Question

This question is irritating me up. In the given question, the -ve charge on O atom in (1) and (4) is delocalised, in (4) the charge is more delocalised then in (1) due to more resonating structures. Therefore, the (4) is least nucleophilic among the following. Then comes the (1) option. In (2) and (3) the charges are localised and hence they have higher nucleophlicity then 1 and 4. But among (2) and (3), in (2) although the O atom is more electronegative, there is a CH3 group showing +I effect, also the atom is sp3 hybridised, the less the %s character the less stable with the -ve charge on the atom is. On the other hand, in (3) there is no +I group and also the N atom is sp hybridised, %s character more hence more stable the atom is in holding the -ve charge. So the order of nucleophilicity must be 2>3>1>4. But the answer says 3>2>1>4. Help me out and please let me know where I am missing.

Answer 1

Nucleophilicity can be influenced by various factors, including the presence of an alkyl group, the hybridization state, and the ability of an atom to stabilize a negative charge. Electronegativity and the ability to donate lone pairs through resonance also play crucial roles, which could explain the observed order of nucleophilicity as 3>2>1>4 option (3).

Step-by-step explanation:

It appears there might be a misunderstanding regarding the nucleophilicity of the species in question. In options (2) and (3), the presence of an alkyl group with a +I effect and the hybridization state of the atom bearing the negative charge influence nucleophilicity.

While it is true that an sp3-hybridized oxygen as in (2) should theoretically be more nucleophilic due to lower s-character and thus less effective at stabilizing a negative charge compared to an sp-hybridized nitrogen as in (3), nucleophilicity is also greatly affected by other factors such as solvation and steric hindrance, which were not addressed in the original assessment.

According to Lewis structures and resonance stability criteria, when evaluating the potential of resonance structures, it's crucial to consider that structures with a negative charge on the more electronegative atom are generally more stable. Since nitrogen is less electronegative than oxygen, the negative charge is better stabilized on oxygen, which can lead to a reevaluation of nucleophilicity.

Additionally, nitrogen's lower electronegativity can make it more willing to donate its lone pair of electrons, increasing its nucleophilicity. This may partially explain why option (3) can be seen as more nucleophilic than (2). The concepts outlined in your references underline the relative influence of electronegativity, resonance, and formal charge distribution on stability and reactivity.