Question

(b) The amino acid lysine has the following values of pK : PK1 =2.2, pK2 = 8.9, PKR = 10.5 (the pK, of the amino group in the side chain). At pH 7 it has the structure shown below (6). Draw the structure that you would expect for lysine at pH 9.7, indicating the charges as appropriate. HZN 0 HINA (2 marks) (c) Phenol (C6H5OH) and 2,4,6-trichlorophenol (C6H2C13OH) are both weak acids with pK, values of 9.89 and 6.21, respectively, at 298.15 K. (i) Write down the dissociation reaction and corresponding expression for the equilibrium constant for 2,4,6-trichlorophenol. What is the conjugate base of this acid? (3 marks) (ii) Which of these two acids is the stronger? Considering the difference in electron density of the OH group in each molecule and the effect that this will have on the acidic hydrogen, suggest a reason for the difference in the strength of the two acids. (3 marks)

please answer part (ii)

Answer 1

Rit's 0 yo at pH 7 H 1 at higher pH, depootanation takes place to give The at pH_9.7 IN

© (i) OH pla on = 8 +repla +40 10 conjugate base of phenot phenol 애 TOT Ol +40 pla 2, 4, 6-richlosphenol conjugate base C. Among phenol and 2,4,6-trichlorophenol ; 2,4,6-bichloo- -phenol is a stronger acid. It can be explained based on two facts. (i) more the pika value cesses will be the acidic strength, hence 2,4,6-tichlorophenol has lesses plea valule than phenol, therefore it is more stooges acid than phenol. (ii) And also in 2,4,6-trichlorophenol, strong election withdro- helps to stabilize negative charge, therefore -wing chlorine atom helps to stabilize negative cha it acts as much stronger acid than phenol