Chemical Shift In NMR Spectroscopy

The Organic Chemistry Tutor
10 Dec 201815:26
EducationalLearning
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TLDRThis educational video explores the concept of chemical shift in NMR spectroscopy, denoted by delta, which is the ratio of the observed shift in Hertz to the spectrometer's operating frequency in MHz. It explains how electronegativity affects chemical shifts, using halogenated hydrocarbons as examples to illustrate how more electronegative atoms result in higher shifts. The video also covers the impact of electron-withdrawing groups, the position of protons relative to these groups, and the chemical shifts of various functional groups, providing a foundational understanding of NMR spectroscopy for students.

Takeaways
  • ๐Ÿ” The chemical shift (ฮด) is the ratio of the observed shift in hertz to the operating frequency in megahertz, scaled to a range of 0 to 12 by multiplying by 10 to the 6, and is measured in parts per million (ppm).
  • ๐Ÿงฒ Chemical shift values on the x-axis of an NMR spectrum indicate the relative shielding of protons from an external magnetic field, with upfield being towards the right and downfield towards the left.
  • ๐ŸŒŸ Tetramethylsilane (TMS) is a common reference signal in NMR spectroscopy, with a silicon atom and four methyl groups.
  • ๐ŸŒŠ The presence of electronegative atoms like halogens causes protons to have a higher chemical shift, appearing downfield due to the deshielding effect.
  • โš–๏ธ Halogens' electronegativity influences the chemical shift of compounds, with fluorine causing the highest shift, followed by chlorine, bromine, and iodine.
  • ๐Ÿ”ข Chemical shift values for specific compounds are provided, such as methyl fluoride at 4.3 ppm, methyl chloride at 3.1 ppm, and so on.
  • ๐ŸŒฟ The chemical shift of protons in methane is significantly lower at about 1.0 ppm, showing the impact of halogens on increasing the shift.
  • ๐Ÿ”„ The number of electron-withdrawing groups attached to a carbon atom increases the chemical shift of protons, as seen with chloroform at 7.3 ppm compared to dichloromethane at 5.3 ppm.
  • ๐Ÿ“ The position of protons relative to an electron-withdrawing group affects their chemical shift, with those closest to the group having the highest shift.
  • ๐Ÿ”‘ The type of carbon atom (primary, secondary, tertiary) to which a proton is attached influences its chemical shift, with tertiary carbons causing greater deshielding.
  • ๐Ÿ“Š Common functional groups have characteristic chemical shifts, such as carboxylic acids (10-12 ppm), aldehydes (9-10 ppm), and benzene rings (6.5-8.5 ppm).
Q & A
  • What is the chemical shift represented by?

    -The chemical shift is represented by the symbol delta (ฮด) and is the ratio of the observed chemical shift in hertz to the operating frequency of the spectrophotometer in megahertz.

  • How is the chemical shift value adjusted to fall between 0 and 12?

    -To adjust the chemical shift value to fall between 0 and 12, it is multiplied by 10 to the power of 6.

  • What are the units used on the x-axis for chemical shift values?

    -The units used on the x-axis for chemical shift values are parts per million (ppm).

  • What is TMS and why is it used as a reference signal in NMR spectroscopy?

    -TMS stands for Tetramethylsilane, a silicon atom with four methyl groups. It is used as a reference signal in NMR spectroscopy because it is a stable compound with a known and consistent chemical shift.

  • Why does the presence of bromine in methyl bromide result in a higher chemical shift compared to TMS?

    -The presence of bromine, which is more electronegative than silicon, results in a higher chemical shift in methyl bromide because the protons are next to an electron-withdrawing group, causing them to appear downfield in the NMR spectrum.

  • Which halogens have the highest and lowest chemical shifts in their respective methyl compounds?

    -Methyl fluoride has the highest chemical shift, and methyl iodide has the lowest among the halogens in their respective methyl compounds.

  • What is the chemical shift of the protons in methane compared to methyl iodide?

    -The chemical shift of the protons in methane is significantly lower at about 1.0 ppm compared to methyl iodide, which is around 2.2 ppm.

  • How does the number of electron-withdrawing groups affect the chemical shift of a proton?

    -The more electron-withdrawing groups present, the greater the deshielding effect on the proton, resulting in a higher chemical shift.

  • Why do the protons in nitropropane have different chemical shifts?

    -The different chemical shifts in nitropropane are due to the varying distances of the protons from the electron-withdrawing nitro group, with those closest to the group having the highest chemical shift.

  • In the context of NMR spectroscopy, what does 'upfield' and 'downfield' refer to?

    -'Upfield' in NMR spectroscopy refers to the right side of the spectrum where protons that are shielded from an external magnetic field appear. 'Downfield' refers to the left side of the spectrum where protons that are deshielded appear.

  • Why do protons attached to a tertiary carbon have a higher chemical shift than those attached to a primary carbon?

    -Protons attached to a tertiary carbon have a higher chemical shift because the carbon is more electronegative and has more electron-withdrawing groups (other carbon atoms) pulling electrons away, resulting in greater deshielding.

  • What are some common chemical shift ranges for specific functional groups in NMR spectroscopy?

    -Some common chemical shift ranges include carboxylic acid protons (10 to 12 ppm), aldehyde protons (9 to 10 ppm), benzene ring protons (6.5 to 8.5 ppm), and protons on a halogenated carbon (2 to 4.5 ppm depending on the halogen).

Outlines
00:00
๐ŸŒ€ Understanding Chemical Shifts in NMR Spectroscopy

This paragraph introduces the concept of chemical shift, denoted by delta (ฮด), as the ratio of the observed shift in hertz to the operating frequency of the spectrophotometer in megahertz, scaled to a range between 0 and 12 by multiplying by 10 to the power of 6. It explains how chemical shift values are represented on the x-axis in parts per million (ppm) and uses tetramethylsilane (TMS) as a reference signal. The paragraph discusses the impact of electronegativity on chemical shifts, exemplified by comparing methyl bromide and TMS, and explains that electron withdrawal results in a higher (downfield) chemical shift. It poses a question regarding the chemical shift of protons in methyl bromide versus methyl chloride, noting that the former should have a higher shift due to bromine's higher electronegativity. The paragraph concludes with a general trend among halogens, where fluorine compounds would exhibit the highest shifts, followed by chlorine, bromine, and iodine, attributing this to the inductive effect.

05:02
๐Ÿ” Effects of Electron Withdrawing Groups on Proton Chemical Shifts

The second paragraph delves into how the position of protons relative to electron-withdrawing groups influences their chemical shifts. Using nitropropane as an example, it illustrates that the protons closest to the nitro group, a potent electron-withdrawing group, will exhibit the highest chemical shift and appear most downfield in an NMR spectrum. Conversely, those furthest away, like the methyl group at the end of the molecule, will have the lowest shift. The paragraph provides specific chemical shift values for different types of protons in nitropropane and discusses the impact of the number of electron-withdrawing groups on the chemical shift, with trichloromethane showing a higher shift than dichloromethane and methyl chloride due to the increased number of chlorine atoms pulling electrons away from the protons.

10:05
๐Ÿ“Š Comparing Proton Deshielding in Relation to Carbon Chain Structure

This paragraph explores the relationship between the chemical shift of protons and their attachment to carbons of different structural levelsโ€”primary, secondary, and tertiary. It explains that protons attached to tertiary carbons are more deshielded than those on secondary or primary carbons due to the higher electronegativity of carbon compared to hydrogen, which results in greater electron withdrawal. The paragraph uses butanone as an example to discuss which protons will be most deshielded, highlighting the importance of understanding the structural context of protons in determining their chemical shifts. It also provides approximate chemical shift ranges for methyl, methylene, and methane protons to emphasize the point.

15:05
๐Ÿ“š Common Functional Groups and Their Chemical Shifts

The final paragraph provides a brief overview of the chemical shifts associated with common functional groups, urging viewers to consult their textbooks for a comprehensive list. It mentions specific chemical shift ranges for carboxylic acids, aldehydes, benzene rings, methyl groups attached to benzene rings, halogens attached to carbons with protons, and protons directly attached to double and triple bonds. The paragraph concludes by reminding viewers of the importance of knowing these values for exams and ends the video with an invitation to subscribe to the channel.

Mindmap
Keywords
๐Ÿ’กChemical Shift
Chemical shift, denoted by the symbol delta, is a concept central to the video's theme. It refers to the variation in the resonance frequency of a nucleus in a magnetic field due to its chemical environment. Defined as the ratio of the observed frequency shift to the spectrometer's operating frequency, it is typically expressed in parts per million (ppm). The video explains how chemical shift values are calculated and how they are represented on the x-axis of an NMR spectrum, with examples like methyl bromide and tetramethylsilane (TMS).
๐Ÿ’กNMR Spectroscopy
Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique used to determine the structure and dynamics of molecules. The video uses NMR spectroscopy as a context to discuss chemical shifts, explaining how the presence of different functional groups and the electronegativity of atoms can affect the resonance frequency of protons in a molecule, thus influencing the chemical shift.
๐Ÿ’กElectronegativity
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. In the script, electronegativity is used to explain why certain atoms, such as halogens, cause a higher chemical shift in NMR spectra. For example, the video states that bromine is more electronegative than silicon, leading to a higher chemical shift for protons in methyl bromide compared to TMS.
๐Ÿ’กParts Per Million (ppm)
Parts per million (ppm) is a unit used to express the ratio of the chemical shift to the operating frequency of the spectrometer. The video describes how chemical shift values are scaled to fall between 0 and 12 by multiplying by 10^6, resulting in ppm, which is the standard unit for reporting chemical shifts in NMR spectroscopy.
๐Ÿ’กDeshielding
Deshielding is a phenomenon in NMR spectroscopy where a nucleus experiences a weaker shielding from the external magnetic field due to the presence of electronegative atoms or groups. The video explains that deshielded protons appear downfield in the NMR spectrum, as seen with the chemical shift of methyl bromide compared to TMS.
๐Ÿ’กUpfield and Downfield
In the context of NMR spectroscopy, upfield and downfield refer to the position of a signal on the spectrum relative to the reference signal. The video clarifies that upfield is towards the right side of the spectrum and corresponds to lower chemical shifts, while downfield is towards the left and indicates higher chemical shifts, associated with deshielding.
๐Ÿ’กHalogens
Halogens are a group of elements in the periodic table known for their high electronegativity. The video uses halogens to illustrate the inductive effect on chemical shifts, explaining that the presence of a halogen atom attached to a carbon will increase the chemical shift of the protons on that carbon, with fluorine causing the greatest shift.
๐Ÿ’กInductive Effect
The inductive effect is a concept in organic chemistry where the electron-withdrawing nature of a group influences the electron distribution along the carbon chain. The video describes how the inductive effect causes protons attached to a carbon with a more electronegative atom to have a higher chemical shift, thus appearing downfield in the NMR spectrum.
๐Ÿ’กFunctional Groups
Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The video mentions several functional groups, such as carboxylic acids, aldehydes, and benzene rings, and provides typical chemical shift ranges for protons associated with these groups.
๐Ÿ’กMethylene and Methane Protons
Methylene and methane protons refer to the hydrogen atoms attached to methylene (-CH2-) and methane (-CH3) groups, respectively. The video explains that the chemical shift of these protons is influenced by the number of carbon atoms to which they are attached, with methane protons attached to tertiary carbons having higher chemical shifts due to greater deshielding.
Highlights

Chemical shift, represented by delta, is the ratio of the observed chemical shift in hertz to the spectrophotometer's operating frequency in megahertz, multiplied by 10^6 to get a value between 0 and 12.

Chemical shift values are represented on the x-axis of an NMR spectrum in ppm (parts per million).

TMS (tetramethylsilane) is used as a reference signal in NMR spectroscopy.

Methyl bromide has a higher chemical shift than TMS due to bromine's higher electronegativity compared to silicon.

Protons next to an electron-withdrawing group have a higher chemical shift and appear downfield on the NMR spectrum.

Shielded protons appear upfield (right side) on the NMR spectrum, while deshielded protons appear downfield (left side).

Methyl fluoride has the highest chemical shift among halogenated methanes, followed by methyl chloride, methyl bromide, and methyl iodide due to the inductive effect.

The chemical shift values for various halogenated methanes are: methyl fluoride (4.3 ppm), methyl chloride (3.1 ppm), methyl bromide (2.7 ppm), and methyl iodide (2.2 ppm).

The presence of more electron-withdrawing groups on a carbon atom increases the chemical shift of its protons.

The chemical shift of protons increases with the number of electron-withdrawing groups: trichloromethane (7.3 ppm), dichloromethane (5.3 ppm), methyl chloride (3.1 ppm), and methane (1.0 ppm).

The position of protons relative to an electron-withdrawing group affects their chemical shift, with protons closer to the group having a higher shift.

In nitropropane, protons near the NO2 group have a higher chemical shift than those farther away: signal A (4.4 ppm), signal B (2.1 ppm), and signal C (1.0 ppm).

In butanone, the methylene protons have a higher chemical shift than methyl protons due to their position relative to the carbonyl group.

Protons attached to tertiary carbons have higher chemical shifts than those on secondary or primary carbons due to increased deshielding.

Protons on functional groups have characteristic chemical shifts: carboxylic acids (10-12 ppm), aldehydes (9-10 ppm), benzene (6.5-8.5 ppm), methyl groups on benzene rings (2.1-2.3 ppm), and alkenes (4.5-6.5 ppm).

Transcripts
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