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I am told that because the methyl group is electron donating in the conjugate base of acetic acid, this destabilizes the conjugate base by exacerbating the existing negative formal charge on the deprotonated oxygen, while in formic acid the electron donating methyl is absent in lieu of a hydrogen, which is neither withdrawing nor donating.

Is this reasoning correct, and are there any other reasons why formic acid might be stronger than acetic acid?

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    $\begingroup$ I don't have an authoritative reference to the effect, but I strongly suspect that its small size is probably a major factor. That potentially results in formation of more efficient hydrogen-bond networks and superior solvation. $\endgroup$ – Greg E. Aug 28 '14 at 17:08
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    $\begingroup$ Yes, the electron donation effect is pretty small. $\endgroup$ – Dissenter Aug 28 '14 at 17:09
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We are discussing the following equilibrium

enter image description here

We can make the acid a stronger acid by pushing the equilibrium to the right. To push the equilibrium to the right we can

  1. destabilize the starting acid pictured on the left side of the equation, and \ or

  2. stabilize the carboxylate anion pictured on the right side of the equation.

Comparing acetic acid ($\ce{R~ =~ CH3}$) to formic acid ($\ce{R~ =~ H}$), the methyl group is electron releasing compared to hydrogen. Therefore the methyl group will stabilize the dipolar resonance form of the starting acid where there is a partial positive charge on the carbonyl carbon. This should stabilize the starting acid. Further, this electron releasing ability of the methyl group will tend to destabilize the resultant carboxylate anion which already has a full unit of negative charge.

Therefore, because the methyl group 1) stabilizes the starting acid and 2) destabilizes the carboxylate anion product, the methyl group will push the equilibrium to the left, compared to the case where the methyl group is replaced by a hydrogen. Consequently, acetic acid is a weaker acid than formic acid.

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  • $\begingroup$ I get the point you are trying to get across, but it seems weird that you chose exactly those resonance forms for the free acid and those different ones for the carboxylate. The resonance structure with the positive charge on $\ce{OH}$ should contribute more than the $\ce{C+ -O-}$ one. $\endgroup$ – Jan Jun 25 '16 at 17:03
  • $\begingroup$ @Jan wrote "The resonance structure with the positive charge on $\ce{OH}$ should contribute more than the $\ce{C^{+}-O^{-}}$ one." Jan, why do you suspect that? Carbon is less electronegative than oxygen and I thought the position of the carbonyl carbon in $\ce{^13C}$ nmr suggests a fair amount of positive charge on carbon. $\endgroup$ – ron Jun 25 '16 at 17:53
  • $\begingroup$ Because all-octet resonance structres as $\ce{^{- }O-C={O+}H}$ generally contribute more to the overall structure than those without. Also, the carboxyl carbon’s NMR shift is about $30$ to $40~\mathrm{ppm}$ lower than that of a ketone, showing that the carbon’s positive partial charge is lower thanks to the neighbouring hydroxyl group. $\endgroup$ – Jan Jun 25 '16 at 18:04
  • $\begingroup$ What's the partial positive charge on the carbonyl carbon in acetone, around 40%? Reducing that value to account for replacing a methyl group with a hydroxyl would still (IMO) leave significant positive charge on the carbon. $\endgroup$ – ron Jun 25 '16 at 19:09
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Yes the reasoning is correct . The more the charge is increased on a molecule the more it gets destabilised.

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The strength of carboxylic acid can be discussed by positive inductive effect. Here, in ethanoic acid there is an electron releasing inductive effect from the alkyl group. The larger the positive inductive effect, the lesser will be the strength of an acid. Similarly in formic acid there is no alkyl group and hence no inductive effect takes place. So formic acid is a stronger acid than ethanoic acid.

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    $\begingroup$ Welcome to chemistry.se! If you have questions about how to beautify your posts, have a look at the help center. Do you want to know more about this site, please take the tour. $\endgroup$ – Martin - マーチン Mar 2 '15 at 6:57
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This is because of increasing positive inductive effect of alkyl group attached to the $\ce{-COOH}$ group in acid. In ethanoic acid electron releasing group ($\ce{CH3}$) increase the electron density on the carboxylic group $\ce{-COOH}$. But, methanoic acid doesn't contain electron releasing group and there is no inductive effect.The larger the positive inductive effect, the lesser will be the strength of an acid. Hence, methanoic acid (formic acid) is stronger than ethanoic acid.

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