Strength of a base

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.
D

deleted647690

There was a question I came across in my TBR orgo book
"Which of the following types of compounds is the MOST basic?"
A. Primary alcohols
B. Esters
C. Secondary amines
D. Tertiary thiols

It says that the answer is C because N is always more basic than the S in thiol. However, I thought N was more electronegative than S, and therefore more acidic, and less basic?

Members don't see this ad.
 
S is more acidic and N is more basic. A good example to think of is Cysteine (-CH2-S-H) with a pKa ~ 8.0 and Lysine (-(CH2)4-NH3) with a pKa ~ 10.5 (basic).
 
  • Like
Reactions: 1 user
As a general rule, electronegativity increases up and to the right in the periodic table. Acidity increases down and to the right. The trend for basicity is the opposite of the acidity trend. Basicity increases up and to the left.
 
Wait but doesn't increased electronegativity mean that there will be more electron withdrawing, therfore, the propensity to donate electrons will be less, so it will act less like a nucelophilic base and more like an electrophilic acid? Therefore, according to this table of of EN values, N has a higher EN than S and is therefore more acidic. Meaning that N is less basic than S....sorry if I'm misunderstanding something

http://sciencenotes.org/list-of-electronegativity-values-of-the-elements/
 
Members don't see this ad :)
You cannot compare the two in a vacuum like that. Amines are more nucleophilic precisely because it is more electronegative.
N and S are bonded to -R or -H or w.e. N is able to draw more electrons in their bonds making it easier to donate the lone pair.
 
You cannot compare the two in a vacuum like that. Amines are more nucleophilic precisely because it is more electronegative.
N and S are bonded to -R or -H or w.e. N is able to draw more electrons in their bonds making it easier to donate the lone pair.
I think I'm misunderstanding what nucleophilic means. I thought if it was more EN---->pulls e- more=------>more likely to accept electrons---->act as electrophile===acid
 
If something is EN, it means that it has the tendency to pull electron density towards itself. An excess of electron density will bring a negative charge to that atom. This negatively charged atom can act as a nucleophile (meaning it likes the nucleus of another atom--which is positively charged) and attack an electrophile (low electron density).
 
I think I'm misunderstanding what nucleophilic means. I thought if it was more EN---->pulls e- more=------>more likely to accept electrons---->act as electrophile===acid
In general, an atom must form a bond with some other atoms. "pull e- more" refers to the electrons within those bonds. If the atom is negative enough, it can strip the shared electron away from it own.
HA -> [H+] + [A-]
But for this reaction to happen, the products have to be stable. If HA were say NH3, it wouldn't work because [NH2-] is an impossible product.
But either way, you must agree that [A-] and [NH2-] have nucleophilic tendency. So just think of NH3 as "almost [NH2-].
With that in mind, let's compare -SH and -NH, clearly the latter is more likely to form an "almost nucleophile" and is therefore more nucleophilic.
 
Thank you guys for your answers. I have found something online that states that the ability of a nucleophile to accept an H bond increases going down the periodic table; so that is confusing, because that means S would be a stronger nucleophile. There is also the idea that, as EN increases, nucleophilicity decreases. So that would mean N is a stronger nucleophile. Because of their placement on the table, aren't there two competing trends?
However, I guess another part to consider is steric hindrance. A tertiary thiol is much bulkier than a secondary amine, meaning it is less nucleophilic.

secondaryaminegroup.jpg
imgb0002.png
 
Top