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Relative acidity of p-chlorobenzoic acid and p-flurobenzoic acid
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Malcolm Sargeant
Relative acidity of p-chlorobenzoic acid and p-flurobenzoic acid
For acidity of para substituted benzoic acids, -I effect is helpful and +R effect decreases the acidity.
In 4-fluorobenzoic acid, the +R effect is prominent in $\ce{F}$ due to stronger $\ce{2p}\pi - \ce{2p}\pi$ overlap with nearest carbon. Thus, due to stronger +R effect of $\ce{F}$ than -I effect, it creates a negative charge on your starred carbon. This charge delocalises in the carbonyl carbon and the electrophilicity is thus decreased of that carbon. Releasing hydrogen also doesn't help much as the negative charge created over $\ce{O}$ of $\ce{-OH}$ group can only delocalise a little.
Whereas in the $\ce{Cl}$, there is much weaker $\ce{3p}\pi-\ce{2p}\pi$ overlap. So, +R effect of $\ce{Cl}$ is much weaker and due to a relatively higher -I than its +R, there is more positive character on the starred carbon due to $\ce{Cl}$.
And, thus, in reality, acidity of 4-chlorobenzoic acid is a little higher than that of 4-fluorobenzoic acid.
For acidity of para substituted benzoic acids, -I effect is helpful and +R effect decreases the acidity.
In 4-fluorobenzoic acid, the +R effect is prominent in $\ce{F}$ due to stronger $\ce{2p}\pi - \ce{2p}\pi$ overlap with nearest carbon. Thus, due to stronger +R effect of $\ce{F}$ than -I effect, it creates a negative charge on your starred carbon. This charge delocalises in the carbonyl carbon and the electrophilicity is thus decreased of that carbon. Releasing hydrogen also doesn't help much as the negative charge created over $\ce{O}$ of $\ce{-OH}$ group can only delocalise a little.
Whereas in the $\ce{Cl}$, there is much weaker $\ce{3p}\pi-\ce{2p}\pi$ overlap. So, +R effect of $\ce{Cl}$ is much weaker and due to a relatively higher -I than its +R, there is more positive character on the starred carbon due to $\ce{Cl}$.
And, thus, in reality, acidity of 4-chlorobenzoic acid is a little higher than that of 4-fluorobenzoic acid.
I am saying that in $\ce {Cl} $, $-I $ effect is relatively stronger than $+R $ effect. I am not comparing $\ce{F } $ with $\ce{Cl } $ in -I effect.More
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In the case of halogens(on ortho/para), the -I effect dominates over the +R effect and hence due to this the negative charge of carboxylate anion is better stabilized by the flourine substituted compound then the chlorine substituted compound. Therefore, the p- flouro benzoic acid is more acidic than the p- chloro benzoic acid.
In the case of halogens(on ortho/para), the -I effect dominates over the +R effect and hence due to this the negative charge of carboxylate anion is better stabilized by the flourine substituted compound then the chlorine substituted compound. Therefore, the p- flouro benzoic acid is more acidic than the p- chloro benzoic acid.
For acidity of para substituted benzoic acids, -I effect is helpful and +R effect decreases the acidity.
In 4-fluorobenzoic acid, the +R effect is prominent in $\ce{F}$ due to stronger $\ce{2p}\pi - \ce{2p}\pi$ overlap with nearest carbon. Thus, due to stronger +R effect of $\ce{F}$ than -I effect, it creates a negative charge on your starred carbon. This charge delocalises in the carbonyl carbon and the electrophilicity is thus decreased of that carbon. Releasing hydrogen also doesn't help much as the negative charge created over $\ce{O}$ of $\ce{-OH}$ group can only delocalise a little.
Whereas in the $\ce{Cl}$, there is much weaker $\ce{3p}\pi-\ce{2p}\pi$ overlap. So, +R effect of $\ce{Cl}$ is much weaker and due to a relatively higher -I than its +R, there is more positive character on the starred carbon due to $\ce{Cl}$.
And, thus, in reality, acidity of 4-chlorobenzoic acid is a little higher than that of 4-fluorobenzoic acid.
For acidity of para substituted benzoic acids, -I effect is helpful and +R effect decreases the acidity.
In 4-fluorobenzoic acid, the +R effect is prominent in $\ce{F}$ due to stronger $\ce{2p}\pi - \ce{2p}\pi$ overlap with nearest carbon. Thus, due to stronger +R effect of $\ce{F}$ than -I effect, it creates a negative charge on your starred carbon. This charge delocalises in the carbonyl carbon and the electrophilicity is thus decreased of that carbon. Releasing hydrogen also doesn't help much as the negative charge created over $\ce{O}$ of $\ce{-OH}$ group can only delocalise a little.
Whereas in the $\ce{Cl}$, there is much weaker $\ce{3p}\pi-\ce{2p}\pi$ overlap. So, +R effect of $\ce{Cl}$ is much weaker and due to a relatively higher -I than its +R, there is more positive character on the starred carbon due to $\ce{Cl}$.
And, thus, in reality, acidity of 4-chlorobenzoic acid is a little higher than that of 4-fluorobenzoic acid.
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In the case of halogens(on ortho/para), the -I effect dominates over the +R effect and hence due to this the negative charge of carboxylate anion is better stabilized by the flourine substituted compound then the chlorine substituted compound. Therefore, the p- flouro benzoic acid is more acidic than the p- chloro benzoic acid.
In the case of halogens(on ortho/para), the -I effect dominates over the +R effect and hence due to this the negative charge of carboxylate anion is better stabilized by the flourine substituted compound then the chlorine substituted compound. Therefore, the p- flouro benzoic acid is more acidic than the p- chloro benzoic acid.
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