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12.8 Infrared Spectra of Some Common Functional Groups

2 min read•may 7, 2024

is a powerful tool for identifying in organic compounds. By analyzing absorption patterns, we can determine the presence of , , , and based on their unique vibrational frequencies.

like , , , and have distinct IR absorption patterns. These patterns help distinguish between similar structures and provide valuable information about a molecule's composition and bonding.

Infrared Spectroscopy and Functional Group Identification

IR absorptions of functional groups

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  • Alkenes ([C=C](https://www.fiveableKeyTerm:C=C)[C=C](https://www.fiveableKeyTerm:C=C))
    • Weak absorption around 1600-1680 cm−1^{-1} caused by of the carbon-carbon double bond
    • Out-of-plane of the bonds at 800-1000 cm−1^{-1} (cis-2-butene)
  • Alkynes (C≡CC\equiv C)
    • Weak absorption at 2100-2260 cm−1^{-1} resulting from stretching of the carbon-carbon triple bond
    • Absence of signal at 3300 cm−1^{-1} which distinguishes from terminal alkynes (2-butyne)
  • Alcohols ([O−H](https://www.fiveableKeyTerm:O−H)[O-H](https://www.fiveableKeyTerm:O-H))
    • Strong, broad absorption at 3200-3600 cm−1^{-1} attributed to stretching of the O-H bond
    • Stretching of the bond at 1050-1150 cm−1^{-1} (ethanol)
  • Alkanes (C−HC-H)
    • Strong absorption at 2850-3000 cm−1^{-1} caused by stretching of the C-H bonds
    • Bending vibrations of the C-H bonds at 1450-1470 cm−1^{-1} and 1370-1390 cm−1^{-1} (propane)

Functional group analysis in IR spectra

  • Identify functional groups by their characteristic absorption frequencies
    • Compare observed peaks to known absorption ranges for each functional group (alcohols: 3200-3600 cm−1^{-1})
  • Absence of expected peaks indicates the absence of the corresponding functional group
    • No strong, broad absorption at 3200-3600 cm−1^{-1} suggests the compound lacks an O−HO-H group (hexane)
  • Presence of unexpected peaks may indicate the presence of additional functional groups
    • A strong peak at 1700-1750 cm−1^{-1} suggests the compound contains a carbonyl group (acetone)
  • Intensity of absorption peaks is related to the change in during

Carbonyl Compounds and Their IR Absorptions

Carbonyl compounds in IR spectroscopy

  • Aldehydes (R−CHOR-CHO)
    • Strong stretching of the [C=O](https://www.fiveableKeyTerm:C=O)[C=O](https://www.fiveableKeyTerm:C=O) bond at 1720-1740 cm−1^{-1}
    • Weak stretching of the C-H bond at 2700-2800 cm−1^{-1} and 2820-2900 cm−1^{-1} (propanal)
  • (R−CO−R′R-CO-R')
    • Strong stretching of the C=OC=O bond at 1705-1725 cm−1^{-1}
    • Absence of C-H stretching at 2700-2900 cm−1^{-1} which distinguishes from aldehydes (2-butanone)
  • Carboxylic acids (R−COOHR-COOH)
    • Strong, broad stretching of the O−HO-H bond at 2500-3300 cm−1^{-1}
    • Strong stretching of the C=OC=O bond at 1700-1730 cm−1^{-1}
    • Stretching of the C-O bond at 1210-1320 cm−1^{-1} (acetic acid)
  • Esters (R−COO−R′R-COO-R')
    • Strong stretching of the C=OC=O bond at 1735-1750 cm−1^{-1}
    • Stretching of the C-O bond at 1000-1300 cm−1^{-1}
    • Absence of O-H stretching which distinguishes from carboxylic acids (ethyl acetate)

Principles of Infrared Spectroscopy

  • Infrared spectroscopy is a type of that uses in the infrared region
  • It measures the absorption of infrared light by molecules, which causes molecular vibrations
  • is a modern technique that improves the quality and speed of IR spectra collection

Key Terms to Review (30)

Absorption Spectroscopy: Absorption spectroscopy is a technique that measures the absorption of electromagnetic radiation by a sample as a function of wavelength or frequency. It is a powerful analytical tool used to identify and quantify the composition of materials by analyzing the specific wavelengths of light they absorb.
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.
Bending: Bending refers to the change in the angle between chemical bonds in a molecule, resulting in a distortion of the molecule's geometry. This term is particularly relevant in the context of infrared (IR) spectroscopy, where the bending of molecular bonds can produce characteristic absorption bands in the IR spectrum.
C-H: C-H, or carbon-hydrogen, refers to the covalent bond between a carbon atom and a hydrogen atom. This bond is a fundamental structural component in organic chemistry, as it is present in a wide range of organic compounds and plays a crucial role in their properties and reactivity.
C-O: The C-O bond refers to the covalent bond formed between a carbon (C) atom and an oxygen (O) atom. This bond is a fundamental structural feature in many organic compounds and plays a crucial role in understanding the infrared spectra of common functional groups.
C=C: The carbon-carbon double bond, denoted as C=C, is a fundamental structural feature in organic chemistry. It represents two covalent bonds between two carbon atoms, creating a double bond that is central to understanding concepts like degree of unsaturation and the infrared spectra of functional groups.
C=O: The C=O functional group, also known as the carbonyl group, consists of a carbon atom double-bonded to an oxygen atom. This group is a key structural feature in many organic compounds and plays a crucial role in their chemical and physical properties.
C≡C: The carbon-carbon triple bond, represented as C≡C, is a covalent bond in which three pairs of electrons are shared between two carbon atoms. This type of bond is found in various organic compounds and is an important structural feature in the context of infrared spectra of functional groups.
Carbonyl Compounds: Carbonyl compounds are a class of organic compounds that contain a carbon-oxygen double bond (C=O), known as the carbonyl group. This functional group is found in a variety of important molecules, including aldehydes, ketones, carboxylic acids, esters, and amides, which are all integral to many organic chemistry topics and reactions.
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+).
Cm⁻¹: cm⁻¹ is a unit of wavenumber, which is the reciprocal of the wavelength of electromagnetic radiation. It is commonly used to express the frequency of infrared and Raman spectroscopy, providing information about the vibrational modes of molecules.
Dipole Moment: Dipole moment is a measure of the separation of electric charge within a molecule or chemical bond. It represents the magnitude and direction of the unequal distribution of positive and negative charges, which can influence the behavior and properties of a molecule.
Dipole moment (𝜇): A dipole moment is a measure of the separation of positive and negative electrical charges within a molecule, indicating the polarity of a bond or molecule. It is quantitatively expressed in units of Debye (D) and results from differences in electronegativity between bonded atoms.
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.
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.
Fourier Transform Infrared Spectroscopy (FTIR): Fourier Transform Infrared Spectroscopy (FTIR) is an analytical technique used to identify and study the molecular structure of organic and inorganic compounds. It works by measuring the absorption of infrared radiation as it passes through a sample, providing information about the chemical bonds and functional groups present in the material.
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.
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.
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.
O-H: The O-H functional group, also known as the hydroxyl group, consists of an oxygen atom bonded to a hydrogen atom. This group is a key structural feature in many organic compounds and plays a crucial role in their physical and chemical properties, particularly in the context of infrared spectra and the structure and properties of amines.
Stretching: Stretching refers to the vibration of chemical bonds within a molecule that causes changes in the bond length, leading to the absorption or emission of infrared radiation. This term is particularly relevant in the context of understanding the infrared spectra of common functional groups, as the stretching vibrations of specific bonds can provide valuable information about the molecular structure.
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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