F, Cl, Br, I - stronger acid

[wuhsabee]

Hey guys,

I have a question from today's MCAT Question of the Day.

The question was this:
For the compound XCH2CH2COOH, which of the following, if substituted for X, would result in the strongest acid?

A.) F
B.) Cl
C.) Br
D.) I

I was conflicted between F and I, because I thought that to determine which is the strongest acid takes into account both size and electronegativity.

In this case, F is more electronegative, which will make the compound more acidic due to inductive effects, but I is larger, which will also make the compound more acidic because the size of the atom dictates how well the extra electron charges gained from an acid/base reaction are spread out, which will increase stability of the conjugate base/increase acidity.

M Prep gave this explanation: The X component will cause inductive effects, drawing electrons from the oxygen making it easier for a hydrogen to leave. We expect the most electronegative X to cause the most inductive effects. Fluorine is most electronegative and thus would cause the compound to be the strongest acid, thus (A) is the correct answer.



How come they are only taking into account inductive effects/electronegativity, but not size?

Thanks in advance for the help.

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[Hadi7183]

Because the acidic hydrogen is not directly connected to X, so the size of X doesn't affect the acidity in this case.

 

[AlteredScale]

Hey guys,

I have a question from today's MCAT Question of the Day.

The question was this:
For the compound XCH2CH2COOH, which of the following, if substituted for X, would result in the strongest acid?

A.) F
B.) Cl
C.) Br
D.) I

I was conflicted between F and I, because I thought that to determine which is the strongest acid takes into account both size and electronegativity.

In this case, F is more electronegative, which will make the compound more acidic due to inductive effects, but I is larger, which will also make the compound more acidic because the size of the atom dictates how well the extra electron charges gained from an acid/base reaction are spread out, which will increase stability of the conjugate base/increase acidity.

M Prep gave this explanation: The X component will cause inductive effects, drawing electrons from the oxygen making it easier for a hydrogen to leave. We expect the most electronegative X to cause the most inductive effects. Fluorine is most electronegative and thus would cause the compound to be the strongest acid, thus (A) is the correct answer.



How come they are only taking into account inductive effects/electronegativity, but not size?

Thanks in advance for the help.

Size is dependent on first order acids (HF, HBr). What you are assessing is the acidity of a Carboxylic acid. Remember that electronegative atoms have a "pulling" action that make alpha hydrogens more acidic. The same goes for COOH acids. The most electronegative atom will increase the acidity of the hydrogen in COOH.

 

[wuhsabee]

Thanks guys!!