MAKE CONNECTIONS Which chemical group is most likely to be respon...

Question

MAKE CONNECTIONS Which chemical group is most likely to be responsible for an organic molecule behaving as a base?
a. hydroxyl
b. carbonyl
c. amino
d. phosphate

Accepted Answer

Welcome back. The next question says, which of the following functional groups acts as the strongest base? So a good starting point here is to remind ourselves of what a base does in solution. We recall that a base accepts hydrogen ions from solution or donates o h minus ions, whereas an acid donates h+ions or accepts o h minus ions. So with that in mind, let's look at our answer choices and see which functional groups acts as bases or acids, and then of the bases, which is the strongest base, which is what we're looking for. So choice a is a carbonyl group. I'm gonna draw that in blue here. It consists of a carbon double bonded to an oxygen, and this can act as a base in solution. That oxygen with its lone pairs is quite electronegative, so it can pick up a hydrogen ion from solution. However, that leaves a positive charge on the central carbon here, and carbon's not very happy, with a positive charge. So this form is relatively unstable. And when an a compound acts as a base, the it forms its conjugate acid. And when the conjugate acid is unstable as in this situation, that means the original compound is a weak base. It's not all that likely to act as a base since that resultant conjugate acid is unstable. So we're not gonna eliminate the carbonyl group, because it can act as a base but we're kind of set it aside and see if we have a stronger candidate further down. So let's look at our next choice, the carboxyl group. We'll do that in green. I'm just gonna put that over here. Has a carbon double wanted to an oxygen, but then an o h group attached on the other side. And this compound actually acts as an acid. That hydrogen can be donated to solution as an h+ion. And with the result that we get this, carbon still with its double bond to an oxygen and then a negatively charged oxygen over here. And that form is actually quite stable due to the phenomenon of resonance. You might recall that in a benzene ring, how you have the, electrons shared around the ring due to those alternating double and single bonds. And in this case, this can exist in these two forms with a double bond on different oxygens and the electrons here shared among the carbon and 2 oxygens. That makes this form quite stable, meaning that the carboxyl group will definitely act as an acid, not a base. So we can go ahead and eliminate choice b because we're looking for a strong base. Now we'll look down at the hydroxyl group, choice c. Let me draw that in purple. That consists of an oxygen bonded to a hydrogen. And this oxygen here with its double bonds, as we see again, allows us to act as a base, can pick up another hydrogen, from solution as a pick up a hydrogen ion, but that actually leaves a positive charge on this central oxygen. And oxygen being electronegative is definitely unhappy with that positive charge making this conjugate acid form unstable, which as we saw means that the original compound is a weak base. So, again, we'll sort of set that aside. We're looking for a strong strongest base. So choice d will go on to that. The amino group has a nitrogen with 2 hydrogens bonded to it, n h 2. As we recall, nitrogen has 5 valence electrons that leaves 2 more sitting as this lone pair on the nitrogen, and that lone pair is very happy to pick up an electron less hydrogen ion from solution. And that gives us the quite stable, positively charged n h three ion. Nitrogen is not as electronegative. It's pretty content sitting here with a positive charge. So this is a very stable conjugate acid, meaning that n h two is definitely the strongest base of our three choices here. So we're gonna pick go ahead and eliminate our 2 weak bases, a and c, carbonyl and hydroxyl, and we will choose the amino group choice d as our answer. Hope to see you in the next video.

MAKE CONNECTIONS Which chemical group is most likely to be responsible for an organic molecule behaving as a base? a. hydroxyl b. carbonyl c. amino d. phosphate

주요 개념

Functional Groups

Basicity

Amino Group

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