oChem Question

[passDAThope]

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I was wondering if anyone knows how to tell between a list of compounds which is the strongest acid or base? When I took organic chem two years ago we barely talked about determining a strong acid and never went over determining a strong base.

From what I remember is that stronger the acid is based on the stability of its conjugate base. I have only about five examples from my notes. I know that lower electronegative groups such as like a carbon chain donate election density thus lowering the stability of the conjugate base. Thus, an acid with a longer carbon chain the less strong it is. On the other hand, a halide such as F or Cl pull electron density away thus increasing the stability of the conjugate base thus increasing the stability of its conjugate base. Also the more electronegative the atom the more pull it will have, so an acid compound with an F will be a stronger acid than that of the same compound with Cl. Also the positioning of the halide effects the stability of the conjugate base ( the closer the halide is to the lone piar the greater pull it has).

Does anyone know what else causes a compound to be a stronger acid than another. Also how do you determine a strong base cause I don't have any info on it. Any help is much appreciated.

 

[passDAThope]

Bump. Does anyone know how to tell if an organic compound is a strong acid or base? Please😀. Your help is much appreciated.

 

[Shunwei]

Bump. Does anyone know how to tell if an organic compound is a strong acid or base? Please😀. Your help is much appreciated.

For inorganic acids, it is worthwhile to just understand that the halogenic acids, along with nitric, sulfuric, and perchloric acids, are strong acids. For organic acids, I'd suggest understanding a combination of resonance stabilization of the conjugate base, through-space and through-bond effects, and a good understanding of kw = ka x kb, if either of these values are given for the organic acid in question.

 

[Ocean5]

For inorganic acids, it is worthwhile to just understand that the halogenic acids, along with nitric, sulfuric, and perchloric acids, are strong acids. For organic acids, I'd suggest understanding a combination of resonance stabilization of the conjugate base, through-space and through-bond effects, and a good understanding of kw = ka x kb, if either of these values are given for the organic acid in question.

Could you please give some example of Halogenic acids?

 

[passDAThope]

For inorganic acids, it is worthwhile to just understand that the halogenic acids, along with nitric, sulfuric, and perchloric acids, are strong acids. For organic acids, I'd suggest understanding a combination of resonance stabilization of the conjugate base, through-space and through-bond effects, and a good understanding of kw = ka x kb, if either of these values are given for the organic acid in question.

Doesn't the the larger the Ka or Kb value mean the stronger the acid or base? Well I'm referring to say if they give you a list of organic compounds and ask you which is the stronger base or acid. I know resonance plays a part in it but not quite sure to what extent. But I do know atoms like F and Cl are take electron density and tert, sec, primary, etc. give electron density. Since I don't have any info on it, I have no idea on determining a strong base.

 

[Shunwei]

Doesn't the the larger the Ka or Kb value mean the stronger the acid or base? Well I'm referring to say if they give you a list of organic compounds and ask you which is the stronger base or acid. I know resonance plays a part in it but not quite sure to what extent. But I do know atoms like F and Cl are take electron density and tert, sec, primary, etc. give electron density. Since I don't have any info on it, I have no idea on determining a strong base.

What I mean by the ka and kb is simply that if they give you the kb of an organic acid and it's small, it will be obvious that the ka will be big and hence the acid is strong (but the pka will be small). I would probably argue that resonance might be the most important factor for organic acids because naturally the more resonance structures there are the more delocalized the negative charge will be when a proton is lost--which is good for strong acidity. Through-bond and through-space effects are also important inductive factors for an organic acid, as both deal with distorting electron density. You can easily read up on this in your chem book.

 

[passDAThope]

Shunwei, I really appreciate your help. There are these two problems that I have on this book that I'm having trouble understanding maybe you could help 😀. The first is:

Which is the most basic

A)C6H5-NH2
B)BrCH2-NH2
C)FCH2-NH2
D)CH3-NH2
E)ClCH2-NH2

answer is D

Second question:
Find the correct ranking in order of incresing acid strength

1)phenol
2)ethanoic acid (CH2COOH)
3) methanoic acid (CHOOH)
4) ethanol (CH3CH2OH)

The answer is 4,1,3,2. I understand that four is last because their is no resonance and the CH3CH2 donates electron density. I understand that the second weakest is the phenol because although it is resonance stabilized the whole benzene ring donates electron density. The next two I'm confused on because I figured 3 would be a stronger acid than 2. Since they are both carboxylic acids I think of them as being having equal resonance stability. The only reason I thought 3 is stronger is because it doesn't have the CH3 which donates electron density decreasing the stability of the conjugate base. So what gives?

 

[Shunwei]

Shunwei, I really appreciate your help. There are these two problems that I have on this book that I'm having trouble understanding maybe you could help 😀. The first is:

Which is the most basic

A)C6H5-NH2
B)BrCH2-NH2
C)FCH2-NH2
D)CH3-NH2
E)ClCH2-NH2

answer is D

Second question:
Find the correct ranking in order of incresing acid strength

1)phenol
2)ethanoic acid (CH2COOH)
3) methanoic acid (CHOOH)
4) ethanol (CH3CH2OH)

The answer is 4,1,3,2. I understand that four is last because their is no resonance and the CH3CH2 donates electron density. I understand that the second weakest is the phenol because although it is resonance stabilized the whole benzene ring donates electron density. The next two I'm confused on because I figured 3 would be a stronger acid than 2. Since they are both carboxylic acids I think of them as being having equal resonance stability. The only reason I thought 3 is stronger is because it doesn't have the CH3 which donates electron density decreasing the stability of the conjugate base. So what gives?

I believe the answer to your second question is wrong. Formic acid is 10 times as acidic as acetic acid (something like pka of 3.7 compared to 4.7). Check some references because I am fairly sure this is the case.

As for the first one, it is quite simple. In all of the answer choices besides (D), you have an electron withdrawing group (the phenyl is considered one). This electron-withdrawing effect "pulls" the nitrogen lone pair and makes it less labile for donation as a Lewis base. The methyl group, on the other hand, donates electron mildly so makes methalamine a better base than just ammonia.

 

[passDAThope]

I believe the answer to your second question is wrong. Formic acid is 10 times as acidic as acetic acid (something like pka of 3.7 compared to 4.7). Check some references because I am fairly sure this is the case.

As for the first one, it is quite simple. In all of the answer choices besides (D), you have an electron withdrawing group (the phenyl is considered one). This electron-withdrawing effect "pulls" the nitrogen lone pair and makes it less labile for donation as a Lewis base. The methyl group, on the other hand, donates electron mildly so makes methalamine a better base than just ammonia.

In the book it says that 4,1,3,2 is the correct answer. So do you agree with me that the answer should be 4,1,2,3.

Now on the first question is it just the opposite of a strong acid. Like the compounds with an electron withdrawing group (pulls electron density) makes it a strong acid thus a weak base? My only problem with what you said is that I the longer the carbon chain the more it donates electron density which is why I would have chosen A over D. So why exactly is the phenyl an electron with drawing group?

Edit: If the phenyl is an electron withdrawing group then in the second question wouldn't number 1 be a pretty strong acid because once it loses the the H- form the OH it has lots of resonance structures. Man I'm so confused.

 

[Sillygirl]

For acid and base strength look at the conjugate base. Then look at 5 factors and see if any of them contribute: Electronegativity, atom size, resonance, s character and inductive effects. If these delocalize charge then it'll make your base or conjugate base a weaker base and hence a stronger acid.

 

[passDAThope]

For acid and base strength look at the conjugate base. Then look at 5 factors and see if any of them contribute: Electronegativity, atom size, resonance, s character and inductive effects. If these delocalize charge then it'll make your base or conjugate base a weaker base and hence a stronger acid.

I know that the more the electronegative the atom the more electron density it pulls which I guess delocalizes the charge. Resonance I guess also delocalizes the the charge. Using this I still don't understand the second question that I posted. I know compound 4 is the the least acidic because it doesn't have any on the factors mentioned above plus the carbon chain attached to the O donates electron density causing it to be the weakest acid. Compounds 1,2, and 3 all have resonance. Compound 2 has that the CH3 group attached the carboxlyic acid (which donates electron density), which should make it less strong of an acid than compound 3. Since Shunwei said that that Phenyl groups withdraw electron density, then the compound 1 should be the strongest acid. Can someone please tell whats wrong with my thinking because I'm so confused.😕
Also, does a compound with more resonance structures make it more acidic than one with less resonance structures?