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I know that the standard textbook answer is that a polar protic solvent is able to better stabilize/solvate carbocations through hydrogen bonding.

I understand that a solvent such as water or methanol is capable of hydrogen bonding, but what about the substrate? The substrate is a hydrocarbon; how is the substrate capable of reciprocal hydrogen bonding?

Is this stabilization better described as a dipole-ion interaction rather than a hydrogen bond interaction because I don't see how a hydrocarbon can form reciprocal hydrogen bonding interactions.

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    $\begingroup$ The standard explanation for the rate enhancement of SN1 in a polar protic solvent is hydrogen bonding to the leaving group, which stabilizes the building negative charge in the transition state (i.e., not the carbocation). I'm not familiar with the term reciprocal hydrogen bonding. Can you provide an example of a compound capable of this? You are right that a hydrocarbon cannot participate in hydrogen bonds. $\endgroup$
    – jerepierre
    Aug 8, 2014 at 15:24
  • $\begingroup$ I know water can participate in hydrogen bonding because each water molecule has a partially positive end and a partially negative end. So water molecules can hydrogen bond among themselves. But what about carbocations ... is this dipole/ion interaction a true form of hydrogen bonding? $\endgroup$
    – Dissenter
    Aug 8, 2014 at 15:53
  • $\begingroup$ Hydrogen bonding is a very specific type of interaction where a polarized X-H bond interacts with lone pair. The carbocation doesn't fit either component of that definition (in any traditional sense anyway). $\endgroup$
    – jerepierre
    Aug 8, 2014 at 17:14
  • $\begingroup$ what is reciprocal hydrogen bonding? $\endgroup$
    – RE60K
    Aug 8, 2014 at 17:33

1 Answer 1

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I know that the standard textbook answer is that a polar protic solvent is able to better stabilize/solvate carbocations through hydrogen bonding.

In an $\ce{S_{N}1}$ reaction, a polar protic solvent can stabilize the leaving group through both hydrogen bonding and solvent dielectric effects. While these same solvent dielectric effects can also stabilize the carbocation intermediate, I don't think hydrogen bonding plays a role in the stabilization of the carbocation itself. First off, any protic molecule close enough to the carbocation to hydrogen bond would simply react with it instead of forming a hydrogen bond. Second, since the carbocation is positively charged it cannot form a hydrogen bond. Again, solvent stabilization of the carbocation occurs through dielectric (dipolar) effects of the medium.

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  • $\begingroup$ Can you explain the left half of this picture? 50.57.82.128/sites/default/files/userfiles/48/uploaded_images/… $\endgroup$
    – Dissenter
    Aug 8, 2014 at 17:13
  • $\begingroup$ Not really, I see water hydrogen bonding with itself, but I don't see any dashed lines between water and the carbocation indicating any type of interaction. So I am puzzled by their statement "H-bonds stabilize the carbocation." $\endgroup$
    – ron
    Aug 8, 2014 at 17:22
  • $\begingroup$ Yeah I don't understand it either. $\endgroup$
    – Dissenter
    Aug 8, 2014 at 17:23
  • $\begingroup$ What about this can you explain this? o.quizlet.com/6zi2Vb-RZvTlH5S.YMksMg_m.png $\endgroup$
    – Dissenter
    Aug 8, 2014 at 17:39
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    $\begingroup$ "...effective solvents because anions form hydrogen bonds with the hydrogen atom" - that's fine, that's H-bonding with the nucleophile and leaving group. "cations complex with the nonbonding electrons of the oxygen atom" well, that's OK I guess, I'd prefer to say "cations are stabilized by dipolar interaction with the nonbonding electrons on oxygen, as well as through the dielectric properties of the solvent as a whole". Complex, stabilization - kind of similar, I guess. $\endgroup$
    – ron
    Aug 8, 2014 at 20:18

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