2-12-13
Chem 3125 Experiment #2 – The Study of Elimination Reactions Using
Gas Chromatography. The Study of Elimination Reactions Using Gas
Chromatography, Devin Latimer, J. Chem Ed, Vol 80, p. 1183, 2003
Introduction
Elimination and substitution reactions (E1,E2, SN1, SN2) are among the most fundamental
organic reactions. While each of these reaction types is relatively simple in mechanism,
the 2 competing pathways complicate matters. In this experiment, you will investigate the
effects of structural features of the alkyl halide and the base/nucleophile used on the
outcome of the reaction.
You will work in groups of 4-8 with each person completing a reaction between one of
three alkyl halides and one of two bases, and the product distribution of each reaction will
be analyzed by gas chromatography (GC). The GC data will be compiled by each
student and then shared with the group. The effect of different bases/nucleophiles and
primary versus secondary alkyl bromides gives a range of results that demonstrate the
effects of substrate and nucleophiles on product distribution.
Reactions
Br
H3C
+
+
Na
R
O
-
+
K
H3C
Br
+
CH3
H3C
H3C
+
Na
K
R
O
-
+
Br
+
CH3
+
Na
K
R
O
-
+
R = Me, Et, t-Butyl
Required Chemicals
Reagent
1-bromopentane
2-bromopentane
2-bromo-2-methylbutane
Na Methoxide
Na Ethoxide
K t-Butoxide
MTBE
Saturated NH4Cl
Molarity/MW
151.04
151.04
151.04
1.5 M
1.5 M
1.5 M
Not relevant
Amount
10 drops, ~ 0.5 mL
10 drops, ~ 0.5 mL
10 drops, ~ 0.5 mL
1.2 mL
1.2 mL
1.2 mL
10 mL
2*10 mL
2-12-13
Student Procedure
You will be assigned a brominated alkane and a base for your particular
elimination/substitution reaction. Note: the sodium methoxide, sodium ethoxide and
potassium tert-butoxide salts will decompose on prolonged exposure to moisture in the
air and so should be kept closed to the atmosphere as much as possible. In order to
minimize contamination with water the condenser and RB flask should be dried in the
oven for 20 minutes prior to setup. To minimize water contamination, setup your
apparatus after drying before adding chemicals. In a 50 mL RB flask set up for reflux
with heating mantel (hot water bath alternative) with a drying tube, add 1.2 mL of the 1.5
M sodium methoxide, sodium ethoxide or potassium tert-butoxide salt and stir with 10
drops of the brominated alkane (approximately 4.0 mmoles) heated at reflux for one hour.
After the solution is cooled to room temperature, the product is transferred to separatory
funnel and 10 mL of MTBE is added to wash the reaction flask. The organic layer is then
washed with saturated aqueous ammonium chloride (2 x 10 mL) and water (1 x 10 mL).
The organic layer is transferred to a dry 50 mL flask and dried with anhydrous
magnesium sulfate. A 2-4 mL sample is put in a sample vial for GC analysis.
Analysis of Results
You will be given a spreadsheet with GC data of expected olefin products, 1-, 2- & 3pentene, MTBE, 1-bromopentane, 2-bromopentane, and expected substitution products.
You will need to compare these to the GC data of your product mixture. A table of
boiling points versus retention time should be created for the GC chromatograms. Since
MTBE is present in such high levels, the scale is expanded to show products. Be sure to
look for impurities in the GC traces of the starting alkyl bromides and not confuse them
with your products.
Gas chromatography gives relative wt. % of each component as measured in peak area of
desired peaks. You will need to fill out the table below and calculate moles of alkyl
halide and products (Assume you have 100 grams to get relative moles). Convert the
relative GC wt. % of each component to relative mole fraction by dividing by molecular
weight of the component. Then total moles to determine mole fraction of each
component. You then will need to calculate overall conversion of starting alkyl bromide
and % yield of each product. For your conversion calculations, assume all the product
came from alkyl halide to get total moles of products and reactants. Conversion is then,
moles of product/total moles *100. Share this data with your group. Relative product
yields are calculated only in percent of reacted bromoalkane for the olefin and ether
products (moles olefin/total moles ether and olefin).
2-12-13
Component
Relative wt. % Relative mmoles
GC wt. % #
Wt.%/MW
Normalized mole %,
conversion/unreacted
1-Pentene/2-methyl1-butene
Trans 2-Pentene/2methyl-2-butene
Cis 2-Pentene
Alkyl-Pentyl Ethers
1 or 2-bromopentane
Total moles
Calculate % total conversion – Percent total conversion is simple summary of conversion
to olefins and ethers.
Calculate % conversion to olefin & ether – Yield of olefins (ethers) is simply the %
conversion olefin/(% conversion olefin + % conversion ether)*100.
Ratio % of 1-Pentene – Wt. % 1-Pentene/(total wt. % of all olefins, 1-pentene/trans 2pentene/cis 2-pentene) *100. Only calculated for products from 2-bromopentane!
If desired the gc area % can be converted to weight % using relative response factors
before completing the calculations.
After sharing data with groups you will need to fill in table below. Rationalize the
product distribution of each reaction based on your understanding of the relative
reactivities of the bromides with each respective base.
Regarding the table below
1. You should enter % conversions for each component.
2. Guidance above for % conversion to products, would be: total moles products(olefin
& ether)/total moles*100). (e.g. if there were 90 mmoles products and 100 total moles
of products and starting material, % conversion would be 90%).
3. The conversion % for olefin and ether should add up to 100, % olefin = mole
olefin/(mole olefin + mole ether) * 100.
Halide/Base
% Alkyl
Olefin % Ether %
Ratio 1Combo
bromide
Pentene(butane)/total
Reacted
pentene(butane) *
100
1-Br/MeO
2-Br/MeO
2Br2Me/MeO
1-Br/EtO
2-Br/EtO
2Br2Me/EtO
1-Br/t-BuO
2-Br/t-BuO
2Br2Me/t-BuO
The sum of olefin plus ether = 100%
2-12-13
Pre-lab Questions:
1. Draw and name all of the expected products from the reactions used in this
experiment. Comment on the relative amounts of each expected.
2. Explain what is meant by ‘retention time’ in GC and compare this with ‘retention
factor (or Rf)’ in TLC.
Post lab questions/Report
Lab report should include a discussion for each combination of alkyl halide and base why
the products are produced and whether E2/Sn2 or E1/Sn1 was primary mechanism
Mechanism for only your products is required in the lab report.
Example
1-bromopentane/methoxide – Primary product is ether because of the primary carbon
which favors substitution and small nucleophile. Primarily occurs by E2/Sn2.
2-bromopentane/methoxide
2-bromo-2-methylbutane/methoxide
1-bromopentane/t-butoxide
2-bromopentane/ t-butoxide
2-bromo-2-methylbutane/ t-butoxide
2-12-13
Instructors Notes
The sodium bromide is readily observed precipitating out of solution as the reaction
proceeds.
Products from the ethanol reaction sometimes contains ethanol which can interfere with
the olefin separation. A simple solution is to add ~0.2-0.4 mL of water to the GC vial
and shaking removes most of the ethanol. If the student does the washing well, the
amount of ethanol is low. You can avoid this entirely by simply using methoxide and not
ethoxide. However, depending on your GC column you may obtain different
interferences.
The ethanol we utilized is denatured with a mixture of methanol and isopropanol.
Therefore, you normally find the ether products from both methanol and the ethanol in
the reaction products from reactants with Na ethoxide. We just had students lump total
ethers from both methanol and ethanol together.
The percent conversion does not enter into any of the explanations of the data and may be
deleted from the summary table.
In the analysis of the products from 2-bromopentane, toluene is often seen since the
method concentrates it in the product. In our GC method it came right before 2bromopentane.
In the current experimental procedure, the limiting reagent is actually the alkoxide 1.8
mmoles alkoxide versus ~4 mmoles of alkyl bromide. However, we obtained excellent
conversions. We tried one or two experiments with higher levels of alkoxide, with
similar results.
Chemicals
1-bromopentane(CAS # 110-53-2), 2-bromopentane(107-81-3), 2-bromo2methylbutane(507-36-8), 21 wt. % sodium ethoxide (141-52-6) in ethanol (64-17-5), 30
wt.% Na Methoxide(CAS # 124-41-4) in methanol (67-56-1), t-butyl alcohol(75-65-0),
potassium t-butoxide (865-47-4)and ammonium chloride (12125-02-9) were obtained
from Sigma-Aldrich. Methyl tert-butyl ether (1634-04-4) was obtained from JT Baker.
Methanol and denatured ethanol (~4.5 % by volume of methanol & isopropyl alcohol)
were obtained from Pharmco through a local vendor. 1-pentene (109-67-1) and cis &
trans 2-pentene (109-68-2) were prepared by distillation from a acid catalyzed
dehydration of 2-pentanol (6032-29-7). This produced a typical distribution of the
pentenes as well as an impurity of the di-2-pentyl ether. The 1.5 Molar solution were
prepared by dilution of the concentrated reagent or dissolution of the pure alkoxide. The
alcohols should be dried over molecular sieves to minimize effect of water. Preparation
of the alkoxide from original metals can also be utilized.
Sodium methoxide , sodium ethoxide and are strongly basic solutions and should be
handled with care. Reagents,; 1-bromopentane, 2-bromopentane, Ethanol, methanol and
t-butanol. Methyl t-butyl ether is highly flammable
2-12-13
Gas Chromatography method
Gas chromatograph used was a Varian CP-3800 with flame ionization detector. Column
was DB-624 (6 % cyanopropyl-phenyl, 94% dimethylpolysiloxane) from J&W Scientific,
30 meters long, 0.25 mm ID, film thickness of 1.4µm. Injection temperature was 200 C,
with a split ratio of 10 to 1, Helium flow rate 1 mL/min constant flow, initial column
temperature was 50 C for 4 minutes, ramp at 50 C/min to 150 C hold for 2 minutes, ramp
at 40 C/min to 190 C and hold for 2 minutes. Total run time was 11 minutes. Detector
was at 200 C. Attenuation was decreased from 12 to 9 from 4.95-5.50 minutes while the
MTBE eluted. This increased the relative size of the product peaks relative to solvent.
Responsive factors were not determined due to lack of pure materials, but pedagogically
this will have a minimal impact on the comparison of the results. The 2-bromopentane
contained small amounts of toluene identified by GC/MS. In the product chromatograms,
the reaction of the 2-bromopentane reduced this peak substantially; however the toluene
was carried through and showed up as a sizable peak in many of the product
chromatograms.
Retention times key products and reactants
Component
1-pentene
2-methyl-1-butene
trans 2-pentene
cis-2-pentene
2-methyl-2-butene
Methyl 2-methylbutyl ether
Methyl2-pentyl ether
Methylpentyl ether
Ethyl 2-pentyl ether
Ethyl pentyl ether
2-bromo-2-methylbutane
2-bromopentane
1-bromopentane
t-butylpentyl ether
Retention times,
minutes
3.73
3.86
4.00
4.14
4.21
6.25
6.29
6.70
6.82
7.34
7.05
7.45
7.92
8.34
Methanol
3.04
Ethanol RT
3.9
t-butanol
4.8
MTBE
5.0
Di-2-pentyl ether
8.94
Toluene
7.3
1-pentanol
7.44
Boiling points of products and similar products
2-12-13
Boiling Points, C
1-pentene
trans 2-pentene
cis-2-pentene
methyl 2-pentyl ether
Methylpentyl ether
2-ethoxy pentane
Ethyl pentyl ether
2-bromopentane
1-bromopentane
Di-t-butyl ether
t-butyl, 2-pentyl ether
30
37
37
87
94
99
114
117
130
t-Butyl ethyl ether
t-Butyl propyl ether
t-Butyl butyl ether
i-propyl pentyl ether
68
92
119
121
di-propyl ether
Butyl propyl ether
Di 2-butyl ether
Dibutyl ether
85
112
116
137
Dipentyl ether
182
101
Identification of Ethers
The six different ethers produced were identified by comparison of boiling points and
confirmed by GC/MS analysis of the reaction mixtures; Methyl pentyl ether, ethyl pentyl
ether, methyl 2-pentyl ether, methyl 2-methyl-2-butyl ether, ethyl 2-pentyl ether, and tbutyl pentyl ether.
2-12-13
Examples of Data from each variation are below.
GC data corrected for Relative Response factor – GC wt. % * RRF for ether
GC Data
1-pentene
2-Methyl-1butene
trans 2-pentene
cis-2-pentene
2-Methyl2butene
Methyl 2methylbutyl
ether
Methyl2-pentyl
ether RRF = 1.40
Methylpentyl
ether RRF = 1.40
Ethyl 2-pentyl
ether RRF = 1.31
Ethyl pentyl
ether RRF = 1.31
2-bromo-2methylbutane
2-bromopentane
1-bromopentane
t-butylpentyl
ether RRF = 1.20
1-Br
MeO
2-Br
MeO
2BrMe
MeO
1-Br
EtO
2-Br
EtO
1-Br
tBuO
2-Br
tBuO
2-BrMe
tBuO
GC wt.%
1.46
GC wt.%
2.44
GC wt.%
GC wt.%
1.57
GC wt.%
12.7
GC wt.%
4.30
GC wt.%
31.63
GC wt.%
11.11
44.69
7.61
2.33
29.55
4.46
7.66
4.12
28.11
30.66
63.78
16.91
1.99
117.85
1.74
26.12
88.71
0.06
9.77
33.04
3.37
2.18
0.13
30.22
14.30
25.45
Legend – 1-Br =1-Bromopentane; 2-Br = 2-Bromopentane; 2-BrMe = 2-Bromo-2methylbutane; MeO = NaMeO/MeOH; EtO = NaEtO/EtOH; tBuO = Kt-BuO/BuOH;
2-12-13
Relative Moles
Relative moles
1-pentene
2-Methyl-1butene
trans 2-pentene
cis-2-pentene
2-Methyl-2butene
Methyl 2methylbutyl ether
Methyl2-pentyl
ether
2-Methyl-2butene
Methyl 2methylbutyl ether
Methylpentyl
ether
Ethyl 2-pentyl
ether
Ethyl pentyl ether
2bromomethylbuta
ne
2-bromopentane
1-bromopentane
t-butylpentyl
ether
Summary total
moles
Summary moles
product
1-Br
MeO
Relative
Moles
2.09
2-Br
MeO
Relative
Moles
3.49
2BrMe
MeO
Relative
Moles
1-Br
EtO
Relative
Moles
2.24
2-Br
EtO
Relative
Moles
18.14
1-Br
tBuO
Relative
Moles
6.14
2-Br
tBuO
Relative
Moles
45.19
15.87
2-BrMe
tBuO
Relative
Moles
63.84
10.88
3.33
42.21
6.37
10.94
5.89
40.16
43.80
62.53
16.58
1.95
115.54
0.00
1.70
0.00
25.61
0.00
0.00
76.48
0.04
6.47
21.90
1.44
2.23
0.09
9.48
20.03
0.00
17.68
0.00
119.86
56.17
118.60
80.51
95.73
33.29
82.04
114.12
117.63
34.27
118.56
80.42
94.29
23.82
62.01
107.64
2-12-13
Percent conversion
Percent conversion
1-pentene
2-Methyl-1-butene
trans 2-pentene
cis-2-pentene
2-Methyl-2butene
Methyl 2-methylbutyl
ether
Methyl2-pentyl ether
Methylpentyl ether
Ethyl 2-pentyl ether
Ethyl pentyl ether
2-bromomethylbutane
2-bromopentane
1-bromopentane
t-butylpentyl ether
% 1-Pentene/2-Meethyl
1- butene
Conversion %
Conversion Olefin
conversion Ether
1-Br
MeO
% conv
1.7
2-Br
MeO
% conv
6.2
2BrMe
MeO
% conv
1-Br
EtO
% conv
2-Br
EtO
% conv
1-Br
tBuO
% conv
2-Br
tBuO
% conv
0.0
13.4
1.9
19.0
18.5
55.1
2-BrMe
tBuO
% conv
53.8
19.4
5.9
44.1
6.7
13.3
7.2
33.9
36.9
52.7
29.5
2.0
96.4
2.0
94.3
39.0
1.5
1.9
100.0
98.1
1.8
98.2
26.8
0.1
19.7
61.0
51.6
48.4
28.3
100.0
47.3
52.7
100.0
99.9
2.8
97.2
24.4
28.5
53.1
27.2
98.5
70.8
29.2
100.0
71.5
25.8
74.2
72.9
75.6
100.0
0.0
59.3
94.3
100.0
0.0
2-12-13
Sample Gas Chromatograms of Reactants, products and by-products
Figure #1 – GC of alcohols & MTBE
µV
pe
ta
n
1-
34,000
32,000
M
et
ha
no
36,000
ol
TB
E
alcohols1.DATA
l
38,000
no
l
30,000
Is
o
pr
op
a
28,000
26,000
24,000
la
lc
oh
ol
22,000
20,000
ut
y
18,000
l
tB
16,000
M
To
l
ue
n
e?
et
ha
no
14,000
12,000
10,000
O
W
NK
N
U
4,000
??
??
??
?
??
N
8,000
6,000
2,000
0
-2,000
RT [min]
2.5
3
3.5
4
Name
Methanol
ethanol
UNKNOWN
Isopropanol
t Butyl alcohol
MTBE
??
???
????
UNKNOWN
Toluene?
1-pentanol
4.5
5
5.5
6
6.5
7
7.5
Time [Min] Area [µV.Min] Area % [%]
3.07
559.8
0.425
3.97
28916.6
21.935
4.47
433.3
0.329
4.58
1552.2
1.177
4.83
2680
2.033
4.98
2017.2
1.53
5.69
88.4
0.067
6.09
63.9
0.048
6.58
20
0.015
7.19
24.2
0.018
7.3
98.4
0.075
7.52
95375
72.347
Figure #2 – GC of pentenes, 1-bromopentane, di-2-pentyl ether, 2-pentanol.
8
8.5
9
2-12-13
µV
3,800,000
1-br old pentenes1.DATA
ta
n
e
3,600,000
en
3,400,000
te
n
e
3,000,000
1Br
om
op
3,200,000
en
2,800,000
-2
-p
2,600,000
s
2,400,000
ci
2,200,000
ne
2,000,000
200,000
r
le
th
e
2pe
ns
D
E
i2
-p
e
en
ty
tra
nt
en
M
TB
400,000
pe
600,000
1
800,000
STH 10.00
1,000,000
SPW 0.20
1,200,000
2-
1,400,000
nt
an
pe
n
1,600,000
ol
te
1,800,000
0
RT [min]
0
0.5
1
1.5
2
2.5
Name
1 pentene
trans 2- pentene
cis - 2- pentene
UNKNOWN
MTBE
2-pentanol
UNKNOWN
UNKNOWN
UNKNOWN
1-Bromopentane
UNKNOWN
Di 2-pentyl ether
UNKNOWN
Figure #3 – 2-bromopentane
3
3.5
4
4.5
5
5.5
6
6.5
7
Time [Min] Area [µV.Min] Area % [%]
3.75
4008.4
1.338
4.01
44745.8
14.936
4.15
27746.8
9.262
4.87
368.4
0.123
4.97
2435.4
0.813
6.80
40785.4
13.614
7.12
2211.3
0.738
7.46
277.3
0.093
7.68
551.4
0.184
8.01
166269.9
55.501
8.63
208.7
0.07
8.97
9164.2
3.059
9.04
805.5
0.269
7.5
8
8.5
9
9.5
10
10.5
2-12-13
µV
2 bromo pentane1.DATA
op
en
ta
ne
70,000
65,000
TB
E
2Br
om
60,000
M
55,000
50,000
45,000
40,000
35,000
30,000
25,000
To
lu
en
e
20,000
15,000
10,000
5,000
0
-5,000
RT [min]
4.5
5
5.5
Name
UNKNOWN
MTBE
UNKNOWN
UNKNOWN
UNKNOWN
UNKNOWN
Toluene
2-Bromopentane
UNKNOWN
6
6.5
7
7.5
8
8.5
Time [Min] Area [µV.Min] Area % [%]
4.39
241
0.219
4.98
3356.7
3.051
5.7
133
0.121
6.05
23.6
0.021
6.64
33.6
0.031
6.76
79.1
0.072
7.32
173
0.157
7.49
105770
96.142
8.8
204.7
0.186
9
9.5
10
10.5
2-12-13
Figure #4 – Reaction product Methoxide with 2-bromo-2-methylbutane
600,000
µV
methoxide 2-br-2Mebutane3.DATA
eb
ut
yl
et
he
r
550,000
2M
et
hy
l-2
-M
500,000
450,000
2M
e2Bu
te
ne
400,000
350,000
300,000
eB
ut
an
e
TB
E
2Br
-2
-M
50,000
M
100,000
2M
e1Bu
te
ne
150,000
SPW 0.20
200,000
STH 10.00
250,000
0
-50,000
RT [min]
0
0.5
1
1.5
Name
2-Me-1-Butene
2-Me-2-Butene
MTBE
UNKNOWN
2-Methyl-2-Mebutyl
ether
2-Br-2-MeButane
Total
2
2.5
3
3.5
Time [Min]
3.86
4.21
4.97
6.16
6.25
7.05
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
Quantity
Area
Area %
[% Area]
Height [µV] [µV.Min]
[%]
11.11
73266.4
4219.2
11.112
28.11
239661
10673
28.109
12.06
61763.8
4580.8
12.064
0.64
9470.7
243.1
0.64
45.56
0.06
669168.9
547.7
17299.5
24.1
45.561
0.063
100
1070998.5
37969.5
100
9.5
10
10.5