#gabbarsinghtuturials #pradeepkshterpal # khan academy Follow on:https://www.facebook.com/profile.php?id=100007151580463 Follow on:https://mobile.twitter.com/Khanp331Zahid Acidity of Carboxylic Acids
January 19, 2016 0 Comments
Carboxylic acid is an organic compound containing carboxyl group (COOH) attached to an alkyl or aryl group. They react with metals and alkalis to generate carboxylate ions. These reactions of carboxylic acids indicate their acid nature. Acidity of carboxylic acids is higher in comparison to simple phenols as they react with weak bases like carbonates and bicarbonates to liberate carbon dioxide gas.
acidity of carboxylic acids
Carboxylic acids dissociatein water to form carboxylate ion and hydronium ion. The carboxylate ion formed is stabilized through resonance by effective delocalization of the negative charge.
acidity of carboxylic acids
Carboxylic acids are weaker than mineral acids but are strongest among the organic compounds. Acidity of carboxylic acid is higher than alcohols and even phenols. As discussed above, carboxylate ion, the conjugate base of carboxylic acid is stabilized by two equivalent resonance structures in which the negative charge is effectively delocalized between two more electronegative oxygen atoms. On the other hand in case of phenols,
On the other hand in case of phenols, negative charge is less effectively delocalized over one oxygen atom and less electronegative carbon atoms in phenoxide ion. Therefore, the carboxylate ion exhibits higher stability in comparison to phenoxide ion. Hence, the carboxylic acids are more acidic than phenols.
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The acidity of carboxylic acids further depends on the nature of substituent alkyl or aryl group attached to the carboxyl group. An electron withdrawing group ensures effective delocalization of negative charge through resonance or inductive effect. Thus, electron withdrawing groups increase the stability of the conjugate base formed and hence the acidity of carboxylic acids. On the other hand, electron donating groups destabilizes the conjugate base formed and hence decreases the acidity of carboxylic acids. A general trend can be seen as:
CF3COOH ,
CCl3COOH,CHCL2COOH,NO2CH2COOH NC-CH2COOH
Due to the resonance effect, phenyl or vinyl groups too, increase the acidity of carboxylic acids in spite of decreasing the acidity due to induction effect.
acidity of carboxylic acids
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Acidity of Carboxylic Acids
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The pKa ‘s of some typical carboxylic acids are listed in the following table. When we compare these values with those of comparable alcohols, such as ethanol (pKa = 16) and 2-methyl-2-propanol (pKa = 19), it is clear that carboxylic acids are stronger acids by over ten powers of ten! Furthermore, electronegative substituents near the carboxyl group act to increase the acidity.
Compound
pKa
Compound
pKa
HCO2H
3.75
CH3CH2CH2CO2H
4.82
CH3CO2H
4.74
ClCH2CH2CH2CO2H
4.53
FCH2CO2H
2.65
CH3CHClCH2CO2H
4.05
ClCH2CO2H
2.85
CH3CH2CHClCO2H
2.89
BrCH2CO2H
2.90
C6H5CO2H
4.20
ICH2CO2H
3.10
p-O2NC6H4CO2H
3.45
Cl3CCO2H
0.77
p-CH3OC6H4CO2H
4.45
Why should the presence of a carbonyl group adjacent to a hydroxyl group have such a profound effect on the acidity of the hydroxyl proton? To answer this question we must return to the nature of acid-base equilibria and the definition of pKa , illustrated by the general equations given below. These relationships were described in an previous section of this text.
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