Factors that Influence Acidity-- Bond Polarity v. Electronegativity

[ColgateRaider08]

Hi all,

This is my first post to SDN, so here goes.

I recently came across the following two statements while studying for the MCAT:

Increasing electronegativity leads to an increase in acid strength.
Decreasing bond polarity causes and increase in acid strength.

I am a bit confused by this. How can both of these statements be true? I always thought that electronegativity and bond polarity tend to go hand-in-hand. The more electronegative something is, the more it tends to draw electrons to itself, thus making the bond more polar. What am I missing?

Any clarification would be greatly appreciated. Thanks! 🙂

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[ColgateRaider08]

Hi all,

I had accidentally posted this in the "MCAT" forum rather than the "MCAT Study Question Q&A" sub-forum, so here is my second attempt to ask this question... 🙄

I recently came across the following two statements while studying for the MCAT:

Increasing electronegativity leads to an increase in acid strength.
Decreasing bond polarity causes and increase in acid strength.

I am a bit confused by this. How can both of these statements be true? I always thought that electronegativity and bond polarity tend to go hand-in-hand. The more electronegative something is, the more it tends to draw electrons to itself, thus making the bond more polar. What am I missing?

Any clarification would be greatly appreciated. Thanks! 🙂

 

[loveoforganic]

The second statement doesn't seem correct to me.

 

[NickNaylor]

The second statement doesn't seem correct to me.

Agreed. Increasing electronegativity -> increasing bond polarity -> increasing acidity. The second statement is incorrect.

 

[orgohacks]

Hi all,

I had accidentally posted this in the "MCAT" forum rather than the "MCAT Study Question Q&A" sub-forum, so here is my second attempt to ask this question... 🙄

I recently came across the following two statements while studying for the MCAT:

Increasing electronegativity leads to an increase in acid strength.
Decreasing bond polarity causes and increase in acid strength.

I am a bit confused by this. How can both of these statements be true? I always thought that electronegativity and bond polarity tend to go hand-in-hand. The more electronegative something is, the more it tends to draw electrons to itself, thus making the bond more polar. What am I missing?

Any clarification would be greatly appreciated. Thanks! 🙂

I can see how this can be confusing. One way to clarify it would be to look at two different situations.

1) For carboxylic acids: halogenated derivatives of acetic acid, XCH2COOH, the acidity of the COOH will increase in the order I < Br < Cl < F . This follows electronegativity exactly. Note here that we are talking about the increasing ability of these halides to stabilize negative charge.

2) For hydrogen halides HX, acidity increases in the order F < Cl < Br < I . This is the INVERSE of electronegativity, or if you prefer, decreasing bond polarity. This trend in acidity directly correlates with decreasing strength of the H-X bond: the H-I bond is the weakest (least polar) therefore HI is the most acidic. Another way of looking at it is that the I- ion is the most stabilized negative ion, since it is larger and the charge is dispersed over a greater volume.

hope this helps - James

 

[Jackson-Ave-Dental]

TBR makes this very clear.

When considering the atom directly connected to the acidic hydrogen...
If comparing from the same row, you use electronegativity. The greater the electronegativity, the more acidic

If comparing from the same column, you use size. The larger the atomic radius, the more acidic.

 

[loveoforganic]

I can see how this can be confusing. One way to clarify it would be to look at two different situations.

1) For carboxylic acids: halogenated derivatives of acetic acid, XCH2COOH, the acidity of the COOH will increase in the order I < Br < Cl < F . This follows electronegativity exactly. Note here that we are talking about the increasing ability of these halides to stabilize negative charge.

2) For hydrogen halides HX, acidity increases in the order F < Cl < Br < I . This is the INVERSE of electronegativity, or if you prefer, decreasing bond polarity. This trend in acidity directly correlates with decreasing strength of the H-X bond: the H-I bond is the weakest (least polar) therefore HI is the most acidic. Another way of looking at it is that the I- ion is the most stabilized negative ion, since it is larger and the charge is dispersed over a greater volume.

hope this helps - James

I'm going to argue with you a bit. The acidity for HX goes in that order due to the size of the valence orbital the electron is displaced to. The effect is large enough to outdo the lesser electronegativity. If somehow iodine had the same electronegativity as fluorine, the acidity of HI would be markedly higher. The electronegativity increases acidity holds true - there are just other factors at play that need to be considered.

Also, bond strength = homolysis.

 

[Don Draper]

I'm going to argue with you a bit. The acidity for HX goes in that order due to the size of the valence orbital the electron is displaced to. The effect is large enough to outdo the lesser electronegativity. If somehow iodine had the same electronegativity as fluorine, the acidity of HI would be markedly higher. The electronegativity increases acidity holds true - there are just other factors at play that need to be considered.

Also, bond strength = homolysis.

well yeah, he was talking about the inductive effect at first, then he simplified the idea for the second part.

Obviously if you have a weaker conj base (more stable) you have a stronger acid.

pKa + pKb = 14 for the pair...

 

[orgohacks]

I'm going to argue with you a bit. The acidity for HX goes in that order due to the size of the valence orbital the electron is displaced to. The effect is large enough to outdo the lesser electronegativity. If somehow iodine had the same electronegativity as fluorine, the acidity of HI would be markedly higher. The electronegativity increases acidity holds true - there are just other factors at play that need to be considered.

Also, bond strength = homolysis.

Essentially "size of the valence orbital" provides one with the correct answer. I don't think we're in fundamental disagreement here.

I'm not sure about the meaning of the third and fourth sentences. I don't see it as two separate effects (size of valence orbitals and electronegativity) - I see it as one effect. The electronegativity is going to be inversely related to the distance of the valence electrons from the nucleus.

Bond strength = homolysis, true, but in the case of HX it also works if you look at them as point charges. The fluoride ion is smaller whereas on the iodide ion the charge will be more diffuse. Ionic radii differences mean that you'll have a shorter (and therefore stronger) HF bond versus HI.

Another factor that comes up with F- is that it is an absolute beast for hydrogen bonding. I'm not sure of the numbers, but I'd think that there would be a huge entropy cost of going from HF --> H+ + F- due to the tremendous solvent shell of water molecules that will surround F- . Hydrogen bonding isn't really a factor with Cl-, Br- and I-.

 

[loveoforganic]

Your explanation tripped me up somewhere and I was having a brainfart with those sentences. We're in agreement!

 

[Don Draper]

Essentially "size of the valence orbital" provides one with the correct answer. I don't think we're in fundamental disagreement here.

I'm not sure about the meaning of the third and fourth sentences. I don't see it as two separate effects (size of valence orbitals and electronegativity) - I see it as one effect. The electronegativity is going to be inversely related to the distance of the valence electrons from the nucleus.

Bond strength = homolysis, true, but in the case of HX it also works if you look at them as point charges. The fluoride ion is smaller whereas on the iodide ion the charge will be more diffuse. Ionic radii differences mean that you'll have a shorter (and therefore stronger) HF bond versus HI.

Another factor that comes up with F- is that it is an absolute beast for hydrogen bonding. I'm not sure of the numbers, but I'd think that there would be a huge entropy cost of going from HF --> H+ + F- due to the tremendous solvent shell of water molecules that will surround F- . Hydrogen bonding isn't really a factor with Cl-, Br- and I-.

orgohacks seem to have orgo-game...

oh, and his screenname is orgohacks. so yeah.

 

[loveoforganic]

He does indeed 👍

 

[orgohacks]

He does indeed 👍

Thanks - don't stop disagreeing with me though 🙂... I'm fallible, and it helps me think.