Question

Label the most acidic hydrogen in each molecule and justify your choice by using appropriate \(\mathrm{p} K_{\mathrm{a}}\) values. (a) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (b) \(\mathrm{HSCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{C}=\mathrm{CH}\) (d) \(\mathrm{HOCCH}_{2} \mathrm{CH}_{2} \mathrm{SH}\) (e) CC(O)C(=O)O (f) \(\mathrm{H}_{3} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{COH}^{+}\) (g) \(\mathrm{H}_{3} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{CO}^{-}\) (h) \(\mathrm{HSCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Short answer

(b) Thiol and amine are present in the molecule. Which functional group has a lower pKa value, and therefore the most acidic hydrogen? (c) Alcohol and alkene are present in the molecule. Which functional group has a lower pKa value, and therefore the most acidic hydrogen? (d) Carboxylic acid and thiol are present in the molecule. Which functional group has a lower pKa value, and therefore the most acidic hydrogen? (e) Carboxylic acid and alcohol are present in the molecule. Which functional group has a lower pKa value, and therefore the most acidic hydrogen? (f) Ammonium ion and alcohol are present in the molecule. Which functional group has a lower pKa value, and therefore the most acidic hydrogen? (g) Amine and carboxylate ion are present in the molecule. Which functional group has a lower pKa value, and therefore the most acidic hydrogen? (h) Thiol and alcohol are present in the molecule. Which functional group has a lower pKa value, and therefore the most acidic hydrogen?

Step-by-step solution

Identify functional groups in each molecule

Before identifying the most acidic hydrogen, we need to recognize the functional groups present in each molecule. (a) alcohol, amine (b) thiol, amine (c) alcohol, alkene (d) carboxylic acid, thiol (e) carboxylic acid, alcohol (f) ammonium ion, alcohol (g) amine, carboxylate ion (h) thiol, alcohol

Use pKa values to identify the most acidic hydrogen and justify the choice

The most acidic hydrogen will be the one with the lowest pKa value (i.e., more likely to be deprotonated). We will use general pKa values for different functional groups to compare which hydrogen is more acidic. (a) Most acidic hydrogen: \(\ce{-OH}\) (alcohol, pKa = 15-18) because the amine group has a higher pKa value (~38). (b) Most acidic hydrogen: \(\ce{-SH}\) (thiol, pKa = 10-12) because the amine group has a higher pKa value (~38). (c) Most acidic hydrogen: \(\ce{-OH}\) (alcohol, pKa = 15-18) because the \(\ce{-C-H}\) on alkene has a higher pKa value (~44). (d) Most acidic hydrogen: Carboxylic acid \(\ce{-OH}\) (pKa ~ 4-5) because the thiol group has a higher pKa value (10-12). (e) Most acidic hydrogen: Carboxylic acid \(\ce{-OH}\) (pKa ~ 4-5) because the alcohol group has a higher pKa value (15-18). (f) Most acidic hydrogen: Ammonium ion \(\ce{-NH3+}\) (pKa ~ 9) because the alcohol group has a higher pKa value (15-18). (g) Most acidic hydrogen: Carboxylate ion \(\ce{-CO-}\) (pKa ~ 4-5) because the amine group has a higher pKa value (~38). (h) Most acidic hydrogen: \(\ce{-SH}\) (thiol, pKa = 10-12) because the alcohol group has a higher pKa value (15-18).