I was reading about nucleophilicity and basic strength orders. I came across this interesting set of three compounds $\ce{NH3},$ $\ce{N2H4}$ and $\ce{NH2OH}$ where I am not able to apply the rules I have learnt.

The correct order of basic strength is given to be:

$K_\mathrm b$ value: $\ce{NH3} > \ce{N2H4} > \ce{NH2OH}$

The rules I know are:

  1. If the donor atom is same, then order of nucleophilicity is same as the order of basicity.
  2. Alpha effect: if the adjacent atom attached to the donating atom has lone pair of electrons, then nucleophilicity is enhanced.

When I try to conclude the nucleophilicity order, I am getting contradicting results with the above rules. Also, I wonder whether the −I effect due to the presence of highly electronegative atom may have a role to play.

So, what is the correct order of nucleophilicity of $\ce{NH3},$ $\ce{N2H4}$ and $\ce{NH2OH}?$

Is it theoretically possible to conclude the correct order based on deciding which factor will dominate in a given situation?


My doubt is pertaining to specifically these three compounds.Many have said that this question is a duplicate of Is there a general consensus on the causes of the alpha-effect? But, I am not able to get the order of nucleophilicity that I need from the answers to that question.I am not able to find the information on order of nucleophilicity of these three compounds in the books I use.So, it would be helpful if someone clarifies the position of $\ce{NH2OH}$ in this.

  • 2
    $\begingroup$ Does this answer your question? Is there a general consensus on the causes of the alpha-effect? $\endgroup$
    – Mithoron
    Jun 29 '20 at 18:12
  • $\begingroup$ @Mithoron I have already seen that question.But, I am not able to get the answer to my question over there $\endgroup$ Jun 29 '20 at 18:58
  • $\begingroup$ Well, nucleophilicity has little to do with basicity, that's why alpha effect was coined. $\endgroup$
    – Mithoron
    Jun 29 '20 at 19:03
  • 1
    $\begingroup$ But what would be the correct order then?I saw this somewhere:N2H4 >NH3>NH2OH first one eventhough Kb is less than ammonia due to lone pair-lone pair repulsion and larger size it becomes a better nucleophile and -I effect of oxygen dominate over alpha effect.Is this correct? $\endgroup$ Jun 29 '20 at 19:08
  • 1
    $\begingroup$ chemistry.stackexchange.com/questions/83072/… $\endgroup$
    – Mithoron
    Jun 29 '20 at 19:18

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