[M - 15]+ is a mass spectrometry fragment ion that forms when the molecular ion loses a methyl group, CH3. In Organic Chemistry, it shows up in fragmentation patterns that help identify alcohols and phenols.
[M - 15]+ is a fragment ion in organic mass spectrometry that is 15 amu lighter than the molecular ion because the molecule has lost a methyl group, CH3. You will usually see it written as a peak at m/z equal to M minus 15, where M is the molecular ion peak.
In Organic Chemistry, this is part of the fragmentation story, not a separate molecule you isolate in a flask. The mass spectrometer ionizes the compound, then the molecular ion breaks apart in predictable ways. If one of those breakups removes a methyl group, the leftover cation appears as the [M - 15]+ peak.
This pattern is common for alcohols and phenols that have alkyl substituents attached to the carbon skeleton. Those side groups can be cleaved during fragmentation, especially when the positive charge can stay on a more stable part of the molecule. The spectrum does not just tell you the molecular weight, it gives clues about which pieces were easier to lose.
A useful way to think about [M - 15]+ is that it often suggests a methyl substituent somewhere in the structure, but it does not automatically tell you exactly where that methyl group sits. You still have to compare it with the molecular ion peak, other fragment peaks, and the rest of the structure. For example, a phenol with a methyl group on the ring can show [M - 15]+ when CH3 is lost, alongside other fragments from the aromatic ring.
The peak is strongest when that cleavage gives a reasonably stable carbocation-like fragment. If a molecule fragments in several competing ways, [M - 15]+ may be present but not the base peak. That is why mass spectra are read as patterns, not as one-peak answers. You match the loss, the remaining mass, and the rest of the spectrum to the structure you are trying to identify.
[M - 15]+ matters because mass spectrometry in Organic Chemistry is often about reading breakup patterns, not just finding a molecular weight. If you can recognize a 15-unit loss, you can start connecting a spectrum to a methyl substituent and narrow down the identity of an unknown alcohol or phenol.
It also helps you separate structures that share the same formula but fragment differently. Two compounds can have the same molecular ion peak and still give different intensities or combinations of fragments, so [M - 15]+ becomes one more clue in the structure puzzle.
This term also fits into the bigger logic of fragmentation. You can ask what group was lost, what piece kept the charge, and why that fragment is stable enough to appear on the spectrum. That is the same kind of reasoning you use for other cleavage patterns in spectroscopy.
In lab reports and problem sets, recognizing [M - 15]+ can be the difference between a vague guess and a structure that is actually supported by evidence.
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Visual cheatsheet
view galleryMolecular Ion (M+)
The molecular ion is the starting point for reading a mass spectrum, because [M - 15]+ is measured relative to it. First you find the M+ peak, then you check whether a smaller peak sits 15 mass units lower. Without the molecular ion, you lose the reference needed to interpret the fragmentation pattern.
Fragmentation
Fragmentation is the overall process that creates smaller charged pieces after ionization. [M - 15]+ is one specific fragmentation outcome, so understanding fragmentation helps you see why this peak appears at all. Different compounds break in different ways, and the distribution of peaks tells you which pathways were favored.
Alkyl Cleavage
Alkyl cleavage is the bond-breaking step that can remove a methyl or other alkyl group from the molecular ion. [M - 15]+ is what you get when the fragment loses CH3 specifically. In alcohols and phenols, alkyl cleavage often gives structural clues about side chains attached to the oxygen-bearing or aromatic portion.
Alpha Cleavage
Alpha cleavage breaks the bond next to a functional group, especially when that gives a more stable cation. It is a common alternative pathway in alcohol spectra, so it can compete with the formation of [M - 15]+. Comparing the peaks from alpha cleavage and methyl loss helps you see which fragmentation route is more favored.
A mass spectrometry question may give you a molecular ion peak and several fragment peaks, then ask you to identify the compound or the functional group pattern. When you see a peak 15 units below M+, look for a methyl loss and ask whether the structure has a CH3 group that could be cleaved. For alcohols and phenols, that clue often shows up alongside other fragmentation peaks, so you use it with the full spectrum rather than by itself.
In a lab report or quiz, you might also be asked to explain why a particular fragment ion is formed. A good response names the lost group, identifies the remaining charged fragment, and connects the peak to the likely structure.
[M - 15]+ is a fragment ion, while M+ is the intact molecular ion. M+ tells you the molecular weight of the whole compound, and [M - 15]+ tells you that the molecule broke apart and lost CH3. If you mix them up, you will misread the spectrum and the structure clue.
[M - 15]+ is a mass spectrometry fragment ion that sits 15 amu below the molecular ion because the compound lost CH3.
In Organic Chemistry, this peak often shows up in the spectra of alcohols and phenols with alkyl substituents.
You read [M - 15]+ as part of a fragmentation pattern, not as a stand-alone proof of one exact structure.
The peak is most useful when you compare it with the molecular ion and the other fragment peaks in the spectrum.
A methyl-loss fragment can narrow down the structure, but you still need the rest of the spectrum to make a strong identification.
[M - 15]+ is a fragment ion in mass spectrometry that forms when the molecular ion loses a methyl group, CH3. The remaining positively charged fragment is 15 mass units lighter than the original molecule. In Organic Chemistry, this is a clue that the compound may be an alcohol or phenol with a methyl substituent.
Alcohols and phenols often have alkyl groups that can break off during ionization and fragmentation. If a methyl group is lost, the spectrum shows a peak at M minus 15. That peak is one of several clues you use to map out the structure.
No. The molecular ion is the intact ionized molecule, while [M - 15]+ is a fragment made after the molecule loses CH3. If you only look at [M - 15]+, you miss the mass of the whole compound, so you always compare it back to M+.
Start with the molecular ion peak, then check whether a fragment appears 15 units lower. If it does, look for a methyl group that could be lost and compare that clue with other peaks in the spectrum. The best identification comes from the full fragmentation pattern, not just one fragment.