Chem 231 Homework Assignment 2

Due Feb. 20

 

The purpose of this homework assignment is to calculate conditions (e.g. pH) needed to separate two similar compounds using a liquid-liquid extraction with water and octanol as the solvents (this also will be close to the types of distributions expected between water and a C18 stationary phase).

 

You will need to create three sets of calculations for the following pairs of compounds:

1) phenol and 1-aminobutane (n-butylamine)

2) 2-methylphenol and 3-nitrophenol

3) pyridine and 1-aminobutane (n-butylamine)

 

A demonstration Excel program is available online. (older version of Excel)

 

a) Find the Ka and octanol-water partitioning coefficient for each compound. An online resource is: http://www.vcclab.org/lab/alogps/, which gives the logKow values To use this, start the ALGORPS program. This program requires you to use the SMILES routine a link under SMILES is given to the Wikipedia page. For example, phenol is c1(O)ccccc1. This gives a logKow value (listed as average logP) value of 1.53. You will also need to find the pKa values. These are listed in the Harris textbook mentioned in the class resources website (there are also online sites that give this value, such as Wikipedia, for a limited number of molecules). Another available source is: http://openmopac.net/pKa_table.html. For example, for phenol, the pKa listed in the Harris text is 9.997 (it is listed as hydroxybenzene). In your report, be sure to give your source of data.

 

b) Use a spreadsheet program to calculate the fraction of each compound in the non-ionized form (α), the distribution coefficient (D), and the fraction of compound left in the aqueous phase (q) as a function of pH (from pH = 1 to 14). Assume that equal volumes of each solvent will be used.

For this assignment, remember that D must consider the amount of compound in the ionized form and α = [HA]/([HA] + [A-]) for weak acids and α = [B]/([B] + [BH+]) for weak bases. You should be able to use the Ka equations to create an equation for α in which [H+] is the only variable.

 

c) Determine at which pH the two compounds can be separated with at least 90% of one compound in one solvent and less than 10% of the other compound in the same solvent. If more than one extraction is necessary, indicate how many extractions are needed.

 

Excel solutions

 

Turn in a copy of the spreadsheet(s) plus a statement on point c for each pair.