this question relates to the conversion of alcohols to alkyl halides. I understa
ID: 896620 • Letter: T
Question
this question relates to the conversion of alcohols to alkyl halides. I understand that primarily the hydroxy group needs to be protonated by an acid in order to conver it into a lgood leaving group(weak negativey charged base/stable nonreactive). However, even after that the comparison of basicity of R-OH2+ with respect to X- does not satisfy the conditions need for a SN mechanisms. We learned that a reaction favors SN meachisms if and only if the leaving group(H2O) is weaker base/ more stable-nonreactive with respect to the nucleophile(X-). One way we can measure their basicity is to compare the pka of their Conjugate Acids. H3O+ has a pka of -1.7, while HCl has a pka of -7. This implies that HCl is the stronger acid or Cl- is the weaker base/most stable-nonreactive, not complying with the rules of SN mechanisms. PLease explain why is the reaction can still undergo if it does not.
Explanation / Answer
Nucleophiles are also bases. Though the converse is also true, the two terms nucleophile and base are not synonymous. Remember the earlier is related to the rate of the reaction and the latter is related to the equilibrium of an acid-base reaction. So, the nuclophilicity is the kinetic property, hence you can't always relate to the basicity. Here in this case, H3O+ is a good leaving group due to it's bulkiness, at the same time Cl- has diffuse electron clouds which makes it as a better nucleophile.
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