1
$\begingroup$

I found a nucleophilicity order as

$\ce{HO-} > \ce{CH3-O-} > \ce{(CH3)2CH-O-} > \ce{(CH3)3C-O-}$

The reason was given that bulky groups reduce nucleophilicity. But why?

The order of the $+I$ effect is $\text{tertiary} > \text{secondary} > \text{primary}$. Due to the greater $+I$ effect, the electron on oxygen must experience greater repulsion and thus the order of neucleophilicity must be reversed. Also, according to the given order of nucleophilicity a tertiary alcohol is more acidic than a primary alcohol. But I have read that presence of more $-I$ effect increase acidity of alcohols like $\ce{Cl-CH2-OH}$ is more acidic than $\ce{CH3-OH}$. Similarly due to more $+I$ effect in $\ce{(CH3)3C-OH}$, it should be less acidic than $\ce{(CH3)2CH-OH}$ which should be less acidic than $\ce{CH3-CH2-OH}$.

$\endgroup$
  • 3
    $\begingroup$ Nucleophilicity is a kinetic property. It is about how good a species is at attacking a carbon centre. If the species in question is very large it just can't get close enough to the electrophilic centre to react. $\endgroup$ – bon Feb 5 '17 at 11:26
  • 1
    $\begingroup$ Then what about acidity of alcohols? Is tertiary alcohols more acidic than primary alcohol. $\endgroup$ – Avi Feb 5 '17 at 11:28
  • $\begingroup$ No. Acidity is a thermodynamic property relating to the relative stabilities of the acid and the conjugate base. Basicity and nucleophilicity are not the same thing. $\endgroup$ – bon Feb 5 '17 at 11:33
  • $\begingroup$ Does this mean that in this case Basicity order is opposite of the given order? $\endgroup$ – Avi Feb 5 '17 at 11:42
  • 1
    $\begingroup$ Yes it does. It is not unusual for the order of basicity to be different to the order of nucleophilicity. $\endgroup$ – bon Feb 5 '17 at 12:26
4
$\begingroup$

$$\Large\color{red}{\text{nucleophilicity} \ne \text{basicity}}$$

Please memorise firmly that nucleophilicity and basicity, albeit related in many ways, are different physico-chemical properties. Most importantly, basicity is a thermodynamic property while nucleophilicity is a kinetic property. Therefore, it is not uncommon for compound $\ce{A}$ to be more basic but less nucleophilic than compound $\ce{B}$.

This is the case here, too. It is unquestioned and supported by the corresponding $\mathrm{p}K_\mathrm{a}$ that tert-butanolate is the strongest base, followed by isopropanolate followed by ethanolate. This is due to the $+I$ effects you correctly outlined.

Nucleophilicity does not parallel this series since the it does not abide by the same rules. For a compound to be nucleophilic, the nucleophilic atom’s lone pairs must be able to actually attack, i.e. reach, the electrophilic site it wishes to attack. This means two molecules interacting and not just one molecule interacting with a proton which is so small it can even tunnel. In tert-butanolate, you have three α-methyl groups that take up space and make said interaction harder. Due to this steric interaction, tert-butanolate is an absolutely lousy nucleophile (while it is a great base).

It is similar for isopropanolate which still has too much steric hindrance to be an effective nucleophile although the conditions are better than for tert-butanolate.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.