I was trying to compare the reactivity between acetaldehyde and benzaldehyde. After some googling, I found out that benzaldehyde is less reactive than acetaldehyde because the benzene ring can stabilize the partially positive Carbon through resonance. However, if I apply the same logic to benzoic acid vs acetic acid, I find that benzoic acid should be less acidic than acetic acid, but that's not true. What am I missing?
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1$\begingroup$ Does this answer your question? Why is benzoic acid a stronger acid than acetic acid? $\endgroup$– MithoronCommented Aug 23, 2021 at 13:04
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$\begingroup$ @Mithoron I saw that question before. The answer did not satisfy me. In the comments, A.K said "The aromatic ring is so thermodynamically stable that the phenyl ring donating electrons to the carboxylate is not a major resonace structure. Though it is enough to dictate meta directed electrophilic aromatic substitution", by this argument, can't it be said that the phenyl ring should not stabilize the carbonyl carbon in benzaldehyde through resonance? $\endgroup$– Tahsin ChoudhuryCommented Aug 24, 2021 at 9:17
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1 Answer
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The reactivity of an aldehyde group (or a ketone group for that matter) depends on the strength of the partial positive charge on the relevant carbonyl carbon. Thus, when comparing the reactivity of benzaldehyde and acetaldehyde, it is a valid argument that benzaldehyde is less reactive than acetaldehyde because the adjacent phenyl ring can stabilize the partially positive charge on carbonyl carbon through resonance (decreasing the strength of positive charge). That is possible because carbonyl group and phenyl ring is directly conjugated.
However, that same argument is not valid when you compare the reactivity of benzoic acid and acetic acid. That's because the reactive center of acidity is not the carbonyl carbon anymore as it was for aldehydes and ketones. The reactivity of carboxylic acid is depend on the strength of the $\ce{O-H}$ bond on carboxylic group, a bond away from carbonyl carbon. Since it is not directly conjugated to the other group attached to the carbonyl carbon, the resonance is not playing the role as that is in aldehyde and ketones. The inductive effect is the one playing strong role in this case. The more the electron withdrawing ability of the group attached to $\ce{COOH}$ the better the acidity. Thus, when comparing the electron withdrawing ability of phenyl and methyl groups, $\ce{Ph \gt CH3}$. Therefore, benzoic acid is more acidic than acetic acid by that reason alone.
answered Aug 22, 2021 at 14:42
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$\begingroup$ "The reactivity of carboxylic acid is depend on the strength of the O−H bond on carboxylic group, a bond away from carbonyl carbon" but O-H bonds in carboxylic acids are weak because of the partial charge developed in the carbonyl carbon. So if resonance can stabilise the carbonyl carbon, then won't the O-H bond be stronger? $\endgroup$ Commented Aug 24, 2021 at 9:14
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$\begingroup$ I'm confused as to why direct conjugation is so important, because in my point of view, the acidity of the group mainly depends on the stability of the carbonyl carbon. O-H group is present in alcohol too, but they aren't very acidic. $\endgroup$ Commented Aug 24, 2021 at 9:22
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$\begingroup$ @Tahsin Choudhury: Remember, the acetic hydrogen is equally distributed between two oxygen in $\ce{COOH}$ group. You may need more reading on this subject. $\endgroup$ Commented Aug 24, 2021 at 16:48
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$\begingroup$ Sorry, what do you mean by that? Could you share some resources to know more about it? $\endgroup$ Commented Aug 25, 2021 at 2:39