GC
Analysis of Butene Isomers
Student’s
Name
Institutional
Affiliation
Table of Contents
GC
Analysis of Butene Isomers
Purpose
The main objective of
this laboratory experiment was to prepare butene isomers from dehydration of 1-
and 2-butanols using heat and concentrated sulphuric (VI) acid and from the dehydrobromination
of organic molecules of 1- and 2-bromobutane using potassium hydroxide
solution, ethanol with heating. By performing this experiment, we also aimed to
understand the elimination reaction involving E1 and E2 mechanisms and
comprehend the GC analysis of the products for the documentation of the
quantity of the major and minor butene isomer from after elimination reaction.
The will help in understanding the analysis of GC and its industrial
applications of the experiment.
Introduction
Elimination reaction is
important in the manufacture of butene isomers. The reaction is either
dehydrohalogenation or dehydration. In dehydrobromination, bromobutane in
presence of concentrated ethanol and potassium or sodium hydroxides are heated
to produce gaseous by-products of butene isomers. On the other hand,
dehydration of 1- or 2-butane in the presence of concentrated sulphuric (VI)
acid or concentrated phosphoric (V) acid produces gaseous byproducts of butene.
Because of the less density compared to water, and slightly soluble in water,
the gaseous components are collected in the “over-the-water” method through
bubbling the gases in an inverted test-tube or boiling tube corked with a
rubber septum. The gaseous products are then analyzed using the conventional
method, GC after injection of the gaseous mixture into the GC capillary tube.
Using the inert gas (mobile phase) and silica coatings of the stationary phase,
the isomers butene are eluted corresponding to the boiling points of each
isomer as they reach the flame ionization
detector (FID). Retention time (tR)
in minutes is the time an eluent takes to elute from the GC (Mohrig et al.,
2014). In the detector, signals are displaced as peaks in the picoAmp form a
chromatogram if plotted against tR (Mohrig et al., 2014, p. 292).
The GC computer software represents peak areas of each isomer corresponding to
each amount of the products (Mohrig et al., 2014). Hence, the number of by-products
of a reaction can be identified and each given in percentage ratios.
Observations
After the GC
analysis of the products of 1- and 2-butanol under strong acidic environment
(Concentrated Sulphuric (VI) acid) with heating through (Dehydration reaction),
the three products are formed and are displayed on the GC monitor.
Table
1:
GC analysis of butene isomers from dehydration reaction of 1-butanol and Concentrated
Sulphuric (VI) acid with heat.
|
1-butanol byproducts
|
Retention time(Rt)
(Min)
|
Area pA
|
Height
|
Area %
|
|
1 Isomer
|
1.55300
|
1392.58179
|
1440.54150
|
9.56754
|
|
2-Isomer
|
1.59700
|
8666.57031
|
9033.75586
|
59.54249
|
|
3-Isomer
|
1.64000
|
4496.11914
|
4240.09961
|
30.88997
|
Table
2:
GC analysis of butene isomers from elimination reaction of 2-butanol using
Concentrated Sulphuric (VI) acid and heat (Dehydration reaction)
|
2-butanol byproducts
|
Retention time(Rt) in
Minutes
|
Area pA
|
Height
|
Area %
|
|
1 Isomer
|
1.550
|
315.03412
|
361.17862
|
6.66155
|
|
2 Isomer
|
1.595
|
2933.59448
|
3172.76782
|
62.03230
|
|
3 Isomer
|
1.637
|
1480.51160
|
1466.69922
|
31.30615
|
Table
3:
Results of GC analysis of products of elimination reaction of 1-bromobutane
using strong base, Potassium hydroxide, ethanol with heating
(Dehydrobromination reaction)
|
2-bromobutane
|
Retention time (Rt) in
Minutes
|
Area pA
|
Height
|
Area %
|
|
I Isomer
|
1.552
|
2433.96606
|
2622.00122
|
27.27015
|
|
2 Isomer
|
1.597
|
5022.93555
|
5419.86230
|
56.27696
|
|
3 Isomer
|
1.639
|
1468.48389
|
1395.07690
|
16.45289
|
Table
4:
GC analysis of butene isomers of elimination reaction of 1-bromobutane using
strong base, Potassium hydroxide, ethanol with heating (Dehydrobromination
reaction)
|
1-bromobutane
|
Retention time (Rt) in
Minutes
|
Area pA
|
Height
|
Area %
|
|
1-butene
|
1.564
|
1.31881e4
|
1.25790e4
|
1.000e2
|
Table
5:
Physical Properties of isomers of butene
|
Physical
Properties
|
1-butene
|
Trans-butene
|
Cis-butene
|
|
Physical state
|
Gaseous
|
Gaseous
|
Gaseous
|
|
Boiling Point (0C)
|
-6.3
|
0.8
|
3.7
|
|
Collection Method
|
Over-Water
|
Over-Water
|
Over-Water
|
(Hayes et al., 2016, p. 3-78)
E1 Mechanism of Butyl Alcohol (Dehydration)
Elimination
Reaction of Butyl alcohol such as 2-methyl-2-propanol produces a C-C pi bond in
the E1 elimination reaction following dehydration (unimolecular elimination)
(Mondal, 2018). The systematic reaction commences by protonation of the tert-alcohol
group found on the structure of 2-methyl-2-propanol by the strong acid
(Sulphuric (VI) acid to form a hydroxonium ion bond. Subsequently, the bond
leaves the group to form a tert-carbocation. The beta hydrogen found on one
carbon over is then confronted by Sulphuric (VI) acid to form a pi bond, thus
leading to satisfaction of the carbocation (Ashenhurst, 2020). This
reaction is only possible in the presence of acidic conditions and increased
temperatures (Heat) in the absence of a base. The figure 1 below shows
systematic mechanisms
Step
1:
Protonation of oxygen on hydroxide group
Figure
1
Step
2:
Breaking C-O bond allow release of water molecule (H2O) resulting in
carbocation intermediate.
Figure
2
Step
3:
Deprotonation by Water molecule (H2O) from carbon adjacent to the carbocation
center occasioning the creation of carbon-carbon double bond.
Figure
3E2 Mechanism of Butyl bromide (Dehydrobromination)
Elimination of
Butyl bromides such as 2-bromo-2-methylpropane using a strong base such as
potassium hydroxide to produce 1-butene and 2-butene follow E2 mechanism
through dehydrobromination. In this case, the electrons located on the oxygen
atom found on the structure of potassium hydroxide attacks the beta proton
found at a carbon away from where the bromide group is located, resulting in
the repulsion of the electrons to the C-C bonds hence the formation of pi bonds
(Carrascosa et al.,2017). This leads to the bromide group leaving the alkyl
group and the alkyl group occuring in an anti periplanar geometry (Mondal,
2018). Besides, E2 elimination for the base-induced reaction is a one-step
reaction compared to E1 elimination. The systematic step for the formation of
butene isomers is shown below.
Figure
4:
E2 elimination reaction of butyl bromide using a strong base, potassium
hydroxide.
Figure
4
Step
1:
Breaking of the C-Br bond allows bromide ion to leave resulting in carbocation
intermediate
R=H,
CH3CH2-
Figure 5
Step
2:
Deprotonation by base-induced elimination from the carbon atom adjacent to the
carbocation leads to the creation of pi bond.
R=H, CH3CH2-
Figure 6Discussion and Analysis
In the elimination
reactions above, dehydrobromination
of 1- bromobutane is a byproduct of 1-butene.
The electrons on the hydroxide molecule of strong base attack the carbon bound
away from the carbon close to where bromide is bonded resulting in bromide ion
leaving the group, thus creating an anti periplanar to the beta hydrogen to
form butene (Carrascosa et al., 2017). The result is a “Zaitsev product” having
the position of the butene at the most substituted position.
From the elimination
reactions illustrated above, the hypotheses of the experiment are that majority
of products will be the Zaitsev products because of the application of
unhindered base (KOH) in E2 reaction and sulphuric (VI) acid (Strong acid) and
the hydrogen as the periplanar in the E1 reaction. Nevertheless, if a bulky
basic compound that is mostly sterically hindered were to be used with no
periplanar hydrogen molecule, then Hoffman products would have been expected to
be the major products in E2 and E1 respectively. The best method to verify the
formation of stable products of Zaitsev is GC based on the hypotheses of
formation of 2-butene for both E1 and E2 elimination being major. This is done
by observing the peaks, the calculation of peak areas, and the number of peaks
formed as illustrated in the results below.
GC Analysis
From table 1-3
above; and based on the hypotheses, Isomer 1 had the shortest retention time (tR),
followed by Isomer 2 and then 3. In comparison with the boiling points,
1-butene should elute first because it is highly volatile and has a lesser
boiling point as shown in Table 5 compared
to 2-butene isomers. Following closely is the elution of Trans-butene and finally Cis-butene.
The first Isomer is 1-butene, the second Isomer is Trans-butene, and the third isomer is a Cis-butene.
Moreover, the pA
areas that are equivalent to the amount of each isomer was identified using the
GC analysis. In this case, the elution time and percentage peak areas show that
the major isomer of butene was under E1 and E2 elimination. In the dehydration
reaction of 1-butanol, 1-butene had a
relative amount equivalent to 9.56754%, Trans-butene
relative amount was 59.54249%, and Cis-butene
amount was indicated as 30.88997%. For 2-butanol undergoing dehydration
reaction, 1-butene showed to have a relative amount of 6.66155%, Trans-butene
with 62.03230%, and Cis-butene indicating a relative amount of 31.30615%. In
the dehydrobromination of 1-bromobutane, only a single product was produced
with a relative amount of 1.000e2%. However, the dehydrobromination of
2-bromobutane produced three isomers with relative amounts of 27.27015%
representing 1-butene, Trans-butene relative amount was 56.27696%, with
Cis-butene having the relative amount of 16.45289%.
Conclusion
In conclusion, in terms
of purity of the byproducts, the E1 elimination reaction showed a pure form
reaction mechanism as compared to the E2 reaction. For instance, as illustrated
from the percentage areas and elution time (tR), the elution was
early with a small area of 6.66155%. In comparison, 1-butene produced in E2
elimination reaction of dehydrobromination
of 2-bromobutane resulted in
early elution of 1-butene but with a larger area of 27.27015%. Therefore, a
one-step formation of products in the E1 reaction is recommended for use
compared to the multi-step E2 elimination reaction. However, the GC of E2
elimination suggested that the products of the elimination reactions were
separated efficiently based on the boing points. As indicated, the results
produced by the GC showed a coherent association with the elimination mechanism
of base-induced dehydrohalogenation of alkyl halides such as butyl bromides and
acid-stimulated dehydration of organic alcohols such as butyl alcohols.
References
Ashenhurst, J. (2020). The E1 reaction and its mechanism – Master
organic chemistry. Master Organic
Chemistry. Retrieved from https://www.masterorganicchemistry.com/2012/09/19/the-e1-reaction/
Carrascosa, E., Meyer, J., Zhang, J.,
Stei, M., Michaelsen, T., Hase, W. L., Yang, L., &
Wester, R. (2017). Imaging dynamic fingerprints of competing E2 and SN2
reactions. Nature Communications, 8(1). doi:10.1038/s41467-017-00065-x
Haynes, W. M., Lide, D. R., & Bruno, T. J.
(2016). CRC handbook of chemistry and physics: a ready-reference book
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& Hammond, C. (2014). Laboratory techniques in organic chemistry.
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