Please Help Dat Friday morning! Number 81 on 2009 DAT

[StayFlossin]

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So number 81 on the 2009 DAT says that the answer is A. I am confused why answer D isn't the correct answer since I is more acidic than F in the trend for acidity. Anyone who could clear this up would be greatly appreciated. They are all COOH's so I used exclamation points.


F-CH2COOH Says this is the correct answer. Please explain why

Cl-CH2COOH

Br-CH2COOH

I-CH2COOH

 

[Dwight Beet Farm]

I hope you know that some people hasn't purchased the 2009 DAT. You have to post the question! Here is the trend, if I knew what they asked about,
When determining acid strength: Remember this trend C N O F
Cl
Br
I
Acid strength increases with increasing electronegativity from left to right and increases down a group. Now, If you have HCl, HBr, HF, and HI and you need to rank from most to least acidic then simply the one with the largest size wins and the least size would be the least acidic. It would be HI>HBr>HCl>HF. With the given acids, one need to focus on size rather than electronegativity since all the acids given fall in the same group. However, if they give you bunch of acids that have an N, O, F
in them then your point of comparison would be electronegativity.

 

[Dwight Beet Farm]

I hope you know that some people hasn't purchased the 2009 DAT. You have to post the question! Here is the trend, if I knew what they asked about,
When determining acid strength: Remember this trend C N O F
Cl
Br
I
Acid strength increases with increasing electronegativity from left to right and increases down a group. Now, If you have HCl, HBr, HF, and HI and you need to rank from most to least acidic then simply the one with the largest size wins and the least size would be the least acidic. It would be HI>HBr>HCl>HF. With the given acids, one need to focus on size rather than electronegativity since all the acids given fall in the same group. However, if they give you bunch of acids that have an N, O, F
in them then your point of comparison would be electronegativity.

The Cl, Br, and I has to to under the F just like the periodic table. SDN switched it.

 

[StayFlossin]

I hope you know that some people hasn't purchased the 2009 DAT. You have to post the question! Here is the trend, if I knew what they asked about,
When determining acid strength: Remember this trend C N O F
Cl
Br
I
Acid strength increases with increasing electronegativity from left to right and increases down a group. Now, If you have HCl, HBr, HF, and HI and you need to rank from most to least acidic then simply the one with the largest size wins and the least size would be the least acidic. It would be HI>HBr>HCl>HF. With the given acids, one need to focus on size rather than electronegativity since all the acids given fall in the same group. However, if they give you bunch of acids that have an N, O, F
in them then your point of comparison would be electronegativity.

Look now because what you said is exactly what I thought but this leads to the wrong answer

 

[Dwight Beet Farm]

Okay I see. In this problem, I would deprotonate the acids first. After deprotonation, all of the acids will have the same resonance structure for their carboxylate group (draw it). Then look at which one has the most electronegative atom attached to it. Flourine in this case. This is pretty much boils down to induction dipole. Since the resonance is distributed between two oxygen in the carboxylate, and since the F is most electronegative, it would have the most stabilizing effect on the resonance structure rendering this to be the weakest base which makes it the strongest acid.

 

[StayFlossin]

Okay I see. In this problem, I would deprotonate the acids first. After deprotonation, all of the acids will have the same resonance structure for their carboxylate group (draw it). Then look at which one has the most electronegative atom attached to it. Flourine in this case. This is pretty much boils down to induction dipole. Since the resonance is distributed between two oxygen in the carboxylate, and since the F is most electronegative, it would have the most stabilizing effect on the resonance structure rendering this to be the weakest base which makes it the strongest acid.

Okay I understand that, But wouldn't atom size be more important than dipole induction? i.e. Br being larger than F?

 

[Dwight Beet Farm]

Size is going to increase the bond length and thus there is a fairly larger distance between the carboxyl group and the halogen with the greater size. Thus, it cannot stabilize the resonance bc of the increased distance. Whats why you need the one with the smallest size/more electrognegative since it will have the least distance and thus can stabilize the resonance form. This is how I memorize it. If there is a possible resonance next to a halogen, I think about electronegativity

 

[nskakavac]

Size would be important if the negative charge was on the atom directly. For example, when comparing HF and HBr and their acidity, HBr would be the most acidic due to it being the bigger atom and thus being able to stabilize the negative charge better. In your case, the negative charge is directly on the oxygen in every answer choice, therefore they will all have the same size. However, the stabilization will be different. This negative charge will be stabilized by the halogens that are in proximity. The more electronegative the halogen is, the more electron density it will pull away from the oxygen atom thus stabilizing it more. In your problem, the answer choice would be A since it will be the most stable due to the fluorine pulling the most electron density away from the oxygen atom that has the negative charges.

 

[100%permwinner]

As the above post implied, the more electronegative the element, the greater its ability in withdrawing electron density through sigma bonds, thus weakens O-H bond more.