8
$\begingroup$

Which compound is more basic of the two?

A: 1,2‐dihydropyridine; B: 1,4‐dihydropyridine

My thinking: In compound A, the lone pair of nitrogen are in resonance with two other double bonds. The resonance chain here seems to be longer as compared to in compound B. However, in compound B, the lone pairs of nitrogen seem to be more 'busy' because of the two double bonds on its either side,when compared to compound A (@YUSUFHASAN provided this second logic).

So, which of the two compounds should be more basic? (The main idea is that, the lesser 'busy' lone pair electrons of nitrogen means more 'free' electrons to pick up $\ce{H^+},$ which is what bases do.)

Do note that this question is supposed to be answered qualitatively. (By this I mean, not looking at tabular data from research papers or books!)

$\endgroup$
6
$\begingroup$

In compound $\ce{A}$ , lone pair on $\ce{N}$ is in conjugation with two $\ce pi$ bonds (as shown below). Therefore this lone pair is delocalized (resonance structures in the figure below) to a greater extent.

In compound $\ce{B}$ , lone pair on $\ce{N}$ is in conjugation with a single $\ce pi$ bond , while the other pi bond is not in conjugation or is cross conjugated.

Since lone pair on $\ce{N}$ in compound $\ce{B}$ is less delocalized then in compound $\ce{A}$ , compound $\ce{B}$ is more basic then compound $\ce{A}$.

enter image description here

$\endgroup$
  • $\begingroup$ So what you're saying is that, cross-conjugation is of not much importance when it comes to delocalization of the lone pair electrons? So whenever there are same number of electrons, and if they are in regular conjugation (as in my compound A) the delocalization will always be more than a cross conjugated system (as in my compound B)? I would love to draw some conclusions from your answer :) $\endgroup$ – McSuperbX1 Jul 30 at 4:19
  • $\begingroup$ General tech note: bright green color is usually not a good choice for presentation, especially combined with the red color. The majority of occurrences of color blindness is related to green color, and, especially when combined with the red color, this makes reading such illustrations for people suffering from deuteranopia quite troublesome. If possible, avoid using any color and stick to black-and-white images: this is a good tone for any scientific/technical publication. Also, on the nitpicking side, try to use font of the same size (and typeface) on the pictures and in the text. $\endgroup$ – andselisk Jul 30 at 4:36
  • 2
    $\begingroup$ Although the OP said that he wanted to do this qualitatively... Yet I would be grateful if you could pull up some pKa values for both these compounds above to support your rationale and prove mine wrong $\endgroup$ – YUSUF HASAN Jul 30 at 8:46
  • $\begingroup$ @ChakravarthyKalyan could you please repsond to my comment above? Thank you! I would love to have a conclusion drawn :-) $\endgroup$ – McSuperbX1 Jul 30 at 9:53
  • $\begingroup$ @McSuperbX1 , I am currently in work area , which forbades me to answer. I will respond , when I am back to my PC . $\endgroup$ – Chakravarthy Kalyan Jul 30 at 12:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.