Relative Acidity

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Emma Li 2C
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Relative Acidity

Postby Emma Li 2C » Wed Dec 06, 2017 11:46 am

For the example Lavelle gave during the lecture, Why is trichloroacetic acid (CCl3OOH, pKa=0.52) a stronger acid than acetic acid (CH3COOH, pKa=4.75)?

Thank you!

Hannah Guo 3D
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Re: Relative Acidity

Postby Hannah Guo 3D » Wed Dec 06, 2017 12:15 pm

Cl atoms are more electronegative than H atoms, so electrons are more delocalized in CCl3COOH. We know that the electrons in strong acids are easier to be delocalized, resulting in higher yield of H30+.
Another way to think about this is that since CCl3COOH has a lower pKa value and pKa=-log[Ka], CCl3COOH has a greater Ka value, which means that CCl3COOH is easier to be deprotonated and it is more acidic.

Hope it helps!

Abigail Urbina 1K
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Re: Relative Acidity

Postby Abigail Urbina 1K » Wed Dec 06, 2017 12:20 pm

The CCl3 group in trichloroacetic acid that is bonded to the carboxyl group, -COOH, is more electronegative (electron-drawing) than the CH3 group in acetic acid that is bonded to the carboxyl group. With respect to the definition of a Lewis acid, an electron acceptor, this would make CCl3 more acidic. Cl is a highly electronegative atom.

Victoria Draper 1G
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Re: Relative Acidity

Postby Victoria Draper 1G » Wed Dec 06, 2017 12:38 pm

Another general rule that will be helpful when given the pKa values of two molecules is that the lower the pKa the strong the acid is. Therefore, since the CCl3OOH has a significantly lower pKa value it is considered to be a stronger acid than CH3COOH.

Hope this helps!

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