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