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12.7 Interpreting Infrared Spectra

4 min read•may 7, 2024

Infrared spectroscopy is a powerful tool for identifying in organic molecules. By analyzing the absorption of specific wavelengths, chemists can deduce structural information about compounds, distinguishing between similar molecules and deducing complex structures.

Understanding IR spectra is crucial for characterizing organic compounds. From in to C=O vibrations in carbonyls, each functional group has a unique spectral fingerprint. This knowledge enables chemists to unravel molecular structures and confirm synthetic products.

Interpreting Infrared Spectra

Functional groups in IR spectra

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  • O-H stretching vibrations
    • Alcohols and absorb in the range of 3200-3600 cm−1^{-1} (methanol, ethanol)
    • exhibit a broad absorption band from 2500-3300 cm−1^{-1} due to (acetic acid, benzoic acid)
  • vibrations
    • and display between 3300-3500 cm−1^{-1} (methylamine, acetamide)
  • vibrations
    • show absorption peaks in the range of 2850-3000 cm−1^{-1} (hexane, cyclohexane)
    • absorb between 3010-3100 cm−1^{-1} due to the presence of sp2^2 hybridized carbons (ethene, 1-butene)
    • exhibit C-H stretching vibrations from 3000-3100 cm−1^{-1} (benzene, toluene)
    • have a characteristic absorption peak around 3300 cm−1^{-1} resulting from the C-H stretching of the sp hybridized carbon (ethyne, 1-butyne)
  • vibrations
    • and absorb strongly between 1690-1760 cm−1^{-1} (acetaldehyde, acetone)
    • Carboxylic acids show a strong C=O stretching band from 1700-1730 cm−1^{-1} (formic acid, propionic acid)
    • display a characteristic absorption peak in the range of 1735-1750 cm−1^{-1} (ethyl acetate, methyl benzoate)
    • Amides exhibit C=O stretching vibrations between 1640-1690 cm−1^{-1} (formamide, acetamide)
  • vibrations
    • Alkenes absorb in the range of 1620-1680 cm−1^{-1} due to the presence of C=C double bonds (1-pentene, cyclopentene)
    • Aromatic compounds show characteristic absorption bands between 1450-1600 cm−1^{-1} resulting from the conjugated C=C bonds (naphthalene, anthracene)
  • vibrations
    • Alkynes exhibit a strong absorption peak in the range of 2100-2260 cm−1^{-1} due to the C≡C triple bond (1-hexyne, diphenylacetylene)
  • vibrations
    • Alcohols, ethers, and esters display absorption bands between 1050-1300 cm−1^{-1} corresponding to the C-O single bond stretching (ethanol, diethyl ether, ethyl acetate)

Comparison of similar compounds

  • Alcohols vs phenols
    • Alcohols exhibit a strong, broad O-H stretching peak around 3300-3400 cm−1^{-1} due to intermolecular hydrogen bonding (1-propanol, 2-butanol)
    • Phenols display a sharp O-H stretching peak between 3200-3600 cm−1^{-1} and characteristic aromatic C=C stretching peaks around 1450-1600 cm−1^{-1} (phenol, 4-methylphenol)
  • Aldehydes vs
    • Aldehydes show unique C-H stretching vibrations from 2700-2900 cm−1^{-1} and a strong C=O stretching peak around 1720-1740 cm−1^{-1} (propanal, benzaldehyde)
    • Ketones lack the aldehyde C-H stretching vibrations and exhibit a strong C=O stretching peak between 1705-1725 cm−1^{-1} (2-butanone, cyclohexanone)
  • Primary vs secondary vs tertiary alcohols
    1. Primary alcohols have a strong, broad O-H stretching peak and a strong C-O stretching peak around 1050 cm−1^{-1} (1-butanol, 1-hexanol)
    2. Secondary alcohols display a strong, broad O-H stretching peak and a medium intensity C-O stretching peak around 1100 cm−1^{-1} (2-propanol, 2-pentanol)
    3. Tertiary alcohols exhibit a strong, broad O-H stretching peak and a weak C-O stretching peak around 1150 cm−1^{-1} (2-methyl-2-propanol, 2-methyl-2-butanol)

Structural deduction from IR data

  • Identify the presence or absence of key functional groups based on their characteristic absorption bands (carboxylic acid in acetic acid, ester in ethyl acetate)
  • Determine the relative number of hydrogens attached to sp3^3, sp2^2, and sp hybridized carbons by comparing the intensities of C-H stretching peaks (more sp3^3 C-H in hexane compared to 1-hexene)
  • Distinguish between conjugated and non-conjugated systems by observing the shift in C=O and C=C stretching frequencies
    • lowers the frequency of C=O and C=C stretching vibrations (conjugated C=O in benzoic acid vs non-conjugated C=O in acetic acid)
  • Recognize the presence of hydrogen bonding by observing the broadening and shifting of O-H and N-H stretching bands (broad O-H stretching in ethanol due to intermolecular hydrogen bonding)
  • Identify the presence of in molecules by the absence of certain vibrational modes
    • lack IR active asymmetric stretching and bending vibrations (no IR active asymmetric stretching in carbon dioxide)

Principles of IR Spectroscopy

  • IR spectroscopy is based on the absorption of by molecules, causing
  • The relates the absorption of light to the concentration of the absorbing species and path length
  • (FTIR) is a modern technique that improves the quality and speed of IR measurements
  • Different functional groups absorb IR radiation at characteristic frequencies, allowing for their identification in organic compounds

Key Terms to Review (39)

Absorption Bands: Absorption bands are specific regions within the infrared (IR) spectrum where a molecule absorbs particular wavelengths of infrared radiation. These distinct absorption patterns are a result of the molecule's unique vibrational and rotational energy levels, which are characteristic of its chemical structure and functional groups.
Alcohols: Alcohols are organic compounds containing a hydroxyl (-OH) functional group attached to a saturated carbon atom. They are widely used in various chemical reactions and have diverse applications in industry, medicine, and everyday life.
Aldehydes: Aldehydes are a class of organic compounds characterized by the presence of a carbonyl group (C=O) with a hydrogen atom attached to the carbon. They are important intermediates in many chemical reactions and have a wide range of applications in various industries, from pharmaceuticals to fragrances.
Alkanes: Alkanes are a class of saturated hydrocarbons composed entirely of single-bonded carbon and hydrogen atoms. They are the simplest organic compounds and serve as the foundation for many other organic molecules and reactions.
Alkenes: Alkenes are a class of unsaturated organic compounds characterized by the presence of a carbon-carbon double bond. They are an important functional group in organic chemistry, with a wide range of applications and reactivity. Alkenes are closely related to the topics of chirality, isomerism, electrophilic addition reactions, halogenation, hydration, the E2 reaction, infrared spectroscopy, 13C NMR spectroscopy, alcohol preparation, and the Wittig reaction.
Alkynes: Alkynes are a class of organic compounds characterized by the presence of a carbon-carbon triple bond. They are an important family of hydrocarbons with unique chemical properties and applications in various fields, including organic synthesis, materials science, and fuel production.
Amides: Amides are a class of organic compounds that contain a carbonyl group (C=O) bonded to a nitrogen atom. They are derived from carboxylic acids and can be considered the result of replacing the hydroxyl group (-OH) of a carboxylic acid with an amino group (-NH2). Amides are important functional groups in many organic molecules, including proteins, and play a crucial role in various chemical reactions and processes.
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 Compounds: Aromatic compounds are a class of organic compounds characterized by the presence of one or more benzene rings in their structure. These compounds exhibit unique chemical and physical properties that set them apart from other organic molecules.
Beer-Lambert Law: The Beer-Lambert law, also known as Beer's law, is a fundamental relationship in spectroscopy that describes the attenuation of light as it passes through a medium. It establishes a direct correlation between the concentration of an absorbing species in a solution and the amount of light absorbed by that solution.
C-H Stretching: C-H stretching refers to the vibrational mode of the carbon-hydrogen bond in organic molecules, which can be detected and analyzed using infrared (IR) spectroscopy. This characteristic absorption band provides valuable information about the presence and nature of C-H bonds in a compound.
C-O Stretching: C-O stretching refers to the vibrational mode of the carbon-oxygen bond in organic compounds, which can be observed and analyzed using infrared (IR) spectroscopy. This term is particularly relevant in the context of understanding the spectroscopic properties of alcohols, phenols, and ethers.
C=C Stretching: C=C stretching refers to the vibrational mode of a carbon-carbon double bond in a molecule, which involves the periodic stretching and contraction of the double bond along the bond axis. This vibrational mode is an important characteristic in the analysis of organic compounds using infrared (IR) spectroscopy.
C=O Stretching: C=O stretching refers to the vibration of the carbon-oxygen double bond, which is a characteristic absorption in the infrared (IR) spectrum. This vibrational mode is particularly important in the analysis and identification of organic compounds containing carbonyl groups.
C≡C Stretching: C≡C stretching refers to the vibrational mode of a carbon-carbon triple bond, where the two carbon atoms move towards and away from each other along the bond axis. This characteristic vibration is an important feature in the interpretation of infrared (IR) spectra.
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+).
Centrosymmetric Molecules: Centrosymmetric molecules are a class of molecules that possess a center of symmetry, also known as an inversion center or a center of inversion. This means that for every atom in the molecule, there exists an identical atom located on the opposite side of the molecule, equidistant from the center of symmetry.
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.
Electromagnetic Radiation: Electromagnetic radiation is a form of energy that propagates through space and matter in the form of oscillating electric and magnetic fields. It encompasses a wide spectrum of wavelengths and frequencies, including visible light, infrared, ultraviolet, X-rays, and radio waves, among others.
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.
Esters: Esters are a class of organic compounds formed by the reaction between a carboxylic acid and an alcohol, resulting in the replacement of the hydroxyl group (-OH) of the acid with an alkoxy group (-OR). Esters are ubiquitous in nature and play a crucial role in various chemical processes and applications.
Fingerprint region: In infrared spectroscopy, the fingerprint region refers to the area of an infrared spectrum, typically below 1500 cm^-1, characterized by a complex pattern of peaks unique to each compound. This region is used to identify specific compounds by comparing their spectra with known references.
Fingerprint Region: The fingerprint region, also known as the fingerprint or the far-infrared region, is a critical part of an infrared (IR) spectrum that provides unique information about the molecular structure of a compound. This region, typically located between 500-1500 cm^-1, is characterized by the presence of numerous, complex, and overlapping absorption bands that are specific to the vibrations of individual bonds and functional groups within a molecule.
Fourier Transform Infrared Spectroscopy: Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify and quantify the chemical composition of a sample by measuring its interaction with infrared radiation. It provides detailed information about the molecular structure and functional groups present in a compound.
Functional Groups: Functional groups are specific arrangements of atoms within a molecule that determine the chemical reactivity and physical properties of that molecule. These groups play a crucial role in understanding and predicting the behavior of organic compounds.
Hydrogen Bonding: Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom covalently bonded to a highly electronegative element, such as nitrogen, oxygen, or fluorine, experiences an attractive force with another nearby highly electronegative element. This intermolecular force is stronger than a typical dipole-dipole interaction and has a significant impact on the physical and chemical properties of many organic compounds.
Ketones: Ketones are organic compounds characterized by a carbonyl group (C=O) bonded to two other carbon atoms within the molecule. They are formed by the oxidation of secondary alcohols.
Ketones: Ketones are a class of organic compounds containing a carbonyl group (C=O) bonded to two alkyl or aryl groups. They are characterized by the presence of a carbonyl carbon flanked by two carbon atoms. Ketones are important in various organic chemistry topics, including chirality, oxidation reactions, mass spectrometry, infrared spectroscopy, and NMR spectroscopy.
Molecular Vibrations: Molecular vibrations refer to the oscillatory motion of atoms within a molecule around their equilibrium positions. This dynamic behavior of molecules is a fundamental aspect of spectroscopy and is crucial for understanding infrared and other types of molecular spectra.
N-H Stretching: N-H stretching refers to the vibration of the nitrogen-hydrogen bond in organic compounds, particularly in the context of infrared spectroscopy and the analysis of amines. This vibration occurs when the N-H bond is stretched and compressed, resulting in a characteristic absorption band in the infrared spectrum.
O-H Stretching: O-H stretching refers to the vibration of the covalent bond between an oxygen atom and a hydrogen atom, which occurs when the bond is stretched and compressed during molecular vibrations. This phenomenon is particularly important in the context of spectroscopic techniques used to analyze organic compounds.
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.
Sp Hybridization: sp Hybridization is a concept in organic chemistry that describes the formation of hybrid atomic orbitals through the combination of one s orbital and one p orbital, resulting in the creation of two equivalent sp hybrid orbitals. This hybridization is particularly important in understanding the structure and bonding patterns of certain organic compounds, such as alkynes.
Sp² Hybridization: sp² hybridization is a type of orbital hybridization in which one s orbital and two p orbitals of an atom combine to form three equivalent sp² hybrid orbitals. This hybridization is commonly observed in carbon compounds with trigonal planar geometry, such as in the benzene ring and carbonyl groups.
Sp³ Hybridization: sp³ hybridization is a type of atomic orbital hybridization that occurs in molecules with a central atom that forms four covalent bonds, such as methane (CH₄). It involves the mixing of one s orbital and three p orbitals to create four equivalent sp³ hybrid orbitals, which allow the central atom to participate in tetrahedral molecular geometries.
Symmetry: Symmetry refers to the balanced and proportional arrangement of the parts of an object or system. It is a fundamental concept in chemistry that describes the spatial arrangement and orientation of atoms or molecules within a compound.
Wavenumber: Wavenumber is a fundamental concept in spectroscopy that represents the number of waves per unit distance, typically measured in inverse centimeters (cm^-1). It is an important parameter in understanding the interaction between matter and electromagnetic radiation, particularly in the context of infrared (IR) and Raman spectroscopy.
Wavenumber, 𝛎˜: Wavenumber is a measure of the number of wavelengths per unit distance, typically expressed in inverse centimeters (cm\(^{-1}\)). In the context of infrared spectroscopy, it quantifies the frequency at which molecules absorb infrared light, helping identify molecular structures.
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12.8 Infrared Spectra of Some Common Functional Groups