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Chapter 19q: Practice Quiz

  • Page ID
    205971
  • Acidity of Carboxylic Acids

    Question 1

    For each of the following pair of molecules, choose which would be more acidic and explain why.

    carboxylic acids Q1.png

    Answer

    Remember that acidity is based on the stability of the conjugate base. Whichever molecule has a more stable conjugate base will be more stable.

    benzoic acid.png

    The amino group is electron donating by resonance which would destabilize the carboxylate (conjugate base of the acid) making the para-aminobenzoic acid less acidic than benzoic acid.

    para nitro benzoic acid.png

    The nitro group is electron withdrawing by resonance which would stabilize the carboxylate making the para-nitroobenzoic acid more acidic than benzoic acid.

    fluoropropanoic acid.png

    The fluoro group is more electronegative than chloro, therefore will inductively stabilize the carboxylate better than the chloro group will. 

    dichloropropanoic acid.png

    The 1,1-dichloropropanoic acid has two groups that withdraw electron density inductively thereby stabilizing the carboxylate more than the mono chloro molecule.

    question 2

    For the following amino acid, draw the structure under acidic, neutral and basic conditions.

    carboxylic acids Q2.png

    Answer

    carboxylic acids Q2 answers.png

    Under acidic conditions both the carboxylate and the amine are protonated and the molecule has an overall charge of +1.

    Under neutral conditions, the amine is protonated while the carboxylate is deprotonated giving the overall charge neutral.

    Under basic conditions, the amine is not protonated and neither is the carboxylate. The molecule will have an overall charge of -1.

     

    question 3

    What makes glycine different from all other amino acid residues that constitute proteins?

    Answer

    glycine.png

    As can be seen in the structure, glycine has two hydrogens on the alpha carbon making glycine achiral. All other amino acids have one hydrogen and one different group which makes the alpha carbon a stereocenter.

    Contributors  

    • Layne Morsch (University of Illinois Springfield)