The Modern Student laboratory: Chromatography GC/MS Experiments for the Organic Chemistry Laboratory 1. E2 Elimination of 2-Bromo-2-methyloctane Michael ~ o v a k , Julie ' Heinrich, Kristy A. Martin, John Green, and Scott Lytle Miami University, Oxford, OH 45056
Gas ChromatographyNass Spectrometry (GCNS) has become an indispensable tool offhenucd rrwawh since its introduction in 1957 1 1 1 . Capllla~-v ~ W / M S is oarticularlv useful for the separation of~mult~component mixtures df volatile organics, including mixtures of isomers. Because reliable instruments equipped with capillary columns can be purchased for considerably less than $100,000, it has become practical to introduce this technology into undergraduate laboratory courses.
Both experiments allow students to obtain hands-on expcrlcncc with GC MS in a realistic situation in which analvsisofframnrntation oatterns 1s essential to thesuccessful interpretation of the experimental results. These experiments generate isomeric mixtures of products that can be successfully analyzed by capillary GC/MS. They also demonstrate the power--and the limitationsof G C N S and show how reaction conditions can affect product distributions in two mechanistically well-understond reactions.
Introducing a Valuable Instrumental Technique at the Sophomore Level Ameasure of the increasing use of GCIMS e a u i ~ m e nin t undergraduate laboratories can be found in data pnnidrd bv the National Science Foundation's Instrumentation for Laboratory Improvement (ILI) Program. I n fiscal year 1989, 10 of 116 successful proposals to the ILI program from chemistry departments were for the purchase of GCIMS equipment (2). I n fiscal year 1991, 14 of 131 successful proposals involved G C N S (2). Although G C N S eauipment has become increasinelv availableffor undergraduate use, it does not appear to he used in the sophomore organic laboratory to a significant extent. The better-known laboratory texts mention the technique only briefly! Over the past two years we have attempted to incorporate the GCIMS technique into the sophomorelevel B.S. organic laboratory course & Miami University. We have found that in small courses (15-30 students) the technique can he rapidly mastered by the students. It can increase their understanding of both mass spectrometry and the organic chemistry principles demonstrated by the laboratory experiment.
The E2 Elimination The E2 elimination of alkyl halides in alcohol solvents is known to he sensitive to steric effects (3,4). Although the Zaitsev products predominate when OH-or EtO- is used a s the base, a bulky base such a s t-BuO- may lead to a product mixture containing a higher yield of the thermodynamically less stable alkenes (3).I n this experiment the students use an elimination procedure first outlined by Brown and Moritani (3).
..
-"
The Product Mixture
The substrate chosen for the experiment, 2-hromo-2methyloctane yields two products 1):
ceq
2-methyl-1-octene 2 2-methyl-2-octene3
Ta 7 7 +
3
(1)
The two alkenes are readily separated from each other a s well a s the injection solvent, CH2C12,on a capillary column containing a cross-linked methyl silicone gum stationary phase. The two alkenes can also be distinguished from each other by their mass spectra. Elimination in OH that the KOH/ E ~ O Hand in ~ - B ~ O W ~ . B ~illustrates product distribution is sensitive to steric effects.
Hands-on Experience and Analysis of Fragmentation Patterns
I n this and the following article we describe two capillary G C N S experiments that were designed for, and tested in, our B.S. sophomore organic laboratory course: E2 Elimination of 2-Bmmo-2-methyloctane Friedel4rafts Alkylation ofp-Xylene 'Author to whom correspondence should be addressed. 'A recent full bibliographic and abstract search of GC(w)MSand Gas(w)Chromatography(w)Mass(w)Spectrometty on CA ON LINE generated over 7.000 citations covering the period from 1967 to the present. 3We examined five laboratory texts published since 1989. Only two even mentioned GCIMS, and none presented experiments involving GCIMS.
2
1
Two-step Synthesis of the Substrate The synthesis of 1from 1-hromohexane 4 can be accomplished in two steps (eq 2).
4
=yH
2. Me2C0 3. H*IH20
CHCh
(2)
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Volume 70 Number 4 April 1993
A103
The Modern Student lclboratory: Chromatogrclphy Synthesizing the Intermediate Alcohol
The intermediate alcohol, 2-methyl-2-octanol 5 is commercial?~available, but quite expensive (Wiley Organics, Coshocton, OH; no. 43812; $18 for 2mL). It can be obtained in good yield from the Grignard reaction of l-hexylmagnesium bromide with acetone (5). Synthesizing Tertiary Alkyl Bromides
The traditional reagents for generating tertiary alkyl bromides from the corresponding alcohols-concentrated HBr (5)or PBr3 (6)-work well for low molecular weight bromides, but do not give reliable yields for higher molecular weight bromides in the hands of sophomore organic students in a 2- or 3-h laboratorv oeriod. For the low molecular weight bromides, this is due to the very low solubilitv of the alcohol in the aaueous solution of the acid. For t
