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Effect of phenyl and vinyl substituents on Acidity of carboxylic
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Mats Andersson
Effect of phenyl and vinyl substituents on Acidity of carboxylic
When the cases of carboxilic acids are considered, we must note that the conjugation of $\ce{COO-}$ group is so strong in itself, that it's conjugation with the rest of the compound is rendered relatively weak. In such cases, the inductive effect is dominant.
Let us take the example of benzoic acid. The carbon to which $\ce{COOH}$ is attached shall be named as the key-carbon $\ce{C}$1. The conjugate base would be $\ce{PhCOO-}$, and the +I(Inductive effect) of $\ce{COO-}$ group will be dominant on $\ce{C}$1. Any group attached to this compound will further increase or decrease the acidic strength of the compound as a whole, depending upon the attached group's contribution towards electron density at $\ce{C}$1.
For example, in case of para-nitrobenzoic acid, electron density at $\ce{C}$1 decreases due to $\ce{NO2-}$'s strong -R (Mesomeric) Effect(partial positive charge is developed on $\ce{C}$1 which is stabilised by $\ce{COO-}$'s +I). Hence it turns out to be a strong acid.
The opposite case is that of para-methoxybenzoic acid. The electron density at $\ce{C}$1 increases due to +R of $\ce{OCH3}$ group and makes the conjugate base unstable, hence acid weaker. This can be cross checked by the $\mathrm{p}K_\mathrm{a}$ values of above mentioned two compounds as well.
Now coming to the compounds mentioned in the question, which are phenyl and vinyl, one must note that both of these would reduce the charge density on $\ce{C}$1 because of the $\mathrm{sp^2}$ hybridised Carbon atom which contributes to the overall electro-negativity of these two groups.
Long story short, the resonance effect between $\ce{COO-}$ and the rest of the compound is suppressed by a relatively stronger Inductive effect which works mutually between $\ce{COO-}$ and the rest of the compound.
When the cases of carboxilic acids are considered, we must note that the conjugation of $\ce{COO-}$ group is so strong in itself, that it's conjugation with the rest of the compound is rendered relatively weak. In such cases, the inductive effect is dominant.
Let us take the example of benzoic acid. The carbon to which $\ce{COOH}$ is attached shall be named as the key-carbon $\ce{C}$1. The conjugate base would be $\ce{PhCOO-}$, and the +I(Inductive effect) of $\ce{COO-}$ group will be dominant on $\ce{C}$1. Any group attached to this compound will further increase or decrease the acidic strength of the compound as a whole, depending upon the attached group's contribution towards electron density at $\ce{C}$1.
For example, in case of para-nitrobenzoic acid, electron density at $\ce{C}$1 decreases due to $\ce{NO2-}$'s strong -R (Mesomeric) Effect(partial positive charge is developed on $\ce{C}$1 which is stabilised by $\ce{COO-}$'s +I). Hence it turns out to be a strong acid.
The opposite case is that of para-methoxybenzoic acid. The electron density at $\ce{C}$1 increases due to +R of $\ce{OCH3}$ group and makes the conjugate base unstable, hence acid weaker. This can be cross checked by the $\mathrm{p}K_\mathrm{a}$ values of above mentioned two compounds as well.
Now coming to the compounds mentioned in the question, which are phenyl and vinyl, one must note that both of these would reduce the charge density on $\ce{C}$1 because of the $\mathrm{sp^2}$ hybridised Carbon atom which contributes to the overall electro-negativity of these two groups.
Long story short, the resonance effect between $\ce{COO-}$ and the rest of the compound is suppressed by a relatively stronger Inductive effect which works mutually between $\ce{COO-}$ and the rest of the compound.
When the cases of carboxilic acids are considered, we must note that the conjugation of $\ce{COO-}$ group is so strong in itself, that it's conjugation with the rest of the compound is rendered relatively weak. In such cases, the inductive effect is dominant.
Let us take the example of benzoic acid. The carbon to which $\ce{COOH}$ is attached shall be named as the key-carbon $\ce{C}$1. The conjugate base would be $\ce{PhCOO-}$, and the +I(Inductive effect) of $\ce{COO-}$ group will be dominant on $\ce{C}$1. Any group attached to this compound will further increase or decrease the acidic strength of the compound as a whole, depending upon the attached group's contribution towards electron density at $\ce{C}$1.
For example, in case of para-nitrobenzoic acid, electron density at $\ce{C}$1 decreases due to $\ce{NO2-}$'s strong -R (Mesomeric) Effect(partial positive charge is developed on $\ce{C}$1 which is stabilised by $\ce{COO-}$'s +I). Hence it turns out to be a strong acid.
The opposite case is that of para-methoxybenzoic acid. The electron density at $\ce{C}$1 increases due to +R of $\ce{OCH3}$ group and makes the conjugate base unstable, hence acid weaker. This can be cross checked by the $\mathrm{p}K_\mathrm{a}$ values of above mentioned two compounds as well.
Now coming to the compounds mentioned in the question, which are phenyl and vinyl, one must note that both of these would reduce the charge density on $\ce{C}$1 because of the $\mathrm{sp^2}$ hybridised Carbon atom which contributes to the overall electro-negativity of these two groups.
Long story short, the resonance effect between $\ce{COO-}$ and the rest of the compound is suppressed by a relatively stronger Inductive effect which works mutually between $\ce{COO-}$ and the rest of the compound.
When the cases of carboxilic acids are considered, we must note that the conjugation of $\ce{COO-}$ group is so strong in itself, that it's conjugation with the rest of the compound is rendered relatively weak. In such cases, the inductive effect is dominant.
Let us take the example of benzoic acid. The carbon to which $\ce{COOH}$ is attached shall be named as the key-carbon $\ce{C}$1. The conjugate base would be $\ce{PhCOO-}$, and the +I(Inductive effect) of $\ce{COO-}$ group will be dominant on $\ce{C}$1. Any group attached to this compound will further increase or decrease the acidic strength of the compound as a whole, depending upon the attached group's contribution towards electron density at $\ce{C}$1.
For example, in case of para-nitrobenzoic acid, electron density at $\ce{C}$1 decreases due to $\ce{NO2-}$'s strong -R (Mesomeric) Effect(partial positive charge is developed on $\ce{C}$1 which is stabilised by $\ce{COO-}$'s +I). Hence it turns out to be a strong acid.
The opposite case is that of para-methoxybenzoic acid. The electron density at $\ce{C}$1 increases due to +R of $\ce{OCH3}$ group and makes the conjugate base unstable, hence acid weaker. This can be cross checked by the $\mathrm{p}K_\mathrm{a}$ values of above mentioned two compounds as well.
Now coming to the compounds mentioned in the question, which are phenyl and vinyl, one must note that both of these would reduce the charge density on $\ce{C}$1 because of the $\mathrm{sp^2}$ hybridised Carbon atom which contributes to the overall electro-negativity of these two groups.
Long story short, the resonance effect between $\ce{COO-}$ and the rest of the compound is suppressed by a relatively stronger Inductive effect which works mutually between $\ce{COO-}$ and the rest of the compound.
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