HNMR Practice 4

Determining the Structure of C₄H₈O from ¹H NMR and ¹³C NMR Spectra

The molecular formula is C₄H₈O, and the ¹H NMR and ¹³C NMR spectra shown below.

Step 1: Analyze the Molecular Formula

The molecular formula is C₄H₈O.

Degree of Unsaturation (DU):

\[ \text{DU} = \frac{2C + 2 - H}{2} = \frac{2(4) + 2 - 8}{2} = 1 \]

A DU of 1 indicates either one ring or one double bond (C=C or C=O). Given the presence of oxygen, a carbonyl group (C=O) is likely.

Step 2: Examine the ¹H NMR Spectrum

The ¹H NMR spectrum shows:

• A triplet at 1.2 ppm.
• A quartet at 2.3 ppm.
• A singlet at 3.7 ppm.

Key Observations:

• Chemical Shifts:
  • The signal at 1.2 ppm is in the aliphatic region, suggesting protons on a methyl group (-CH₃).
  • The signal at 2.3 ppm is slightly downfield, suggesting protons on a carbon adjacent to a carbonyl group (e.g., -CH₂-CO-).
  • The signal at 3.7 ppm is further downfield, suggesting protons on a carbon adjacent to an oxygen atom (e.g., -O-CH₃).
• Splitting Patterns:
  • A triplet indicates coupling with two equivalent neighboring protons (n+1 rule, where n = 2).
  • A quartet indicates coupling with three equivalent neighboring protons (n+1 rule, where n = 3).
  • A singlet indicates no coupling with neighboring protons.

Step 3: Examine the ¹³C NMR Spectrum

The ¹³C NMR spectrum shows four signals.

This indicates four distinct carbon environments in the molecule.

Step 4: Deduce the Structure

a) Triplet at 1.2 ppm:

• This signal corresponds to protons on a terminal methyl group (e.g., -CH₃).
• The triplet indicates these protons are coupled to two equivalent neighboring protons.

b) Quartet at 2.3 ppm:

• This signal corresponds to protons on a carbon adjacent to a carbonyl group (e.g., -CH₂-CO-).
• The quartet indicates these protons are coupled to three equivalent neighboring protons.

c) Singlet at 3.7 ppm:

• This signal corresponds to protons on a carbon adjacent to an oxygen atom (e.g., -O-CH₃).
• The singlet indicates no coupling with neighboring protons.

d) Combining the Information:

• The molecular formula (C₄H₈O) and the DU of 1 suggest a carbonyl-containing compound (e.g., ester or ketone).
• The splitting patterns and chemical shifts are consistent with an ester structure.
• The data fits methyl propanoate (CH₃-CH₂-CO-O-CH₃):
  • The -CH₃ protons (attached to -CH₂-) appear as a triplet at 1.2 ppm.
  • The -CH₂- protons (adjacent to the carbonyl) appear as a quartet at 2.3 ppm.
  • The -O-CH₃ protons appear as a singlet at 3.7 ppm.

Step 5: Verify the Structure

• Molecular Formula: C₄H₈O matches methyl propanoate.
• ¹H NMR Spectrum:
  • Triplet at 1.2 ppm: -CH₃ protons (3 protons).
  • Quartet at 2.3 ppm: -CH₂- protons (2 protons).
  • Singlet at 3.7 ppm: -O-CH₃ protons (3 protons).
• ¹³C NMR Spectrum:
  • Four signals corresponding to the four distinct carbon environments in methyl propanoate:
    1. Carbonyl carbon (C=O): ~170–180 ppm.
    2. -CH₂- carbon adjacent to the carbonyl: ~30–40 ppm.
    3. -CH₃ carbon (attached to -CH₂-): ~10–20 ppm.
    4. -O-CH₃ carbon: ~50–60 ppm.

Step 6: Conclusion

The compound is methyl propanoate (CH₃-CH₂-CO-O-CH₃). The NMR spectra are consistent with its structure, and the molecular formula matches.

Summary of Key Points

1. Molecular Formula (C₄H₈O): Suggests a carbonyl-containing compound (DU = 1).
2. ¹H NMR Spectrum:
   • Triplet at 1.2 ppm: -CH₃ protons.
   • Quartet at 2.3 ppm: -CH₂- protons adjacent to the carbonyl.
   • Singlet at 3.7 ppm: -O-CH₃ protons.
3. ¹³C NMR Spectrum:
   • Four signals corresponding to the four distinct carbon environments in methyl propanoate.
4. Structure: Methyl propanoate is the only ester with C₄H₈O that fits the data.

Final Answer

The compound is methyl propanoate (CH₃-CH₂-CO-O-CH₃).