Help - Determining Relative Acidity? (Orgo)

[PreMedSS]

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Hey all, I need some help for my organic chemistry exam on monday. Given in this picture are 4 organic compounds, and we're asked to list them in order from MOST acidic, to LEAST acidic. The correct answer is "3>4>1>2"

What I don't understand about this is why in the world is compound 1 more acidic than compound 2? I understand that Bromine may be more electronegative, but on the other hand Iodine is large based on the periodic table. Help please?

 

[Amba]

3 is a carboxylic acid and will be most acidic. Br is more electronegative than I, so it will pull more electron density than I; therefore, 1>2. Having the bromine closer to the acidic hydrogen will have more of an effect than when it's farther away. Hence, 4>1.

Put all that together and you get 3>4>1>2. Hope that helped.

 

[PreMedSS]

3 is a carboxylic acid and will be most acidic. Br is more electronegative than I, so it will pull more electron density than I; therefore, 1>2. Having the bromine closer to the acidic hydrogen will have more of an effect than when it's farther away. Hence, 4>1.

Put all that together and you get 3>4>1>2. Hope that helped.

hey, i thought that despite bromine's electronegativity, the size of the Iodine would lead to it being more acidic because it would be able to help spread the negative charge over a larger region of space. According to Wade 7th Edition "Acidity increases down the column, as the size of the element increases."

That's what's got me so confused about it lol 😕

 

[Amba]

hey, i thought that despite bromine's electronegativity, the size of the Iodine would lead to it being more acidic because it would be able to help spread the negative charge over a larger region of space. According to Wade 7th Edition "Acidity increases down the column, as the size of the element increases."

That's what's got me so confused about it lol 😕

I believe that is more applicable to the actual molecule that the acidic hydrogen is bonded to. Ex: HI vs. HBr. The size of I vs. Br is essentially irrelevant in your problem since they are two or three bonds apart. In cases like your problem, look at electronegativities.

 

[PreMedSS]

I believe that is more applicable to the actual molecule that the acidic hydrogen is bonded to. Ex: HI vs. HBr. The size of I vs. Br is essentially irrelevant in your problem since they are two or three bonds apart. In cases like your problem, look at electronegativities.

okey dokey. thanks alot sir 🙂

 

[gettheleadout]

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