DAT Destroyer Ochem 24 2011

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I attached the problem as a picture. I know that the carboxyl group has the most acidic proton, but how would you rank them from greatest to least?

E>A>D>C>B is what I would say.

Carbonyl group has the most acidic proton; proton off the benzyl ring is second; then d because it's a double alpha proton but also because its closer to the carboxylic acid than c; c is next because it's an alpha proton and then lastly b.

e>a>d>c>b
e is the most acidic proton and b is the least.
to check for acidity, you actually have to check their conjugate base, and compare them. (remember: the stronger the acid, the weaker, or more stable, it's conjugate base will be.)
you remove the proton (H) to get the conjugate base, and compare the bases to each other.
first, you check for "charge". in this case all the conjugate bases that you form are going to have a negative charge, because you removed the hydrogen.
second, you look at the atom that carries the charge. The more electronegative and/ or larger atom will be able to hold the negative charge better, therefore more stable. and more stable means weaker base. (so weaker conjugate base = stronger acid). using this step, you will eliminate choices b,c, & d because they will have the negative charge on Carbon vs. Oxygen for a & e.
step 3: now if you have go further, you check for resonance. in this case, the negative oxygen for carboxylic acid group can be stabilized by the other oxygen, and for the alcohol group (choice a), the resonance will be using the Carbons on the benzene ring, but since having the negative charge on an Oxygen is better (more stable) than Carbon, the conjugate base of the carboxylic acid group (choice "e") will be more stable, therefore the Hydrogen "e" is more acidic than "a".
therefore "e" is the strongest acid. then "a". and after that using the resonance roue, you can arrange the rest the same way.
just in case if you had to go more further,
step 4: you look at dipole induction, ( too see if any electronegative atom close to the atom with negative charge can stabilize it)
and step 5: you look at the orbitals. ex. an sp carbon with negative charge (Carbanion) is more stable than a an sp2 Carbanion.

I hope that helped.

This question was really bothering me a lot, i was getting confused with Q 23 as well. They are somewhat similar. How did u answer Q #23 i dont get it!

I have the 2010 version of the Destroyer, the questions (numbers) are different. Please provide the question.

This question is asking about which proton is most acidic. I said A because it is closer to the benzene ring and its is resonance stabilized.
But in Question 24, the original question in this post is asking about the lowest pka value and lowest pka value means more acidic. So, in Q 24 i said proton A is most acidic becasue it is resonance stabilized as well just the same as Q23.
But from reading your answers you guys are saying the carboxylic acid proton is most acidic right? I thought aldehyde proton in Q23 is also most acidic.

I am confused. !

Photo on 23 2012-07-10 at 11.53.jpg

From the most acidic to the least: aromatic acid-carboxylic acid- alcohol-ketone or/and alsehyde-alkyne-alkene-alkane