3.3.15.2 Carbon-13 NMR Spectroscopy

• Carbon-13 NMR spectroscopy is used to determine the number and types of carbon environments
• What information does carbon-13 NMR provide about a molecule?

  Types of carbon environments
• What happens to the chemical shift when shielding occurs?

  Shifts to lower values
• Chemical shifts in carbon-13 NMR depend on the degree of shielding or deshielding of carbon atoms.
• What is used as the standard reference for chemical shifts in carbon-13 NMR spectroscopy?

  TMS
• What type of groups cause deshielding in carbon-13 NMR spectroscopy?

  Electron-withdrawing groups
• The type of carbon environment affects the chemical properties and reactivity of a molecule.


  True
• What isotope is targeted in carbon-13 NMR spectroscopy?

  Carbon-13
• What is the standard reference for chemical shifts in carbon-13 NMR spectroscopy?

  TMS
• Shielding in carbon-13 NMR shifts the signal to higher ppm values.

  False
• Match the carbon environment with its chemical shift range:

  Alkyl Halide (C-X) ↔️ 20-80 ppm

  Alcohol (C-OH) ↔️ 50-90 ppm

  Carboxyl (C=O) ↔️ 160-220 ppm

  Ethers (C-O-C) ↔️ 60-100 ppm
• What is a primary (1°) carbon atom bonded to?

  One other carbon
• What is the chemical shift range for alkyl carbon atoms in ppm when analyzing carbon-13 NMR spectra?

  0-50
• What is the third step in determining the structure of an organic molecule using carbon-13 NMR data?

  Connect fragments to form a structure
• The natural abundance of carbon-13 is approximately 1.1%
• Match the carbon environment with its chemical shift range:

  Alkyl (C-C) ↔️ 0-50 ppm

  Alkyl Halide (C-X) ↔️ 20-80 ppm

  Alcohol (C-OH) ↔️ 50-90 ppm

  Carboxyl (C=O) ↔️ 160-220 ppm
• Steps to analyze a carbon-13 NMR spectrum

  1️⃣ Identify the number of peaks

  2️⃣ Determine the chemical shift range of each peak

  3️⃣ Identify the corresponding carbon environment

  4️⃣ Use the data to propose a molecular structure
• Electron-donating groups cause deshielding in carbon-13 NMR.

  False
• Deshielding shifts the signal to higher values on the NMR spectrum.

  True
• Match the type of carbon atom with its definition:

  Primary (1°) ↔️ Carbon bonded to one other carbon

  Secondary (2°) ↔️ Carbon bonded to two other carbons

  Tertiary (3°) ↔️ Carbon bonded to three other carbons

  Quaternary (4°) ↔️ Carbon bonded to four other carbons
• By matching the observed signals in the spectrum to chemical shift ranges, you can identify the types of carbon environments present in the molecule.
• The weak signal from carbon-13 requires more sophisticated instrumentation and longer acquisition times
• What is used as a standard reference in carbon-13 NMR spectroscopy?

  Tetramethylsilane (TMS)
• What is the chemical shift range for alkyl carbon atoms in ppm?

  0-50
• Different carbon environments in organic molecules refer to the various positions and bonding arrangements
• Steps for analyzing carbon-13 NMR spectra

  1️⃣ Identify the number of signals

  2️⃣ Determine the chemical shift ranges

  3️⃣ Match signals to carbon environments

  4️⃣ Determine connectivity
• The chemical shift range for alcohol carbon atoms in carbon-13 NMR is 50-90
• What is the proposed structure of a molecule with signals at 25 ppm, 55 ppm, and 180 ppm?

  CH₃CH₂OCH₂COOH
• Carbon-13 NMR targets the carbon-13 isotope.

  True
• In carbon-13 NMR, chemical shifts depend on the electronic environment
• Deshielding shifts the signal to higher chemical shift values.

  True
• Match the carbon environment with its chemical shift range:

  Alkyl (C-C) ↔️ 0-50 ppm

  Alkyl Halide (C-X) ↔️ 20-80 ppm

  Alcohol (C-OH) ↔️ 50-90 ppm

  Ether (C-O-C) ↔️ 60-100 ppm
• Shielding occurs when electron-donating groups increase the electron density around the carbon atom, shifting the signal to lower values
• A primary carbon is bonded to only one other carbon
• What is the first step in analyzing a carbon-13 NMR spectrum?

  Interpreting chemical shifts
• The natural abundance of carbon-13 is approximately 1.1%.


  True
• Order the steps involved in analyzing a carbon-13 NMR spectrum:

  1️⃣ Record the spectrum

  2️⃣ Identify chemical shifts

  3️⃣ Match shifts to carbon environments

  4️⃣ Analyze the carbon backbone
• Deshielding in carbon-13 NMR occurs when electron-withdrawing groups decrease electron density
• Chemical shifts in carbon-13 NMR are independent of the electronic environment of carbon atoms.

  False
• A tertiary (3°) carbon atom is bonded to three other carbons