---
title: "Cation Radical | Organic Chemistry"
description: "Cation radical in Organic Chemistry is a positively charged species with one unpaired electron, often formed by ionization in mass spectrometry."
canonical: "https://fiveable.me/organic-chem/key-terms/cation-radical"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 1"
---

# Cation Radical | Organic Chemistry

## Definition

A cation radical is a molecule that has lost one electron, so it carries a positive charge and has an unpaired electron. In Organic Chemistry, you see it most often as a mass spectrometry ion.

## What It Is

A cation radical in Organic Chemistry is a species that has lost one electron, leaving it both positively charged and short one electron pair, so it has an unpaired electron. You will usually see it written as a molecular ion with a radical cation character, especially in mass spectrometry.

That combination matters because this is not just a regular ion. A simple cation has a positive charge, but a cation radical is also a radical, which means it has an unpaired electron and can be much more reactive or fragile than a normal closed-shell ion.

The most familiar place it appears is in ionization during mass spectrometry. When a neutral organic molecule is hit with enough energy to eject an electron, the result can be the molecular ion M+·. That species may be the first detectable ion, and then it may fragment into smaller ions. If you are reading a mass spectrum, the cation radical often corresponds to the molecular ion peak, which gives you a clue about the original compound’s mass.

This fits right into reaction mechanism thinking. Organic Chemistry cares about where the electrons go, and a cation radical is a direct snapshot of that electron movement. Instead of a two-electron process like many acid base or substitution reactions, ionization can remove a single electron and create an odd-electron intermediate.

You should also connect this term to atomic structure. Electrons carry the negative charge in the atom, so removing one changes the charge balance without changing the nucleus. The nucleus stays the same, but the electron count changes, which is why the species becomes positively charged and radical-like at the same time.

## Why It Matters

Cation radicals matter because they show up when Organic Chemistry moves from pure structure drawing into real analysis. If you are interpreting a mass spectrum, recognizing a cation radical helps you identify the molecular ion peak and estimate the molar mass of the unknown.

This term also gives you a cleaner picture of how electron movement works. A lot of organic reactions are taught as two-electron processes, but ionization can remove only one electron. That small difference changes everything about the reactivity of the species and the kinds of fragments it can form.

It also connects structure to stability. Some cation radicals survive long enough to be seen clearly, while others fragment fast. That difference depends on how the molecule can spread out charge and radical character, which is why certain aromatic or conjugated systems often give stronger, more useful signals than simple unstable molecules.

When you are given an unknown compound, the ability to spot a cation radical in a spectrum or mechanism step gives you a starting point for identifying the molecule instead of guessing from fragments alone.

## Connections

### Ionization

Ionization is the step that creates the cation radical in mass spectrometry. A neutral molecule loses an electron and becomes an odd-electron ion, which is what gives you the molecular ion signal. If you understand ionization, you can separate the original molecule from the fragment ions that appear later.

### Mass Spectrometry

Mass spectrometry is the main setting where you run into cation radicals in Organic Chemistry. The ion source creates the molecular ion, and the analyzer separates ions by mass-to-charge ratio. When a spectrum includes an M+· peak, that often points to the cation radical formed from the original compound.

### [Homolytic Bond Cleavage](/organic-chem/key-terms/homolytic-bond-cleavage)

Homolytic bond cleavage splits a bond so each atom gets one electron, which creates radicals. A cation radical is not the same as a homolytic cleavage product, but both involve unpaired electrons. Comparing them helps you keep straight whether a process gives neutral radicals or a charged radical ion.

### [Heterolytic Bond Cleavage](/organic-chem/key-terms/heterolytic-bond-cleavage)

Heterolytic bond cleavage sends both bonding electrons to one atom, making ions without producing a radical. That makes it a useful contrast with cation radicals, which do have an unpaired electron. If you can tell which kind of electron movement is happening, you can predict the products more accurately.

## On the AP Exam

A quiz question might show a mass spectrum and ask you to identify the molecular ion peak, or it might ask which species forms after a molecule loses one electron. In those problems, you are looking for the odd-electron ion, often written M+·, and using it to connect the spectrum back to the original structure.

You may also be asked to compare ionization products to radicals from bond cleavage. The move is simple: decide whether the process removed an electron or split a bond, then check whether the product is charged, radical, or both. If you can track charge and electron count, you can usually rule out the wrong choice quickly.

## Cation radical vs carbocation

A carbocation is positively charged but does not have an unpaired electron, so it is a closed-shell ion. A cation radical has both a positive charge and an unpaired electron, which makes it an odd-electron species. In organic mechanisms and mass spectrometry, that difference changes stability and fragmentation.

## Key Takeaways

- A cation radical is a positively charged species with one unpaired electron.
- In Organic Chemistry, it most often appears when a molecule is ionized in mass spectrometry.
- The familiar notation is M+·, which points to the molecular ion of the compound.
- Because it has an unpaired electron, a cation radical can fragment differently from a regular carbocation.
- Watching charge and electron count together helps you identify this species in mechanisms and spectra.

## FAQs

### What is a cation radical in Organic Chemistry?

A cation radical is an ion that has lost one electron, so it is both positively charged and radical-like because it has an unpaired electron. In Organic Chemistry, you usually meet it in mass spectrometry as the molecular ion, written M+·. That makes it a useful clue for the identity of an unknown compound.

### Is a cation radical the same as a carbocation?

No. A carbocation has a positive charge but no unpaired electron, so it is not a radical. A cation radical has both a positive charge and an unpaired electron, which changes how it behaves and how it fragments.

### Where do cation radicals show up in Organic Chemistry?

They show up most often in mass spectrometry after ionization removes one electron from a molecule. You may also see the idea when discussing odd-electron intermediates or electron-transfer processes. In spectra, the molecular ion peak is often the first place to look.

### How do you recognize a cation radical in a spectrum?

Look for the molecular ion written as M+·, which means the species has one positive charge and one unpaired electron. If the peak is present, it can give you the mass of the intact molecule before fragmentation. If it is weak or missing, the molecule may be fragmenting quickly after ionization.

## Related Study Guides

- [1.1 Atomic Structure: The Nucleus](/organic-chem/unit-1/atomic-structure-nucleus/study-guide/CsxGpH2mNTAFREpb)
- [6.2 How Organic Reactions Occur: Mechanisms](/organic-chem/unit-6/organic-reactions/study-guide/DZhJwGlaW4bNLFum)

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