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My textbook claims that -

Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: primary > secondary > tertiary.

Now I have a problem with the first line.

Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base.

The substituted ammonium cation is a conjugate acid (because its lost a proton/$H^+$). So if the conj. acid is more stable then the corresponding amine should be more acidic and hence less basic. So is the textbook wrong or am I missing something here?

Citation: Page 399, Chapter 13, NCERT Chemistry Part 2

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    $\begingroup$ "Greater is the X, stronger is the Y" is grammatically incorrect. You should write "The greater the stability of the ammonium cation, the stronger the corresponding amine is as a base"; or "The more stable an ammonium cation is, the more basic the corresponding amine is"; or even just "A more stable ammonium cation leads to a more basic amine". $\endgroup$ Jan 20, 2022 at 10:51
  • $\begingroup$ Thats exactly what's written in the textbook. I literally copy-pasted the text. Besides, your comment does not answer my Q. $\endgroup$
    – HarshDarji
    Jan 20, 2022 at 10:58
  • $\begingroup$ Comments aren't supposed to answer questions. I didn't attempt to answer. Your book is at least factually correct, though. Surely you are familiar with conjugate bases of weaker acids being more stable. The same is true here, except that base and acid are swapped. $\endgroup$ Jan 20, 2022 at 12:20
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    $\begingroup$ @orthocresol I thought this is a place where ppl ask questions and get their doubts clarified. I know its not a homework solution website but asking doubts and seeking for a solution should not be looked down upon. And the irony about your comment is that you said that my textbook has wrong grammar, but you essentially just copy-pasted those lines and changed a couple of words (which I dont think made any difference). $\endgroup$
    – HarshDarji
    Jan 20, 2022 at 13:45
  • $\begingroup$ @everyone All I want is clarification. What makes an primary amine more basic than secondary and tertiary amine. What does that first line mean? I am most definitely sure that I am misinterpreting something here. And I've asked for help regarding that in the Q as well. I dont see a reason why someone needs the entire textbook to tell what the meaning of one line is. But whatever, I still gave it. Now please. Answer my question. I rlly need to clarify this doubt. Thanks very much to whoever who acknowledged and accepted my request. $\endgroup$
    – HarshDarji
    Jan 20, 2022 at 13:54

1 Answer 1

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The sum of the $\mathrm{p}K_\mathrm{b}$ value of a base and the $\mathrm{p}K_\mathrm{a}$ value of its conjugate acid is a constant (at a constant temperature), and is equal to $\mathrm{p}K_\mathrm{w}$ (which is $14$ at room temperature). The more stable the conjugate acid, the higher the $\mathrm{p}K_\mathrm{a}$ of the conjugate acid $\implies$ lower $\mathrm{p}K_\mathrm{b}$ of the amine $\implies$ more basic amine.

(For those who are reading this without reading the textbook, the quoted lines are part of an explanation of the order of basicity of amines; obviously the order of basicity of aliphatic amines does not follow the order primary > secondary > tertiary, as the quoted lines seem to imply out of context)

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  • $\begingroup$ But doesnt high stability of conjugate acid imply high acidic nature of the corresponding compound? And hence low basic nature? $\endgroup$
    – HarshDarji
    Jan 20, 2022 at 16:45
  • $\begingroup$ If by "corresponding compound" you are talking about the base, then no; high stability of the conjugate acid does not imply high acidic nature of the base. $\endgroup$ Jan 20, 2022 at 16:51
  • $\begingroup$ So weaker conjugate acid (ie. more stable substituted amm. cation) => its a strong base. Which means its not very stable. Right? $\endgroup$
    – HarshDarji
    Jan 20, 2022 at 16:56
  • $\begingroup$ You should be more specific as to what exactly you are referring to by "its"... do you want to talk in stackexchange chat? It might be easier to clear up your confusion there instead of leaving multiple comments.. $\endgroup$ Jan 20, 2022 at 16:59
  • $\begingroup$ "Its" = strong base $\endgroup$
    – HarshDarji
    Jan 20, 2022 at 18:18

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