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7.10 The Hammond Postulate

  • Page ID
    44305
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    Objective

    After completing this section, you should be able to use the Hammond postulate to explain the formation of the most stable carbocation during the addition of a protic acid, HX, to an alkene.

    Key Term

    Make certain that you can define, and use in context, the key term below.

    • Hammond postulate

    Study Notes

    The transition state shown in Figure 7.16 on page 248 of the textbook could be represented as follows:

    The hatched line (/ / / / / / / /) represents a partially formed bond or a partially broken bond, and the two symbols indicate that the positive charge is shared between the incoming hydrogen and the carbon that ultimately carries the whole charge. You could still argue that a developing tertiary carbocation is more stable than a developing secondary carbocation, because the presence of a greater number of alkyl groups in the former allows for better dispersal of charge.

    Now, back to transition states. Chemists are often very interested in trying to learn about what the transition state for a given reaction looks like, but addressing this question requires an indirect approach because the transition state itself cannot be observed. In order to gain some insight into what a particular transition state looks like, chemists often invoke the Hammond postulate, which states that a transition state resembles the structure of the nearest stable species. For an exergonic reaction, therefore, the transition state resembles the reactants more than it does the products.

    image038.png

    If we consider a hypothetical exergonic reaction between compounds A and B to form AB, the distance between A and B would be relatively large at the transition state, resembling the starting state where A and B are two isolated species. In the hypothetical endergonic reaction between C and D to form CD, however, the bond formation process would be much further along at the TS point, resembling the product.

    image040.png

    The Hammond Postulate is a very simplistic idea, which relies on an assumption that potential energy surfaces are parabolic. Although such an assumption is not rigorously true, it is fairly reliable and allows chemists to make energetic arguments about transition states by employing arguments about the stability of a related species. Since the formation of a reactive intermediate is very reliably endergonic, arguments about the stability of reactive intermediates can serve as proxy arguments about transition state stability.

    The Hammond Postulate and the SN1 Reaction

    the Hammond postulate suggests that the activation energy of the rate-determining first step will be inversely proportional to the stability of the carbocation intermediate. The stability of carbocations was discussed earlier, and a qualitative relationship is given below:

    sn1diag.gif

    Carbocation
    Stability
    CH3(+) < CH3CH2(+) < (CH3)2CH(+) CH2=CH-CH2(+) < C6H5CH2(+) (CH3)3C(+)

    Consequently, we expect that 3º-alkyl halides will be more reactive than their 2º and 1º-counterparts in reactions that follow an SN1 mechanism. This is opposite to the reactivity order observed for the SN2 mechanism. Allylic and benzylic halides are exceptionally reactive by either mechanism.

    Contributors


    7.10 The Hammond Postulate is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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