One question about the organic chem from old exam

[vDDmaniaC]

---

Members do not see this ad.

Hi, guys.
I am going over all the previous exams before actually taking the exam.
There is one question that I don't know how to deal with.
It might easy for you so I ask.
The question is to find the strongest acid from the following:
1. CH3-CH3
2. CH3-O-CH3
3. CH3-CH2-O-H
4. CH3-COOCH3 (ester)
5. CH3-CH2-N(CH3)2

The answer is number 3. But I don't know why.
Do I need to determine acid/base based on activating or deactivating group? Will you please give me a brief explanation about how to deal with this kind of question?

Thanks in advance!

 

[smiley007]

Hm, it's been a while, but I believe the strongest acid is one that can will most readily give up its proton?

1) Can't be most acidic because it is most stable in that form. If you take away a proton from CH3-CH3, that leaves CH3-CH2, and the CH2 has a negative sign..and carbon is not very electronegative. A negative charge on it would destabilize it, so it's not likely to give up the proton.

2) This also can't be the strongest acid due to the reasons for #1. There would be a negative charge on the carbon. Unfortunately, oxygen does not have d-shells like phosphorus would, so the extra electrons on carbon cannot form a double bond with oxygen to get rid of the negative charge.

4) Same reasons as #2.

5) Same reasons as #1.

Therefore, #3 would most readily give up its proton. Oxygen is highly electronegative and would readily accept the electrons and negative charge. As for the activating and deactivating groups...I have no idea. I always thought they were only important in E1/E2.

Please correct me if I'm wrong.

 

[needa20]

# 3

oxygen is the most electo neg out of O, C, N. therefore does not mind the Neg charge when it donates the H+ (proton). Carbon def does not want to give up a proton and N will, but O is more electro neg. Look at the periodic table, F is most electro neg, so O is closer to F then N or C

at least thats the way i look at it.

 

[klutzy1987]

The answer is 3.
The reson for this is that we will never form a alkane anion. They are among the most powerful bases known to man. Therefore, since all of the other choices would have a negative charge on a fully saturated carbon (i.e. no double or trilpe bonds) they would all be completely nonacidic. However Oxygen can handle a negative charge, and although ethoxide is a very strong base, it is nowhere near as strong as an alkane anion.

 

[doc3232]

Hi, guys.
I am going over all the previous exams before actually taking the exam.
There is one question that I don't know how to deal with.
It might easy for you so I ask.
The question is to find the strongest acid from the following:
1. CH3-CH3
2. CH3-O-CH3
3. CH3-CH2-O-H
4. CH3-COOCH3 (ester)
5. CH3-CH2-N(CH3)2

The answer is number 3. But I don't know why.
Do I need to determine acid/base based on activating or deactivating group? Will you please give me a brief explanation about how to deal with this kind of question?

Thanks in advance!

The answer is 3.
The reson for this is that we will never form a alkane anion. They are among the most powerful bases known to man. Therefore, since all of the other choices would have a negative charge on a fully saturated carbon (i.e. no double or trilpe bonds) they would all be completely nonacidic. However Oxygen can handle a negative charge, and although ethoxide is a very strong base, it is nowhere near as strong as an alkane anion.

Yes, as klutzy said you don't want charges on Carbons, but for the ester you need to know that the charge will not be on a carbon but on the Oxygen through resonance. The first carbon on the left of CH3COOCH3 will deprotonate and basically the Oxygen takes the Hydrogen and then can it deprotonate from there. Ketol enol tautomerization. CH2=C (O-)CH3
where there is a double bond between those carbons and the Oxygen is holding the neg charge.
Just trying to point out which would be the second most acidic molecule.

Acidic from most to least 3>4>2>1>5 (2 and 1 are about the same acidity)
5 is actually quite basic and would be least acidic.