Dehydration of an Alcohol, 4-methyl-2-pentanol

Dehydration of an Alcohol, 4-methyl-2-pentanol Introduction For dehydration of alcohols, the mechanism starts with protonation of the alcohol since OH - is a poor leaving group while water is an excellent leaving group. This protonation is rapid and reversible and produces the oxonium ion, which contains a positively charged oxygen atom, further polarizing the C-O bond. Dehydration of the oxonium ion involves the loss of water either through elimination or substitution upon reaction with a base or nucleophile, respectively. While the substitution reaction involves the exchange of the alcohol group for another functional group, the elimination reaction forms an alkene by removing a proton from the beta carbon position together with the leaving group and forming a double bond. For secondary or tertiary alcohols, the first step is the rate determining step in which the oxonium ion first passes through a transition state that produces a carbocation intermediate and an elimination of water. Proton transfer with a base yields the final product, an alkene. Since the rate depends on the concentration of only one chemical entity, the leaving group, in the rate-determining step this is called an elimination unimolecular, or E1, reaction. The ease of dehydration for the E1 mechanism is tertiary then secondary alcohols (primary alcohols are generally unreactive towards E1), which parallels the relative stabilities of carbocations. For primary alcohols, ionization of the oxonium ion would produce an unstable primary carbocation intermediate, so an alternative bimolecular mechanism applies. Since it is a concerted process, and the rate depends on the concentrations of both the substrate and the base, it is called an elimination bimolecular, or E2, reaction. Because elimination and substitution are competing pathways, the choice of reagent can be used to prevent substitution; for this reason, in this experiment, sulfuric acid, rather than HX (whose conjugate base is a nucleophile and can trap the carbocation to yield the substitution product), is used as the acid catalyst. Since the starting alcohol in this experiment, 4-methyl-2-pentanol, is disubstituted, it will undergo an E1 reaction. It follows the E1 mechanism where the first step is protonation by the acid catalyst, followed by carbocation formation (which is the dehydration step), and finally double bond formation in which the regeneration of the catalyst takes place and carbocation rearrangements can occur. The equation for this reaction is: (CH3)2CHCH2CH(CH3)OH + H2SO4 → (C6H12)n Equation 1 Multiple isomeric alkenes can form due to an unsymmetrical carbocation intermediate that can yield different products based on relative transition state energies, sterics, and rearrangements like hydride shifts; five different products are expected in the dehydration of 4-methyl-2-pentanol, as shown in figure 1: Figure 1. Five expected products of acid catalyzed dehydration of 4-methyl-2-pentanol. For many elimination reactions, such as this one, the beta carbon positions are not identical, so the double bond can form in different regions of the molecule, producing two different regiochemical outcomes. While both products are generally formed, the more substituted alkene is usually the major 1 product, called the Zaitsev product, because the lower-energy transition state leads to the more substituted, stable alkene. The less-substituted product, called the Hofmann product, is minor. Thus, a trisubstituted Zaitsev product like2-methyl-2-pentene is expected to be in higher yield than the other four products. Experimental Procedure A stirbar, 4.0 mL of 4-methyl-2-pentanol, and 2.5 mL of 9 M sulfuric acid were added to a 25mL roundbottom flask, and the liquids were thoroughly mixed by gentle swirling. The flask was equipped for fractional distillation, and an ice-water bath was used to cool the receiving flask. Joints of the fractional distillation apparatus were well-secured, and the opening of the stillhead was blocked with parafilm in order to prevent escape of the low-boiling product. The mixture was heated at a head temperature of 85oC throughout the reaction. When the liquid stop boiling, after about 2.5 mL of liquid remains in the flask, heating was discontinued, and the distillate was collected and weighed. Two drops of distillate were added to a GC vial along with diethyl ether, and gas chromatography was performed on the sample. Observations The starting material was clear and colorless as was the product. The reaction mixture was boiling/refluxing, and boiling started to occur around 70oC; the product was low-boiling. Results Weight of product: 1.547 g Weight of starting material, 4-methyl-2-pentanol: vol. x density = 4.0 mL Percent yield: g = 3.208 g ( 0.802 1 mL ) 1.547 g ×100 % = 48.22% yield 3.208 g Figure 2. GC chromatograph of the products of dehydration of 4-methyl-2-pentanol. Table 1. Relevant information from GC chromatograph of the products of dehydration of 4-methyl-2pentanol. Peak Retention Time (min) Area under the curve (%) 1 1.53 5.987 2 1.589 76.155 3 1.703 3.213 4 1.824 13.298 5 1.931 1.347 2 Discussion The area under the curve indicates the percentage of products obtained. Based on the information from Table 2, assignments of the peaks in the chromatograph of Figure 1 and product ratios are as follows: Peak 1: 4-methyl-1-pentene (5.987%) Peak 2: trans-4-methyl-2-pentene and cis-2-methyl-2-pentene (76.155%) Peak 3: 2-methyl-1-pentene (3.213%) Peak 4: 2-methyl-2-pentene (13.298%) Peak 5: diethyl ether (solvent for analysis)??? The assignment was not entirely as expected since less substituted alkenes, such as the 4-methyl2-pentenes, did predominate over more substituted alkenes, such as 2-methyl-2-pentene. In fact, the major product, with a 76.2% product ratio, is the two 4-methyl-2-pentenes, but this can be explained because they are the initial alkenes to form without carbocation rearrangement. Furthermore, the formation of these alkenes is highly favored over the formation of the other, less substituted alkene 4methyl-1-pentene, that can be formed without carbocation rearrangement. Of the other products, however, the most substituted alkene 2-methyl-2-pentene greatly predominates at a 13.3% ratio. Although more highly substituted, this product did not come in as great a yield as the 4-methyl-2pentenes because its carbocation had to have undergone a hydride shift to form the final alkene; thus, the 4-methyl-2-pentenes were able to form more rapidly. For this same reason, 4-methyl-1-pentene with a ratio of 5.99% slightly predominates over 2-methyl-1-pentene with a ratio of 3.2%, despite being less substituted; the latter product had to have undergone two hydride shifts in order to be formed. Conclusion This experiment showed that the alkene product distribution is not always predictable based on Zaitsev’s rule alone due to the possibility of intermediate carbocation rearrangement, such as through hydride shift. Nonetheless, Zaitsev’s rule can still be used as a guideline for predicting products in the dehydration of alcohol, because in this experiment, the trisubstituted alkene dominated over the other two minor, less substituted products. This experiment also showed that sulfuric acid does serve as an acid catalyst for dehydration of an alcohol, as the expected peaks for the products appeared on the GC chromatograph, and the percent yield was a little under one half (48.2%). Finally, this experiment helped better understanding in the mechanism for dehydration of an alcohol using sulfuric acid because the products could only be explained through the mechanism, especially through understanding of the transition state and the structure of the carbocation intermediate. 3