Acid-Base determination of organic compounds

Question

I'll start this question with the problem that led me to ask this question...

USNCO 2009 Problem 8 part c:

Part c. states that all four of the compounds above are more basic than ammonia, and the solution explains that it is because the "carbon-containing groups" that are attached to the nitrogen are "better electron donors" than just hydrogen (in ammonia). This "inductive effect" causes the lone pair of the nitrogen to attract the proton ($\ce{H^+}$) better, or as the solution said, "donate the electron pair" to the proton better.

This got me thinking.

My thought process was that the polarity of the hydrocarbon group and the nitrogen overall makes the atom extremely polar towards nitrogen. What I mean here is that the nitrogen has a large negative dipole. This also, in corollary, means that nitrogen has more electrons. In turn, it would attract the proton more strongly.

However, I do not know if my thought process is correct.

This is the official ACS solution to the problem in question:

What does it mean they are "better electron donors?" What is this inductive effect? If you can provide it, are there any other rules that are important pertaining to this and is there a source where I could learn these rules?

Thank you in advance.

Note: this is NOT homework, and my thought process for part c HAS been specified.

Source: https://www.acs.org/content/dam/acsorg/education/students/highschool/olympiad/pastexams/2009-usnco-exam-part-ii.pdf

Answer 1

There is a slight error in your sentence. The nitrogen does not have more electrons, but it has a higher electron density because of it's neighbours. But I think this is how you meant it. Often it is described in terms of donating electrons, whilst these electrons are not donated in a cation/anion sense, so I think electron density is a better to grasp description.

Anyway, with being "better electron donors", it is meant that they are more readily able to donate part of their electron density, just like you describe with your description of polarization.

The inductive effect is the ability of an atom or functional group to donate part of it's electron density through $\sigma$-bonds. So in this we are talking about the electronegativity of a group. It is something you have already encountered in chemistry, but this time it has received the proper name 'inductive effect'.

What you would want to be reading, is about Hammett plots. Hammett plots are a tool to picture the ability of substituents on benzoic acid to influence the acidity. Originally Hammett used hydrogen as the reference substituent. The cool thing is that you can compare the effect of substituents on a different molecule, with the effect they have on benzoic acid. I can advise you to get to the library and find 'Modern physical organic chemistry' by Anslyn and Dougherty. Especially chapter 8.2 about Substituent Effects and chapter 8.3 about Hammett Plot's as a tool will be useful.