Relative Acid Strength

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Relative Acid Strength

Postby karinaseth_1A » Mon Dec 02, 2019 11:54 pm

In homework question 6C.21, we are asked to say why there is a difference in acid strengths of formic acid (HCOOH) and acetic acid (C3COOH). Why is formic acid stronger? Shouldn't acetic acid be stronger as it has a carbon atom (not including the carboxyl group) with higher electronegativity which means higher electron withdrawing ability?

Ryan 1K
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Re: Relative Acid Strength

Postby Ryan 1K » Tue Dec 03, 2019 1:16 am

The methyl group is electron releasing group, meaning that it is more likely to donate an electron. This is because the carbon is slightly more electronegative than the hydrogen, and its partial negative charge repulses its remaining electron.

Since it is more likely to donate an electron compared to the single hydrogen that is present in formic acid, it leads to a less stable anion product, which is already negatively charged, so adding the methyl group will compound on that negative charge. A more destabilized product means the equilibrium will shift to the left, and the KA will decrease, and the pKa will increase. As such, the methyl group causes the pKa to be higher, meaning acetic acid is a weaker acid than formic acid.

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