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Organic Chemistry
Table of Contents
Unit 13 Overview: NMR Spectroscopy for Structure Determination
13.1 Nuclear Magnetic Resonance Spectroscopy
13.2 The Nature of NMR Absorptions
13.3 Chemical Shifts
13.4 Chemical Shifts in 1H NMR Spectroscopy
13.5 Integration of 1H NMR Absorptions: Proton Counting
13.6 Spin–Spin Splitting in 1H NMR Spectra
13.7 1H NMR Spectroscopy and Proton Equivalence
13.8 More Complex Spin–Spin Splitting Patterns
13.9 Uses of 1H NMR Spectroscopy
13.10 13C NMR Spectroscopy: Signal Averaging and FT–NMR
13.11 Characteristics of 13C NMR Spectroscopy
13.12 DEPT 13C NMR Spectroscopy
13.13 Uses of 13C NMR Spectroscopy

Carbon-13 NMR spectroscopy is a powerful tool for identifying unique carbon environments in molecules. By analyzing chemical shifts and signal patterns, chemists can deduce structural information about organic compounds.

This technique relies on the principles of nuclear magnetic resonance, using the carbon-13 isotope to generate spectra. Understanding factors like electronegativity, hybridization, and molecular symmetry helps interpret these spectra and uncover molecular structures.

13C NMR Spectroscopy

Interpretation of 13C NMR spectra

  • Each non-equivalent carbon atom in a molecule produces a separate signal in the 13C NMR spectrum
    • Non-equivalent carbons are in different chemical environments (different bonding arrangements or neighboring atoms)
    • Equivalent carbons have the same chemical shift and appear as a single signal (symmetrical molecules or identical substituents)
  • The number of signals in a 13C NMR spectrum corresponds to the number of non-equivalent carbons in the molecule
    • Count the signals to determine the number of unique carbon environments (ethanol has 2 signals, indicating 2 non-equivalent carbons)
  • Chemical shift values can be used to identify specific types of carbons
    • Alkanes: 0-50 ppm (methane at 0 ppm, longer chains at higher values)
    • Alkenes and aromatic compounds: 100-150 ppm (benzene at 128 ppm, ethylene at 123 ppm)
    • Alkynes: 60-90 ppm (acetylene at 74 ppm)
    • Alcohols and ethers: 50-90 ppm (methanol at 49 ppm, diethyl ether at 66 ppm)
    • Aldehydes: 190-200 ppm (formaldehyde at 193 ppm)
    • Ketones: 170-210 ppm (acetone at 206 ppm)
    • Carboxylic acids: 160-185 ppm (acetic acid at 178 ppm)
    • Esters: 160-180 ppm (ethyl acetate at 171 ppm)
    • Amines: 30-60 ppm (methylamine at 27 ppm)

Factors affecting chemical shifts

  • Electronegativity of neighboring atoms influences the chemical shift of a carbon
    • Electronegative atoms (O, N, F, Cl) deshield the carbon, causing a downfield shift to higher ppm values
    • Electropositive atoms (alkyl groups) shield the carbon, causing an upfield shift to lower ppm values
  • Hybridization of the carbon atom affects the chemical shift
    • $sp^3$ hybridized carbons have the lowest chemical shift (0-50 ppm) due to greater shielding
    • $sp^2$ hybridized carbons have higher chemical shifts (100-150 ppm) due to less shielding
    • $sp$ hybridized carbons have chemical shifts between $sp^3$ and $sp^2$ (60-90 ppm)
  • The combined effect of electronegativity and hybridization determines the final chemical shift of a carbon
    • An $sp^3$ carbon bonded to an electronegative atom will have a higher chemical shift than a simple alkane carbon
    • An $sp^2$ carbon in an aromatic ring will have a different shift than an $sp^2$ carbon in an isolated alkene

Molecular symmetry in NMR signals

  • Molecules with high symmetry have fewer non-equivalent carbons and, therefore, fewer signals in the 13C NMR spectrum
    • Benzene (C6H6) has six equivalent carbons due to its high symmetry, resulting in a single signal at 128 ppm
    • Cyclohexane (C6H12) has only one signal due to its highly symmetric chair conformation
  • Molecules with low symmetry have more non-equivalent carbons and, consequently, more signals in the 13C NMR spectrum
    • 1,2-Dichlorobenzene (C6H4Cl2) has four non-equivalent carbons due to its lower symmetry, resulting in four distinct signals
    • 2-Butanol (C4H10O) has four signals due to its asymmetric structure and lack of symmetry
  • Symmetrical substitution patterns lead to fewer signals compared to asymmetrical substitution patterns
    • 1,4-Dimethylbenzene has two signals (two sets of equivalent carbons), while 1,2-dimethylbenzene has four signals (four non-equivalent carbons)
    • para-Substituted benzenes generally have fewer signals than ortho- or meta-substituted benzenes

Advanced 13C NMR Techniques

  • Nuclear magnetic resonance (NMR) spectroscopy utilizes the carbon-13 isotope for 13C NMR analysis
  • Spin-spin coupling between carbon atoms can provide additional structural information
  • Proton decoupling is often employed to simplify 13C NMR spectra by removing carbon-proton coupling
  • Relaxation time of carbon nuclei affects signal intensity and acquisition time
  • Fourier transform NMR allows for rapid data collection and improved sensitivity in 13C NMR experiments

Key Terms to Review (44)

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+).
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.
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.
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.
Electronegativity (EN): Electronegativity is a measure of an atom's ability to attract and hold onto electrons when it is part of a compound. The higher the electronegativity value, the more strongly an atom can pull electrons towards itself.
Hybridization: Hybridization is a fundamental concept in chemistry that describes the process of mixing atomic orbitals to form new hybrid orbitals, which are used to explain the geometry and bonding patterns of molecules. This term is closely related to the development of chemical bonding theory, valence bond theory, and molecular orbital theory, as well as the structure and properties of various organic compounds.
Electronegativity: Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. It is a fundamental concept in understanding the nature and strength of chemical bonds, as well as predicting the polarity and reactivity of molecules.
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.
Benzene: Benzene is a planar, aromatic hydrocarbon compound with the chemical formula C6H6. It is a key building block in organic chemistry and has a unique resonance structure that contributes to its stability and reactivity.
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.
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.
Ethers: Ethers are a class of organic compounds characterized by an oxygen atom connected to two alkyl or aryl groups. They are widely used in various chemical processes and have diverse applications in industry, medicine, and everyday life.
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.
Cyclohexane: Cyclohexane is a saturated, cyclic hydrocarbon compound with the chemical formula C6H12. It is a key component in understanding various aspects of organic chemistry, including the naming and stability of cycloalkanes, conformational analysis, and its role in the structure and properties of aromatic compounds and steroids.
Molecular Symmetry: Molecular symmetry refers to the arrangement and orientation of atoms within a molecule that allows for the identification of symmetry elements such as planes, axes, and centers of symmetry. This concept is crucial in understanding the conformations of molecules, their handedness, and the characteristics of nuclear magnetic resonance (NMR) spectroscopy.
2-Butanol: 2-Butanol is a secondary alcohol with the chemical formula CH3CH2CHCH3. It is an isomer of 1-butanol, with the hydroxyl group (-OH) attached to the second carbon atom in the butane chain. This positioning of the hydroxyl group gives 2-butanol unique properties and reactivity compared to other butanol isomers.
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.
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.
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.
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.
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.
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.
Spin-Spin Coupling: Spin-spin coupling, also known as J-coupling, is a phenomenon in nuclear magnetic resonance (NMR) spectroscopy where the magnetic moments of adjacent nuclei interact with each other, leading to the splitting of NMR signals. This interaction provides valuable information about the structure and connectivity of molecules.
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.
Nuclear Magnetic Resonance: Nuclear magnetic resonance (NMR) is a powerful analytical technique that uses the magnetic properties of atomic nuclei to provide detailed information about the structure and composition of chemical compounds. This technique is widely used in organic chemistry to identify and characterize organic molecules.
Relaxation Time: Relaxation time is a fundamental concept in nuclear magnetic resonance (NMR) spectroscopy that describes the time it takes for the nuclear spin system to return to its equilibrium state after being perturbed by a radiofrequency (RF) pulse. This relaxation process is crucial for understanding the characteristics and applications of both 1H NMR and 13C NMR spectroscopy.
Shield: In the context of 13C NMR spectroscopy, a shield refers to the shielding effect that surrounding electrons have on the carbon nucleus, which influences the chemical shift observed in the spectrum. The shielding effect is a key characteristic that helps provide information about the chemical environment and structure of the carbon-containing molecule.
Downfield Shift: A downfield shift, also known as a deshielding effect, refers to the phenomenon in nuclear magnetic resonance (NMR) spectroscopy where the signal for a particular nucleus, such as carbon-13 (13C) or hydrogen (1H), appears at a higher ppm (parts per million) value on the NMR spectrum. This shift is caused by a decrease in the effective shielding of the nucleus, resulting in a higher frequency of the observed signal.
Upfield Shift: An upfield shift, in the context of 13C NMR spectroscopy, refers to the phenomenon where the resonance signal of a carbon nucleus appears at a higher frequency (lower chemical shift value) on the NMR spectrum compared to the expected position for that type of carbon. This shift is influenced by the electronic environment surrounding the carbon atom.
Sp^3 Hybridized Carbons: sp^3 hybridized carbons are a type of carbon atom that has formed four equivalent bonds by combining one s orbital and three p orbitals. This results in a tetrahedral geometry around the carbon atom with bond angles of approximately 109.5 degrees.
Equivalent Carbons: Equivalent carbons are carbon atoms in a molecule that have the same chemical environment and, as a result, exhibit the same chemical shift in a 13C NMR spectrum. These carbons are indistinguishable from one another and produce a single signal in the NMR spectrum.
1,2-Dichlorobenzene: 1,2-Dichlorobenzene is an aromatic organic compound with the chemical formula C6H4Cl2. It is a colorless liquid with a characteristic sweet, almond-like odor and is used in various industrial applications. In the context of 13.11 Characteristics of 13C NMR Spectroscopy, 1,2-dichlorobenzene serves as an important reference compound for understanding the chemical shifts and coupling patterns observed in the 13C NMR spectra of aromatic compounds.
Ortho-Substituted Benzenes: Ortho-substituted benzenes refer to aromatic compounds where a substituent group is attached to the benzene ring in the position adjacent to another substituent. This positioning of the substituents has significant implications for the characteristics of 13C NMR spectroscopy.
Deshield: Deshielding refers to the reduction or removal of the shielding effect experienced by a nucleus in a molecule, resulting in a change in the chemical shift observed in NMR spectroscopy. This term is particularly relevant in the context of 13C NMR spectroscopy and DEPT 13C NMR spectroscopy.
Fourier Transform NMR: Fourier transform NMR (FT-NMR) is a technique in nuclear magnetic resonance (NMR) spectroscopy that utilizes a Fourier transform algorithm to rapidly acquire and process NMR signals, providing detailed information about the chemical structure and properties of molecules.
Proton Decoupling: Proton decoupling is a technique used in 13C NMR spectroscopy to simplify the complex carbon-13 spectra by removing the splitting patterns caused by the coupling between carbon and hydrogen nuclei. This allows for the observation of pure carbon-13 signals, making the spectra easier to interpret and analyze.
Meta-Substituted Benzenes: Meta-substituted benzenes refer to aromatic compounds where the substituent groups are positioned at the 1 and 3 positions on the benzene ring, creating a 1,3-disubstitution pattern. These compounds exhibit unique characteristics that are particularly relevant in the context of 13C NMR spectroscopy.
Sp^2 Hybridized Carbons: sp^2 hybridized carbons are a type of carbon atom that has three sp^2 hybrid orbitals and one remaining p orbital. This configuration is commonly found in alkenes, aromatic compounds, and other planar molecules.
Non-Equivalent Carbons: Non-equivalent carbons are carbon atoms within a molecule that have distinct chemical environments, resulting in unique signals in the 13C NMR spectrum. These carbons do not share the same chemical shift, multiplicity, or coupling patterns, making them distinguishable in the spectroscopic analysis.
Sp Hybridized Carbons: sp Hybridized Carbons refer to carbon atoms that have undergone sp hybridization, a type of orbital hybridization where the carbon's s and p orbitals combine to form two equivalent sp hybrid orbitals. This hybridization is particularly relevant in the context of 13C NMR Spectroscopy, as the unique electronic environment of sp hybridized carbons influences their chemical shifts and coupling patterns in the 13C NMR spectrum.
1,4-dimethylbenzene: 1,4-dimethylbenzene, also known as p-xylene, is an aromatic hydrocarbon compound with two methyl groups attached to a benzene ring in the para position. It is an important industrial chemical and a precursor for various organic compounds.
Para-Substituted Benzenes: Para-substituted benzenes refer to aromatic compounds where a substituent is attached to the benzene ring in the para position, or the 1,4 orientation. This structural arrangement has important implications for the compound's physical and chemical properties, particularly in the context of 13C NMR spectroscopy.
Carbon-13 Isotope: The carbon-13 isotope, denoted as $^{13}$C, is a naturally occurring stable isotope of carbon with a higher mass compared to the more abundant carbon-12 isotope. This isotope plays a crucial role in nuclear magnetic resonance (NMR) spectroscopy, particularly in the analysis and characterization of organic compounds.
1,2-dimethylbenzene: 1,2-dimethylbenzene, also known as o-xylene, is an aromatic hydrocarbon compound with two methyl groups attached to the benzene ring in the ortho position. It is an important industrial chemical and a component of gasoline and other fuels.