24.4 Basicity of Arylamines
2 min read•may 7, 2024
Arylamines, like , are weaker bases than alkylamines due to of their lone pairs. This makes them less likely to accept protons. Substituents on the aromatic ring can significantly impact , with increasing it and decreasing it.
Understanding basicity is crucial for predicting their reactivity in organic reactions. Factors like , inductive effects, and substituent positioning all play roles. This knowledge helps chemists design and control reactions involving these important compounds in various applications.
Basicity of Arylamines
Basicity of arylamines vs alkylamines
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- Arylamines are less basic than alkylamines
- Aniline () is about times less basic than ()
- Resonance stabilization of the arylamine lone pair reduces its basicity
- The lone pair on arylamines can delocalize into the aromatic ring (resonance)
- This delocalization is not possible for alkylamines (no conjugated system)
- Protonation of arylamines disrupts this resonance stabilization
- The resulting has a localized positive charge on nitrogen
- This localized charge is less stable than the delocalized lone pair in the neutral molecule
Substituent effects on arylamine basicity
- Electron-donating groups (EDGs) increase the basicity of arylamines
- Examples: -OH (), -OR (), -NH2 (aniline), -NHR, -NR2, -alkyl
- EDGs increase electron density on the aromatic ring and the nitrogen atom through resonance and/or inductive effects
- This enhanced electron density stabilizes the protonated form, increasing basicity
- Electron-withdrawing groups (EWGs) decrease the basicity of arylamines
- Examples: -NO2 (), -CN (), -SO3H (), -COOH (), -COR, -COOR, -halogens ()
- EWGs decrease electron density on the aromatic ring and the nitrogen atom through resonance and/or inductive effects
- This reduced electron density destabilizes the protonated form, decreasing basicity
Ranking of substituted anilines
- Relative basicity of substituted anilines follows this general trend:
- p-NH2 ()
- p-OH ()
- p-OCH3 ()
- p-alkyl ()
- p-H (aniline)
- p-Cl ()
- p-COOH ()
- p-NO2 ()
- Resonance effects dominate over inductive effects in determining basicity
- p-aminoaniline is more basic than p-methoxyaniline despite the greater of -OCH3
- The -NH2 group can participate in resonance donation more effectively than -OCH3 (two lone pairs vs one)
- Steric effects can also influence basicity
- o-substituted anilines are usually less basic than their p-substituted counterparts
- Bulky o-substituents hinder and protonation of the amino group ()
Factors affecting arylamine basicity and nucleophilicity
- Conjugation: Extends the π-electron system, influencing basicity and reactivity
- : Electron-donating or withdrawing effects through σ-bonds
- : Related to basicity but also affected by steric factors and solvent
- : Quantitative measure of acid strength, inversely related to basicity
- Solvation: Stabilization of charged species by solvent molecules, affecting overall basicity
Key Terms to Review (30)
Aniline: Aniline is an aromatic organic compound with the chemical formula C6H5NH2. It is a colorless, oily liquid with a distinctive, unpleasant odor. Aniline is an important precursor in the synthesis of various dyes, pharmaceuticals, and other organic compounds, and it plays a significant role in the context of naming aromatic compounds, amines, and understanding their basicity.
Anisole: Anisole is an aromatic organic compound with the chemical formula C6H5OCH3. It consists of a benzene ring with a methoxy group (OCH3) attached, and it is a colorless, flammable liquid with a characteristic sweet, anise-like odor.
Aryl Ammonium Ion: The aryl ammonium ion is a positively charged species formed when an aromatic amine, such as aniline, is protonated. This ion is an important intermediate in various organic reactions and plays a crucial role in understanding the basicity of arylamines.
Arylamine: An arylamine is a functional group in organic chemistry that consists of an amino group (-NH2) attached directly to an aromatic ring, such as a benzene ring. Arylamines are an important class of compounds with diverse applications in various fields, including pharmaceuticals, dyes, and polymers.
Basicity: Basicity is a measure of the strength or ability of a chemical species to accept a proton (H+) and form a conjugate acid. It is a fundamental concept in organic chemistry that plays a crucial role in understanding the reactivity and properties of various organic compounds, including those involved in SN2 reactions, aromatic heterocycles, amines, and their reactions.
Basicity constant, Kb: The basicity constant, \(K_b\), measures the strength of a base in solution, specifically how well an amine can attract and hold a proton (H+). It quantitatively expresses the equilibrium between the amine in its basic form and its corresponding protonated form in solution.
Benzenesulfonic Acid: Benzenesulfonic acid is an organic compound with the chemical formula C6H5SO3H. It is an aromatic sulfonic acid derived from benzene, and it serves as an important intermediate in the synthesis of various organic compounds, particularly dyes and pharmaceuticals.
Benzoic Acid: Benzoic acid is an aromatic carboxylic acid with the chemical formula C6H5COOH. It is a white crystalline solid that is widely used as a food preservative and in the production of various organic compounds.
Benzonitrile: Benzonitrile is an aromatic nitrile compound with the chemical formula C6H5CN. It is a colorless, flammable liquid with a distinctive almond-like odor. Benzonitrile is an important organic compound that is relevant in the context of spectroscopy of carboxylic acids and nitriles, as well as the basicity of arylamines.
Chlorobenzene: Chlorobenzene is an aromatic organic compound consisting of a benzene ring with a chlorine atom substituted onto it. It is a colorless, flammable liquid with a characteristic almond-like odor. Chlorobenzene is an important industrial chemical with applications in the synthesis of various other organic compounds.
Conjugation: Conjugation refers to the overlap or sharing of atomic orbitals, resulting in the delocalization of electrons across a system of connected atoms. This concept is central to understanding resonance, the stability of certain molecules and ions, and the interpretation of various spectroscopic techniques in organic chemistry.
Cyclohexylamine: Cyclohexylamine is an organic compound with the chemical formula C$_6$H$_{11}$NH$_2$. It is a primary amine derived from the cyclic hydrocarbon cyclohexane, and it exhibits the characteristic properties of aliphatic amines, including basicity and nucleophilicity.
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.
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.
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.
P-Aminobenzoic Acid: p-Aminobenzoic acid, also known as PABA, is an organic compound that consists of a benzene ring with a carboxyl group and an amino group attached at the para positions. It is an important precursor in the biosynthesis of folic acid and has various applications in the pharmaceutical and personal care industries.
P-Aminophenol: p-Aminophenol is an organic compound consisting of a phenol group with an amino group attached to the para position on the benzene ring. It is an important intermediate in the production of various pharmaceuticals, dyes, and other chemical products.
P-Anisidine: p-Anisidine is an aromatic amine compound with the chemical formula CH3O-C6H4-NH2. It is a derivative of aniline with a methoxy group (-OCH3) attached to the para position of the benzene ring. p-Anisidine is an important precursor in the synthesis of various pharmaceuticals and dyes, and its basicity properties are relevant in the context of 24.4 Basicity of Arylamines.
P-Chloroaniline: p-Chloroaniline is an aromatic amine compound with a chlorine substituent at the para position on the benzene ring. It is an important intermediate in the synthesis of various dyes, pharmaceuticals, and other organic compounds.
P-Nitroaniline: p-Nitroaniline is an organic compound with the chemical formula C₆H₆N₂O₂. It is a yellow crystalline solid that is commonly used as a precursor in the synthesis of various dyes, pharmaceuticals, and other chemical products. In the context of 24.4 Basicity of Arylamines, p-nitroaniline is an important example that demonstrates the effects of substituents on the basicity of aromatic amines.
P-Phenylenediamine: p-Phenylenediamine is an aromatic diamine compound with two amino groups attached to a benzene ring in the para position. It is an important intermediate in the synthesis of various dyes, pigments, and other organic compounds, and its basicity is a key consideration in the context of 24.4 Basicity of Arylamines.
P-Toluidine: p-Toluidine is an aromatic amine compound with the chemical formula CH3C6H4NH2. It is an isomer of toluidine, with the methyl group (-CH3) attached to the para position of the benzene ring, relative to the amino group (-NH2).
Phenol: Phenol is an aromatic organic compound with a hydroxyl group (-OH) attached directly to a benzene ring. It is a key structural feature in many important organic molecules and plays a significant role in various chemical reactions and properties across several topics in organic chemistry.
PKa: pKa, or the acid dissociation constant, is a measure of the strength of an acid in a solution. It represents the pH at which a particular acid is 50% dissociated into its conjugate base. This value is crucial in understanding the behavior and properties of acids, bases, and their reactions in organic chemistry.
Resonance Stabilization: Resonance stabilization is a phenomenon where the delocalization of electrons in a molecule or ion leads to a more stable configuration compared to a single Lewis structure. This concept is crucial in understanding the behavior and properties of various organic compounds, including their acidity, basicity, reactivity, and stability.
Solvation: Solvation is the process by which solute particles or molecules become surrounded by solvent molecules, forming a solvated species. This interaction between the solute and the solvent is a crucial aspect of various chemical processes, including noncovalent interactions between molecules and the basicity of arylamines.
Steric Hindrance: Steric hindrance, also known as steric strain or steric effect, refers to the repulsive forces that arise between atoms or groups of atoms in a molecule due to their physical size and spatial arrangement. This phenomenon can significantly impact the stability, reactivity, and conformations of organic compounds.