Characterisation of Organic Compounds IV: Practice Questions
1. A sample of an organic compound was analysed and it was found from the elemental analysis as: C = 53.46%, H = 6.98%, and O = 39.56%.
a. Determine its empirical formula
The sample was also analysed in spectroscopy and figures below show the spectrum of mass spectrometry, IR, H NMR, and C-13 spectroscopy.
b. Determine the molecular formula of the sample. Hence, calculate the double bond equivalents.
c. What functional groups are consistent with both IR spectrum and molecular formula?
d. How many non-equivalent carbon environments can be determined from the C-13 NMR spectrum?
e. What other structural information can be obtained from the C-13 NMR spectrum?
f. How many non-equivalent hydrogen environments can be determined from the H-1 NMR spectrum?
g. What other structural information can be obtained from the H-1 NMR spectrum? (Hint: Ignore the small peaks)
h. Suggest a structure consistent with ALL the spectral data presented and your answers to the above questions.
Click here to see the structure.
2. Another sample was analysed and it was found the elemental analysis as: C = 64.85%, H = 6.35%. Given the spectra analysis from this sample as shown below.
a. Determine the empirical formula.
b. Determine the molecular formula. Hence, calculate the number of double bond equivalents.
c. Calculate the extinction coefficient for 250 nm.
d. What functional groups are consistent with both the IR spectrum and molecular formula?
e. How many non-equivalent carbon environments can be determined from the C-13 NMR spectrum?
f. What other structural information can be obtained from the C-13 NMR spectrum?
g. How many non-equivalent hydrogen environments can be determined from the H-1 NMR spectrum?
h. What other structural information can be obtained from the H-1 NMR spectrum?
i. Suggest a structure consistent with all the spectral data presented and your answers to the question above.
Click here to see the structure.
3. There were 6 samples of organic compounds that have molecular formula C5H10O2, and all of them are isomeric with the descriptions as follows:
Determine the structures of all isomers by using the information given above and analysing the H-1 and C-13 NMR spectra given below.
a. Compound A (see answer)
b. Compound B (see answer)
c. Compound C (see answer)
d. Compound D (see answer)
e. Compound E (see answer)
f. Compound F (see answer)
4. [18]Annulene is an aromatic compound containing 18 carbon atoms. The annulene molecule has an almost planar structure with 6 inner hydrogens (Hin) and 12 outer hydrogens (Hout). The H-1 NMR spectra of [18]annulene at 213 K and 383 K are shown below.
a. In the spectrum obtained at 213 K, estimate the area ratio of the peaks at 9.3 and -3.0 ppm.
b. Explain why the spectrum obtained at 383 K has only one singlet peak while that obtained at 213 K has two multiplet peaks.
c. Estimate the position of the singlet peak in the spectrum obtained at 383 K.
a. Determine its empirical formula
The sample was also analysed in spectroscopy and figures below show the spectrum of mass spectrometry, IR, H NMR, and C-13 spectroscopy.
c. What functional groups are consistent with both IR spectrum and molecular formula?
d. How many non-equivalent carbon environments can be determined from the C-13 NMR spectrum?
e. What other structural information can be obtained from the C-13 NMR spectrum?
f. How many non-equivalent hydrogen environments can be determined from the H-1 NMR spectrum?
g. What other structural information can be obtained from the H-1 NMR spectrum? (Hint: Ignore the small peaks)
h. Suggest a structure consistent with ALL the spectral data presented and your answers to the above questions.
Click here to see the structure.
2. Another sample was analysed and it was found the elemental analysis as: C = 64.85%, H = 6.35%. Given the spectra analysis from this sample as shown below.
b. Determine the molecular formula. Hence, calculate the number of double bond equivalents.
c. Calculate the extinction coefficient for 250 nm.
d. What functional groups are consistent with both the IR spectrum and molecular formula?
e. How many non-equivalent carbon environments can be determined from the C-13 NMR spectrum?
f. What other structural information can be obtained from the C-13 NMR spectrum?
g. How many non-equivalent hydrogen environments can be determined from the H-1 NMR spectrum?
h. What other structural information can be obtained from the H-1 NMR spectrum?
i. Suggest a structure consistent with all the spectral data presented and your answers to the question above.
Click here to see the structure.
3. There were 6 samples of organic compounds that have molecular formula C5H10O2, and all of them are isomeric with the descriptions as follows:
- None of the compounds is branched
- There is no O-H absorbance in the IR spectra
- In each compound, one oxygen atom is sp2- and the other is sp3-hybridised.
Determine the structures of all isomers by using the information given above and analysing the H-1 and C-13 NMR spectra given below.
a. Compound A (see answer)
b. Compound B (see answer)
c. Compound C (see answer)
d. Compound D (see answer)
e. Compound E (see answer)
f. Compound F (see answer)
4. [18]Annulene is an aromatic compound containing 18 carbon atoms. The annulene molecule has an almost planar structure with 6 inner hydrogens (Hin) and 12 outer hydrogens (Hout). The H-1 NMR spectra of [18]annulene at 213 K and 383 K are shown below.
b. Explain why the spectrum obtained at 383 K has only one singlet peak while that obtained at 213 K has two multiplet peaks.
c. Estimate the position of the singlet peak in the spectrum obtained at 383 K.
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