Iodination of alkanes using iodine (I₂) is usually ...

Question

Iodination of alkanes using iodine (I₂) is usually an unfavorable reaction. (See Problem 4-17 , for example.) Tetraiodomethane (CI₄) can be used as the iodine source for iodination in the presence of a free-radical initiator such as hydrogen peroxide. Propose a mechanism (involving mildly exothermic propagation steps) for the following proposed reaction. Calculate the value of ΔH for each of the steps in your proposed mechanism.

The following bond-dissociation energies maybe helpful:

The following bond-dissociation energies maybe helpful:

Accepted Answer

Welcome back everyone in the eye donation of ethane shown below. Tetra bio methane ci four is used as the source of iodine in the presence of a free radical initiator such as hydrogen peroxide. Propose a mechanism for the reaction and determine the value of entropy change for each of the steps in your mechanism using appropriate bond dissociation energies. So we're given our reaction below. So we have dimethyl reacting with our tetra bio methane to produce ethyl iodide and H. C. I three, which is the compound iota form and were given a chart with our different bonds involved and the bond association entropy for the overall reactions for the mechanism. So let's begin by highlighting how this mechanism even occurs. So, according to our chart, we start with our peroxide which is H. 0-0. H. In condensed form. And we should recognize that the first step of this mechanism is going to be an initiation because we have to generate a radical. And so in an initiation step, recall that heat is required. So recall that we represent that with a delta sign over our arrow for heat required. And so he is going to be our energy supply to each of our atoms here in this home elliptic cleavage. That will occur with this bond. And that makes sense as to why we're given an entropy change of plus 213 kg per mole because that is the energy supply to our atoms and as a result will form to most of O. H radical products where we have a radical on our oxygen's and again note that this positive value for entropy means that we had bonds that are broken and energy is absorbed by our atoms. And so now in our next step of our mechanism which will say is step two, we can use one mole of our hydroxide radicals to react with one mole of our molecule. I bonded to a carbon bonded to three iodine atoms here on the other end. And recall that a compound with a radical being our hydroxide radical here is very reactive. And so this is the next step of our mechanism which will consider our first propagation step in which we'll have a radical attack. And we should actually draw it in this way so that it's easier to show. But our oxygen is going to attack the I died where the I died will use one of the electrons in its bond that it shares with carbon to form a bond with our oxygen. And this is going to result in a another radical formed back on carbon here from the remaining electron in the bond. And as a result will form the falling products where we now have oxygen sharing a bond with I died. And then we have a radical of carbon produced. And so notice that we have a bond that is broken here between iodide and carbon supplying an absorption of energy to our carbon atom to create our carbon radical. And according to our chart to break that bond we require a total of positive 188 kg joules of mol of energy that is required slash absorbed And then notice that we also had a bond formed which will say is at this point Between oxygen and I died. And according to our given chart from the prompt to form that bond we have an entropy that is going to be actually negative since energy will be released from the formation of a bond being negative kg jewels Permal. And note that again, energy is released because the potential potential energy of our atoms once a bond is formed will decrease. So that release of energy will result due to there now being a shared electron cloud between oxygen and I did. And so because we have two things happening here, a bond being broken and a bond being formed, we need to calculate our net entropy. So our net entropy for step two is going to equal the sum of our two n therapies above which is positive 1 88 plus negative 2 30 for leaving us with a net negative 46 killed jules per mole of entropy for step two in our mechanism. And that means that we should recall a negative net entropy for a reaction means that the reaction is primarily exo thermic. So step two is a very exotic thermic step and now let's move on to step three of our mechanism in which we have H three CH two C. Bought it to h reacting with our radical generated from before we have the carbon radical bonded to three iodine atoms. So likewise before our very reactive radical is going to attack our hydrogen where hydrogen will use one of the electrons in its bond with carbon to form a bond with our carbon radical, leaving behind another carbon radical. And so we have again a bond that is broken and then a bond formed between carbon and hydrogen. And so as a result, we have the following product where we have an ethyl radical CH three CH two with the radical on the carbon being the CH two and then our second product is going to be I three C. With now this carbon bonded to hydrogen. And so according to our chart, in order to break the bond between carbon and hydrogen, we have an entropy of energy required slash absorbed equal to positive 423 kg jewels Permal. And then for our bond broken being between carbon Or sorry, our bond formed between carbon and hydrogen, we have an entropy change according to our chart of -418 killed jules per mole. Since again, energy is released from the formation of this bond between carbon and hydrogen. And so we need to calculate our net entropy for step three which is going to equal the sum of our two entities from bonds broken and bonds formed, which results in a value of positive five kg per mole. And since entropy is positive, our net entropy for step three, we can say that Step three is primarily an endo thermic step or reaction, meaning that a majority of Step three involved needing a supply of energy in order to break our first reactant bond. And now let's move on to step four being our final step of our mechanism. Note that Step three was our second propagation step since we had another radical attack. And step four is going to be our third propagation since we have another radical to react with. And so we would begin with our radical we generated from before which was our ethyl radical with the radical on the ch two carbon. And this is going to react with another molecule of I bonded to carbon bonded to three iodine atoms. And so yet again our very active radical carbon will attack our ID ID, which will use one of its electrons in his bond with carbon to form a bond with carbon Leaving behind another radical CI three. And so as a result we have a bond that sorry, this should be purple for bond broken between iodine and carbon. And then we have a bond form between carbon and I died from our reactant. So we would form the following products where we have a church three C H two C bonded to I died. And then plus our carbon CI three radical. And according to our chart, our entropy change for our bond broken between died and carbon is equal to positive 420 or sorry positive kill it jules Permal. And then for our bond broken between carbon and I died we have a net entropy according to our chart equal to and sorry just to make a correction before I continue. So for our bond broken between I died and carbon going back to our chart. That is for this molecule here which corresponds to actually 188. So one positive 188 kg joules per mole. Since energy is going to be absorbed in the breaking of a bond. And then for again our entropy change of bond formed between our carbon radical and our I died will be according to our last point of our chart being a value of negative 238 kg joules per mole of energy that is released from the lowering of our potential energy of carbon and I died. Now being in a bond with a shared electron cloud. And so now for step four we need our net entropy by adding these two entities together net delta H. For step four which is equal to a result of negative 50 kg joules per mole. And because our net entropy is negative for step four that means that this step is primarily eggs a thermic involving the release of energy. And so now we just need to supply a overall entropy change for the given overall reaction above in the prompt and so recall that the net entropy change of our overall reaction not for a mechanism but for the overall reaction being our mechanism entirely is going to be only based on our propagation steps. So note that we had three total propagation steps in which from our first propagation step we found a net entropy change of negative 46 kg jules Permal which will be added to Our second propagation step entropy change which we found to be as positive five kg Permal and then added to our propagation entropy for our third propagation step which is here as negative 50 kila jules per mole. And so this all makes up the sum of our entropy change for the reaction overall and this will equal a value of negative 91 kg jewels Permal as our net entropy change for our overall reaction. And so what we have highlighted in yellow represents our entropy change for the overall reaction as well as steps one through four compensating our entire mechanism for the coordination of ethane. So our final answer is highlighted in yellow correspond to choice A as our final correct answer to complete this example. So I hope this helped and let us know if you have any questions

Key Concepts

Free Radical Mechanism

Bond Dissociation Energy (BDE)

Enthalpy Change (ΔH)

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