In short
An electron-ionization mass spectrum is a record of how an ionized molecule falls apart, and it falls apart along predictable paths: next to oxygen (alpha cleavage), by hydrogen transfer to a carbonyl (McLafferty rearrangement), and by unzipping a cyclohexene ring (retro-Diels–Alder). Knowing them lets you explain a base peak rather than merely match it, and reject a library hit whose fragments make no chemical sense. For terpenes, whose isomers fragment alike, the retention index still does much of the identifying.
What you will learn
- Explain alpha cleavage, McLafferty rearrangement and retro-Diels–Alder fragmentation and predict the ions each gives
- Rationalize the base peaks of limonene, linalool and alpha-pinene from their structures
- Use a predicted fragment to accept or reject a library match
- Say why isomeric monoterpenes are identified by retention index as much as by spectrum
Before this lesson: Reading a Mass Spectrum, Retention Indices and Identification, Coelution, Isomers and False Identifications
Before you start
This builds directly on reading a mass spectrum, retention indices and coelution and false identifications. You should be able to work out a molecular formula's mass and draw skeletal structures.
The mechanisms
Alpha cleavage
Ionization usually removes an electron from a lone pair, here on oxygen. The bond to the next carbon then breaks, and the positive charge stays on a fragment stabilized as an oxonium ion. In alcohols, the largest alkyl group is commonly lost most readily.
McLafferty rearrangement
In an ionized aldehyde, ketone or ester with a hydrogen on the γ-carbon, that hydrogen transfers to the carbonyl oxygen through a six-membered arrangement, the α–β bond breaks, and a neutral alkene is lost. Straight-chain aldehydes give the enol ion at m/z 44; methyl ketones at m/z 58. The ion's mass therefore reports how the carbonyl is substituted.
Retro-Diels–Alder
An ionized cyclohexene can split into a diene and an alkene, the reverse of the reaction that would have made the ring. Which half carries the charge depends on which is better at stabilizing it.
Worked cases
| Molecule | M+ | Base peak | Mechanism |
|---|---|---|---|
| Limonene, C10H16 | 136 | 68 | Retro-Diels–Alder: the ring splits into two C5H8 halves, one keeping the charge |
| Linalool, C10H18O | 154 (weak) | 71 | Alpha cleavage at the tertiary carbinol carbon, losing the larger chain and leaving a C4H7O+ oxonium ion |
| α-Pinene, C10H16 | 136 | 93 | Opening of the strained four-membered ring and loss of a C3H7 radical, leaving a C7H9+ ion |
Limonene and α-pinene have the same molecular mass and share many ions (93 and 121 appear in both), but their base peaks differ because their skeletons break differently. Other monoterpene hydrocarbons are much less distinguishable, which is why Adams's reference tables give retention indices beside spectra and why retention index is an equal partner in identification.
Using mechanisms to judge a match
A library search returns similarity, not identity. Ask three questions of each proposed match: does the molecular ion (or a plausible loss from it, such as M − 18 for water from an alcohol) appear? Is the base peak explained by a mechanism the structure allows? Is the retention index within the tolerance your method supports? A match that fails the second question is suspect however high its score; a match that passes all three, on two columns of different polarity, is about as good as GC-MS gets without an authentic standard.
Where it stops
- Enantiomers give identical spectra. Only a chiral column separates them.
- Rearrangements compete. Ions can form by more than one route; a rationalization is a plausible account, not a proof.
- Coeluting peaks give blended spectra, whose ions belong to two molecules.
- Authentic standards decide. Spectrum plus retention index makes an identification "tentative" in most reporting conventions until a standard is run under the same conditions.
Open questions
Machine-learning spectrum prediction and structure elucidation are advancing quickly, but their performance on isomer-rich natural products is still argued over. Whether they outperform an analyst with retention indices on, say, monoterpene hydrocarbons is a fair, testable question.
Practical exercise: Rationalize five reference spectra
- From the NIST Chemistry WebBook, open the electron-ionization spectra of limonene, linalool, α-pinene, linalyl acetate and one straight-chain aldehyde of your choice.
- For each, record M+ (present, weak or absent), the base peak and one other prominent ion.
- Explain each base peak and the other ion with a named mechanism, drawing the bond that breaks. For linalyl acetate, account for the loss of acetic acid (60).
- For your aldehyde, predict the McLafferty ion before you look, then check.
- Note one ion you cannot explain, and say what you would need to settle it.
You have done this well if each explanation names a mechanism and shows the bond broken, your predicted McLafferty ion was written down before you looked, mass arithmetic is correct to the unit, and you are honest about the ion you could not explain.
Common mistakes
- Trusting the match score. It measures similarity of patterns, not chemical sense.
- Expecting a molecular ion. Tertiary alcohols and many esters show little or none.
- Identifying isomers by spectrum alone. Use retention index on two columns.
Key terms
- Alpha cleavage
- Breaking of a bond next to the atom carrying the charge or radical site after ionization, typically beside an oxygen. It gives a resonance-stabilized cation.
- McLafferty rearrangement
- A fragmentation of carbonyl compounds with a hydrogen three carbons away: the hydrogen moves to the oxygen through a six-membered arrangement and an alkene is lost.
- Retro-Diels–Alder fragmentation
- The reverse of a Diels–Alder reaction in an ionized cyclohexene ring, splitting it into a diene and an alkene, one of which keeps the charge.
- Diagnostic ion
- A fragment whose mass points to a specific structural feature, used to confirm or reject a proposed identity.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.