Dewar and Healy (1982)

All life depends on Carbon-Carbon bonds. Carbon is reactive enough for some reactions to occur but not so reactive that it can’t form stable compounds. Carbon is the goldilocks level of reactivity and Dewar explains why. Using computational methods he authored, Dewar examines the heat of formation for a variety of Carbon and Silicon molecules as well as the bond lengths of transition states. I think a grasp of an SN2 reaction coordinate diagram would be very useful in understanding the paper and the results.

In the event you lost your paper for class tomorrow or just prefer a digital copy, here’s a link to the paper.

http://pubs.acs.org/doi/pdf/10.1021/om00072a029
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6 Responses to “Dewar and Healy (1982)”

  1. Hunter Burgin Says:

    To my understanding, figure two shows the minimum energy reactions for three different SN2 reaction with the first being X- + RX, the second being Y- + RX, and the third being X- + SiH3Cl. This graph poses several immediate questions in my mind, the first being how exactly can you recognize (and kinetically record) a reaction occurring when the nucleophile is identical to the leaving group as in equation a. I was also curious as to what exactly is happening in equation c where the intermediate is clearly very favorable and low in energy. What causes the intermediate to overcome the energy barrier and result in the higher energy product X-SiH3 + Cl-?

  2. Antony Says:

    I was thinking this paper was going to go another direction. In a way it gave me a “42”. It was interesting to see how the inability of carbon to form hypervalent compounds account for its “tameness”. This is a great follow up to seeing Olah talk about how reactive carbon can be.

    I have a question about compounds 2 and 4. Are they the same thing? Is one just an intermediate of the other?

    Also, on a more serious note: Do you think the lack of sterics of smaller molecule might allow for hypervalent C O or N molecules? Such as H, Li, or Be?

  3. Daniel Begay Says:

    It’s definitely difficult exploring new territory when dealing with computational chemistry as you need to have some type of preliminary results to base them off. So it is interesting to see that don’t try to replicate the same reactions in Table II with there results in Table I. When it comes to explaining the the strength of the the transition state, they look to the bond lengths. If I’m gathering this correctly, the longer the bond length, the weaker the transition state? So it is easily destabilized? While the stronger the transition state, the shorter the bond? Does this explain why in Table I when they state that the bond length are 25% longer, CN- CH3F is still relatively the same length?

  4. Adam Settimo Says:

    I feel like I’m a little confused with the automatic jump to the conclusion that the relative reactivities of Si and C being attributed to access to the 3d AO’s for Si? The energy gap for between the 3 p and 3 d is quite substantial. Why was this considered before sterics?

  5. Kevin Greenwood Says:

    Three-center bonds have been on my mind since we discussed borane earlier in the semester, though it had not occurred to me to think of an SN2 transition state as being one. Like Daniel, I am a little caught up with the bond lengths and energies given in table 1.

    In the fourth line, it was calculated that the bond length for methyl fluoride is slightly longer than the bond of methyl cyanide. The bond enthalpies show that this reaction is overall endothermic and not really favorable, but the bond length data shows a shorter bond, indicating the nitrile group is pulling harder on the carbon center than the fluoride was, despite its larger size and smaller electronegativity.

    Something similar is occurring in the sixth line, where the bond enthalpies show an overall exothermic reaction for fluoride displacing bromide on methane. However, the bond lengths show a shorter bond between carbon and bromine then between carbon and fluorine. This data is contrasts to the bottom line of table one, where the formation of dimethyl ether from methyl bromide shows an exothermic reaction and a decrease in bond length between carbon and the nucleophile, which one could reasonable expect for this reaction.

    Why does this data seem strange for one set of calculations, but reasonable for another? What is it about fluoride that would give these results?

  6. Josh Ellsworth Says:

    I can appreciate the use of bond lengths to talk about relative stability, and that Halogen substitutions provide a convenient platform to compare the bond lengths of Carbon and Silicon. Why was there no comparison made between alkanes and silanes? When building a life form it’s important to have multiple repeating units, never mind chirality and all that other stuff. Wouldn’t it have been just as easy, or at least complimentary, to conduct a study on the lability of the C-C bond as compared to the Si-Si bond?

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