Challenging structure elucidation examples using combined NMR, MS, IR data

📚 Quick Study Guide

• 🧪 NMR Spectroscopy: Chemical shift values ($\delta$) indicate the electronic environment of nuclei. Integration gives relative numbers of nuclei. Splitting patterns (multiplicity) reveal neighboring nuclei. 2D NMR (COSY, HSQC, HMBC) help correlate nuclei.
• 📊 Mass Spectrometry: Molecular ion peak (M+) gives the molecular weight. Fragmentation patterns provide structural clues (e.g., loss of common fragments like $H_2O$, $CH_3$). High-resolution MS gives accurate mass for elemental composition.
• 🔥 Infrared Spectroscopy: Identifies functional groups based on characteristic absorption frequencies. Important absorptions: O-H (3200-3600 $cm^{-1}$), C=O (1650-1800 $cm^{-1}$), C-H (2850-3100 $cm^{-1}$), N-H (3300-3500 $cm^{-1}$).
• 🧮 Degrees of Unsaturation (DoU): Calculated using the formula: $DoU = \frac{2C + 2 + N - H - X}{2}$, where C = carbons, N = nitrogens, H = hydrogens, and X = halogens. It indicates the number of rings and/or $\pi$ bonds in the molecule.
• 💡 General Strategy:
  • Determine the molecular formula from MS data (if provided).
  • Calculate the DoU.
  • Analyze IR to identify key functional groups.
  • Use NMR to piece together the carbon-hydrogen framework.
  • Use MS fragmentation to confirm structural assignments.

Practice Quiz

1. A compound shows a molecular ion peak at m/z = 100 in its mass spectrum. The IR spectrum shows a strong absorption at 1720 $cm^{-1}$. $^{1}$H NMR shows a single peak. What is the most likely structure?

   1. CH3CH2CH2COCH3
   2. (CH3)2CHCOCH3
   3. (CH3)3CCHO
   4. (CH3)4C

2. A compound has a molecular formula of C6H12O. Its IR spectrum shows a broad absorption at 3300 cm$^{-1}$. $^{1}$H NMR shows a doublet at ~1 ppm, a multiplet at ~4 ppm, and a broad singlet at ~2 ppm. What type of compound is it?

   1. An aldehyde
   2. A ketone
   3. An alcohol
   4. An ether

3. A compound has a molecular formula of C4H8O. Its $^{1}$H NMR spectrum shows a triplet at δ 1.0 ppm (3H), a singlet at δ 2.1 ppm (3H), and a quartet at δ 2.4 ppm (2H). What is the structure?

   1. Butanal
   2. 2-Butanone
   3. Diethyl ether
   4. 1-Butanol

4. A compound gives the following data: Molecular ion at m/z 74; strong IR absorption at 3300 cm$^{-1}$ and 3400 cm$^{-1}$; $^{1}$H NMR: δ 3.6 (2H, triplet), δ 1.6 (2H, quintet), δ 0.9 (3H, triplet). What is the compound?

   1. Ethanol
   2. Propanol
   3. 1-Butanol
   4. 2-Butanol

5. What information does the integration of a peak in a ¹H NMR spectrum provide?

   1. The number of neighboring hydrogens.
   2. The chemical shift of the hydrogen.
   3. The relative number of hydrogens responsible for the signal.
   4. The coupling constant.

6. A compound has a molecular formula of C7H8O. The IR spectrum shows a broad peak around 3300 cm$^{-1}$. The ¹H NMR shows several signals including a broad singlet at δ 4.6 ppm. What functional group is likely present?

   1. Aldehyde
   2. Carboxylic acid
   3. Alcohol
   4. Ester

7. Which spectroscopic method is most useful for determining the molecular weight of an unknown compound?

   1. IR Spectroscopy
   2. ¹H NMR Spectroscopy
   3. Mass Spectrometry
   4. UV-Vis Spectroscopy