3
$\begingroup$

I'm particularly looking in the context of alkyl lithiums compared to vinyl-lithiums and alkynyl-lithiums.

Why is it that shifting from sp3 to sp2, and even further to sp carbanions, reduces the basicity and allows for nucleophilic attack to occur preferably?

I understand that a greater s-character means the negative charge is held closer to the nucleus and therefore further stabilised, decreasing its drive to obtain a proton, but wouldn't this also decrease its drive to attack an electrophile?

$\endgroup$
4
  • 1
    $\begingroup$ The C-Li bond is so reactive in alkyl lithium compounds, that they tend to form aggregates except in solvents with donor atoms (like THF). This aggregate only exposes the alkyl groups to the outside, and buries the nucleophilic Li-C bonds inside. My guess is that this type of aggregation does not happen in alkynes, so they are more anionic in character, and their nucleophilic part is exposed. $\endgroup$
    – S R Maiti
    Apr 3, 2021 at 15:32
  • $\begingroup$ Basicity decreases but acidity increases when going from $\mathrm{sp^3}$-$\ce{C-H}$ to $\mathrm{sp^2}$-$\ce{C-H}$ to $\mathrm{sp}$-$\ce{C-H}$ regardless of it is an organo-lithium compound or not. $\endgroup$ Apr 3, 2021 at 19:38
  • 1
    $\begingroup$ @ShoubhikRMaiti aggregation does occur in $sp^2$ and $sp$ substrates, for example allyl lithium is highly aggregated ($n>=10$). My guess would be that there is a slight drop in nucleophilicity as the charge is better stabilised since it is held in a lower energy orbital, but the change is very small the change in nucleophilicity isn't really noticeable. All organolithium compounds are very reactive so a small drop/increase in reactivity isn't easily noticeable. $\endgroup$
    – H.Linkhorn
    Apr 3, 2021 at 19:58
  • $\begingroup$ @H.Linkhorn I don't know if allyl lithium can be really considered sp2, because if you consider the C-Li bond covalent, then it is sp3. Also I found this paper: www2.chem.wisc.edu/areas/reich/papers/… . It seems that sp2 (vinyl lithium) and sp (lithium acetylides) do aggregate, but the number is low, usually dimer or tetramer. So, I guess my original point still stands. There's more aggregation in alkyl substrates, usually n=4,6,8 etc. $\endgroup$
    – S R Maiti
    Apr 3, 2021 at 21:41

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.