Is NH2 electron donating or electron withdrawing? [closed]

Question

Can anyone explain why NH2 shows -I in inductive effect and +R in resonance effect? I mean NH2 is ready to take electrons in inductive effect (with its electronegativity) but in the resonance effect, it is ready to donate electrons(with its lone pair) then how are there 2 different type of qualities for NH2?

Answer 1

You did not specify it in your question, but this topic is usually taught in the context of electrophilic aromatic substitution reactions, and I will use them as an example.

Let's say you go to the lab and perform a bromination reaction using benzene and aniline as your substrates. You will see that the reaction with benzene will require a catalyst and maybe some heat to proceed. Meanwhile, the aniline will be triply brominated just by adding Br2 to it. So you can be 100% sure that NH₂ is activating for electrophilic aromatic substitution reactions.

Now, you are not satisfied with an experimental result only, and want to explain it, so you test more substituents. You will notice that some of them speed the reactions, some make them slower. In fact, substituents in these two groups seem to have some properties in common.

For instance, substituents that bond to aromatic rings via an sp³ O or N seem to always speed the reaction. If the O/N is bonded to a carbonyl, the effect is less pronounced. Alkyl groups also speed the reactions, to a lesser extent.

Meanwhile, halogens, CF₃, and groups with pi bonds to electronegative atoms (carbonyl, sulfonyl, nitro) make the reactions slower.

This is what we know for sure.

Now, we can try to rationalize these experimental results. Decades ago, scientists postulated that these observed effects could be separated into resonance and inductive contributions (sterics is also important). +R, -R, +I, -I are part of a theory, something we create to explain experimental results.

Molecular orbital theory is a different approach, and you could say something like "the HOMO of aniline has a different energy compared to benzene".

So what does the theory say about NH₂? There is an electron pair on N that is conjugated with the aromatic ring. This conjugation increases the electron density on the ring, making it more reactive towards electrophiles (species that "want" electrons). We call this a +R effect.

What about the -I effect? N is more electronegative than C, and we know that in the absence of pi electrons, more electronegative groups (e.g. CF₃) tend to slow reactions. The -I effect of N will be more clear when you study nucleophilic aromatic substitutions with benzyne, for which the effect of pi electrons is null.

Bonus question: what is the product of the nitration of aniline? Consider the effect of the acidic environment on N and how it affects its electron-donating ability.