15.6b Interpreting NMR Example 2 | Organic Chemistry

Chad's Prep
20 Sept 201807:43
EducationalLearning
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TLDRThe video script presents a detailed analysis of a chemical compound with the formula C11H1402. The speaker begins by calculating the compound's degree of unsaturation, which is found to be 5, indicating a combination of pi bonds and rings. A benzene ring is identified as a likely component due to its contribution to the unsaturation count. The script then delves into the interpretation of the compound's carbon and proton nuclear magnetic resonance (NMR) spectra. Key observations include the identification of a t-butyl group, a carboxylic acid signal, and a para-substituted benzene ring. The final structure is deduced by piecing together the fragments, with the benzene ring serving as the central piece, flanked by the carboxylic acid and t-butyl group at the ends. The summary underscores the systematic approach to solving complex spectral analysis problems in organic chemistry.

Takeaways
  • 🧮 **Degrees of Unsaturation**: The compound has a molecular formula of C11H14O2, which translates to 5 degrees of unsaturation, indicating a combination of pi bonds and/or rings.
  • 🔍 **Benzene Ring Presence**: A quick way to achieve 4 degrees of unsaturation is through a benzene ring, which is later confirmed by the analysis.
  • 📉 **Carbon Spectrum Analysis**: The carbon spectrum reveals signals in the alkane and aromatic regions, with one carbonyl carbon downfield of 160 ppm.
  • 🌟 **Aromatic Hydrogens**: The H NMR spectrum shows four signals, with one downfield indicative of a carboxylic acid hydrogen, and the rest in the aromatic region.
  • 🔢 **Integration Ratios**: The integration of the H NMR signals is in the ratio of 1:2:2:2:2:9, which corresponds to the number of hydrogens in the molecule.
  • 🏔️ **t-Butyl Group Identification**: The alkane signal representing nine hydrogens suggests the presence of a t-butyl group attached to a carbon capable of free rotation.
  • ⚖️ **Molecular Formula Confirmation**: All atoms in the molecular formula are accounted for, including the carboxylic acid group and the t-butyl group.
  • 🔄 **Symmetry in Aromatic Substitution**: The para substitution on the benzene ring is indicated by the symmetry and the presence of two doublets in the H NMR spectrum.
  • 🔗 **Connecting Fragments**: The benzene ring, with multiple attachment points, is placed in the middle of the molecule, with the carboxylic acid and t-butyl group at the ends.
  • 🧩 **Puzzle Assembly Approach**: The process of determining the structure is likened to assembling a puzzle, emphasizing the importance of having all fragments before connecting them.
  • 📚 **Exam Strategy**: The speaker suggests common practices for exams, such as looking for methyl groups first in the alkane region and considering symmetry in aromatic substitutions.
Q & A

  • What is the molecular formula given for the second compound in the transcript?

    -The molecular formula given for the second compound is C11H1402.

  • How many degrees of unsaturation does the compound have?

    -The compound has 5 degrees of unsaturation, which can be a combination of pi bonds and rings.

  • What is a quick way to achieve 4 degrees of unsaturation?

    -A quick way to achieve 4 degrees of unsaturation is to have a benzene ring.

  • How many signals are found in the alkane range in the carbon spectrum?

    -There are two signals found in the alkane range in the carbon spectrum.

  • What does the carbon signal downfield of 160 indicate?

    -The carbon signal downfield of 160 indicates a carbon double-bonded to an oxygen, likely a carbonyl group.

  • How many total signals are there in the H NMR spectrum?

    -There are a total of four signals in the H NMR spectrum.

  • What does the signal downfield of 10 in the H NMR spectrum typically indicate?

    -The signal downfield of 10 in the H NMR spectrum typically indicates a carboxylic acid hydrogen.

  • What is the integration ratio of the signals in the H NMR spectrum?

    -The integration ratio of the signals in the H NMR spectrum is 1:2:2:2:2:2:9.

  • How many equivalent hydrogens are represented by the singlet signal in the alkane region?

    -The singlet signal in the alkane region represents nine equivalent hydrogens, likely three equivalent methyl groups attached to the same carbon.

  • What substitution pattern on the benzene ring results in only two signals in the aromatic region?

    -Para substitution on the benzene ring results in only two signals in the aromatic region due to symmetry.

  • How many hydrogens does a para-substituted benzene ring have?

    -A para-substituted benzene ring has four hydrogens, with two hydrogens being equivalent on each of the non-substituted carbons.

  • How are the carboxylic acid group and the t-butyl group positioned relative to the benzene ring in the final structure?

    -In the final structure, the carboxylic acid group and the t-butyl group are attached to opposite ends of the benzene ring, with the benzene ring in the middle due to its multiple attachment points.

Outlines
00:00
🔍 Analyzing the Compound's Structure and Degrees of Unsaturation

The first paragraph introduces the molecular formula C11H1402 and emphasizes the importance of calculating the degrees of unsaturation, which is found to be 5. This suggests a combination of pi bonds and rings. The presence of a benzene ring is inferred due to its contribution to the unsaturation count. The carbon spectrum reveals signals in the alkane and aromatic regions, with one carbonyl carbon. The H NMR spectrum shows four signals, including one for a carboxylic acid, two for aromatic hydrogens, and one in the alkane region. The integration ratio of 1:2:2:2:2:2:9 is discussed, indicating the ratio of hydrogen atoms. A t-butyl group is identified in the alkane region, and the uniqueness of each fragment in assembling the molecule is highlighted.

05:03
🧬 Determining the Benzene Substitution Pattern

The second paragraph focuses on the benzene ring's substitution pattern. It is established that the benzene ring is para-disubstituted, as indicated by the two doublets in the aromatic region of the H NMR spectrum. The symmetry of the para-substituted benzene ring results in only two unique sets of hydrogens, which is consistent with the observed signals. The paragraph also discusses how to integrate the carboxylic acid and t-butyl group into the molecule, concluding that the benzene ring, with its multiple attachment points, must be central, while the other two groups are positioned at the ends of the molecule.

Mindmap
Keywords
💡Degrees of Unsaturation
The degrees of unsaturation is a calculation used in organic chemistry to determine the number of pi bonds and rings present in a molecule. It is defined as half the product of twice the number of carbon atoms plus twice the number of nitrogen atoms minus the number of hydrogen atoms. In the video, the compound C11H1402 has a calculated degree of unsaturation of 5, which suggests a combination of pi bonds and rings. This concept is central to understanding the structure of the compound being discussed.
💡Benzene Ring
A benzene ring is a type of aromatic ring structure with the molecular formula C6H6, consisting of six carbon atoms joined in a hexagonal ring with alternating single and double bonds. It is a key structural element in many organic compounds. In the video, the presence of a benzene ring is inferred from the degree of unsaturation and confirmed through the analysis of the carbon spectrum, which is crucial for deducing the overall structure of the compound.
💡Carbonyl Carbon
A carbonyl carbon refers to a carbon atom double-bonded to an oxygen atom, which is characteristic of carbonyl groups found in aldehydes, ketones, and carboxylic acids. In the context of the video, the carbonyl carbon is identified by its signal downfield of 160 in the carbon spectrum, indicating its presence in the compound and contributing to the overall molecular structure.
💡H NMR (Proton Nuclear Magnetic Resonance)
H NMR is a type of spectroscopy that provides detailed information about the hydrogen atoms in a molecule, including their chemical environment and how they are connected to other atoms. The video discusses the interpretation of H NMR spectra, which reveals the presence of different types of hydrogen atoms, such as those in alkane, aromatic, and carboxylic acid regions, essential for piecing together the molecular structure.
💡Chemical Shifts
Chemical shifts in NMR spectroscopy refer to the resonance frequencies of nuclei relative to a standard in a magnetic field. They provide information about the electronic environment surrounding a nucleus, which can be used to infer structural details. In the video, chemical shifts are used to identify different types of hydrogen atoms, such as those in the alkane and aromatic regions, helping to deduce the structure of the compound.
💡Integration
Integration in NMR spectroscopy is a measure of the area under a signal peak, which corresponds to the number of nuclei of the same type contributing to that signal. It is used to determine the ratio of different types of hydrogen atoms in a molecule. In the video, the integration ratio of 1:2:2:2:2:9 is given, which helps to identify the number of hydrogen atoms in different parts of the molecule.
💡Methyl Groups
Methyl groups are the simplest alkyl groups with the chemical formula CH3. They are often found attached to other carbon atoms in organic molecules. In the video, the presence of a singlet peak in the alkane region with an integration of 9 suggests the presence of three equivalent methyl groups attached to the same carbon, which is a key part of the molecular structure.
💡Carboxylic Acid
A carboxylic acid is an organic compound containing the carboxyl group (-COOH). It is characterized by an acidic hydrogen atom that can be replaced by metal ions in neutralization reactions. In the video, the carboxylic acid is identified by its signal downfield of 10 in the H NMR spectrum and is a component of the compound's structure.
💡Para Substitution
Para substitution refers to the arrangement of two substituents on opposite sides of a benzene ring, with two carbon atoms between them. This results in symmetry, causing certain hydrogen atoms to be equivalent and thus appearing as fewer signals in the NMR spectrum. In the video, the para substitution pattern is deduced from the aromatic region of the H NMR spectrum, showing two doublets, which helps in determining the structure of the benzene ring.
💡Aromatic Hydrogens
Aromatic hydrogens are the hydrogen atoms attached to the carbon atoms in an aromatic ring, such as benzene. Their chemical environment is distinct from hydrogens in aliphatic or alkane regions due to the delocalization of pi electrons. In the video, the aromatic hydrogens are identified by their chemical shifts in the H NMR spectrum, which aids in understanding the substitution pattern on the benzene ring.
💡T-Butyl Group
A t-butyl group, or tertiary butyl group, is a structural unit derived from tert-butane, with the formula C4H9. It is characterized by three methyl groups attached to a central carbon atom. In the video, the t-butyl group is inferred from the alkane signal with nine equivalent hydrogens, which is a significant part of the final molecular structure.
Highlights

Calculate the degrees of unsaturation for the compound with formula C11H1402 to determine the presence of pi bonds and rings.

Quickly achieve 4 degrees of unsaturation by having a benzene ring in the molecule.

Examine the carbon spectrum to identify signals in the alkane, aromatic, and carbonyl regions.

Total of 7 signals in the carbon spectrum, including 2 alkane, 4 aromatic, and 1 carbonyl.

Analyze the H NMR spectrum with 4 signals, including a carboxylic acid hydrogen and aromatic hydrogens.

Integration of signals in the H NMR spectrum reveals a 1:2:2:2:2:2:9 ratio, summing to 14 hydrogens.

Identify a singlet peak in the alkane region representing 9 equivalent hydrogens from 3 methyl groups attached to the same carbon.

The adjacent carbon to the methyl groups has no hydrogens attached, indicating a quaternary carbon.

Assemble the puzzle pieces by first confirming all atoms and functional groups are accounted for in the molecular formula.

The benzene ring, with multiple attachment points, is the central piece of the molecule.

The carboxylic acid and t-butyl groups, with single attachment points, must be at the ends of the molecule.

Determine the substitution pattern of the benzene ring based on the number of signals in the aromatic region of the H NMR spectrum.

Para substitution is indicated by 2 doublets representing 2 equivalent hydrogens each in the aromatic region.

Ortho and meta substitution patterns would result in 4 unique signals in the aromatic region, which is not observed.

Confirm the presence of all 11 carbons, 14 hydrogens, and the carboxylic acid group in the molecular formula before assembling the structure.

Assemble the molecule by attaching the carboxylic acid to one end and the t-butyl group to the other, with the benzene ring in the center.

The final structure is depicted in the provided H NMR and carbon spectra.

Transcripts

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Related Tags
Organic ChemistryNMR SpectroscopyMolecular StructureBenzene RingChemical AnalysisDegrees of UnsaturationPara SubstitutionAromatic HydrogensCarboxylic AcidT-butyl GroupChemical ShiftsMolecular Puzzle