Organic Chemistry Homework

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CVEN 4424 Environmental Organic Chemistry

Problem Set 2 Organic Acids and Bases

For some questions, all of the information and data is presented here. Some questions come from chapters in Schwarzenbach et al. (2017)
Environmental Organic Chemistry – they are labeled as “P XX.xx” where XX is the chapter number and xx is the problem number. Some
questions require review and use of data in journal publications available from the library.

For all questions, (1) cite sources of data, (2) consider significant figures in your answers, and (3) highlight your answers in boxes. Minor
point deductions for missing data citations, clearly inappropriate significant figures, and unboxed answers.

1. P 4.1 (SGI, 2017) Assessing the speciation of organic acids and bases in natural waters. In addition to calculating the fraction of the
compounds present as the neutral species, draw log C-pH graphs showing the speciation of the compounds over the pH range of 0 to 14
and a concentration range of 10-14 to 1 M for a total concentration of 10-2 M of each compound.

2. For the following compounds, draw the compound and estimate the pKa values using the Hammett approach. Check the estimated
pKa values with those provided in SGI (2017; Appendix C). For each compound, calculate the fraction in the acidic form (αa) at pH 6 using
your estimated pKa value.
(a) 2,3-lutidine (requires making an assumption about one of the Hammett σ values)
(b) tetrachloroguaicol (no longer in Appendix C; pKa = 6.26)
(c) 2,4-dinitro-o-cresol

3. Consider 4-aminopyridine, a compound used to manage symptoms of multiple sclerosis, and its pKa value of 9.11 at 25°C.
(a) To which nitrogen functional group, the heterocyclic nitrogen or the amino group, does the pKa value pertain? Hint: try estimating
the pKa of the heterocyclic nitrogen as pyridine with an amino substituent (requires another assumption about the Hammett σ).
(b) What pKa would you expect for the other functional group (hint: should be the amino group)? You’ll need to consult Table III in the
Tomasik and Johnson (1976) paper on the Extra Materials page of the Canvas course site. This is a challenging problem.
(c) Draw the compound in the protonated and deprotonated form in an acid dissociation reaction.

https://canvas.colorado.edu/courses/11125/pages/extra-materials

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1. a) Calculate the fraction of pentachlorophenol (PCP), 3,4-dimethylaniline (DMA) present at 24 degrees C as neutral species in a raindrop (pH = 4.0) and in lake water (pH = 8.0).
PCP pKa = 4.75, DMA pKa = 5.28

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b)