Source: Baeyer-Villiger Lab Report, Organic Chemistry II, University of Michigan
Tags: IR spectroscopy, mass spectrometry, MS fragmentation, lactone IR, ester carbonyl stretch, purification, recrystallisation, melting point, product purity, organic chemistry lab
Difficulty: Intermediate | Prerequisites: Part 1 of these notes (Mechanism and Theory), basic IR and MS interpretation from lecture
Once a Baeyer-Villiger reaction is complete, the real lab work begins: isolating the product, confirming its identity, and assessing purity. This document covers the spectroscopic evidence (IR and MS) used to verify that you made the right product, how to interpret the data, and how to troubleshoot common purification problems. This is the practical, post-reaction half of the experiment, where characterisation skills matter as much as the chemistry itself.
The IR spectrum of the lactone product should show a strong C=O stretch near 1735 cm⁻¹ (ester/lactone region) and the disappearance of the ketone carbonyl near 1715 cm⁻¹. Mass spectrometry confirms the molecular ion and key fragmentation pattern. The main contaminant is 3-chlorobenzoic acid, and impure product is typically the result of incomplete separation during workup.
IR spectroscopy (infrared spectroscopy)
A technique that measures the absorption of infrared light by molecular bonds. Different functional groups absorb at characteristic frequencies (reported in cm⁻¹), so IR is used to identify which functional groups are present in a sample. Think of it as a fingerprint scanner for bond types.
Carbonyl stretch (C=O stretch)
The strong absorption in the IR spectrum caused by the stretching vibration of a C=O double bond. Ketones absorb near 1715 cm⁻¹; esters and lactones absorb at a slightly higher frequency, near 1735–1750 cm⁻¹. The shift tells you whether you have a ketone or an ester.
Mass spectrometry (MS)
A technique that ionises molecules and separates the resulting ions by their mass-to-charge ratio (m/z). It gives you the molecular weight and, from the fragmentation pattern, structural information about the molecule.
Molecular ion peak (M⁺)
The peak in the mass spectrum corresponding to the intact molecule after losing one electron. For ε-caprolactone, the molecular ion appears at m/z = 114.
Fragmentation pattern
The set of smaller ion peaks produced when the molecular ion breaks apart in predictable ways. Each fragment corresponds to a piece of the molecule, and the pattern helps confirm the structure.
Base peak
The most intense (tallest) peak in the mass spectrum. It is assigned a relative intensity of 100%. It may or may not be the molecular ion.
3-Chlorobenzoic acid (mCBA)
The by-product of the mCPBA oxidant. In the IR, it shows a broad O–H stretch (2500–3300 cm⁻¹) and a carboxylic acid C=O stretch near 1690–1710 cm⁻¹. Its presence in your product indicates incomplete separation. In MS, it appears at m/z = 156/158 (the chlorine isotope pattern).
Recrystallisation
A purification technique where a solid is dissolved in a hot solvent and then allowed to crystallise slowly as the solution cools. Pure crystals form while impurities remain in solution. This is a common method for cleaning up solid Baeyer-Villiger products or by-products.
Melting point (mp)
The temperature at which a solid becomes a liquid. A pure compound has a sharp melting point; impurities depress it and broaden the range. Comparing experimental mp to the literature value is a quick purity check.
Percent error
The deviation of your experimental value from the accepted literature value, expressed as a percentage. Used here for melting point comparison.
What to look for in a clean lactone (ε-caprolactone):
Strong C=O stretch near 1735–1750 cm⁻¹ (ester/lactone carbonyl). This is the single most important peak.
C–O stretches in the 1000–1300 cm⁻¹ region (two bands are typical for esters: the asymmetric and symmetric C–O–C stretches).
Absence of a broad O–H stretch in the 2500–3300 cm⁻¹ region (if this appears, carboxylic acid contamination is likely).
What to look for as evidence of contamination:
A broad O–H stretch centred around 2500–3300 cm⁻¹ indicates 3-chlorobenzoic acid (mCBA) is present.
A second carbonyl peak near 1690–1710 cm⁻¹ (lower than the ester) points to carboxylic acid C=O.
If the starting material (cyclopentanone) is still present, you may see a ketone C=O near 1715 cm⁻¹, which can overlap awkwardly with the ester peak.
How to discuss inconsistencies:
If your IR shows both a lactone carbonyl and a carboxylic acid O–H, the product is contaminated with mCBA.
The presence of both peaks means your separation (typically a base wash to remove the acidic by-product) was incomplete.
ε-Caprolactone (MW = 114.14):
Molecular ion at m/z = 114.
Common fragments: loss of CO₂ (m/z = 114 – 44 = 70, corresponding to the ring-opened fragment after losing the ester CO₂ unit), loss of CH₂=CH₂ (m/z = 114 – 28 = 86, corresponding to loss of an ethylene fragment from the ring).
Additional small fragments at m/z = 42 (ketene, CH₂=C=O) are typical of lactone fragmentation.
3-Chlorobenzoic acid (MW = 156.57):
Molecular ion at m/z = 156, with a characteristic M+2 peak at m/z = 158 due to the ³⁵Cl/³⁷Cl isotope pattern (roughly 3:1 ratio).
Loss of OH (m/z = 139) or loss of CO₂H (m/z = 111) from the carboxylic acid group.
The chlorine isotope doublet is a dead giveaway for the presence of this by-product.
Cyclopentanone (MW = 84.12), if unreacted:
Molecular ion at m/z = 84.
Loss of CO to give m/z = 56 (cyclobutane cation) is a common fragmentation.
Why was the product impure?
The main impurity is 3-chlorobenzoic acid. This is formed stoichiometrically and must be separated.
Unreacted mCPBA (if present) also contaminates, though it reacts with base during workup.
Residual cyclopentanone from incomplete reaction.
Standard separation strategy:
Dissolve the crude product in an organic solvent (e.g., dichloromethane).
Wash with aqueous sodium bicarbonate or sodium hydroxide solution. The mCBA (an acid) deprotonates and moves into the aqueous layer. The neutral lactone stays in the organic layer.
Dry the organic layer, filter, and remove solvent by rotary evaporation.
What could be done differently to improve purity?
Multiple base washes rather than a single wash, to ensure complete removal of mCBA.
A more careful aqueous workup: checking the pH of the wash to confirm all acid has been removed.
Column chromatography as a secondary purification step if recrystallisation is not feasible (caprolactone is a liquid at room temperature, so recrystallisation of the product itself is not possible).
Using a slight excess of mCPBA to drive the reaction to completion, then removing excess peracid with a reducing agent (e.g., sodium sulfite wash) before the base wash.
Functional Group | Frequency (cm⁻¹) | Appearance |
|---|---|---|
Ester/lactone C=O | 1735–1750 | Strong, sharp |
Ketone C=O | 1705–1720 | Strong, sharp |
Carboxylic acid C=O | 1690–1710 | Strong, sharp |
Carboxylic acid O–H | 2500–3300 | Broad, often overlaps C–H region |
Ester C–O–C | 1000–1300 | Two bands, medium intensity |
Compound | M⁺ (m/z) | Key Fragments (m/z) |
|---|---|---|
ε-Caprolactone | 114 | 70 (loss of CO₂), 86 (loss of C₂H₄), 42 (ketene) |
3-Chlorobenzoic acid | 156, 158 (Cl isotope) | 139 (loss of OH), 111 (loss of CO₂H) |
Cyclopentanone | 84 | 56 (loss of CO), 55 (loss of CHO) |
IR and MS are the standard first-pass characterisation tools in every synthetic chemistry lab, from undergraduate teaching to pharmaceutical R&D. The ability to read a carbonyl stretch and identify contamination by a carboxylic acid by-product is exactly the skill used by process chemists when scaling up drug intermediates. In forensic chemistry and environmental testing, MS fragmentation patterns are used to identify unknown compounds in complex mixtures.
Students often assume that any carbonyl peak in the IR confirms the desired product. A carbonyl peak alone is not enough; you need to note its exact position. A peak at 1715 cm⁻¹ could be unreacted ketone, not your lactone product.
A common error is ignoring the broad O–H stretch when interpreting the IR. If that broad feature is present, the sample contains a carboxylic acid impurity, regardless of how clean the carbonyl region looks.
Students sometimes expect the molecular ion to always be the base peak in the mass spectrum. It frequently is not. The base peak may be a fragment, and the molecular ion can be small or even absent for some compounds.
The chlorine isotope pattern (M and M+2 in a roughly 3:1 ratio) is sometimes mistaken for two different compounds. It is one compound with two naturally occurring chlorine isotopes.
⚠️ Lab reports will ask you to discuss your IR spectrum in detail. Be ready to assign the major peaks and explain what each one tells you about the product and any impurities.
⚠️ MS interpretation questions often ask you to explain specific fragment losses. Know how to work backwards from a fragment mass to the structural piece that was lost (e.g., loss of 44 = CO₂, loss of 28 = CO or C₂H₄).
⚠️ The chlorine isotope pattern is a favourite exam topic. If you see a pair of peaks two mass units apart in a roughly 3:1 ratio, flag it as a chlorine-containing compound.
⚠️ Questions about product purity will ask both what the impurities are and how to remove them. Saying "the product was impure" without naming the contaminant and a specific purification fix will not earn full marks.
True or False: A lactone C=O stretch appears at a lower frequency than a ketone C=O stretch.
Answer: False. Lactones (esters) absorb at a higher frequency (~1735–1750 cm⁻¹) than ketones (~1715 cm⁻¹).
Fill in the blank: A broad absorption between 2500 and 3300 cm⁻¹ in the IR spectrum of your product indicates contamination with a ______.
Answer: carboxylic acid (specifically 3-chlorobenzoic acid)
True or False: The molecular ion of ε-caprolactone appears at m/z = 84.
Answer: False. m/z = 84 is cyclopentanone. Caprolactone appears at m/z = 114.
Fill in the blank: A pair of peaks at m/z 156 and 158 in a 3:1 ratio indicates the presence of a ______ atom in the molecule.
Answer: chlorine
True or False: A single base wash is always sufficient to remove all 3-chlorobenzoic acid from the product.
Answer: False. Multiple washes or checking the pH of the aqueous layer is recommended to ensure complete removal.
Q: Your IR spectrum of the Baeyer-Villiger product shows a strong peak at 1740 cm⁻¹ and a broad absorption between 2500 and 3300 cm⁻¹. What does this tell you?
A: The peak at 1740 cm⁻¹ is consistent with the ester/lactone C=O stretch of ε-caprolactone, confirming the product was formed. The broad absorption between 2500 and 3300 cm⁻¹ indicates the presence of a carboxylic acid O–H, meaning 3-chlorobenzoic acid (mCBA) was not fully removed during workup.
Q: In the mass spectrum of your product, you observe a peak at m/z = 114 and another at m/z = 70. Explain both.
A: The peak at m/z = 114 is the molecular ion of ε-caprolactone. The peak at m/z = 70 corresponds to loss of CO₂ (44 mass units) from the molecular ion, a common fragmentation pathway for lactones where the ester linkage cleaves.
Q: You also observe peaks at m/z = 156 and 158 in an approximately 3:1 ratio. What compound is responsible, and how do you know?
A: These peaks correspond to the molecular ion of 3-chlorobenzoic acid. The 3:1 ratio of the peaks two mass units apart is the signature isotope pattern of a compound containing one chlorine atom (³⁵Cl and ³⁷Cl in natural abundance).
Q: Name two specific changes to the experimental procedure that would improve product purity.
A: (1) Perform multiple aqueous sodium bicarbonate washes (rather than a single wash) to ensure complete removal of 3-chlorobenzoic acid. (2) After the base wash, perform an additional wash with aqueous sodium sulfite solution to destroy any unreacted mCPBA, which would otherwise contaminate the organic layer.
Q: Why can you not recrystallise ε-caprolactone to improve its purity?
A: ε-Caprolactone is a liquid at room temperature (mp approximately –1 °C), so it cannot be recrystallised under standard lab conditions. Purification instead relies on liquid–liquid extraction (workup washes) or distillation/column chromatography.
This connects to IR spectroscopy fundamentals from earlier in the course: every functional group transformation you study (reduction, oxidation, substitution) has a corresponding change in the IR spectrum. Learning to spot a ketone-to-ester shift here trains you for every other reaction where the functional group changes.
The MS fragmentation concepts here (loss of CO₂, loss of CO, isotope patterns) apply directly to structure determination problems on exams and in later courses such as spectroscopy or analytical chemistry.
The purification techniques (liquid–liquid extraction, base washes, drying agents) are the same toolkit used in every organic chemistry lab, from Grignard reactions to Diels-Alder syntheses. Mastering them here carries over to every experiment that follows.
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