science diagnosis Q253

whitebrush
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Joined: Thu Oct 19, 2017 7:51 pm

science diagnosis Q253

Postby whitebrush » Thu Oct 19, 2017 7:55 pm

science diagnosis Q253: why D is the least acidic? Bromine is less electron-withdrawing than F and Cl, and so should more likely to lose the bonding with H on carboxyl group, and so should be most acidic?
NS_Tutor_Andrew
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Re: science diagnosis Q253

Postby NS_Tutor_Andrew » Fri Oct 20, 2017 12:31 pm

Hi whitebrush,

Great question (& welcome to the forums!). The key here is to recognize how the inductive effect makes acid stronger. We'll work through the example of a carboxylic acid, but the reasoning applies to organic weak acids in general. When a -COOH group acts as an acid and gives up an H+, it results in -COO-, which is a negatively charged species known as the carboxylate ion. In general, organic molecules don't "like" being charged (this is a bit of a simplification, but it'll do), so the more that negative charge can be stabilized, the more stable the carboxylate ion will be, and the more likely it is that the -COOH group will give up an H+ to form the carboxylate ion -- in other words, it becomes more likely that the -COOH group will act as an acid, making it more strongly acidic. Take the time to make sure you understand that reasoning carefully. In particular, the idea that the strength of an acid is proportional to the stability of its conjugate base is a really important one.

With all of that in mind, it follows pretty naturally that anything you can do to stabilize that negative charge will make an acid stronger. One way for that to happen is to have a highly electronegative substituent that wants to pull electrons towards itself. Since electrons carry negative charge, that will result in a delocalization of the negative charge, which helps stabilize it. A way to think of this is that delocalization allows more atoms in the molecule to 'share the burden' of the negative charge. A highly electronegative substituent close to the carboxylic acid group will do this very effectively, and multiple electronegative substituents will help out even more (as in choices B and C). However, Br in answer choice D is far away from the -COOH group, and as you pointed out, Br is less electronegative than the other halogens, so it will not help to delocalize and stabilize that negative charge as effectively. Therefore, D will be the least strong acid.

Hope this has helped to clarify things, and by all means, keep up the great questions and thorough review!
Andrew D.
Content Manager, Next Step Test Prep.
whitebrush
Posts: 3
Joined: Thu Oct 19, 2017 7:51 pm

Re: science diagnosis Q253

Postby whitebrush » Fri Oct 20, 2017 8:55 pm

I think I am clear now. Thank you sir.
Na

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