15.5a The Chemical Shift in C 13 and Proton NMR | Organic Chemistry

Chad's Prep

20 Sept 201808:40

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TLDRThe video script provides an in-depth explanation of interpreting Nuclear Magnetic Resonance (NMR) spectra, focusing on Carbon-13 (C-13) and Proton (H-1 or H NMR) NMR. It emphasizes that the number of carbon environments in a molecule equals the number of signals in a C-13 NMR spectrum, which is simplified for undergraduates by using spin-decoupled spectra, avoiding complex splitting patterns. The chemical shift, measured in parts per million (ppm), is a key factor in determining the type of carbon environment, such as alkane, alkene, aromatic, or carbonyl. The script also discusses the influence of electronegative atoms and π electrons on the chemical shift, with proximity to these elements causing a signal to appear downfield (to the left on the spectrum). Using propionic acid as an example, the script illustrates how to identify different carbon environments through their chemical shifts. For H NMR, the script explains the importance of chemical shift, the use of TMS (tetramethylsilane) as a reference compound, and how the presence of π electrons and electronegative atoms affect the shift. The example of propionic acid's H NMR spectrum is used to show how different hydrogen environments result in distinct signals, with the carboxylic acid hydrogen appearing downfield due to its proximity to the electronegative oxygen atom.

Takeaways

🧪 **Understanding NMR Spectra**: Carbon-13 (C-13) NMR is often easier to interpret for undergraduates because it typically involves analyzing spin-decoupled spectra, which means no splitting patterns to consider.
📊 **Signal Count**: The number of carbon environments in a molecule is equal to the number of signals observed in the C-13 NMR spectrum.
🔍 **Chemical Shift**: The chemical shift, symbolized by the Greek letter Delta (Δ), is measured in parts per million (ppm) and is influenced by the proximity to π electrons or electronegative atoms.
🧲 **Upfield and Downfield**: Upfield refers to a nucleus that is more shielded by electrons, appearing towards the left on the spectrum, while downfield indicates less shielding and appears more to the right.
🧫 **Reference Compound**: A reference compound is used in NMR to establish a zero point for the spectrum, with the most common being Tetramethylsilane (TMS).
🔑 **Environment Identification**: Chemical shift can help identify the type of carbon environment, such as alkane, alkene, aromatic, carbonyl, or benzene ring carbons.
🌟 **Solvent Peaks**: In an NMR spectrum, solvent peaks, such as those from CDCl3 or DMSO, are often present and should be recognized as not part of the compound being analyzed.
🔬 **Proximities Affecting Shift**: The chemical shift of a proton in proton NMR (H-NMR) is affected by proximity to π electrons and electronegative atoms, with the former being a more significant factor.
📈 **Alkene and Aromatic Shifts**: Alkene and aromatic protons, which are associated with π electrons, typically appear further downfield due to decreased shielding from these electrons.
🔬 **Electronegativity Effect**: Proximity to electronegative atoms pulls electron density away from a nucleus, causing it to be less shielded and appear downfield in the spectrum.
🧬 **Multiplicity in H-NMR**: Unlike C-13 NMR, H-NMR spectra often show splitting patterns due to spin coupling, which provides additional information about the molecule's structure.
🔑 **Equivalence in H-NMR**: Hydrogens in equivalent chemical environments, such as those affected by free rotation or in a symmetrical part of the molecule, will produce the same signal in the H-NMR spectrum.

Q & A

What makes carbon-13 NMR spectroscopy easier to interpret compared to other types of NMR?
-Carbon-13 NMR is easier to interpret because, for undergraduates, it typically involves analyzing spin-decoupled spectra which do not show splitting patterns like doublets or triplets, resulting in less information to process.
How is the number of carbon environments in a molecule related to the number of signals in a carbon-13 NMR spectrum?
-The number of carbon environments in a molecule is equal to the number of signals in the carbon-13 NMR spectrum, which simplifies the interpretation of the spectrum.
What is a chemical shift in NMR spectroscopy, and how is it represented?
-A chemical shift in NMR spectroscopy is the position of a signal relative to a reference compound in the spectrum. It is represented by the Greek letter Delta and is measured in parts per million (ppm).
What is the term used to describe a nucleus that is more shielded by the electrons around it?
-A nucleus that is more shielded by the electrons around it is described as being 'upfield' in the NMR spectrum.
How does the presence of pi electrons affect the chemical shift of a nucleus?
-The presence of pi electrons results in deshielding of the nucleus, causing the signal to appear further downfield (more to the left) in the NMR spectrum.
What reference compound is commonly used in proton NMR (H NMR) spectroscopy, and why is it suitable?
-Tetra-methyl silane (TMS) is commonly used as a reference compound in proton NMR because it has a single type of hydrogen environment and is not very electronegative, which makes it unlikely to have signals appearing to the right of its signal, providing a stable zero point for the spectrum.
How does the proximity to electronegative atoms affect the chemical shift of a proton in proton NMR?
-Proximity to electronegative atoms pulls electron density away from the nucleus, making it less shielded or deshielded, and causing the signal to appear more downfield (to the left) in the proton NMR spectrum.
What is the significance of the number of signals in the H NMR spectrum of propionic acid?
-The number of signals in the H NMR spectrum of propionic acid corresponds to the number of unique hydrogen environments in the molecule. In the case of propionic acid, three signals indicate three unique hydrogen environments.
What does the chemical shift range of zero to around 4.5 ppm typically represent in the proton NMR spectrum of an alkane?
-The chemical shift range of zero to around 4.5 ppm in the proton NMR spectrum of an alkane represents hydrogens that are not near any electronegative atoms and are not associated with pi electrons.
How do the splitting patterns in proton NMR spectra provide additional information about the molecule?
-Splitting patterns in proton NMR spectra provide information about the coupling between protons, which can reveal details about the connectivity and stereochemistry of the molecule.
Why are the signals from the solvent often ignored when analyzing an NMR spectrum of a compound?
-The signals from the solvent are often ignored because they do not pertain to the compound of interest. Solvent signals are usually known and can be easily recognized, allowing the focus to be on the signals that correspond to the compound under study.
What is the term used to describe the effect of pi electrons on the chemical shift in NMR spectroscopy, and how does it influence the position of the signal?
-The term used is 'deshielding'. The presence of pi electrons in a molecule, such as in alkenes or aromatic rings, causes deshielding of the nucleus, which results in the signal appearing further downfield in the NMR spectrum.

Outlines

00:00

🧪 Understanding Carbon-13 NMR Spectroscopy

The first paragraph delves into the basics of interpreting Carbon-13 (C-13) NMR spectra, which is considered easier due to the use of spin-decoupled spectra that simplify the interpretation by eliminating splitting patterns like doublets and triplets. The key takeaways are that the number of carbon environments in a molecule corresponds to the number of signals in the spectrum, and the chemical shift, measured in parts per million (ppm) relative to a reference compound, provides insights into the carbon's environment. The chemical shift is influenced by the proximity to π electrons or electronegative atoms, with upfield signals indicating more electron shielding and downfield signals indicating less. The example of propionic acid is used to illustrate how the three unique carbon environments result in three distinct signals in the spectrum, with the carbonyl carbon appearing significantly downfield due to its connection to an oxygen atom.

05:01

🧬 Proton NMR Spectroscopy: Chemical Shifts and Environments

The second paragraph focuses on Proton (H-1) NMR, which provides more complex spectra due to spin coupling. The chemical shift, again relative to a reference compound (TMS or Tetramethylsilane), is the primary piece of information, indicating the environment of the hydrogen atoms. The chemical shift is influenced by two main factors: proximity to π electrons, which cause downfield shifts due to their motion generating a magnetic field opposing the external field, and proximity to electronegative atoms, which pull electron density away from the nucleus, resulting in less shielding and a downfield shift. The example of propionic acid's H NMR spectrum is used to demonstrate how the three hydrogen environments correspond to three signals, with the carboxylic acid hydrogen showing a significantly downfield signal and the alkane hydrogens appearing upfield, depending on their distance from the electronegative oxygen atom.

Mindmap

Keywords

💡NMR (Nuclear Magnetic Resonance)

Nuclear Magnetic Resonance (NMR) is a spectroscopic technique used to study the magnetic properties of atomic nuclei. In the context of the video, it is used to analyze the structure of organic molecules, specifically through Carbon-13 NMR and Proton NMR, which provide different types of information about a molecule's structure.

💡Spin Decoupled Carbon 13 NMR

Spin decoupled Carbon 13 NMR is a type of NMR where the carbon-hydrogen couplings are removed, simplifying the spectrum. This makes it easier to interpret because there are no splitting patterns such as doublets or triplets, which reduces the complexity of the information to process. It is used to identify the number of unique carbon environments in a molecule.

💡Chemical Shift

The chemical shift in NMR is the position of an NMR signal relative to a reference compound, measured in parts per million (ppm). It indicates the environment of a nucleus within a molecule. A shift towards the left (downfield) signifies less electron shielding, often due to the presence of electronegative atoms or pi electrons, while a shift to the right (upfield) indicates more shielding.

💡Reference Compound

A reference compound in NMR is a substance with a known and stable chemical shift that serves as a benchmark for calibrating the chemical shifts of other compounds in the spectrum. In the video, TMS (tetramethylsilane) is mentioned as a common reference compound due to its non-electronegative nature and single carbon environment.

💡Upfield and Downfield

Upfield and downfield are terms used to describe the relative positions of signals in an NMR spectrum. Upfield refers to signals that are to the left of the reference signal (higher shielding), while downfield refers to signals to the right (less shielding). These terms help in identifying the type of environment a carbon or hydrogen atom is in, such as alkane, alkene, or carbonyl.

💡Alkane, Alkene, and Carbonyl Environments

These are different types of chemical environments for carbon atoms in organic molecules. Alkanes are saturated hydrocarbons, alkenes contain a carbon-carbon double bond, and carbonyls are characterized by a carbon-oxygen double bond. The video explains how the chemical shift in NMR can help distinguish between these environments based on the signal's position.

💡Propionic Acid

Propionic acid is an organic compound with the formula C3H6O2 and is used as an example in the video to illustrate how to interpret an NMR spectrum. It has three carbon atoms, each in a unique environment, resulting in three distinct signals in the carbon 13 NMR spectrum.

💡Proton NMR (H NMR)

Proton NMR, also known as H NMR, is a type of NMR spectroscopy that focuses on the hydrogen atoms in a molecule. It provides information about the different hydrogen environments and their interactions with other atoms. The video discusses how the chemical shift in H NMR can reveal details about the molecule's structure, including the presence of electronegative atoms or pi bonds.

💡Splitting Patterns

Splitting patterns in NMR arise due to the coupling between nuclear spins. They provide information about the number of neighboring protons and their relative positions. In the video, splitting patterns are mentioned as a more complex aspect of interpreting H NMR spectra, which will be discussed after understanding the chemical shifts.

💡Electronegative Atoms

Electronegative atoms are atoms that have a high affinity for electrons, such as oxygen, nitrogen, and halogens. Their presence in a molecule can affect the chemical shift of nearby atoms, causing them to appear downfield in an NMR spectrum. The video explains how the proximity to electronegative atoms influences the chemical shift of hydrogen atoms.

💡Pi Electrons

Pi electrons are electrons that are involved in the formation of a double bond between two atoms. Their movement in a magnetic field can cause deshielding of nearby nuclei, resulting in downfield shifts in the NMR spectrum. The video emphasizes that the proximity to pi electrons is the most significant factor affecting the chemical shift of a proton in H NMR.

Highlights

Focusing on interpreting NMR spectra, starting with carbon-13 NMR which is easier due to the lack of splitting patterns.

Undergraduates typically interpret spin-decoupled carbon-13 NMR spectra.

The number of carbon environments in a molecule equals the number of signals in the spectrum.

Chemical shift is a key concept in NMR, measured in parts per million and symbolized by the Greek letter Delta.

Chemical shift position relative to a reference compound helps determine the type of carbon environment.

Upfield and downfield shifts are influenced by the proximity to pi electrons or electronegative atoms.

The chemical shift can identify the type of carbon, such as alkane, alkyne, alkene, aromatic, or carbonyl.

An example of interpreting propionic acid's carbon-13 NMR spectrum reveals three unique carbon environments.

Solvent signals, such as CDCl3 and DMSO, are commonly seen in NMR spectra and should be ignored when analyzing the compound of interest.

The proximity to electronegative atoms like oxygen causes signals to appear more downfield.

The carbonyl carbon in propionic acid is identified by its downfield signal past 160 ppm.

Proton NMR (H NMR) involves spin-coupled spectra with more complex information processing.

The chemical shift in H NMR is analogous to that in carbon-13 NMR, using TMS as the reference compound.

The presence of pi electrons or electronegative atoms affects the chemical shift of a proton in H NMR.

Shielding and deshielding refer to the electron density around a nucleus, influencing its position in the spectrum.

The H NMR spectrum of propionic acid shows three signals corresponding to three different hydrogen environments.

The carboxylic acid hydrogen in propionic acid is identified by its signal around 12 ppm.

Alkane hydrogens attached to a carbon near an electronegative atom can show up downfield of three in H NMR.

The H NMR spectrum provides information on the number of environments and their chemical shifts, aiding in compound identification.

Transcripts

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Takeaways

Q & A

What makes carbon-13 NMR spectroscopy easier to interpret compared to other types of NMR?

How is the number of carbon environments in a molecule related to the number of signals in a carbon-13 NMR spectrum?

What is a chemical shift in NMR spectroscopy, and how is it represented?

What is the term used to describe a nucleus that is more shielded by the electrons around it?

How does the presence of pi electrons affect the chemical shift of a nucleus?

What reference compound is commonly used in proton NMR (H NMR) spectroscopy, and why is it suitable?

How does the proximity to electronegative atoms affect the chemical shift of a proton in proton NMR?

What is the significance of the number of signals in the H NMR spectrum of propionic acid?

What does the chemical shift range of zero to around 4.5 ppm typically represent in the proton NMR spectrum of an alkane?

How do the splitting patterns in proton NMR spectra provide additional information about the molecule?

Why are the signals from the solvent often ignored when analyzing an NMR spectrum of a compound?

What is the term used to describe the effect of pi electrons on the chemical shift in NMR spectroscopy, and how does it influence the position of the signal?