{"id":7521,"date":"2013-08-30T17:37:02","date_gmt":"2013-08-30T21:37:02","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=7521"},"modified":"2026-05-07T10:49:42","modified_gmt":"2026-05-07T15:49:42","slug":"radical-initiation-why-is-light-or-heat-required","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2013\/08\/30\/radical-initiation-why-is-light-or-heat-required\/","title":{"rendered":"Free Radical Initiation: Why Is “Light” Or “Heat” Required?"},"content":{"rendered":"

Free-Radical Reactions Require Heat Or Light For Initiation (Bond-Breaking)<\/strong><\/p>\n

\"Summary-why<\/a><\/p>\n

If you come across just a few free-radical reactions, you should notice a familiar pattern. Every free-radical reaction that you’ll encounter is accompanied by either “heat” or “light”.<\/p>\n

In fact, this is one of the most important clues to knowing you’re dealing with a free radical reaction!\u00a0\u00a0<\/strong><\/p>\n

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

So why is that? Because the first step in free radical reactions is\u00a0initiation<\/strong> –\u00a0 the homolytic cleavage of bonds to create new free radicals, and this requires an input of energy either from light or heat.<\/p>\n

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

    \n
  1. Bond Breaking Requires An Input of Energy<\/a><\/li>\n
  2. The First Step In Free Radical Chain Reactions Is “Initiation”<\/a><\/li>\n
  3. Notes<\/a><\/li>\n
  4. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Bond Breaking Requires An Input Of Energy<\/h2>\n

    Free radicals are created when a bond undergoes homolytic cleavage<\/strong><\/a> – that is, the bond breaks such that each atom receives the same number of electrons.<\/p>\n

    It’s important to recognize that breaking a bond requires an input<\/b> of energy to the molecule. [Note 1<\/a>]. The energy required for bond cleavage commonly comes from two sources – you guessed it – heat, or light.<\/p>\n

    Take chlorine, for example. The heating of chlorine gas (actually – more commonly, of chlorine dissolved in a solvent) or exposure of chlorine gas to visible light results in significant cleavage of the Cl-Cl bond to deliver two free radicals, as shown below.<\/p>\n

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

    [Why light? recall from general chemistry, E =h\u03bd\u00a0 ; there is a relationship between the frequency of photons of electromagnetic radiation (“light”) and their energy. Photons that collide with molecules impart energy to them; this can be sufficient to break bonds if sufficient conditions are met – see below for more]<\/em><\/span><\/p>\n

    The energy associated with a quantum of light, 48 kcal\/mol at 600 nm (orange<\/em>), 96 kcal\/mol at 300 nm (near-UV<\/em>) is of the same order of magnitude <\/strong>as that of most covalent bonds<\/strong>. [Ref<\/a>]<\/p>\n

    <\/a>2. The First Step In Free Radical Chain Reactions Is “Initiation”<\/h2>\n

    In free radical reactions, this first step – homolytic cleavage of a bond to yield free radicals – is referred to as “initiation”.\u00a0In an “initiation step, the number of free radicals is always increased”.\u00a0<\/strong><\/p>\n

    You might ask: once a bond breaks into two radicals, what’s to stop it from re-combining? Good point! In fact – and this is often neglected in textbooks – it’s more proper to think of bond-breaking and bond-forming as being in equilibrium with each other. As we’ll go into details in the next post, only a very small concentration of free radicals is required for a reaction to take place.<\/p>\n

    \"-in<\/a><\/p>\n

    This first step – initiation – is often quite slow. In fact, free radical reactions are often observed to have an induction period<\/b><\/a>. That is, after all the reagents are mixed together, there is often a variable period of time where no reaction is observed, followed by a sudden – sometimes explosive! – acceleration of the rate.<\/p>\n

    In the case of chlorination of alkanes, as we’ll see, the generation of an unstable chlorine radical is followed by subsequent removal of a hydrogen atom from an alkane followed by chlorination of the carbon free radical, in a chain reaction.<\/p>\n

    In the next post we’ll talk about the three stages of a free radical reaction – initiation, propagation, and termination.\u00a0<\/strong><\/p>\n

    For more specific information about what is actually happening when heat or light interacts with a molecule and how this leads to fragmentation, read on below the fold to Note 2<\/a>.<\/p>\n

    Next Post: Initiation, Propagation, Termination<\/a><\/strong><\/p>\n

    \n

    <\/a>Notes<\/strong><\/h2>\n

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