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I have often read that radicals are electron-deficient and are therefore stabilized by things like electron-donating groups (EDGs) and hyperconjugation. However, today I learned that electron-withdrawing group (EWGs) can also stabilize radicals. Does this mean that radicals are not truly electron deficient?

I thought electron-withdrawing groups destabilized electron-deficient species. Why do we consider radicals to be electron-deficient if they are stabilized by EWGs?

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    $\begingroup$ Probably because of this stupid electron counting. $\endgroup$
    – Mithoron
    Commented Apr 30, 2022 at 0:34
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    $\begingroup$ Cobaltocene is a neutral isolable radical compound which is very electron-rich. There are many other such 19-electron transition metal complexes. $\endgroup$
    – Nicolau Saker Neto
    Commented Apr 30, 2022 at 3:07
  • $\begingroup$ It is a term to state that they have the ability to accept at least one electron in the half- filled [or is it half empty] orbital or to form a bond either by combining with or generating another radical. Stability is difficult to encompass, a mix of O2 [a diradical and stable molecule] and a hydrocarbon gas is stable, but one should avoid a room full of that mix at all costs. Radicals react. $\endgroup$
    – jimchmst
    Commented Apr 30, 2022 at 3:45
  • $\begingroup$ Actually, they can be considered both electron-deficient and rich, depending upon the environment. You can read these articles about the electrophilic and nucleophilic behavior of radicals which can cause non-covalent interaction. doi.org/10.1039/C9CP05374C , doi.org/10.1039/D0CP02619K , doi.org/10.1039/D0CP01933J and doi.org/10.1021/acs.jpca.9b01133 . Hope this helps. $\endgroup$
    – Pro
    Commented May 4, 2022 at 5:06

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Radicals should not always be perceived as "electron-deficient". They are not defined in this way. You should always keep the classification criteria in mind. What makes a radical electron "deficient," in relation to what? Reflect upon this.

Another dangerous term is "stable" or "stability" in general chemistry, what is the meaning of stability? Stable with respect to which criterion?

A radical is fundamentally an entity with an odd number of electrons in an orbial, commonly one, in organic compounds. In other words, an unpaired electron. For metal centers in complexes they can also have unpaired spins but one does not call them radicals, althought both radicals and metals complexes are electron spin resonance spectroscopy (ESR) active. This is the true definition by IUPAC rather than "electron deficient" species. Odd electron species can exist as radical cations, and radical anions. There are neutral radicals as well. When you will study mass spectrometry, you will encounter all the three types.

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    $\begingroup$ I thought it was "at least one unpaired valance electron" rather than "an odd number of electrons". Or are carbene and triplet oxygen not radicals? IUPAC gold book says "possessing an unpaired electron" which I thought meant "at least one". $\endgroup$
    – abligh
    Commented Apr 30, 2022 at 10:24
  • $\begingroup$ Right... better wording is typically odd number of electron or an unpaired spin in an orbital. For most of organic compounds, one odd electron is commonly observed but for metal complexes, the number of unpaired electrons is higher. $\endgroup$
    – ACR
    Commented Apr 30, 2022 at 15:00
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From high school chemistry, almost 30 years ago, the basic explanation that we were given (not overly scientific) is as follows.

Each atom "wants" to have a full outer electron shell. For noble gasses, this is the default because they hit the right number and are full, so they don't react. For everything else, the amount they are short or over dictates how they react.

Covalent bonds:

For each electron short, the atom wants to interact with other atoms that can "share" their electrons so that the outer shell "feels full". Oxygen is -2 electrons so it can connect with another oxygen and both share 2 electrons so that they "feel full". Carbon is -4 electrons so finds some way of connecting with things to make up that deficit, which works in $\ce{CO2}$ by sharing 2 electrons each with the oxygen atoms that want to share 2 each.

Ionic bonds:

Everything in column 1 of the periodic table is +1 in its outer electron shell, and thus highly unstable because they really want to get rid of that extra electron. This makes hydrogen, sodium, and the like to behave (when isolated) as an explosive. This works well for elements in the next-to-last column of the periodic table that are of the -1 variety in their outer electron shell. Sodium will freely give up its unwanted electron to form an ionic bond with chlorine, turning an explosive and a poison gas into harmless table salt.

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