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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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20.8 Spectroscopy of Carboxylic Acids and Nitriles

Citation:

Carboxylic acids and nitriles have unique spectroscopic fingerprints. IR spectroscopy reveals distinct absorption bands, while NMR spectroscopy shows characteristic chemical shifts for these functional groups.

These spectroscopic techniques are crucial for identifying and analyzing organic compounds. By understanding the specific signals, chemists can confidently determine the presence of carboxylic acids and nitriles in unknown samples.

Spectroscopic Identification of Carboxylic Acids

Carboxylic acid IR absorptions (infrared spectroscopy)

  • Carboxylic acids have distinct IR absorptions due to their functional group
    • Broad O-H bond stretch absorption band appears in the range of 2500-3300 cm$^{-1}$ caused by hydrogen bonding between carboxylic acid molecules (acetic acid, benzoic acid)
    • Strong, sharp C=O bond stretch absorption band appears in the range of 1700-1730 cm$^{-1}$ with the exact position depending on conjugation or electron-withdrawing groups (formic acid, propionic acid)
  • Presence of both O-H and C=O stretches strongly indicates a carboxylic acid (butyric acid, phthalic acid)

NMR spectra of carboxylic acids (nuclear magnetic resonance spectroscopy)

  • $^{1}$H NMR helps differentiate carboxylic acids from other carbonyl compounds
    • Broad singlet carboxylic acid proton (COOH) appears in the range of 10-13 ppm due to exchange with solvent or other carboxylic acid molecules (valeric acid, cinnamic acid)
    • Aldehydes have a characteristic singlet at 9-10 ppm for the aldehyde proton (CHO) (acetaldehyde, benzaldehyde)
    • Ketones and esters do not have a signal in this downfield region (acetone, ethyl acetate)
  • $^{13}$C NMR provides additional confirmation of a carboxylic acid
    • Carboxylic acid carbonyl carbon (C=O) appears around 170-185 ppm with the specific position depending on substituents and conjugation (hexanoic acid, salicylic acid)
    • Carbonyl carbons of aldehydes, ketones, and esters typically appear at different positions (propanal, butanone, methyl benzoate)
    • Chemical shift of the carbonyl carbon can provide information about the electronic environment

Spectroscopic Identification of Nitriles

IR and NMR signals of nitriles

  • Nitriles have a characteristic IR absorption due to the C$\equiv$N bond
    • Strong, sharp C$\equiv$N stretch absorption band appears in the range of 2200-2260 cm$^{-1}$ with the position being relatively insensitive to substituents (acetonitrile, benzonitrile)
    • Presence of this distinct absorption strongly indicates a nitrile functional group (propionitrile, phthalonitrile)
  • $^{13}$C NMR helps identify nitriles
    • Nitrile carbon (C$\equiv$N) appears in a characteristic region typically in the range of 110-125 ppm, significantly different from other types of carbons (butyronitrile, phenylacetonitrile)
  • $^{1}$H NMR signals of protons adjacent to the nitrile group may be slightly deshielded
    • $\alpha$-protons next to the nitrile often appear around 2.0-3.0 ppm, with the deshielding effect being less pronounced compared to other electron-withdrawing groups (isobutyronitrile, 3-phenylpropionitrile)

Additional Spectroscopic Techniques

  • Mass spectrometry can provide molecular weight information and fragmentation patterns for carboxylic acids and nitriles
  • Functional group analysis using spectroscopic data helps in the structural elucidation of unknown compounds containing carboxylic acid or nitrile groups

Key Terms to Review (40)

Carboxylic acid derivative: Carboxylic acid derivatives are compounds that contain a functional group which is a modified form of the carboxylic acid group (–COOH), where the hydroxyl part (-OH) is replaced by another atom or group of atoms. These derivatives undergo nucleophilic acyl substitution reactions, where an electron-rich nucleophile attacks the carbonyl carbon, leading to the substitution of the leaving group.
Chemical shift: In nuclear magnetic resonance (NMR) spectroscopy, a chemical shift is a measure of the change in the resonant frequency of a nucleus relative to a standard reference. It provides insights into the electronic environment surrounding a nucleus, helping to identify molecular structures.
Mass spectrometry (MS): Mass spectrometry is an analytical technique used in organic chemistry to determine the mass-to-charge ratio of ions. It helps identify the composition of a sample by generating ions and measuring their mass and charge.
Benzaldehyde: Benzaldehyde is an aromatic aldehyde compound with the chemical formula C6H5CHO. It is a colorless liquid with a characteristic almond-like odor and is widely used in the production of various organic compounds, including pharmaceuticals, flavors, and fragrances.
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.
Acetone: Acetone is a simple organic compound with the chemical formula CH3COCH3. It is a colorless, volatile, flammable liquid that is widely used as a solvent and in various chemical processes. Acetone is a key term that is relevant in the context of several important organic chemistry topics.
Acetaldehyde: Acetaldehyde is a colorless, flammable organic compound with the chemical formula CH3CHO. It is the simplest aliphatic aldehyde and is an important intermediate in various chemical processes and metabolic pathways.
Ethyl Acetate: Ethyl acetate is a versatile organic compound that serves as a common solvent, a flavoring agent, and an important intermediate in various chemical reactions. It is the ester formed by the reaction between acetic acid and ethanol, and its diverse applications make it a crucial compound in both laboratory and industrial settings.
Carboxylic Acid: Carboxylic acids are organic compounds characterized by the presence of a carboxyl functional group (-COOH), which consists of a carbonyl (C=O) and a hydroxyl (-OH) group. They are widely found in nature and play a crucial role in various organic chemistry topics.
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.
Butanone: Butanone, also known as methyl ethyl ketone, is an organic compound with the chemical formula CH3C(O)CH3. It is a colorless liquid with a distinctive sweet, acetone-like odor and is commonly used as a solvent and in the production of various chemicals. Butanone is an important functional group in the context of 3.1 Functional Groups and 20.8 Spectroscopy of Carboxylic Acids and Nitriles.
Methyl Benzoate: Methyl benzoate is an organic compound that consists of a benzene ring with a carboxyl group (-COOH) attached, and a methyl group (-CH3) esterified to the carboxyl group. It is a colorless liquid with a sweet, fruity odor and is commonly used as a flavoring agent and in the production of various chemical compounds.
Acetonitrile: Acetonitrile, also known as methyl cyanide, is a colorless, volatile, and flammable organic compound with the chemical formula CH3CN. It is widely used as a solvent in various chemical reactions and analyses, particularly in the context of organic chemistry and biochemistry.
Mass Spectrometry: Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of ions to identify and quantify the chemical composition of a sample. It provides detailed information about the molecular structure and fragmentation patterns of compounds, making it a powerful tool in organic chemistry and various other fields.
Infrared Spectroscopy: Infrared spectroscopy is an analytical technique that uses the infrared region of the electromagnetic spectrum to identify and characterize the chemical composition of a sample. It provides information about the molecular structure and functional groups present in a compound by analyzing the absorption or emission of infrared radiation.
Nuclear Magnetic Resonance Spectroscopy: Nuclear Magnetic Resonance (NMR) Spectroscopy is an analytical technique that uses the magnetic properties of certain atomic nuclei to determine the structure and composition of organic compounds. It is a powerful tool for identifying and characterizing the chemical environment of specific atoms within a molecule.
Chemical Shift: Chemical shift is a fundamental concept in nuclear magnetic resonance (NMR) spectroscopy that describes the position of a signal in the NMR spectrum relative to a reference signal. It provides information about the chemical environment of a nucleus, allowing for the identification and characterization of different functional groups and molecular structures.
Nitrile: A nitrile is a functional group consisting of a carbon-nitrogen triple bond (C≡N). Nitriles are important in organic chemistry, with applications in the synthesis of various compounds and as precursors to other functional groups.
Propanal: Propanal is the simplest aliphatic aldehyde, with the molecular formula C$_{3}$H$_{6}$O. It is a colorless, volatile liquid with a pungent, fruity odor. Propanal is an important organic compound that is closely related to the topics of naming aldehydes and ketones, nucleophilic addition reactions, spectroscopy, and carbonyl condensation reactions.
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.
Butyric Acid: Butyric acid is a short-chain fatty acid with the chemical formula CH3CH2CH2COOH. It is a colorless, oily liquid with a distinctive unpleasant odor, and is found naturally in butter, parmesan cheese, and the digestive tract of humans and animals.
Functional Group Analysis: Functional group analysis is the process of identifying and characterizing the specific functional groups present in a chemical compound. This analytical technique is crucial in organic chemistry for understanding the reactivity, properties, and potential transformations of molecules.
Propionic Acid: Propionic acid is a short-chain carboxylic acid with the chemical formula CH3CH2COOH. It is a colorless liquid with a pungent odor and is commonly used in the food industry as a preservative and in the production of various chemicals.
Propionitrile: Propionitrile is a nitrile compound with the chemical formula CH3CH2CN. It is an organic compound that is widely used in various industrial and chemical applications, particularly in the context of nitrile chemistry and the spectroscopic analysis of carboxylic acids and nitriles.
IR Absorptions: IR (Infrared) absorptions refer to the characteristic patterns of absorption of infrared radiation by different functional groups or molecular structures within organic compounds. These absorption patterns provide valuable information about the chemical composition and structural features of a molecule, which is particularly useful in the spectroscopic analysis of carboxylic acids and nitriles.
Phthalic Acid: Phthalic acid is a dicarboxylic acid with the chemical formula C6H4(COOH)2. It is an important intermediate in the production of various industrial chemicals and is commonly used in the synthesis of plasticizers, dyes, and pharmaceutical compounds.
O-H Bond Stretch: The O-H bond stretch refers to the vibrational mode of the covalent bond between an oxygen atom and a hydrogen atom, which is a key feature in the spectroscopic analysis of carboxylic acids and nitriles. This bond stretch involves the periodic lengthening and shortening of the O-H bond length, resulting in a characteristic absorption band in the infrared spectrum.
Hexanoic Acid: Hexanoic acid, also known as caproic acid, is a six-carbon saturated fatty acid that is commonly found in various natural sources. It is an important compound in the context of the spectroscopy of carboxylic acids and nitriles, as it exhibits characteristic features that can be analyzed using various analytical techniques.
Butyronitrile: Butyronitrile is a colorless, flammable liquid with a characteristic odor. It is an organic compound that belongs to the class of nitriles, which are functional groups containing a carbon-nitrogen triple bond. Butyronitrile is particularly relevant in the context of spectroscopy of carboxylic acids and nitriles.
Phenylacetonitrile: Phenylacetonitrile is an organic compound with the chemical formula C6H5CH2CN. It is a colorless liquid with a characteristic odor and is commonly used as a precursor in the synthesis of various pharmaceutical and agrochemical products.
3-phenylpropionitrile: 3-phenylpropionitrile is an organic compound consisting of a phenyl group attached to a 3-carbon chain with a nitrile functional group at the end. It is an important compound in the study of spectroscopy of carboxylic acids and nitriles.
C$\equiv$N Bond: The C$\equiv$N bond, also known as the cyano or nitrile bond, is a triple covalent bond between a carbon atom and a nitrogen atom. This type of bond is found in various organic compounds, including nitriles, and is an important structural feature in the context of spectroscopy of carboxylic acids and nitriles.
C=O Bond Stretch: The C=O bond stretch refers to the vibrational mode of the carbon-oxygen double bond, which is a key structural feature in various organic compounds such as carboxylic acids and nitriles. This stretching vibration occurs when the carbon and oxygen atoms move closer and farther apart along the bond axis, causing changes in the bond length.
$^{1}$H NMR: $^{1}$H NMR, or proton nuclear magnetic resonance spectroscopy, is an analytical technique used to determine the structure of organic compounds by identifying the unique hydrogen environments within a molecule. It provides valuable information about the chemical shifts, coupling patterns, and integration of hydrogen signals, which are essential for characterizing the structure of carboxylic acids, nitriles, and other organic functional groups.
Valeric Acid: Valeric acid, also known as pentanoic acid, is a saturated straight-chain carboxylic acid with the chemical formula CH3(CH2)3COOH. It is an important organic compound that is studied in the context of spectroscopy of carboxylic acids and nitriles.
Cinnamic Acid: Cinnamic acid is an organic compound with the formula C6H5CH=CHCOOH. It is a white crystalline solid that occurs naturally in various plants and is used in the production of cinnamaldehyde, a key component of cinnamon flavor and fragrance. Cinnamic acid is particularly relevant in the context of 20.8 Spectroscopy of Carboxylic Acids and Nitriles, as it exhibits characteristic spectroscopic properties that can be used to identify and analyze this class of 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.
Isobutyronitrile: Isobutyronitrile is a nitrile compound with the chemical formula (CH3)2CHCN. It is a colorless liquid with a pungent odor and is used in the production of various chemicals and pharmaceuticals. Isobutyronitrile is particularly relevant in the context of spectroscopy of carboxylic acids and nitriles, as it exhibits characteristic signals in various spectroscopic techniques.
Salicylic Acid: Salicylic acid is a naturally occurring organic compound that is classified as a phenolic acid. It is a key ingredient in many over-the-counter medications and is widely used in the treatment of various skin conditions, such as acne, warts, and psoriasis.
Phthalonitrile: Phthalonitrile is a chemical compound consisting of a benzene ring with two nitrile (-CN) functional groups attached. It is an important precursor in the synthesis of phthalocyanine dyes and pigments, which have applications in various industries, including coatings, plastics, and electronics.