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Organic Chemistry
Table of Contents
Unit 20 Overview: Carboxylic Acids and Nitriles
20.1 Naming Carboxylic Acids and Nitriles
20.2 Structure and Properties of Carboxylic Acids
20.3 Biological Acids and the Henderson–Hasselbalch Equation
20.4 Substituent Effects on Acidity
20.5 Preparing Carboxylic Acids
20.6 Reactions of Carboxylic Acids: An Overview
20.7 Chemistry of Nitriles
20.8 Spectroscopy of Carboxylic Acids and Nitriles

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20.2 Structure and Properties of Carboxylic Acids

Citation:

Carboxylic acids form cyclic dimers through hydrogen bonding, boosting their boiling points. This unique structure sets them apart from similar-sized molecules, making them tougher to separate.

These acids pack a punch compared to alcohols, thanks to their stable conjugate bases. The carboxylate ion's negative charge spreads out, giving carboxylic acids more oomph in the acidity department.

Structure and Properties of Carboxylic Acids

Cyclic dimers of carboxylic acids

  • Carboxylic acids form cyclic dimers through intermolecular hydrogen bonding
    • Hydrogen of one carboxyl group attracts to oxygen of another carboxyl group (formic acid, acetic acid)
    • Two carboxylic acid molecules held together by two hydrogen bonds forming a cyclic dimer
  • Formation of cyclic dimers increases boiling points of carboxylic acids compared to similar-sized molecules
    • More energy required to break additional hydrogen bonds and separate dimers
    • Results in higher boiling points than expected based on molecular weight alone (propanoic acid vs propanol)

Acidity of carboxylic acids vs alcohols

  • Carboxylic acids are stronger acids than alcohols
    • Carboxylic acids have lower $pK_a$ values (typically 4-5) compared to alcohols (typically 16-18)
    • Lower $pK_a$ values indicate higher tendency to donate protons and stronger acidity (acetic acid vs ethanol)
  • Carboxylic acids are stronger acids due to stability of their conjugate base (carboxylate ion)
    • Negative charge on carboxylate ion delocalized through resonance stabilizing it as a more favorable species
  • In contrast, conjugate base of an alcohol (alkoxide ion) has localized negative charge on oxygen
    • Alkoxide ion is less stable and less favorable making alcohols weaker acids (sodium ethoxide vs sodium acetate)
  • The inductive effect of the carbonyl group further increases acidity by withdrawing electron density from the O-H bond

Charge delocalization in carboxylate ions

  • Carboxylate ions (RCOO$^-$) distribute negative charge through resonance
  • Two main resonance structures contribute to delocalization of negative charge:
    1. Single bond between C and O, negative charge on O$^-$
    2. Double bond between C and O, negative charge on the other O$^-$
  • Actual structure of carboxylate ion is a hybrid of these two resonance structures
    • Negative charge evenly distributed between the two oxygen atoms
    • Both C-O bonds have bond order of 1.5 between a single and double bond (acetate ion, benzoate ion)
  • Delocalization of negative charge stabilizes the carboxylate ion
    • Increased stability contributes to stronger acidity of carboxylic acids compared to alcohols

Structure and Reactivity of Carboxylic Acids

  • Carboxylic acids contain a carbonyl group (C=O) directly bonded to a hydroxyl group (-OH)
  • The alpha hydrogen, located on the carbon adjacent to the carbonyl group, can participate in various reactions
  • Carboxylic acids can form various carboxylic acid derivatives through reactions at the carbonyl group
  • The oxygen of the carbonyl group acts as a hydrogen bond acceptor, contributing to the formation of intermolecular hydrogen bonds

Key Terms to Review (28)

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.
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+).
Alkoxide ion, RO–: An alkoxide ion is the conjugate base of an alcohol, formed by the deprotonation of the hydroxyl group (OH) in an alcohol molecule, resulting in a negatively charged oxygen atom bonded to an alkyl group (R). It plays a crucial role in various organic reactions, especially as a strong nucleophile.
Carboxylic acid derivatives: Carboxylic acid derivatives are compounds that feature a functional group derived from the substitution of the hydroxyl (-OH) group in carboxylic acids with other atoms or groups of atoms. They retain the carbonyl (C=O) group but differ in the atom or group attached to the acyl moiety.
Resonance Structures: Resonance structures are a set of contributing structures that describe the delocalization of electrons in a molecule. They represent the different ways in which the atoms in a molecule can be bonded to satisfy the octet rule and create the most stable arrangement of electrons.
Acetate Ion: The acetate ion is the conjugate base of acetic acid, formed when a proton is removed from the carboxyl group. It is a key species in many organic chemistry reactions and processes, particularly those involving carboxylic acids and their derivatives.
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.
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.
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.
Carboxylate Ion: The carboxylate ion is a negatively charged species formed when a carboxylic acid loses a proton (H+). It is a key intermediate in various organic chemistry reactions and plays a crucial role in understanding the structure and properties of carboxylic acids, as well as their reactions and the chemistry of acid anhydrides.
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.
Acetic Acid: Acetic acid is a weak organic acid with the chemical formula CH3COOH. It is a colorless liquid with a distinctive sour odor and is the main component of vinegar. Acetic acid is a versatile compound that plays important roles in various organic chemistry topics, including functional groups, oxidation of alkenes, reduction of carbonyl compounds, naming of carboxylic acids, and the chemistry of esters.
Boiling Points: Boiling point is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid, and bubbles of vapor form inside the liquid. This property is crucial in understanding the behavior and characteristics of various organic compounds, including alkanes, alcohols, carboxylic acids, and amines.
Sodium Ethoxide: Sodium ethoxide is an alkoxide compound with the chemical formula C₂H₅ONa. It is a strong nucleophile and base used in various organic reactions, including the preparation of alkenes, the Wittig reaction, and Claisen condensations.
Charge Delocalization: Charge delocalization refers to the distribution or spreading of an electrical charge across multiple atoms or a molecular structure, rather than being localized on a single atom. This phenomenon is crucial in understanding the stability and reactivity of various chemical species, including carbocations, carboxylic acids, and their substituents.
Alkoxide Ion: An alkoxide ion is a negatively charged species formed when an alkyl group (R-) is bonded to an oxygen atom. It is a key intermediate in various organic chemistry reactions, including the preparation of ethers, nucleophilic addition reactions of aldehydes and ketones, and the hydration of carboxylic acids.
Formic Acid: Formic acid is the simplest carboxylic acid, with the chemical formula HCOOH. It is a colorless, pungent liquid that is found naturally in the venom of ants and is also produced synthetically. Formic acid is an important chemical compound that is relevant in the context of naming carboxylic acids and nitriles, understanding the structure and properties of carboxylic acids, and analyzing the spectroscopy of carboxylic acids and nitriles.
Hydrogen Bond Acceptor: A hydrogen bond acceptor is an atom or functional group that has a lone pair of electrons available to form a hydrogen bond with a hydrogen atom that is covalently bonded to another electronegative atom, such as oxygen or nitrogen. These acceptors play a crucial role in the structure and properties of carboxylic acids.
RCOO-: RCOO- is the carboxylate ion, which is the negatively charged conjugate base of a carboxylic acid. It is a key functional group in the structure and properties of carboxylic acids, as it contributes to the acidity, reactivity, and biological importance of these organic compounds.
Benzoate Ion: The benzoate ion is the conjugate base of benzoic acid, formed when a hydrogen atom is removed from the carboxyl group. It is a common anion found in various food preservatives and pharmaceutical compounds.
Alpha Hydrogen: The alpha hydrogen refers to the hydrogen atom that is bonded to the carbon atom adjacent to a carbonyl group (C=O) in organic molecules. This term is particularly relevant in the context of understanding the structure and properties of carboxylic acids, as well as the acidity of alpha hydrogen atoms and the formation of enolate ions.
Intermolecular Hydrogen Bonding: Intermolecular hydrogen bonding is a type of dipole-dipole attraction that occurs between a hydrogen atom covalently bonded to a highly electronegative element, such as oxygen, nitrogen, or fluorine, and another nearby highly electronegative atom. This type of intermolecular force has a significant impact on the physical and chemical properties of molecules, particularly in the context of carboxylic acids.
Propanol: Propanol, also known as propyl alcohol, is a three-carbon primary alcohol with the chemical formula C3H7OH. It is an important organic compound that is widely used in various industrial and commercial applications, particularly in the context of carboxylic acids and their properties.
Cyclic Dimers: Cyclic dimers are a type of molecular structure formed by the dimerization of carboxylic acid groups. In this process, two carboxylic acid molecules come together and form a cyclic structure through the creation of two covalent bonds between the carbonyl carbon and the hydroxyl oxygen of the adjacent carboxylic acid group.
Propanoic Acid: Propanoic acid, also known as propionic acid, is a carboxylic acid with the chemical formula CH3CH2COOH. It is a colorless liquid with a pungent odor and is widely used in various industries and applications.
Carboxylic Acid Derivatives: Carboxylic acid derivatives are a class of organic compounds that are structurally related to carboxylic acids, but have a different functional group attached to the carbonyl carbon. These derivatives include esters, acyl halides, anhydrides, and amides, and they play crucial roles in various chemical reactions and biological processes.
Sodium Acetate: Sodium acetate is a salt formed by the neutralization of acetic acid with sodium hydroxide. It is a common chemical compound with various applications in organic chemistry and related fields.
Carbonyl group: A carbonyl group is a functional group characterized by a carbon atom double-bonded to an oxygen atom, represented as C=O. This group is pivotal in organic chemistry as it forms the backbone of various important classes of compounds, influencing their chemical properties and reactivity.