![]() Here we’re going to keep H as the variable which is the same, and by examine the trends which influence free radical stability in a new light. If we keep one variable constant and vary the other variable, we can analyze the influence of structure on free radical stability. Low bond dissociation energies reflect the formation of stable free radicals, and high bond dissociation energies reflect the formation of unstable free radicals. The bottom line for this post is that b ond dissociation energy is correlated to free radical stability. Bond Dissociation Energy Is Correlated With Free Radical Stability , and we learned previously that the stability of free radicals decreases as we go from left to right across the periodic table, since O is more electronegative than C and that partially empty orbital is being held more closely to the positively charged nucleus.That leads us to comparing the stability of H 3C What’s different is the identity of the other radical. Note how we’re forming a H radical in both cases. Here’s an example of what we need to look at in this case: Instead of the stability of the ions ( heterolytic cleavage!) we need to look at the stability of the free radicals ( homolytic cleavage). If you’re thinking about BDE’s and acid-base reactions, you’re using the wrong mental model.Īcid base reactions involve heterolytic cleavage and BDE’s are a measure of homolytic cleavage. However when we look at the BDE’s, we see that HO–H is 118 kcal/mol and H 3C–H is 104 kcal/mol. Which has the weaker bond to H ? Thinking back to some of the chemistry we’ve talked about earlier, such as acid base reactions, it might be tempting to say that O–H is weaker than C–H, since we can think of many strong bases which will deprotonate water, but very few that will deprotonate alkanes. Take two molecules – methane (CH 4) and water (H 2O). ![]() Let’s look at a quick representative example. Why Does Water Have A Higher Bond Dissociation Energy Than Methane? The purpose of this post is to help connect the concept of bond dissociation energies with free radical stabilities. Therefore, BDE is essentially a measure of free radical stability. What many people take some time to realize is that BDE is a measure of the energy required for homolytic bond cleavage, and as we discussed earlier, homolytic bond cleavage leads to the formation of free radicals ]. You might be familiar with it already! There’s probably a table of bond dissociation energies in your textbook, usually within the first 100 pages or so. That’s OK, because interestingly, there is one measurement which can help us keep all of these factors straight. You might initially find it hard to keep track of the factors we mentioned. ![]() There were a total of six factors we discussed. The bottom line is that radicals are electron deficient and that any factor which either helps to donate electron density to the half-filled orbital, or to spread that unpaired electron out over a larger volume (a.k.a “delocalize” it) will stabilize the radical. Over the last two posts we’ve been going through the factors which affect the stability of free radicals.
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