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Reactivity of Benzaldehyde vs Acetaldehyde and Benzoic acid vs Acetic acid
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Ahmad Ali
Reactivity of Benzaldehyde vs Acetaldehyde and Benzoic acid vs Acetic acid
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.
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.
@Tahsin Choudhury: Remember, the acetic hydrogen is equally distributed between two oxygen in $\ce{COOH}$ group. You may need more reading on this subject.More
Im 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 arent very acidic.More
"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 wont the O-H bond be stronger?More
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.
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.
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