Chemistry Reference
In-Depth Information
5. Arrange the following groups of molecules in order of increasing basicity. Explain your
results using partial charges and inductive effects.
Initially, when considering inductive effects, we realize that F, O, and Cl all possess
partial negative charges. Therefore, we realize that all of these atoms will pull elec-
tron density from the carboxylic acid, thus stabilizing the anion resulting from
deprotonation and lowering the pK
a
values compared to the baseline acetic acid.
The question now focuses on how strong this effect is for each atom. The
answer is found in the periodic table of the elements and relates to electronegativ-
ities. Of the three atoms in question, F is the most electronegative. Moving to the
second row, Cl is more electronegative than O. Since the most basic compound will
have the least electronegative atoms associated with it,
the order of increasing
basicity is as follows:
Note that this is the opposite sequence as that presented in Chapter 2, Problem 6.
6. Predict the order of protonation of the basic sites on the following molecules. Support
your answers with pK
a
values.
In addressing this problem, it is important to recognize that the order of protonation
depends upon the basicity associated with the respective functional groups. As dis-
cussed above, basicity can be relayed back to the pK
a
values associated with the conju-
gate acids of the respective sites of protonation. Thus conjugate acids with the higher
pK
a
values will be protonated first while conjugate acids with lower pK
a
values will
be protonated last.
According to Appendix 1, the pK
a
value for an acetylene proton is approximately 25,
and the pK
a
value for a carbonyl-protonated amide is approximately 0. Therefore, the
order of protonation is as follows:
