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Answer:
According to their expected pKa values, the order of those acids should be:
1- Cl2CHCOOH is the strongest acid and the lowest pKa.
2- ClCH2COOH is a strong acid, but no more than the first. Medium pKa value.
3- ClCH2CH2COOH is a strong acid, but no more than the two previous acids. High pKa value.
4- CH3CH2COOH is the weakest acid, so the highest pKa value.
Explanation:
The pKa values are the negative logarithm of dissociation constant. It represents the relative strengths of the acids. Stronger acids show smaller pKa values and weak acids present larger pKa value. The stronger the acid, the weaker it's the conjugate base. The larger the pKa of the conjugate base, the stronger the acid. The strength of an acid is inversely related to the strength of its conjugate.
Conjugate bases are the substance that has one less proton than the parent acid. The conjugate base of the acid presented in the problem are:
ClCH2COOH -> ClCH2COO- + H+
ClCH2CH2COOH -> ClCH2CH2COO- + H+
CH3CH2COOH -> CH3CH2COO- + H+
Cl2CHCOOH -> Cl2CHCOO - + H+
Cl2CHCOOH. The negative charge presented on its conjugate base is by resonance and inductive effect. This is the strongest acid.
ClCH2COOH. A negative charge is stabilized by resonance and electron-withdrawing but only one atom is present. So this acid is less strong than the first one.
ClCH2CH2COOH. The negative charge is stabilized by resonance and electron-withdrawing atom but the effect is less compared to the two acids showed previously.
CH3CH2COOH. The negative charge is stabilized by resonance and destabilized due to CH3 group. This is the weakest acid among the problem.
Stronger acids have smaller pKa values and weak acids have larger pKa values. Due to the information present in this problem, Cl2CHCOOH is the strongest acid and the lowest pKa. CH3CH2COOH is the weakest acid, so the highest pKa value.
Finally, we can conclude that according to their expected pKa values, the order of those acids should be:
1- Cl2CHCOOH is the strongest acid and the lowest pKa.
2- ClCH2COOH is a strong acid, but no more than the first. Medium pKa value.
3- ClCH2CH2COOH is a strong acid, but no more than the two previous acids. High pKa value.
4- CH3CH2COOH is the weakest acid, so the highest pKa value.
The ranking of the acids according to expected pKa values is;
Cl2CHCOOH> ClCH2COOH > ClCH2CH2COOH > CH3CH2COOH
Certain electronic effects in organic chemistry must come into play when considering the pKa values of the acids listed. First of all, pKa is a measure of the ability of acids to dissociate in solution.
In considering the acids listed, we must remember that chlorine is highly electronegative. As such, it attracts electrons to itself thereby weakening the O-H bond in the acid and making the acidic proton more labile. This effect diminishes as the distance of the chlorine from the -COOH moiety increases. This is known as inductive effect. Inductive effect in which electrons are withdrawn towards an atom is designated as -I inductive effect.
Hence, structures having more chlorine atoms or in which the chlorine atoms are in close proximity to the -COOH moiety experience greater -I inductive effect an must have a higher pKa. Therefore, the ranking of the acids according to expected pKa values is;
Cl2CHCOOH> ClCH2COOH > ClCH2CH2COOH > CH3CH2COOH
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