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So, an exercise wants me to compare these three amines: enter image description here

With respect to their basicity. I know that $\ce{NO2}$ and $\ce{Br}$ groups with "draw" the electric density to themselves, increasing the acidity of the amine. (This is all the information i have. I'm not a chemistry student / chemical engineering student, so i don't have a great background in organic chemistry). So, i got a few questions:

1 - Is $\ce{HCO}$ a group that will "draw" electric density? Or repel? I'm not sure of that.

2 - Who is stronger (in terms of drawing electric density), $\ce{Br}$ or $\ce{NO2}$? I don't know how to compare these things.

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  • $\begingroup$ Write mesomeric structures... $\endgroup$ – Mithoron Nov 17 '16 at 13:37
  • $\begingroup$ @Mithoron i'm sorry, but o don't know how to that in this case.. $\endgroup$ – Vitor C Goergen Nov 17 '16 at 13:39
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1 - Is $\ce{HCO}$ a group that will "draw" electric density? Or repel? I'm not sure of that.

It is moderately electron withdrawing. Draw resonance structures

Who is stronger (in terms of drawing electric density), $\ce{Br}$ or $\ce{NO2}$ I don't know how to compare these things.

$\ce{NO2}$ is much stronger.

For 4-nitroaniline, $pK_\text{a} = 1.01^{[1]}$

For 4-bromoaniline, $pK_\text{a} = 3.86^{[2]}$

Between the two, 4-bromoaniline is more basic, since as you noted correctly, stronger electron withdrawing substituents increase the acidity of the amine. Since $\ce{-CHO}$ group moderately electron withdrawing (intermediate between bromo and nitro groups), the order of basicity (most basic to least):

4-bromoaniline > 4-aminobenzaldehyde >4-nitroaniline

[1] https://pubchem.ncbi.nlm.nih.gov/compound/7475#section=pH

[2] https://pubchem.ncbi.nlm.nih.gov/compound/7807#section=LogP

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  • $\begingroup$ Thanks a lot. Comparing groups such as $\ce{NO_2}$ and $Br$ will always be like: " you have to remember that X is stronger than Y"? $\endgroup$ – Vitor C Goergen Nov 17 '16 at 19:51

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