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While comparing the relative stabilities of acetic acid and formic acid why don't we consider the 3 possible hyper conjugative structures which would stabilize the positive charge on carbon thus making the conjugate of acetic acid more stable.

My professor told me hyper conjugation does not operate here. But what is the reason behind this?

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    $\begingroup$ Why do you think the carbon has a positive charge on it ? $\endgroup$ – Rajesh Mar 25 '15 at 14:20
  • $\begingroup$ Do you mean "making the conjugate base of acetic acid more stable"? $\endgroup$ – jerepierre Mar 25 '15 at 14:42
  • $\begingroup$ #rajesh. draw the resonating structures and observe it $\endgroup$ – yasir Mar 25 '15 at 22:31
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Hyperconjugation does play a role in your example. Acetic acid ($\mathrm{p}K_\mathrm{a}=4.75$) is a weaker acid than formic acid ($\mathrm{p}K_\mathrm{a}=3.75$). To understand this we must consider the relative stabilities of both the ionized and unionized acids.

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In acetic acid there is a partial positive charge on the carbonyl carbon. Both the inductive effect and resonance effect (e.g. hyperconjugated resonance structures like B) of the methyl group act to stabilize this charge, thereby stabilizing the unionized form of acetic acid. In formic acid, without a methyl group, similar stabilization of its unionized form is not possible.

Now let's look at the ionized acids. The negative charge in the anion is delocalized as shown in resonance structure C. This negative charge on the two oxygens makes them much less electron withdrawing. Consequently, there is much less positive charge on the "carbonyl" carbon in the ionized form. In turn, there is less need for stabilization of positive charge by the methyl group. The methyl group does not stabilize the ionized form as much as it stabilizes the unionized form.

Since the methyl group stabilizes the unionized form of acetic acid more than a hydrogen and since the methyl doesn't stabilize the ionized form much more than a hydrogen, there is a higher energy barrier for acetic acid to ionize compared to formic acid. This means that acetic acid will be a weaker acid than formic acid.

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  • $\begingroup$ so in case of ionized acteic acid there will be no hyper conjugation because there is no need as the positive charge on carbon has already been minimized due to resonance.only +I effect of methyl will operate or the converse will hold, ie no +I effect only the hyper conjugation? $\endgroup$ – yasir Mar 25 '15 at 22:54
  • $\begingroup$ Everything you said up to, "or the converse will hold, ie no +I effect only the hyper conjugation" is correct. In the unionized acid both inductive and hyperconjugation effects occur. $\endgroup$ – ron Mar 25 '15 at 22:58
  • $\begingroup$ so in case of ionized ones only +I of methyl is taken into consideration while in case of unionized ones +I as well as hyper conjugation will take place $\endgroup$ – yasir Mar 25 '15 at 23:06
  • $\begingroup$ Yes, that's correct. $\endgroup$ – ron Mar 25 '15 at 23:25

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