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What are the most common critical thermodynamic and kinetic variables required to convert a secondary or tertiary alcohol to an alkane.? Is bond enthalpy data relevant?

What is the process/processes commonly used to then calculate or determine the temperature and duration it would take to complete the reaction from the thermodynamic and kinetic data?

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  • $\begingroup$ @ron says in the answer to the question about diamonds that from bond activation energy of the c-c bonds it can be calculated that at 25°c it would take diamond a over a billion years to convert to graphite, can someone please explain how this was calculated from the bond enthalpy ? $\endgroup$ – Technetium Jul 20 '15 at 10:27
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    $\begingroup$ There isn't enough information with just the bond energies, which are thermodynamic properties. The question you're asking is about kinetics, which requires activation energy parameters. $\endgroup$ – jerepierre Jul 20 '15 at 16:16
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    $\begingroup$ chemistry.stackexchange.com/a/34202/16683 This is the answer referenced. In his answer, Ron cites a geochemistry worksheet from Cornell; this worksheet in turn cites Pearson et al., 1995: "Pearson et al. (1995) estimated that to convert 1 cc of diamond to graphite at 0.1 Mpa and 1000°C would require 1 billion years, but only a million years would be required at 1200°C." If someone could find that paper, then maybe we could answer this question - I can't seem to find it. $\endgroup$ – orthocresol Jul 20 '15 at 17:10
  • $\begingroup$ Thanks for the comments I'm realy trying to work out how that data was obtained. If it was calculated from the c-c bonds then same style of data could be calculated for the conversion of any material ? $\endgroup$ – Technetium Jul 21 '15 at 3:47
  • $\begingroup$ Why is the question marked down? $\endgroup$ – Technetium Jul 21 '15 at 3:48
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You can't - at least if the only information you have is just the bond energy.

The problem you're running up against here is the difference between thermodynamics and kinetics. The difference can sometimes be subtle, but (roughly) thermodynamics is normally concerned with state functions, whereas kinetics deals with the process of interconversion of states.

The bond enthalpy is a thermodynamic quantity. It's based off a state function. It's the free energy difference between the bond-broken and bond-formed states. It doesn't matter how you go from the bond-formed to bond-broken states, the free energy difference is the same. In order to calculate it, you only need to know information about the two endpoints, not the details about the states which connect them.

In contrast, reaction rates are kinetic quantities. The rate of reaction is highly dependent on the path (reaction mechanism and conditions) you take to interconvert the states. You can't tell anything about the path if all you have is thermodynamic information about the two end points.

That said, you can theoretically calculate reaction rates if you have additional information about the path you're taking. Most notably you would need to know the activation energy of the reaction you're interested in. This is different from the bond energy, and is related to the free energy of the transition state, the hypothetical high-energy intermediate along the reaction pathway. If you have the activation energy along with some other information for a single-step reaction, you can calculate reaction rates for various temperatures with the Arrhenius equation.

Again, these rates are highly dependent on the exact mechanism of the reaction you're examining. Single step versus multi-step, radical versus electron pair transfer mechanisms, the presence of catalysts, etc. all can change the activation energies of the various steps and the reaction rates. So there is no way to give a general answer about bond breakage rates.

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  • $\begingroup$ Thanks for a good answer I'll study the things you mentioned ... $\endgroup$ – Technetium Jul 29 '15 at 10:06

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