2.10 Organic Acids and Organic Bases
2 min read•may 7, 2024
Organic acids and bases are crucial players in chemical reactions. They're defined by specific structural features that determine their behavior. Acids have polarized O-H bonds, while bases have lone electron pairs on nitrogen or oxygen atoms.
Understanding how these compounds interact is key to grasping organic chemistry. Their strength is measured by values, and they form conjugate pairs when reacting. Biological compounds like amino acids showcase both acidic and basic properties, highlighting their importance in living systems.
Organic Acids and Bases
Structural features of organic acids/bases
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- Organic acids contain polarized O-H bond
- Hydrogen partially positive () oxygen partially negative () due to electronegativity difference
- Common functional groups include (-COOH), (Ar-OH), and (-SO3H)
- Organic bases contain lone pair of electrons on nitrogen or oxygen
- Allows acceptance of proton (H+) to form conjugate acid
- Common functional groups include (primary R-NH2, secondary R2NH, tertiary R3N), (Ar-NH2), and (R2C=NR)
Charge stabilization in conjugate bases
- of organic acids form when acid donates proton (H+)
- Resulting negative charge stabilized by electronegativity of atom bearing charge (oxygen better stabilizes than carbon)
- of negative charge spreads it over multiple atoms more stable with more resonance structures
- (RCOO-) stabilized by oxygen electronegativity and resonance between two oxygens
- (ArO-) stabilized by oxygen electronegativity and resonance with aromatic ring
- (RSO3-) stabilized by oxygen electronegativity and resonance among three oxygens
Acid-base theories and quantitative measures
- defines acids as proton donors and bases as proton acceptors
- expands definition to include electron pair acceptors (acids) and donors (bases)
- pKa measures acid strength, with lower values indicating stronger acids
- relates pH, pKa, and concentrations of acid-base conjugate pairs
- resist pH changes and can be prepared using weak acids/bases and their conjugate partners
Acid-base behavior of biological compounds
- Nitrogen-containing compounds
- Amines act as bases lone pair on nitrogen accepts proton (H+) forms ammonium ions (R-NH3+)
- and aromatic nitrogen heterocycles act as bases due to lone pair on nitrogen
- Oxygen-containing compounds
- Alcohols and phenols weakly acidic due to polarized O-H bond can donate proton (H+) to form alkoxide or phenoxide anions
- Carboxylic acids moderately strong acids donate proton (H+) to form carboxylate anions
- Amino acids contain both acidic (carboxyl -COOH) and basic (amino -NH2) groups
- proton transfer from carboxyl to amino group net neutral charge but localized positive (NH3+) and negative (COO-) charges
- pH-dependent behavior
- Low pH protonated amino group neutral carboxyl group
- zwitterionic form predominates
- High pH deprotonated carboxyl group neutral amino group
Key Terms to Review (23)
Amines: Amines are a class of organic compounds derived from ammonia (NH3) by the replacement of one or more hydrogen atoms with alkyl or aryl groups. They are characterized by the presence of a nitrogen atom with a lone pair of electrons, giving them basic properties and the ability to act as nucleophiles in chemical reactions.
Aromatic Amines: Aromatic amines are a class of organic compounds that contain an aromatic ring structure, typically a benzene ring, with one or more amino groups (-NH2) attached. These compounds exhibit unique chemical and physical properties that make them important in various fields, including organic chemistry, biochemistry, and pharmaceutical sciences.
Brønsted-Lowry Theory: The Brønsted-Lowry theory is a model that defines acids and bases in terms of proton donors and proton acceptors. It provides a more comprehensive understanding of acid-base reactions compared to the earlier Arrhenius theory.
Buffer Solutions: Buffer solutions are aqueous solutions that resist changes in pH upon the addition of small amounts of an acid or base. They maintain a relatively stable pH and are essential in various chemical and biological applications, including organic chemistry and biochemistry.
Carboxylate Anion: A carboxylate anion is the negatively charged conjugate base of a carboxylic acid, formed when a carboxylic acid loses a proton. It is a key functional group in organic chemistry that plays an important role in the behavior and reactivity of various organic compounds.
Carboxylic Acids: Carboxylic acids are a class of organic compounds containing a carboxyl functional group (-COOH) attached to an alkyl or aryl group. They are characterized by their acidic properties and play a crucial role in various chemical reactions and biological processes.
Carboxylic acids, RCO2H: Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH), where "R" represents an alkyl or aryl group attached to the carbon atom of the carboxyl group. They are known for being acidic due to the ability of the hydroxyl (OH) part of the carboxyl group to release a proton (H+).
Conjugate Bases: A conjugate base is the species formed when an acid loses a proton (H+) in an acid-base reaction. It is the base that remains after an acid has donated a proton to another species, and it has a negative charge one less than the original acid.
Enamines: Enamines are organic compounds formed by the reaction between a secondary amine and an aldehyde or ketone, characterized by the presence of a nitrogen atom connected to a carbon-carbon double bond. They are the result of nucleophilic addition of amines to carbonyl compounds followed by dehydration.
Henderson-Hasselbalch Equation: The Henderson-Hasselbalch equation is a mathematical expression that relates the pH of a solution to the equilibrium concentrations of the conjugate acid-base pair. It is a fundamental tool used to predict and understand acid-base reactions in organic chemistry, biological systems, and various other applications.
Imidazole: Imidazole is a five-membered aromatic heterocyclic organic compound containing two nitrogen atoms. It is an important structural motif found in various biomolecules and is closely related to the topics of organic acids and bases, aromatic heterocycles, polycyclic aromatic compounds, protection of alcohols, basicity of amines, and heterocyclic amines.
Imines: Imines are organic compounds containing a carbon-nitrogen double bond, formed by the condensation reaction between a primary amine and an aldehyde or ketone. They are important intermediates in organic synthesis and have diverse applications in chemistry and biology.
Isoelectric Point: The isoelectric point (pI) is the pH at which a particular molecule or surface carries no net electrical charge. This is a crucial concept in understanding the behavior of organic acids, organic bases, and amino acids.
Isoelectric point (pI): The isoelectric point (pI) is the pH at which an amino acid or a peptide has no net electric charge. At this pH, the molecule does not move in an electric field during electrophoresis.
Lewis Theory: The Lewis theory, developed by Gilbert N. Lewis, provides a conceptual framework for understanding acid-base reactions and the formation of chemical bonds. It focuses on the sharing and transfer of electron pairs, rather than the traditional Arrhenius definition of acids and bases based on the presence of hydrogen ions (H+) or hydroxide ions (OH-) in aqueous solutions.
Phenols: Phenols are a class of organic compounds containing a hydroxyl (-OH) group attached directly to an aromatic ring. They exhibit both acidic and basic properties and are widely found in various natural and synthetic compounds.
Phenoxide Anion: The phenoxide anion is a negatively charged species formed when a phenol (an aromatic alcohol) loses a hydrogen atom from the hydroxyl group. This anionic species is an important intermediate in various organic reactions and has unique properties that distinguish it from other organic anions.
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.
Pyridine: Pyridine is a heterocyclic aromatic organic compound with the chemical formula C₅H₅N. It is a colorless, volatile liquid with a distinctive unpleasant odor, and it is widely used in the production of various chemicals and pharmaceuticals.
Resonance Delocalization: Resonance delocalization is a phenomenon in organic chemistry where the electrons in a molecule are not confined to a single bond or atom, but are instead spread out or delocalized across multiple bonds and atoms. This delocalization of electrons leads to the stabilization of certain molecular structures and the ability to represent them using multiple resonance structures.
Sulfonate Anion: The sulfonate anion is a functional group consisting of a sulfur atom double-bonded to two oxygen atoms, with a negative charge. It is an important moiety in organic chemistry, particularly in the context of organic acids and organic bases.
Sulfonic Acids: Sulfonic acids are a class of organic acids that contain a sulfonic acid group (-SO3H) attached to a carbon atom. They are an important group of compounds in organic chemistry, particularly in the context of organic acids and organic bases.
Zwitterionic Form: The zwitterionic form is a type of molecular structure where a compound contains both positively and negatively charged functional groups, resulting in an overall neutral charge. This form is particularly important in the context of organic acids and organic bases.