Most Acidic, And Most Basic

[imbackasd]

I understand you have to look at conjugate acid/base but can anyone tell me why these answers are so?


http://oi45.tinypic.com/aws5r8.jpg

Im thinking something to do with aromaticity vs anti aromatic for the first but just cant grasp the proper reasoning for the first?

Also for the 2nd I thought lewis base would be an electron donor thus the middle one can donate its' pi electrons but the 3rd one can not since its' pi electrons are part of aromaticity. Is that a good reasoning?

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[V5RED]

In example 1, the circled structure gains aromaticity when it loses a proton from the carbon that is not yet connected to a double bond. The second structure looks tempting, but it does not fit the Huckel's rule of 4n+2 pi electrons.

In example 2, the nitrogen has a free lone pair of electrons for bonding. The other structure with a nitrogen has its lone pair tied up in conjugation with the double bonds and is aromatic. Using that lone pair to pick up an electrophile woudl break the aromaticity and that is generally not favorable.

TL/DR aromaticity is the name of the game in both examples.

 

[imbackasd]

Okay so my reasoning with the second one is right but I am a little confused with your first answer reasoning.

1. Are you assuming that dot is a radical electron in the first answer?
2. What do you mean it does not fit huckels rule of 4n + 2 for the 2nd one, It has 6 pi electrons, doesnt that mean it is aromatic?

And does being aromatic increase acidity??

 

[V5RED]

Okay so my reasoning with the second one is right but I am a little confused with your first answer reasoning.

1. Are you assuming that dot is a radical electron in the first answer?
2. What do you mean it does not fit huckels rule of 4n + 2 for the 2nd one, It has 6 pi electrons, doesnt that mean it is aromatic?

And does being aromatic increase acidity??

The dot is probably just a smudge or slip of the pencil. It is not a radical.

To be aromatic there are a number of considerations such as the molecule must be planar, it must have 4n+2 pi electrons, said pi electrons must be conjugated in a ring.

In the heptahedral molecule, it would not become aromatic by losing a proton. If the lone pair that was gained added to the pi cloud, the molecule would have 8 pi electrons which fails the 4n+2 test. If they did not add to the pi cloud, then it would not have a fully conjugated ring, so it would not be aromatic.

Being aromatic is a stable condition. If losing a proton will cause a molecule to become aromatic, then that proton is more acidic than a proton on a similar molecule that would not gain aromaticity would be. Aromatic does not mean acidic, but the stability gained by becoming aromatic can be a factor in whether a proton is more acidic than another proton.

 

[imbackasd]

Ahh gotcha, thanks man.

 

[orgohacks]

Yes. Basically, any factor that stabilizes negative charge (electron density) will increase acidity. Aromaticity is one of these factors.

http://www.youtube.com/watch?v=j0yOyyq2iGM