TBR OChem Ch1 Q85

[ranger4]

Which of the following types of compounds are most basic?
A. Primary alcohols
B. Esters
C. Secondary Amines
D. Tertiary Thiols

The answer is C. The explanation states that nitrogen containing compounds are generally more basic than oxygen and sulfur containing compounds of equal hybridization. My question is why? Nitrogen is in between oxygen and sulfur in terms of electronegativity...

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[PiBond]

Which of the following types of compounds are most basic?
A. Primary alcohols
B. Esters
C. Secondary Amines
D. Tertiary Thiols

The answer is C. The explanation states that nitrogen containing compounds are generally more basic than oxygen and sulfur containing compounds of equal hybridization. My question is why? Nitrogen is in between oxygen and sulfur in terms of electronegativity...

I think it's easiest to use POE here. We know that esters aren't at all basic, so eliminate B first. Next, it's easiest to compare atoms in the same column--oxygen and sulfur. Thiols are more acidic (and therefore less basic) than alcohols due to the ability of sulfur to stabilize negative charge better than oxygen. Thus we can eliminate D. Now we're down to A and C. We know that within a row acidity corresponds to electronegativity. Oxygen is more electronegative than nitrogen so the alcohol must be more acidic (and thus less basic) than our nitrogen/amine group. We end up with answer C.

Remember that basicity corresponds to localized charged, or a lone pair in this case. Sulfur is larger so the lone pair is more spread out/diffuse, whereas nitrogen is smaller and therefore has the lone pair more localized on the atom. A more localized lone pair makes it easier for it to abstract a hydrogen. Electronegativities are close here, so size wins out.

Hope that helps.

 

[UrshumMurshum]

I'm not really sure about the answer because I've not yet studied acid/base chapters. My guess would be the electronegativity works against the favor of oxygen + sulfur as bases.

That because Nitrogen is less electronegative, it holds onto its lone pair of electrons less, which permits its lone pair to travel further away from the nucleus than in Oxygen or Sulfur's case. This is what makes nitrogen more basic, its lone pair is more available to take protons.

Also, keep in mind this is a general trend. But there are of course exceptions. Once you get into resonance structures, you'll find the carbonyll group is more basic than an amino group (when both are present) due to resonance stabilization.

 

[ranger4]

I think it's easiest to use POE here. We know that esters aren't at all basic, so eliminate B first. Next, it's easiest to compare atoms in the same column--oxygen and sulfur. Thiols are more acidic (and therefore less basic) than alcohols due to the ability of sulfur to stabilize negative charge better than oxygen. Thus we can eliminate D. Now we're down to A and C. We know that within a row acidity corresponds to electronegativity. Oxygen is more electronegative than nitrogen so the alcohol must be more acidic (and thus less basic) than our nitrogen/amine group. We end up with answer C.

Remember that basicity corresponds to localized charged, or a lone pair in this case. Sulfur is larger so the lone pair is more spread out/diffuse, whereas nitrogen is smaller and therefore has the lone pair more localized on the atom. A more localized lone pair makes it easier for it to abstract a hydrogen. Electronegativities are close here, so size wins out.

Hope that helps.

Thanks! Great answer, makes perfect sense. Urshum, remember nitrogen is more electronegative than sulfur (F > O > N > Cl > Br > I > S > C > H "fonclbrisch"). As PiBond said, atomic size becomes a more significant factor of acid/base strength (larger atoms stabilize negative charge more, reducing basicity) than electronegativity when you're comparing elements within a column of the periodic table.

 

[UrshumMurshum]

Thanks! Great answer, makes perfect sense. Urshum, remember nitrogen is more electronegative than sulfur (F < O < N < Cl < Br < I < S < C < H "fonclbrisch"). As PiBond said, atomic size becomes a more significant factor of acid/base strength (larger atoms stabilize negative charge more, reducing basicity) than electronegativity when you're comparing elements within a column of the periodic table.

That makes sense. Also that's a neat way of remembering it, "fonclbrisch" though I think you have the signs backwards.

 

[ranger4]

That makes sense. Also that's a neat way of remembering it, "fonclbrisch" though I think you have the signs backwards.

Heh, yes I did.