{"id":7452,"date":"2013-08-02T09:00:42","date_gmt":"2013-08-02T13:00:42","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7452"},"modified":"2025-07-06T20:22:32","modified_gmt":"2025-07-07T01:22:32","slug":"3-factors-that-stabilize-free-radicals","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/08\/02\/3-factors-that-stabilize-free-radicals\/","title":{"rendered":"3 Factors That Stabilize Free Radicals"},"content":{"rendered":"

What Factors Affect Free-Radical Stability?<\/strong><\/p>\n

In the last article<\/strong> we introduced free radicals<\/b> – neutral, electron-deficient chemical species with a partially filled orbital – and learned that they are highly reactive intermediates in organic chemistry. (See article – Free Radical Reactions<\/a>)<\/em><\/span><\/p>\n

In this post we’ll cover two of the most important concepts concerning these species: their geometry<\/strong>, and their stability.\u00a0<\/b>It’s this latter concept that we’ll see is particularly important for understanding many free-radical reactions in organic chemistry. [Spoiler: the factors that affect free radical stability are largely the same factors that stabilize carbocations<\/strong> [discussed previously here<\/a>]<\/p>\n

\"summary-factors<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. The Stability of Free Radicals Increases In The Order Methyl < Primary < Secondary < Tertiary<\/a><\/li>\n
  2. Free Radicals Are Stabilized By Delocalization (“Resonance”)<\/a><\/li>\n
  3. The Geometry Of Free Radicals Is That Of A “Shallow Pyramid” Which Allows For Overlap Of The Half-Filled\u00a0p-<\/em>Orbital With Adjacent Pi Bonds<\/a><\/li>\n
  4. The Same Factors Which Stabilize Free Radicals Also Stabilize Carbocations<\/a><\/li>\n
  5. Notes<\/a><\/li>\n
  6. Quiz Yourself!<\/a><\/li>\n
  7. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Stability Of Free Radicals Increases In The Order Methyl < Primary < Secondary < Tertiary<\/h2>\n

    Let’s talk a bit about stability first, and then circle back to their structure. Being electron deficient, you might already have a hunch regarding factors that might stabilize free radicals.\u00a0 Waaaay back<\/a>, we talked about how a considerable portion of organic chemistry can be explained simply by understanding that: 1) opposite charges attract (and like charges repel), and 2) the stability of charges increases if it can be spread out over a greater volume. These still apply here!<\/p>\n

    Electron poor species are stabilized by neighboring atoms that can donate<\/b> electron density. [“if you’re poor, it helps to have rich neighbors”].<\/p>\n

    The most common way to interpret “rich neighbors” here is the observation that increasing the number of alkyl groups <\/b>on the carbon bearing the free radical increases its stability.<\/p>\n

    Radical stability increases in the order methyl < primary < secondary < tertiary<\/b>. [For a second, more conceptually complex example, see the bottom of the post]. [Note 1<\/a>]<\/em><\/span><\/p>\n

    \"stability<\/a><\/p>\n

    <\/a>2. Free Radicals Are Stabilized by Delocalization (“Resonance”)<\/h2>\n

    Secondly, we have also learned that any factor which can lead to the electron deficient site being delocalized<\/strong> [spread out] over a larger area will also stabilize electron poor species. Previously,\u00a0 for example, we’ve seen that the positive charge of a carbocation is considerably stabilized when it is adjacent to a \u03c0 bond.<\/p>\n

    That’s because the carbocation is sp<\/em>2<\/sup> hybridized and bears an empty p orbital<\/strong>,\u00a0 allowing for overlap with the adjacent p<\/em> orbitals and therefore leading the positive charge to be delocalized over multiple carbon atoms, in a manner that is most easily grasped by drawing resonance structures.<\/p>\n

    \"carbocations<\/a><\/p>\n

    Carbocations are flat – so it’s easy to see how the p orbital could be in line with adjacent p orbitals of a double bond. But what about the geometry of free radicals?<\/p>\n

    <\/a>3. The Geometry of Free Radicals Is That Of A “Shallow Pyramid”, Which Allows For Overlap Of The Half-Filled p<\/em>-Orbital With Adjacent Pi Bonds<\/h2>\n

    If we draw out the electrons in a typical alkyl free radical, we see that there are three bonding pairs and a single unpaired electron, for a total of four occupied orbitals. By analogy to, say, amines, we might expect that the hybridization of the molecule to be sp<\/em>3<\/sup> and geometry of a free radical would be trigonal pyramidal.<\/p>\n

    In fact, the geometry of simple alkyl radicals is very close to flat <\/strong>and is often described as a “shallow pyramid”, with only a slight deviation (~ 5\u00b0) from planarity. [Note 2<\/a>]<\/p>\n

    When the free radical is adjacent to a \u03c0 bond<\/strong>, there’s a significant stabilization<\/strong> to be obtained if the p orbitals are all in line so they can overlap<\/strong> [i.e. be “in conjugation”] with each other. Overlap is increased (and the molecule’s energy lowered) if the “shallow pyramid” is flattened out.<\/p>\n

    \"free<\/a><\/p>\n

    It’s a good approximation to think of a free radical adjacent to a \u03c0 bond as being sp<\/em>2<\/sup> -hybridized.<\/strong><\/p>\n

    So what does this all boil down to? The electron-deficient free radical can be delocalized<\/strong> over multiple carbons. Therefore, free radicals are stabilized by resonance.\u00a0<\/strong><\/p>\n

    <\/a>4. The Same Factors Which Stabilize Free Radicals Also Stabilize Carbocations<\/h2>\n

    If you read the article on the stabilization of carbocations (See article – 3 Factors That Stabilize Carbocations<\/a><\/em><\/span>) you might notice something: the same factors which stabilize free radicals are also the same factors which stabilize carbocations!\u00a0<\/strong><\/p>\n

    Quiz time: one of the most stable free radicals known is the triphenylmethyl radical, discovered by Moses Gomberg in 1900. In the absence of oxygen, this radical is indefinitely stable at room temperature. Can you identify the factors which might make this free radical particularly stable?<\/p>\n