Our organic professor did not cover the acid-base chapter, so I am confused how

Question

Our organic professor did not cover the acid-base chapter, so I am confused how to even approach these problems. Please help!

1. Predict whether Ha or Hb is more acidic and provide a detailed (structure-based) explanation for your choice: Ha Hb 2. One molecule below has a pKa of 13 and one has a pKa of 9. Using inductive and resonance argument with structures, predict the pKa for each molecule. O: rO: IV 3. Rank (and label) the following from most basic to least basic. Use structure-based arguments to explain your choice. H2N NH2 H2N VI VII

Explanation / Answer

Acid-base chemistry

1. When the ring strain is large in s bridged system, the protons are least likely to loose from it.

From the two structures shown above, II. has larger ring strain when compared to the bigger ring structure I. Therefore, Ha would be more acidic than Hb.

2. Ketones are more electrophilic systems than esters. In ester the lone pairs on oxygen can act as a donor and thus makes the alpha hydrogens on the carbon in IV less acidic when compared to III. The carboanion once generated in III is more stable due to resonance with the carbonyl than the ester functionality in IV. Thus III will have pKa 9 and IV would have a pKa of 13. (pKa and acidity are inversely related. Higher the pKa, lower is the acidity)

3. Basicity is ability to donate a pair of electrons. In V, the lone pair on N is more easily accessible than either VI or VII. In VII, the lone pair on nitrogen is sterically hindered for donation and thus the ranking of compounds according to decreasing basicity would be, V > VI > VII

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