Chapter 9 — Case Study 1: Determining the Structure of Morphine by NMR

"The structure of morphine was solved over a century — beginning with chemical degradation in the 1800s, becoming firmly known in the 1920s, and being re-confirmed by NMR in the 1950s. Today, a graduate student can verify morphine's structure in an afternoon with modern 2D NMR. The journey from 'unknown alkaloid' to 'fully assigned structure' tells the story of how analytical chemistry has transformed." — paraphrase from a natural products textbook

This case study traces the structure determination of morphine using NMR spectroscopy. We see how the techniques of Chapter 9 — chemical shift, integration, multiplicity, coupling, ¹³C, DEPT, 2D NMR — combine to identify a complex natural product. The story illustrates the power of modern NMR for unknown structure elucidation.

The compound: morphine

Morphine is the principal alkaloid in opium (the latex of Papaver somniferum, the opium poppy). Used as: - A potent analgesic for severe pain (post-surgical, cancer pain). - An anti-cough agent (lower doses). - The starting material for many synthetic opioids (codeine, heroin, hydromorphone, oxycodone, fentanyl-related analogs).

Discovered in 1804 by Friedrich Sertürner (German pharmacist), morphine was the first alkaloid to be isolated. Its biological activity was recognized immediately. Its structure was a mystery for over a century.

The structure (modern view)

Morphine has: - 5 fused rings (a phenanthrene-related scaffold). - 17 carbons, 19 hydrogens, 3 oxygens, 1 nitrogen. - 5 stereocenters (at C5, C6, C9, C13, C14 in standard numbering). - 2 hydroxyl groups (-OH at C3 and C6). - 1 ether bridge (between C4 and C5). - 1 tertiary amine (the central N is methylated). - 1 alkene (C7-C8 double bond). - 1 aromatic ring (the A ring).

Molecular formula: C₁₇H₁₉NO₃. Molecular weight: 285.

Pre-NMR structure determination (1804-1950s)

1804-1900: discovery and early degradation

Sertürner isolated morphine and demonstrated its narcotic properties. By chemical analysis, the molecular formula C₁₇H₁₉NO₃ was determined.

19th-century chemists used chemical degradation to learn about morphine: - Treatment with strong acids decomposes morphine; products give clues about partial structures. - Hofmann elimination (Ch 30) cleaves the amine ring; reveals partial connectivity. - Oxidation gives identifiable smaller fragments.

1925-1955: structure proposed and refined

By the 1920s, several proposed structures had been put forward. Robert Robinson (Oxford, Nobel 1947 for alkaloid synthesis) proposed the now-accepted structure in 1925. Refinement followed via: - X-ray crystallography (Hodgkin, 1955): determined the 3D structure unambiguously. - Total synthesis (Gates, 1952): independently confirmed structure by re-creation.

1950s onward: NMR for verification

Once NMR became available, it became the standard tool for confirming morphine and analogue structures.

NMR analysis of morphine

A modern NMR analysis of morphine in CDCl₃ shows:

¹H NMR (400-600 MHz)

Aromatic region (6.5-7.0 ppm): - Two H's on the aromatic A ring; show as ortho-coupled doublets ($J \approx 8$ Hz).

Vinyl region (5.5-5.7 ppm): - Two H's of the C7-C8 double bond; show as a multiplet (cis alkene, $J \approx 10$ Hz).

Aliphatic region (4-5 ppm): - 1H at ~4.9 ppm (the C-O-H proton, exchangeable). - 1H at ~4.2 ppm (the C-H next to the ether bridge).

3-4 ppm region: - α to nitrogen and α to oxygen protons. - Multiple CHs and CH₂s.

1.5-3 ppm region: - Aliphatic CH₂ and CH protons of the ring system. - N-CH₃ at ~2.5 ppm (3H singlet — diagnostic).

0-1 ppm region: - Few or none.

Total: ~20 distinct ¹H environments (some overlap).

¹³C NMR

17 distinct carbons, each at a different chemical shift: - Aromatic carbons: 110-160 ppm (4-6 carbons). - Vinyl carbons: 130-145 ppm. - Sp³ C-O: 65-90 ppm. - N-CH₃: ~43 ppm (singlet in DEPT 90). - Other sp³ alkyl: 20-50 ppm.

DEPT 135 classifies each carbon as CH₃, CH₂, CH, or quaternary.

2D NMR

For a molecule as complex as morphine, 2D NMR is essential:

COSY: maps which protons are coupled (J-coupled). Constructs the proton "skeleton" of the molecule.

HSQC: connects every ¹H to its directly attached ¹³C. Gives a 2D map of the carbon-proton skeleton.

HMBC: 2-4 bond ¹H-¹³C correlations. Connects across heteroatoms (e.g., from CH₃ to the aromatic C across the ether bridge). Essential for assigning quaternary carbons.

NOESY: through-space correlations (≤5 Å). Establishes 3D stereochemistry (cis vs trans on rings; α vs β orientation).

The result

Combining ¹H, ¹³C, COSY, HSQC, HMBC, and NOESY, a graduate student can verify morphine's complete structure (including all 5 stereocenters) in a single afternoon. This is the standard approach for any complex natural product today.

How NMR solves morphine — a detailed walkthrough

Step 1: count atoms

Integration of ¹H NMR: ~19 H total. From mass spec, MW = 285. C/H/N/O analysis: C₁₇H₁₉NO₃. DoU = (2×17 + 2 + 1 - 19)/2 = 9. So 9 degrees of unsaturation: aromatic ring (4) + vinyl (1) + 4 rings (4) = 9 ✓.

Step 2: identify functional groups

Characteristic chemical shifts: - Aromatic H's at 6.5-7.0: aromatic ring. - Vinyl H's at 5.5-5.7: alkene. - O-H at ~5.0 (broad): hydroxyl. - α to O (~4): C-OH or C-OR. - α to N (~3.5): C-NR₂. - N-CH₃ at 2.5: methylated amine. - Aliphatic 1.5-3: ring CH₂ and CH.

Step 3: assemble fragments

COSY shows which proton is coupled to which. Build the proton-coupling tree: - Aromatic doublet 1 ↔ aromatic doublet 2 (ortho). - Vinyl 1 ↔ vinyl 2 (cis alkene, J = 10 Hz). - Aliphatic CH ↔ aliphatic CH ↔ aliphatic CH (ring chain). - Aliphatic CH ↔ N-CH₃ (slowly via 4J? or via the ring).

Step 4: bridge with HMBC

HMBC shows multi-bond correlations. Crucial for connecting: - N-CH₃ to the ring carbons. - The ether bridge (no H on the O, so HMBC reveals the connection). - Aromatic C to vinyl C (across the ring junction).

Step 5: stereochemistry by NOESY

Cis substituents on the ring give NOE. NOESY of morphine confirms: - The 5 stereocenters in their natural (R, S, R, R, S) arrangement. - The amine ring conformation (chair). - The orientation of the OH groups (axial/equatorial).

After all of this analysis, you have the complete structure of morphine, including stereochemistry, from NMR alone (with mass spec for the molecular formula).

Why NMR is so powerful

A few hours of NMR data, plus the molecular formula from MS, can solve the structure of essentially any organic natural product. This is the working tool of: - Natural product chemists isolating new compounds. - Pharmaceutical chemists verifying synthesis products. - Forensic chemists identifying unknown samples. - Doping control in sports. - Environmental chemists identifying pollutants. - Process chemists monitoring industrial reactions.

What's still missing from NMR

NMR has limitations: - Insolubility: very-large or insoluble molecules are hard. Solid-state NMR helps. - Sensitivity: ¹³C is slow due to low abundance. Hyperpolarization (DNP) is one solution; isotopic labeling (¹³C-enrichment) is another. - Stereochemistry of small molecules: NMR can determine relative stereochemistry but not absolute configuration directly. Chiral shift reagents or chiral chromatography are needed. - Dynamic systems: averaged spectra of fluxional molecules (rapidly equilibrating conformers) can be hard to interpret.

Modern variants and complementary techniques (CD spectroscopy, X-ray crystallography, computational chemistry) fill these gaps.

Take-home

• Morphine's structure was determined by chemical degradation and X-ray (1925-1955); NMR has confirmed and extended these assignments.
• NMR analysis of complex natural products uses ¹H, ¹³C, COSY, HSQC, HMBC, and NOESY together.
• Modern spectroscopic toolkit can verify complete structure (including stereochemistry) in an afternoon.
• Working chemists use NMR as their primary structural tool: pharmaceutical, natural products, forensic, environmental, food chemistry.
• Mastery of Chapter 9 prepares you to read NMR data in any modern publication and to verify structures of compounds you'll encounter throughout the rest of the book.

Further reading

• Friebolin, H. (2010). Basic One- and Two-Dimensional NMR Spectroscopy (5th ed.). Wiley-VCH.
• Silverstein, R. M.; Webster, F. X.; Kiemle, D. J. (2014). Spectrometric Identification of Organic Compounds (8th ed.). Wiley.
• Pretsch, E.; Bühlmann, P.; Badertscher, M. (2009). Structure Determination of Organic Compounds: Tables of Spectral Data (4th ed.). Springer.
• Berger, S.; Braun, S. (2004). 200 and More NMR Experiments: A Practical Course. Wiley-VCH.
• Sanders, J. K. M.; Hunter, B. K. (1993). Modern NMR Spectroscopy: A Guide for Chemists (2nd ed.). Oxford.