively charged, reactive species that forms during the course of an organic reaction, particularly in the context of electrophilic substitution reactions. These intermediates play a crucial role in determining the outcome and regioselectivity of the reaction.
Deactivating groups: In the context of electrophilic aromatic substitution reactions, deactivating groups are substituents attached to a benzene ring that decrease the reactivity of the ring towards an electrophile. They accomplish this by either withdrawing electron density from the ring or by being meta-directing in orientation.
Deactivating Groups: Deactivating groups are substituents on an aromatic ring that decrease the reactivity of the ring towards electrophilic aromatic substitution reactions. These groups make the aromatic ring less susceptible to further substitution by electrophiles.
Electron-Donating Groups: Electron-donating groups are functional groups or substituents that have the ability to donate or contribute electrons to a molecule, typically a benzene ring or other aromatic system. These groups can have a significant impact on the reactivity, stability, and properties of the molecule.
Electron-Withdrawing Groups: Electron-withdrawing groups are functional groups or substituents in a molecule that have a strong affinity for electrons, making them attractive to electrons. This property can significantly influence the reactivity, stability, and spectroscopic properties of the molecule.
Electrophilic aromatic substitution: Electrophilic aromatic substitution is a chemical reaction in which an atom, typically hydrogen, attached to an aromatic system, such as benzene, is replaced by an electrophile. This process preserves the aromaticity of the compound while introducing a functional group.
Electrophilic Aromatic Substitution: Electrophilic aromatic substitution is a fundamental organic reaction in which an electrophile (a species that is attracted to electrons) replaces a hydrogen atom on an aromatic ring, resulting in the formation of a new carbon-electrophile bond. This reaction is crucial in understanding the behavior and reactivity of aromatic compounds, which are prevalent in many organic molecules and have widespread applications.
Electrophilicity: Electrophilicity is a measure of the affinity of a species for electron density, reflecting its ability to attract and bond with electrons. This term is particularly relevant in the context of electrophilic substitution reactions and the nucleophilic addition reactions of aldehydes and ketones.
Friedel-Crafts Alkylation: Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction that allows for the alkylation of aromatic rings. It involves the use of a Lewis acid catalyst, typically aluminum chloride (AlCl3), to facilitate the addition of an alkyl group to the aromatic ring, resulting in the formation of a new carbon-carbon bond.
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.
Inductive effect: The inductive effect is a phenomenon observed in organic chemistry where the polarization of chemical bonds occurs due to the shifting of electrons along a chain of atoms within a molecule, caused by differences in electronegativity. This effect influences the distribution of electron density across the molecule, affecting its reactivity and properties.
Inductive Effect: The inductive effect is an electronic effect in which the unequal sharing of electrons between atoms in a molecule results in a partial charge being transmitted through the bonds of the molecule. This effect can influence the reactivity and stability of various functional groups and intermediates in organic chemistry.
Meta Directing: Meta directing is a concept in organic chemistry that describes the directing effects of substituents on the position of electrophilic aromatic substitution reactions. It refers to the ability of certain functional groups to influence the site of substitution on a benzene ring, typically favoring the meta position relative to the original substituent.
Nitration: Nitration is a chemical reaction in which a nitro group (-NO2) is introduced into an organic compound, typically an aromatic ring structure. This process is widely used in the synthesis of various pharmaceuticals, explosives, and other important chemical products.
Nitro Group: The nitro group (−NO2) is a functional group consisting of a nitrogen atom double-bonded to two oxygen atoms. It is an important substituent in organic chemistry, known for its ability to influence the reactivity and properties of aromatic compounds.
Nitrobenzene: Nitrobenzene is an aromatic organic compound with the chemical formula C6H5NO2. It is a pale yellow oily liquid with a distinctive almond-like odor. Nitrobenzene is an important industrial chemical and an intermediate in the production of various pharmaceuticals, dyes, and other organic compounds.
Nucleophilicity: Nucleophilicity refers to the ability of a species to donate electrons and form a covalent bond with an electrophilic center. It is a key concept in organic chemistry that governs the reactivity and selectivity of many important reactions, including substitution, addition, and elimination reactions.
Ortho-Para Directing: Ortho-para directing refers to the predictable patterns observed in the positioning of substituents during electrophilic aromatic substitution reactions on benzene rings. This term describes how the presence of certain functional groups on the benzene ring can direct the incoming electrophile to specific positions relative to the existing substituents.
Resonance: Resonance is a fundamental concept in organic chemistry that describes the ability of certain molecules to exist in multiple equivalent structures or resonance forms. This phenomenon arises from the delocalization of electrons within the molecule, leading to the stabilization of the overall structure and the distribution of electron density across multiple atoms.
Substituent Effects: Substituent effects refer to the influence that specific functional groups or atoms have on the chemical and physical properties of a molecule. These effects can significantly impact the reactivity, stability, and behavior of organic compounds in various contexts, including conformational analysis, electrophilic and nucleophilic substitutions, and acidity determination.
Toluene: Toluene is an aromatic hydrocarbon compound with the chemical formula C6H5CH3. It is a colorless, flammable liquid with a distinctive sweet odor, and is widely used as a solvent and in the production of various chemical compounds.
Transition State Theory: Transition state theory is a model used to describe the energy changes that occur during a chemical reaction. It explains the formation of an unstable, high-energy intermediate state, known as the transition state, which is the point at which the reactants are converted into products.