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Question

Which of the following compounds would have the smallest value for pKa?


A

CHF2CH2CH2COOH

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B

CH3CH2CF2COOH

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C

CH2FCHFCH2COOH

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D

CH3CF2CH2COOH

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Solution

The correct option is B

CH3CH2CF2COOH


The explanation for the correct option

(B) CH3CH2CF2COOH

  • As pKaincreases, acidic strength will decrease. Thus the compound with the smallest pKavalue will be more acidic.
  • -F is a strong electron-withdrawing group. Thus, it will have a strong -I effect.
  • This strong electron-withdrawing nature of two -F pulls the electron density from -COOH group so that it can easily release its proton H+.
  • Therefore CH3CH2CF2COOH is more acidic and has the smallest pKa value.
  • Hence option B is correct

The explanation for the incorrect options

(A) CHF2CH2CH2COOH

  • -F group is present. But here, -Fis present at third carbon from the carboxyl group.
  • As the distance from the carboxyl group increases, its electron-withdrawing effect will decrease. Thus the acidic strength also gets reduced.
  • Thus pKawill not be the smallest.
  • Hence option A is incorrect

(C) CH2FCHFCH2COOH

  • Here also -F group is present at more distance from the carboxyl group compared to option B.
  • Thus acidic strength will get reduced.
  • Hence option C is incorrect

(D) CH3CF2CH2COOH

  • -F group is at more distance from the carboxyl group compared to option B.
  • Therefore here also acidic strength will get reduced due to the same reason.
  • So more will be the pKa value.
  • Hence option D is incorrect.

Therefore the correct answer is option (B) CH3CH2CF2COOH


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