acid strength

[deleted388502]

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Conceptually really confused about this. Acid strength increases going down the periodic table, so why is the answer here B and not C?

Thanks!!

 

[mcatstuff19]

I think it's because F is more electronegative than Cl. Induction.

 

[ayocaptain628]

i feel like its an inductive effect kind of thing
acid strength aka consider the strength of the conjugate base
after everything is deprotonated, since fluorine is the most electronegative species
CF3 will diffuse the negative charge more .:. stabilize the conjugate base more as opposed to CCl3
more stable conjugate base .:. more acidic

 

[Czarcasm]

Conceptually really confused about this. Acid strength increases going down the periodic table, so why is the answer here B and not C?

Thanks!!

The trend of increased acidity you are referring to is specifically regarding haloacids. This trend of acidity has more to do with the inductive effect. Recall that the acidity strength is often generalized based on the "CARDIO" mneumonic (Charge, Atom, Resonance, (Dipole) Induction, Orbital). In this scenario, the charge, atom, and resonance effects are equal. The next determinant is induction. The strongest inductive effect occurs from the most electronegative atom. Also, the closer and more numerous these electronegative atoms, the stronger the inductive effect. In this case, the leads to answer choice B.

 

[sxld_cu]

Acidity of a carboxylic acid is based on de-protonation and resonance stabilization of the excess negative charge on the oxygen (carboxylate anion). Halogens are withdrawing groups so the more electronegative the atom is, the more negative charge it will draw away from the oxygen, stabilizing the carboxylate anion and increasing acidity. Fluorine is more electronegative than chlorine because electronegativity decreases down a period

I think what you are referring to, when you say that acid strength increases down a period, is the situation when the atom in question is directly bonded to a hydrogen. In this case, as electronegativity decreases down a period, the atom is less likely to hold onto the proton, increasing acidity.

See for ref: http://chemwiki.ucdavis.edu/Organic...id//Base_Reactions/Periodic_trends_in_acidity

 

[deleted388502]

Thanks all! 🙂

 

[The Brown Knight]

for that question, simple answer is inductive effect due to very high electronegativity of the fluorines

"Acid strength increases going down the periodic table" only when the acidic proton is located on the chlorine; this is why you see HI > HBr > HCl > HF
If iodine were on the organic acids above instead of fluorine, its lower electronegativity would exert very small inductive effect and wouldn't be very acidic.

 

[Teleologist]

"Acid strength increases going down the periodic table" only when the acidic proton is located on the chlorine

@The Brown Knight Not every acid has a chlorine. Not sure what you mean here.

 

[Teleologist]

The trend of increased acidity you are referring to is specifically regarding haloacids.

Nope. It also refers to binary Bronsted acids of group 6 elements. Ka(H2O) << Ka(H2S) << Ka(H2Se), etc.

as electronegativity decreases down a period, the atom is less likely to hold onto the proton, increasing acidity.

See for ref: http://chemwiki.ucdavis.edu/Organic_Chemistry/UMM_chemwiki_project/Acid//Base_Reactions/Periodic_trends_in_acidity

Bruh I haven't opened the link but I'm 99% sure the person who wrote whatever's behind that link didn't say what you said. What does electronegativity have to do with holding onto a proton? It doesn't, at least not directly.

You can say that the great EN of the halogens isolates bonding electron density away from the hydrogen, thereby weakening the H-X bond. Or you can argue that the great EN of the halogens magnifies the partial positive character of the hydrogen, making it more reactive, but both suggest that HF should be a strong acid. Except for HF isn't.

 

[NextStepTutor_1]

To piggyback onto what @Czarcasm stated, which was all right, the acidity trend you are referring to (increased acidity as one goes down a periodic column or family) is only applicable if the halogen is directly bound to the protic or acidic hydrogen. In this example, it is not.

This trend is caused by the larger central atom size (reason why HI is a stronger acid than HF) which creates a longer, and thus weaker, bond making the hydrogen more likely to dissociate. The only differences between the answer choices in this question are in regards to substituents bound away from the protic hydrogen - a telltale sign that the compounds' differences in acidity are related to the substituent's inductive effect or lack thereof.