# Nucleophilicity of alcoxy groups

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 $\ce{CH3-CH2-OH}$.

• 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. – bon Feb 5 '17 at 11:26
• Then what about acidity of alcohols? Is tertiary alcohols more acidic than primary alcohol. – Avi Feb 5 '17 at 11:28
• 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. – bon Feb 5 '17 at 11:33
• Does this mean that in this case Basicity order is opposite of the given order? – Avi Feb 5 '17 at 11:42
• Yes it does. It is not unusual for the order of basicity to be different to the order of nucleophilicity. – bon Feb 5 '17 at 12:26

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