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Question

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The answer should be (D) as the inductive effect of 3 methyl groups increases negative charge on oxygen and thus makes it a good nucleophile.But the answer is (A).

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    $\begingroup$ D is the worst nucleophile because of the steric hindrance $\endgroup$ – Waylander Mar 16 '18 at 14:42
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According to me, the answer should be (C) in general cases but this might get changed to A) in special cases.
The best way to judge their nucleophilicity order is to check their basicity. The weak bases are generally better nucleophiles in polar protic solvents . The acidity order of the conjugate acids are $$\ce{CH_3OH < (CH_3)_3COH < NH_3 < CH_4}$$So, the strongest acid is $\ce{CH_3OH}$ and weakest acid is $\ce{CH_4}$. So, weakest base is $\ce{CH_3O^-}$ and strongest is $\ce{CH_3^-}$. So, the maximum nucleophilic character should be of $\ce{CH_3O^-}$,not generally $\ce{CH_3^-}$ as basicity is inversely proportional to nucleophilicity in general cases.
Coming to D), the inductive effect of three methyl groups actually decreases the acidity of tert-butyl alcohol. So, the tert-butoxide ion is more prone to take $\ce{H^+}$ rather than to nucleophilically attack. It also creates steric crowding, and preferably takes easily ionisable hydrogen ion if possible from the compound.So, looking only at increased $\ce{+I}$ effect doesn't tell that its nucleophilicity is increased.
About A), this $\ce{CH_3^-}$ is usually seen in Grignard reagents, whose basic activity is much much greater than its nucleophilic activity. If acidic hydrogen is present in the compound, it preferably takes that. But, if a compound doesn't possess any acidic hydrogen (special case), then its nucleophilic character is more as being less electronegative than $\ce{O}$, it is very less reluctant to keep yhe lone pair on itself like the case in $\ce{CH_3O^- or (CH_3)_3CO^-}$. Thus in the absence of any acidic hydrogen $\ce{CH_3^-}$ is better nucleophile.
So, from the above discussion, it can be concluded that in many of the cases, where the attacked compound doesn't possess any acidic hydrogen, A) is maximum nucleophillic. But in the opposite cases, C) is maximum nucleophilic.

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Nucleophilicity depends upon many other factors other than electron density . A predominating effect is that of steric crowding.
The tertiary butyl carbanion is overcrowded this restricts the ability of the nucleophile to donate its electron pair.Thus making it a bad nucleophile.


Finding out the best nucleophile from the remaining options is pretty simple.
Over here we will use electronegativity as the primary factor for determining the nucleophillic order.
More electronegative an element is stronger the pull on the electrons. Thus the methyl carbanion bearing the negative charge on the carbon is the best nucleophile. (Carbon is the least electronegative element between carbon,nitrogen and oxygen )


Note MasterOrganicChemistry has a well written post, differentiating between nucleophillicity and basicity

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  • $\begingroup$ What about (C)? It doesn't have crowding and has CH3 attached to it which further increases its nucleophilicity. $\endgroup$ – Isha Roy Mar 16 '18 at 14:58
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    $\begingroup$ @IshaRoy Oxygen is more electronegative than carbon so the negative charge is more strongly bound to oxygen(option C) than carbon (option A) $\endgroup$ – Avnish Kabaj Mar 16 '18 at 15:00
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    $\begingroup$ Basicity is not a subset of nucleophilicity. $\endgroup$ – Tan Yong Boon Mar 16 '18 at 15:05
  • $\begingroup$ chemistry.stackexchange.com/questions/20061/… $\endgroup$ – Tan Yong Boon Mar 16 '18 at 15:05
  • $\begingroup$ Edited it . ...... $\endgroup$ – Avnish Kabaj Mar 16 '18 at 15:38

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