CHEM 350 -  Principles of Organic Chemistry I
Prof. T Nalli, Fall 2009, Winona State University

Expt 5. Mechanistic study of an S_N reaction. Competitive nucleophiles. Gas chromatography, refractive index, and proton NMR.

References. Pavia Expt 21. Smith Chapter 7. 

You will carry out experiment 21 (all three parts) in the Pavia experiment. For part A you will join together with another team in a "supergroup" of 6 (same supergroups as in Expt 2 please). One team in your supergroup should carry out the experiment using 1-butanol as the reactant and the other team using 2-butanol.

Gas chromatography and ¹H NMR will be used for the analysis of the part A reactions. Refractive index and ¹H NMR will be used for part B.

Procedure modifications: The reflux apparatus diagramed in the Pavia procedure is close to that which you will be using. However, no drying tube with wet glass wool is necessary at the top since we will be carrying out the reaction in a fume hood. (The top of the reflux condenser is merely left open to the air.) Also, you will use a sand bath (not an aluminum block) as the heat source. The sand bath consists of a thermowell filled with sand and plugged into the variable voltage outlet under each fume hood. Also, I strongly recommend clamping the neck of the flask to the ring stand (rather than clamping the condenser) as that makes securing the flask and condenser together (we do not have the screw joint attachments diagramed) unnecessary because gravity will do the job.

Report

I finished compiling class data for expt 5. The link is on the lab schedule page. Make sure that you not only report your own results but that you also report representative class data for the reaction of all three alcohols. Include an NMR spectrum for all three alcohols as well. Integrate a key pair of peaks in each spectrum so as to determine the RBr:RCl product ratio. Be careful in assigning the key NMR peaks as belonging to the alkyl chloride or alkyl bromide product! Safest is to look up literature spectra for all six of these compounds and use the literature values to assign the peaks.

Also pertaining to expt 5, I gave out GC traces of standard 1.0:1.0 mole ratio solutions of n-BuBr/n-BuCl and s-BuBr/s-BuCl. Use these data as standard values to compare your experimental retention times to. Also, notice that the area ratios in these standard solutions did not come out to exactly 1.0:1.0 hinting strongly at a source of experimental error in the GC measurements.