Glossary

24.10 Spectroscopy of Amines

3 min read•may 7, 2024

Amines, essential organic compounds containing nitrogen, exhibit unique spectroscopic properties. reveals distinct patterns for primary, secondary, and . These patterns provide crucial information about amine structure and bonding.

NMR spectroscopy offers further insights into amine structures. , , and nearby hydrogens display characteristic shifts and integrations. 's and fragmentation patterns round out the toolkit for identifying and analyzing amines.

Spectroscopic Analysis of Amines

Interpretation of IR spectra for amines

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  • (RNH2)
    • Exhibit two distinct N-H stretching absorptions due to symmetric and asymmetric stretching modes
      • appears at lower wavenumbers around 3300-3500 cm-1 (ethylamine)
      • occurs at higher wavenumbers between 3400-3500 cm-1 (propylamine)
  • (R2NH)
    • Display a single N-H stretching absorption as only one N-H bond is present
      • Absorption falls within the range of 3300-3400 cm-1 (diethylamine)
  • Tertiary amines (R3N)
    • Lack N-H stretching absorptions altogether since no N-H bonds are present (triethylamine)

Analysis of amine 1H NMR spectra

  • N-H signals
    • Primary amines (RNH2)
      • Appear as a broad singlet between 1-2 ppm (methylamine)
      • Integrate for 2 hydrogens corresponding to the two protons attached to nitrogen
    • Secondary amines (R2NH)
      • Also present as a broad singlet in the 1-2 ppm range (dipropylamine)
      • Integrate for only 1 hydrogen due to the single N-H proton
    • Tertiary amines (R3N)
      • Completely lack an N-H signal as no protons are directly bonded to nitrogen (tributylamine)
  • N-methyl groups
    • Appear as a sharp singlet between 2.2-2.4 ppm (dimethylamine)
    • Integrate for 3 hydrogens per N-methyl group present in the amine
  • Effect of nitrogen on nearby hydrogens
    • positioned adjacent to nitrogen experience a downfield shift
      • Deshielding effect of nitrogen causes signals to appear at higher ppm values (ethylmethylamine)
    • and those further away from nitrogen are progressively less affected
      • Signals appear closer to their typical aliphatic positions (propylamine)
  • and provide valuable information about the amine structure and environment

Nitrogen rule in amine mass spectrometry

  • Nitrogen rule relates (M+) mass to the number of nitrogen atoms present
    • Odd M+ mass signifies an odd number of nitrogen atoms in the amine (methylamine)
    • Even M+ mass indicates an even number of nitrogen atoms, including the possibility of no nitrogen (ethylamine)
  • Characteristic patterns aid in identifying the type of amine
    • Primary amines (RNH2) often show a loss of ⋅\cdotCH2NH2 radical (ethylamine)
    • Secondary amines (R2NH) typically lose the larger alkyl group as a radical (⋅\cdotR) (diethylamine)
    • Tertiary amines (R3N) lack a characteristic α cleavage pattern
      • Fragmentation depends on the specific alkyl groups present (triethylamine)
  • The in the mass spectrum often corresponds to the most stable fragment ion

Additional Spectroscopic Techniques for Amine Analysis

  • can be used to detect conjugated systems in aromatic amines
  • is a common observed in mass spectrometry of amines with γ-hydrogen atoms

Key Terms to Review (27)

1H NMR Spectroscopy: 1H NMR (Proton Nuclear Magnetic Resonance) Spectroscopy is an analytical technique used to identify and characterize organic compounds by detecting the magnetic properties of hydrogen (proton) nuclei within a molecule. It provides valuable information about the structure, connectivity, and environment of hydrogen atoms in a sample.
Asymmetric Stretch: The asymmetric stretch is a type of molecular vibration that occurs when atoms in a molecule move in opposite directions along the same axis, resulting in a change in the molecule's shape or geometry. This vibrational mode is particularly relevant in the context of spectroscopy of amines, as it can provide valuable information about the structure and bonding of these organic compounds.
Base peak: The base peak is the most intense peak in a mass spectrum, representing the ion that is the most abundantly produced during mass spectrometry. It serves as a reference point with its intensity set at 100% for comparing other peaks in the spectrum.
Base Peak: The base peak is the most intense or tallest peak in a mass spectrum, representing the fragment ion with the greatest abundance. It is a crucial feature in interpreting mass spectra and understanding the fragmentation patterns of molecules.
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.
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.
Coupling Constants: Coupling constants, in the context of nuclear magnetic resonance (NMR) spectroscopy, refer to the quantitative measure of the interaction between nuclear spins within a molecule. They provide valuable information about the connectivity and spatial arrangement of atoms within a compound, which is crucial for understanding its structure and properties.
Fragmentation Pattern: The fragmentation pattern refers to the characteristic breakdown or splitting of a molecule into smaller fragments when subjected to analytical techniques such as mass spectrometry. This pattern provides valuable information about the structure and composition of the original molecule.
IR Spectroscopy: IR spectroscopy is a technique that uses infrared radiation to identify and analyze the molecular structure of organic compounds. It provides information about the vibrational modes of chemical bonds, allowing for the identification of functional groups and the determination of the overall structure of a molecule.
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.
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.
McLafferty rearrangement: The McLafferty rearrangement is a reaction observed in mass spectrometry where a molecule undergoes fragmentation, transferring a hydrogen atom to form a double bond, resulting in a neutral and an ionized fragment. This process aids in identifying the structure of organic compounds by analyzing the resulting mass spectrum.
McLafferty Rearrangement: The McLafferty rearrangement is a type of fragmentation reaction that occurs during mass spectrometry analysis, particularly for organic compounds containing carbonyl groups. It involves the rearrangement of a molecule's structure to form a stable ion, which provides valuable information about the compound's structure.
Molecular ion: A molecular ion is a molecule that has gained or lost an electron, resulting in a charged entity. In the context of mass spectrometry, it's often the starting point for fragmentation processes that help identify the structure of the molecule.
Molecular Ion: The molecular ion, also known as the parent ion, is the intact, positively charged molecule that is formed during the ionization process in mass spectrometry. It represents the molecular weight of the analyte and is the starting point for further fragmentation and analysis.
N-H Signals: N-H signals refer to the characteristic signals observed in the nuclear magnetic resonance (NMR) spectra of compounds containing nitrogen-hydrogen bonds, such as amines. These signals provide important information about the structure and environment of the nitrogen-containing functional groups in organic molecules.
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.
N-methyl groups: N-methyl groups are a type of functional group in organic chemistry where a methyl (CH3) group is attached to a nitrogen atom. These groups are commonly found in various organic compounds, including amines, and have important implications for their spectroscopic properties.
Nitrogen Rule: The nitrogen rule is a principle used in mass spectrometry to determine the number of nitrogen atoms present in an organic compound based on the molecular ion peak and the isotopic peak pattern. It is a crucial concept in interpreting mass spectra, understanding the mass spectrometry of common functional groups, and analyzing the spectroscopy of amines.
Primary Amines: Primary amines are organic compounds containing a nitrogen atom bonded to two hydrogen atoms and one alkyl or aryl group. They are a class of amines that play a crucial role in various organic chemistry topics, including the chemistry of amides, the structure and properties of amines, the synthesis of amines, the reactions of amines, and the spectroscopy of amines.
Secondary Amines: Secondary amines are a class of organic compounds that contain a nitrogen atom bonded to two alkyl or aryl groups. They are characterized by the presence of two carbon-nitrogen bonds, distinguishing them from primary amines which have one carbon-nitrogen bond and tertiary amines which have three carbon-nitrogen bonds.
Symmetric Stretch: A symmetric stretch is a type of molecular vibration where the bonds in a molecule stretch and contract in a symmetrical manner, resulting in no net change in the molecule's dipole moment. This concept is particularly important in the context of understanding the spectroscopy of amines, as it can provide insights into the structure and behavior of these compounds.
Tertiary Amines: Tertiary amines are a class of organic compounds where a nitrogen atom is bonded to three alkyl or aryl groups. They are an important subgroup of amines, which are organic compounds containing a nitrogen atom with a lone pair of electrons.
UV-visible Spectroscopy: UV-visible spectroscopy is an analytical technique that measures the absorption or reflection of ultraviolet and visible light by a sample. It is a widely used method for identifying and quantifying organic compounds, particularly those with conjugated systems, due to their characteristic absorption patterns in the UV-visible region of the electromagnetic spectrum.
α Cleavage: α Cleavage, also known as α-scission, is a type of bond cleavage that occurs in organic chemistry, particularly in the context of mass spectrometry and the fragmentation of molecules. It involves the breaking of a carbon-carbon bond adjacent to a heteroatom, such as nitrogen, oxygen, or sulfur, resulting in the formation of two fragment ions.
α-Hydrogens: α-Hydrogens refer to the hydrogen atoms attached to the carbon atom directly adjacent to a functional group or heteroatom in an organic compound. These hydrogens are of particular importance in the spectroscopy and reactivity of various organic molecules, including amines.
β-Hydrogens: β-Hydrogens refer to the hydrogen atoms located on the carbon atom that is two positions away from a functional group or atom of interest. These hydrogens play a crucial role in the spectroscopic analysis of organic compounds, particularly in the context of amine functional groups.
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