Acid Catalyzed dehydration of 2-Methyl-2-butanol
Abstract
The dehydration of 2-methyl-2-butanol was attained when reacted with concentrated
sulfuric acid. The reaction proceeded through an E2 mechanism, and it yielded to 2-methyl-2butene and 2-methyl-1-butene. The alkene products were found in the expected ratios predicted
by Zaitsev’s law. Nuclear magnetic resonance (NMR) revealed that 2-methyl-2-butene and 2methyl-1-butene were produced in an approximate 4:1 ratio, respectively.
Introduction
Elimination reactions are a type of organic reaction in which two substituents are removed
or eliminated from a molecule in either a one- or two-step mechanism.1 Furthermore, because a
hydrogen and a functional group are eliminated from adjacent carbons, elimination reactions yield
to alkenes to stabilize the electron loss. The one-step mechanism is referred to as an E2 reaction
because the reaction rate is bimolecular, meaning that it depends on the concentrations of both the
nucleophile and the alkyl halide. This is a concerted reaction because bond cleavage and bond
formation occur simultaneously.3 (fig. 1)
Figure 1. E2 reaction mechanism.
By contrast, the two-step mechanism is referred to as an E1 reaction (fig.2) because the
rate at which the reaction occurs depends merely on the concentration of the molecule that contains
a leaving group. Good leaving groups are usually weak bases (such as halide ions) because when
they dissociate through heterolytic bond cleavage, they are stable despite the addition of electrons.
Therefore, as the leaving group becomes a weaker base, the greater the equilibrium constant will
favor the dissociation of the leaving group.2 Once the leaving group is cleaved, a carbocation is
formed, and its positive charge enables a nucleophilic attack from the nucleophile.
Figure 2. E1 reaction mechanism.
When alcohols are treated with strong acids, they undergo E1 reactions because the acid
protonates the hydroxyl group, turning it into water which is a weaker base.4 The water molecule
then leaves the alkane in order to stabilize its charge. The loss of water from a molecule is called
dehydration. More than one elimination product can be formed because a carbocation intermediate
is formed during this reaction, the carbocation undergoes hydride shifts to stabilize its charge. As
a result, two or more alkenes may be formed, and the relative concentrations of each product
depend on their stability (Zaitsev’s rule).
The dehydration of 2-methyl-2-butanol was catalyzed with sulfuric acid, and it yielded to
a mixture of 2-methyl-2-butene and 2-methyl-1-butene (fig. 3). Zaitsev’s rule predicts that 2methyl-2-butene will be the major product given that it is more stable than its resonance structure,
2-methyl-1-butene. The minor product is still expected to be produced; however, its concentration
will be significantly less than the major product because 2-methyl-1-butene is not energetically
favorable.
Figure 3. Dehydration mechanism of 2-methyl-2-butanol.
The products were isolated from the starting materials using fractional distillation because,
as shown in Table 1., the difference in boiling points between the alcohol and the alkenes was not
significant (18oC). Thus, using simple distillation would not have been effective at separating the
reactants from the products. Nuclear magnetic resonance (NMR) imaging was used to determine
the success of the reaction and confirm the predicted concentrations of each product. The signals
obtained in the NMR screening were analyzed and compared with reference NMR screenings of
2-methyl-2-butene and 2-methyl-1-butene (fig.5 and 6).
Figure
5.
NMR
Sprectrum
of
2-methyl-2-butene.
1
Credit: ChemicalBook, 2-Methyl-2-butene (513-35-9) H NMR, 2-Methyl-2-butene(513-35-9) 1H NMR
(chemicalbook.com)
Figure
6.
NMR
Sprectrum
of
2-methyl-1-butene
Credit: Human Metabolome Database, 1H NMR Spectrum (1D, 90 MHz, CDCl3, experimental)
(HMDB0245210), Human Metabolome Database: 1H NMR Spectrum (1D, 90 MHz, CDCl3, experimental)
(HMDB0245210)
Table 1. Properties of Reagents and Products.
Name and Structure
2-methyl-butanol
Formula
C5H12O
Molar Mass
88.1488 g/mol
Boiling Point
102 oC
Sulfuric Acid
H2SO4
98.08 g/mol
337 oC
2-methyl-2-butene
C5H10
70.13 g/mol
31.0 oC
2-methyl-1-butene
C5H10
70.13 g/mol
38.50 oC
Experimental
Materials
Fractional distillation glassware, 3.0 mL of 2-methyl-2-butanol and 15.0 mL of 6M sulfuric
acid as starting reagents. An ice bath was used to prevent the products from evaporating. Solid
sodium carbonate was used to neutralize the distilling flask after the reaction was completed. The
set-up for this procedure is depicted on figure 4.
Figure
4.
Fractional
Distillation
Credit: Zubrick, J. The Organic Chem Lab Survival Manual Distillation; 2015;, pp 164 – 183.
Apparatus
Method
Fractional distillation was carried out with a 50.0 mL round bottom flask used as a distilling
flask. The distilling flask contained 3.0 mL of 2-methyl-2-butanol, to which 15.0 mL of 6M
sulfuric acid was added along with a couple of boiling stones. The distilling flask was placed in a
hot water bath. The water was allowed to boil briefly to initiate condensation. However, its
temperature was adjusted throughout the distillation process to ensure that the vapor temperature
did not exceed 45 oC.
Once enough distillate was collected, the receiving flask was detached from the distilling
head and immediately sealed with a cork and parafilm. The product was analyzed using NMR. The
residue left in the distilling flask was removed from the hot plate and allowed to cool down to
room temperature. Once cooled down the residue was dissolved in 100.0 mL of water and
neutralized with sodium carbonate before washing it down the sink.
Results
The dehydration of 2-methyl-2-butanol was determined to be successful through NMR
analysis. The collected distillate was assumed to have a mixture of the two alkenes, with 2-methyl2-butene being predominant. The NMR analysis of the distillate showed an overlap of both alkenes
(fig. 7). The obtained peaks matched with the individual peaks of either reference spectrum (fig. 5
or 6). The integral ratio of the alkene peaks at around 6 ppm was measured to determine the
respective ratios of the products. It revealed an approximate 4:1 ratio of 2-methyl-2-butene and 2methyl-1-butene, respectively. No additional impurities were found.
Figure 7. NMR analysis of alkene products
Discussion/Conclusion
The acid catalyzed dehydration of 2-methyl-2-butanol was successful, and it followed
Zaitsev's rule, which states that given an elimination reaction, the most substituted alkene product
will be energetically stable and therefore the most favored.5 The overlap of proton signals in the
NMR analysis suggests that both alkenes were present in the collected distillate. From the integral
ratio of the peaks found at around 6 ppm, the ratio between the two products was determined to be
3:1, with 2-methyl-2-butene being more abundant. These concentrations, however, would be
different if a different catalyst was used.6
Using bulky bases (e.g. KOC(CH3)3) yield to non-Zaitsev products in elimination reactions
because their steric effects are greater due to their size and geometry. The oxygen of the base will
eliminate the protons of the less substituted carbon because there will be less electron clouds, and
therefore less repulsive forces.7 An E2 reaction in which a bulky base is used as a catalyst can be
carried out to compare the alkene product ratios of acid-catalyzed dehydration and a base
elimination reaction. Alcohol dehydration with bases is not possible because OH is a strong base.
It is worth noting that strong acids and bulky bases are used for alkene synthesis because
they prevent the formation of a substitution, nucleophilic, bimolecular reaction (SN2). Weak acids
cannot be used as dehydration catalysts because they do not deprotonate as easily. Bulky bases
prevent the formation of a substitution product because the steric hindrance is too strong to bind
the nucleophile to the electrophile. In the laboratory, E2 reactions are essential for the synthesis of
various alkenes such as ligands and those used for the preparation of alkynes.8 In biological
systems, dehydration reactions are one of the most common reactions. They are often used in the
synthesis of symmetric ethers such as those found in the sugars and ATP. Understanding the
mechanisms and parameters of these reactions are important for the comprehension of biochemical
pathways and synthesis as well as properties of complex molecules.9
References
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