HNMR Practice 2

Determining the Structure of C₈H₁₀ from ¹H NMR and ¹³C NMR Spectra

The molecular formula is C₈H₁₀, and the ¹H NMR spectrum shows two signals: a singlet at 7.1 ppm and a singlet at 2.1 ppm. The ¹³C NMR spectrum shows three signals: two between 130–140 ppm (one larger) and one at ~20 ppm. Let’s deduce the structure step by step.

¹HNMR

¹³CNMR

Step 1: Analyze the Molecular Formula

The molecular formula is C₈H₁₀.

Degree of Unsaturation (DU):

\[ \text{DU} = \frac{2C + 2 - H}{2} = \frac{2(8) + 2 - 10}{2} = 4 \]

A DU of 4 suggests the presence of multiple rings and/or double bonds. For example, this could indicate a benzene ring (DU = 4) with additional substituents.

Step 2: Examine the ¹H NMR Spectrum

The ¹H NMR spectrum shows:

• A singlet at 7.1 ppm.
• A singlet at 2.1 ppm.

Key Observations:

• Chemical Shifts:
  • The singlet at 7.1 ppm is in the aromatic region, suggesting the presence of aromatic protons (e.g., benzene ring).
  • The singlet at 2.1 ppm is in the aliphatic region, suggesting the presence of methyl protons (-CH₃).
• Splitting Patterns:
  • The aromatic protons appear as a singlet, indicating that all four aromatic protons are equivalent due to symmetry.
  • The methyl protons appear as a singlet, indicating no adjacent protons (consistent with a -CH₃ group attached to an aromatic ring).

Step 3: Examine the ¹³C NMR Spectrum

The ¹³C NMR spectrum shows:

• Two signals between 130–140 ppm (one larger).
• One signal at ~20 ppm.

Key Observations:

• 130–140 ppm:
  • This region corresponds to aromatic carbons.
  • The presence of two signals (one larger) suggests two distinct aromatic carbon environments, with one being more abundant (e.g., equivalent carbons).
• ~20 ppm:
  • This signal corresponds to aliphatic carbons, specifically a methyl group (-CH₃).

Step 4: Deduce the Structure

a) Aromatic Protons (7.1 ppm):

• The singlet at 7.1 ppm corresponds to four equivalent aromatic protons on a benzene ring.
• The chemical shift and singlet pattern suggest a symmetrically substituted benzene ring.

b) Methyl Protons (2.1 ppm):

• The singlet at 2.1 ppm corresponds to methyl protons (-CH₃).
• The chemical shift suggests the methyl group is attached to an aromatic ring (e.g., toluene-like structure).

c) Combining the Information:

• The molecular formula (C₈H₁₀) and the DU of 4 suggest a benzene ring (C₆H₅) with two additional carbons and five additional hydrogens.
• The presence of a methyl group (2.1 ppm) and aromatic protons (7.1 ppm) suggests a methyl-substituted benzene.
• The ¹³C NMR spectrum shows three signals, indicating symmetry in the molecule.

Step 5: Consider Symmetry

The ¹³C NMR spectrum shows three signals, which suggests a highly symmetric molecule.

For a benzene ring with two substituents, this symmetry is consistent with a para-substitution pattern (1,4-disubstitution).

Step 6: Propose the Structure

The data is consistent with para-xylene (1,4-dimethylbenzene):

\[ \text{para-Xylene: } \text{C}_6\text{H}_4(\text{CH}_3)_2 \]

In para-xylene:

• The aromatic protons (4 protons) appear as a singlet at 7.1 ppm due to symmetry.
• The methyl protons (6 protons) appear as a singlet at 2.1 ppm.
• The ¹³C NMR shows:
  1. Two aromatic carbon signals between 130–140 ppm:
     • One signal corresponds to the equivalent aromatic carbons (4 carbons).
     • The other signal corresponds to the quaternary aromatic carbons (2 carbons).
  2. One methyl carbon signal at ~20 ppm (2 equivalent methyl carbons).

Step 7: Verify the Structure

• Molecular Formula: C₈H₁₀ matches para-xylene.
• ¹H NMR Spectrum:
  • Aromatic protons: 4 protons as a singlet at 7.1 ppm.
  • Methyl protons: 6 protons as a singlet at 2.1 ppm.
• ¹³C NMR Spectrum:
  • Two aromatic carbon signals between 130–140 ppm (one larger due to equivalent carbons).
  • One methyl carbon signal at ~20 ppm.

Step 8: Conclusion

The compound is para-xylene (1,4-dimethylbenzene). The NMR spectra are consistent with its structure, and the molecular formula matches.

Summary of Key Points

1. Molecular Formula (C₈H₁₀): Suggests a benzene ring with additional substituents (DU = 4).
2. ¹H NMR Spectrum:
   • Singlet at 7.1 ppm: Aromatic protons (benzene ring).
   • Singlet at 2.1 ppm: Methyl protons (-CH₃).
3. ¹³C NMR Spectrum:
   • Two aromatic carbon signals between 130–140 ppm (one larger due to symmetry).
   • One methyl carbon signal at ~20 ppm.
4. Structure: Para-xylene is the only symmetric dimethylbenzene that fits the data.

Final Answer

The compound is para-xylene (1,4-dimethylbenzene).