Weak Base Ques

[princesslinda]

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How do you determine which of the following is the weakest base?

A. H-
B. Na2O
C. N3-
D. OH-

I know that hydrides and metal oxides are strong bases, but why is OH- a weaker base than N3-?

 

[BerkReviewTeach]

How do you determine which of the following is the weakest base?

A. H-
B. Na2O
C. N3-
D. OH-

I know that hydrides and metal oxides are strong bases, but why is OH- a weaker base than N3-?

N is less electronegative than O and it's -3 compared to -1 for O, so N3- will donate electrons far more readily than OH-.

Where is this question from? It's a general courtesy to cite the source so others can reference the question too.

 

[Isoprop]

Just being nitpicky here: but I believe N3- is an azide with a -1 charge, not a nitrogen ion with a -3 charge.

OP--the short answer is the periodic trends will predict basicity. Base strength increases going up and to the left on the periodic table. So nitrogen will be more basic than oxygen.

Long answer: oxygen is more electronegative than nitrogen. So O will be more stable with the negative charge than nitrogen. Therefore, the azide, being unhappy with the negative charge, wants to pick up a proton more than the hydroxide. So, azide is more basic. In general, comparing basicity within the same row, you take electronegativity into consideration.

Let's also compare hydroxide (OH-) and bisulfide (SH-). Which is more basic? Well, even though O is more electronegative than S, sulfur is much BIGGER than oxygen. The bigger size of sulfur allows it to accomodate an extra electron better than oxygen. In other words, the negative charge on S is more stable because it has MORE ROOM (in a sense, it is more delocalized). Because SH- is more stable than OH-, the hydroxide will want to pick up a proton and become neutral and it will be more basic. In general, comparing basicity within the same column, you take size into consideration.

This explains why we have this periodic trend.

 

[Erythropoietin]

Just being nitpicky here: but I believe N3- is an azide with a -1 charge, not a nitrogen ion with a -3 charge.

OP--the short answer is the periodic trends will predict basicity. Base strength increases going up and to the left on the periodic table. So nitrogen will be more basic than oxygen.

Long answer: oxygen is more electronegative than nitrogen. So O will be more stable with the negative charge than nitrogen. Therefore, the azide, being unhappy with the negative charge, wants to pick up a proton more than the hydroxide. So, azide is more basic. In general, comparing basicity within the same row, you take electronegativity into consideration.

Let's also compare hydroxide (OH-) and bisulfide (SH-). Which is more basic? Well, even though O is more electronegative than S, sulfur is much BIGGER than oxygen. The bigger size of sulfur allows it to accomodate an extra electron better than oxygen. In other words, the negative charge on S is more stable because it has MORE ROOM (in a sense, it is more delocalized). Because SH- is more stable than OH-, the hydroxide will want to pick up a proton and become neutral and it will be more basic. In general, comparing basicity within the same column, you take size into consideration.

This explains why we have this periodic trend.

This is a great explanation.
I have a question though. Do you think we can compare the basicity of two nitrogen containing compounds? For example, N3- versus NH2-.
Would azide be more basic due to resonance?

 

[Isoprop]

Since azide has resonance forms, it can delocalize those electrons better. This makes azide more stable. The amide does not any resonance forms to delocalize the charge. Since the azide is more stable (resonance delocalizes the electrons), it does not want to pick up a proton as much as the amide. So the NH2- is more basic (less stable) than the N3-.

Another example is why acetate is less basic than ethoxide. CH3COO- has two resonance forms but EtO- has zero. So acetate is more stable.

 

[Erythropoietin]

Since azide has resonance forms, it can delocalize those electrons better. This makes azide more stable. The amide does not any resonance forms to delocalize the charge. Since the azide is more stable (resonance delocalizes the electrons), it does not want to pick up a proton as much as the amide. So the NH2- is more basic (less stable) than the N3-.

Another example is why acetate is less basic than ethoxide. CH3COO- has two resonance forms but EtO- has zero. So acetate is more stable.

Thanks!

 

[bidiboom]

OP--the short answer is the periodic trends will predict basicity. Base strength increases going up and to the left on the periodic table. So nitrogen will be more basic than oxygen.

I think you mean acid..

Actually I have a question mark in my mind about hydride and azide options of the question: both are more basic than hydroxide.. but which one, how far? By the respect of bond strength hydride may be stronger than azide, but by the respect of electronegativity azide is more electronegative.. I couldnt see which one is stronger..

 

[Erythropoietin]

I think you mean acid..

If you have an atom directly bonded to H, then:
within a column, the largest atom will be most acidic (SH > OH)
within a period, the most electronegative will be most acidic (OH > NH)

So when it is directly bonded to H, acidity increases to the right and down; and therefore basicity increases up and left.

When the atom is NOT directly bonded to H, then you can look at inductive effect (ex: CH3F more acidic than CH3NH2). So even in this case, basicity is increasing to the left.

 

[bidiboom]

Yes yes.. I misread 😳, you're right.. thank you..

So hydride should be more basic than azide..

 

[Erythropoietin]

Yes yes.. I misread 😳, you're right.. thank you..

So hydride should be more basic than azide..

Hmm I don't think it's wise to compare the two in terms of basicity but the strong bases are KOH, NaOH, NaOCH3, NaNH2, NaH, t-butoxide, and couple more. But resonance in azide, and in acetate (that Isoprop was talking about), makes the molecule a weak base.

 

[MT Headed]

Y'all are familiar with BEANS, right?

BEANS. Look at a periodic table and write a "B" on the top left, "E" on the top right, "A" on the bottom right, and an "N" and "S" on the bottom left. All those increase in a direction towards that letter.

B=Basicity
E=Electronegativity, ionization Energy, and Electron affinity
A=Acidity
N=Nucleophilicity
S=Size, Shielding

Hope this helps 🙂

from http://forums.studentdoctor.net/showthread.php?t=748851

 

[Erythropoietin]

Y'all are familiar with BEANS, right?



from http://forums.studentdoctor.net/showthread.php?t=748851

Then H- IS more basic than N3 - ?

 

[MT Headed]

Then H- IS more basic than N3 - ?

The BEANS trick is meant for analyzing "similar" compounds. For instance for pKa values, HI < HBr < HCl < HF, or HF < H2O < NH3 < CH4. Comparing azide to hydride does not neatly fit into these trends.

But to answer your question, of course H- is more basic than N3 -. Azide has resonance structures. It can distribute that annoying minus charge amongst many resonance structures and many atoms and therefore is much happier than poor H- which is small and can't distribute that negative charge anywhere. Who is more itchy to grab a proton? Hydride, by a long shot.

HN3, hydrazoic acid, has a pKa of 4.6. So it's a weak acid by inorganic chemistry standards. It's conjugate base, N3-, is also weak. H2, on the other hand, is so weak an acid that it is difficult to even dredge up a pKa value for the molecule. It's a weaker acid than water (pKa 15.7). It's conjugate base, H-, is a strong base.

H- + H2O --> H2 + OH-

So certainly H- is a much stronger base than N3-.

 

[Erythropoietin]

The BEANS trick is meant for analyzing "similar" compounds. For instance for pKa values, HI < HBr < HCl < HF, or HF < H2O < NH3 < CH4. Comparing azide to hydride does not neatly fit into these trends.

But to answer your question, of course H- is more basic than N3 -. Azide has resonance structures. It can distribute that annoying minus charge amongst many resonance structures and many atoms and therefore is much happier than poor H- which is small and can't distribute that negative charge anywhere. Who is more itchy to grab a proton? Hydride, by a long shot.

HN3, hydrazoic acid, has a pKa of 4.6. So it's a weak acid by inorganic chemistry standards. It's conjugate base, N3-, is also weak. H2, on the other hand, is so weak an acid that it is difficult to even dredge up a pKa value for the molecule. It's a weaker acid than water (pKa 15.7). It's conjugate base, H-, is a strong base.

H- + H2O --> H2 + OH-

So certainly H- is a much stronger base than N3-.

Thanks. Btw how long did it take you to walk from Mexico to Canada? 😀

 

[MT Headed]

Thanks. Btw how long did it take you to walk from Mexico to Canada? 😀

http://forums.studentdoctor.net/showthread.php?p=11719131

 

[bidiboom]

Guys thank you both.. this thread shed a pretty good light on my acid/base knowledge and question marks.. and I didnt know BEANS.. thanks for that as well 🙂

 

[Puneet Singh]

Considering the Lewis structure of N3- has two nitrogen atoms hence there are greater number of electron donors available in the molecule , hence it is a better donor of lone pair than OH ions. Also the shape of the azide molecule is linear and it provides this molecule a better approach, hence it acts as a better nucleophile as well
Thus the -OH ions are weak bases as compared to the azide ions

 

[MT Headed]

Considering the Lewis structure of N3- has two nitrogen atoms hence there are greater number of electron donors available in the molecule , hence it is a better donor of lone pair than OH ions. Also the shape of the azide molecule is linear and it provides this molecule a better approach, hence it acts as a better nucleophile as well
Thus the -OH ions are weak bases as compared to the azide ions

Do you have pKa or pKb values to back up your statement? Hydrazoic acid has a pKa of 4.6. Water has a pKa of 15.7. I don't see how N3- could possibly be a stronger base than OH-.

N3- is resonance stabilized and pretty happy even though it has a - charge. It's about as basic as acetate (conj base of vinegar). OH- is unstable like a tasmanian devil, and used in drain cleaners.

 

[Isoprop]

Considering the Lewis structure of N3- has two nitrogen atoms hence there are greater number of electron donors available in the molecule , hence it is a better donor of lone pair than OH ions. Also the shape of the azide molecule is linear and it provides this molecule a better approach, hence it acts as a better nucleophile as well
Thus the -OH ions are weak bases as compared to the azide ions

Unfortunately, this line of reasoning is incorrect even though you arrived at the correct conclusion that azide is a strong base. The number of electron donors is a KINETIC argument, but acidity and basicity are THERMODYNAMIC ideas. Nucleophilicity is also a kinetic concept and should not be used to predict the thermodynamics of a reaction.

 

[MT Headed]

But azide is not a strong base.

Hydrazoic acid (HN3) has a pKa of 4.6. Therefore the pKb of azide ((N3)-) is 9.4.

The pKb of hydroxide (the cutoff between stong base / weak base) is -1.7.

Azide is literally 125 billion times weaker than the definition of a strong base.

======================

Looking at the original question though, I bet that N3- really is nitride. That would follow the trend in the answer choices. The question is asking you to analyze H(-), O(2-), N(3-), and everybody's favorite base OH-.

When you read the question that way, yeah the first three are strong bases, and OH- is sitting right on the border between strong and weak base. It's the weakest base in the list.

 

[BerkReviewTeach]

Just being nitpicky here: but I believe N3- is an azide with a -1 charge, not a nitrogen ion with a -3 charge.

I don't think it's an azide, because azide N=N=N - is a weaker base than OH- and would have been the correct answer to the question. The OP asked why OH- is a weaker base than N3-, which means that if the question they are fielding lists N3- as a stronger base than OH-, then it must be an N with a 3- charge on it that they are referring to.