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The pinacol-pinacolone rearrangement is a typical 1,2-rearrangement reaction of vicinaldiols under acidic conditions. Herein our research team reacted 2,3-dimethyl-2,3-butanediol (pinacol) and sulfuric acid to discover whether the respective pinacolone product could be synthesized, and to determine a mechanistic pathway for the reaction. The product was analyzed via IR and 1H NMR spectroscopy to identify and confirm functional groups as well as the number of different kinds of protons present in the product. Based on the data, it was evident that 3,3-dimethyl-2-butanone (pinacolone) was formed.
Our conclusion was that the reaction proceeded via an acid-catalyzed dehydration mechanism.
The pinacol rearrangement has been well studied over time,1,2,3 and has served as a standard topic in most undergraduate organic textbooks. It refers to the acid-catalyzed transformation of 1,2-diols to ketones or aldehydes by 1,2 migration of a C-C or C-H bond toward the vicinalcarbocation. Although its use is limited because of disadvantages such as poor regio- and diastereoselectivity, and unpredictable side reactions, it has remained an active area of research for synthetic chemists over the past decades, focused on expanding the utility and application of this fascinating series of reactions.
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The present work was undertaken in order to determine whether when reacted with sulfuric acid, pinacol would produce pinacolone. Careful analysis of the product helped confirm the synthesis of the pinacolone product, and a mechanistic approach to the acid-catalyzed reaction of pinacol was established.
Proposed Acid-Catalyzed Dehydration Mechanism for Pinacol Reaction
Based on our IR and 1H NMR spectroscopy data and research of our predecessors,5,6,7 we concluded that the product of our acid-catalyzed synthesis was 3,3-dimethyl-2-butanone (Scheme 1).
It was evident that the strong acid was needed to convert the poor -OH leaving group to a better one, H2O. It became evident that if both of the hydroxyl groups in the reactant were not the same, then the group which yields a more stable carbocation participates in the reaction. The motivation for this rearrangement is the relative stability of the resultant oxonium ion, which has a complete octet configuration at all centers, as opposed to the preceding carbocation. Additionally, the concentration of the acid had a mechanistic impact. According to current research, the pinacol rearrangement is specific to 1,2 diols.4
Such an approach established that the reaction of 2,3-dimethyl-2,3-butanediol with sulfuric acid indeed produced pinacolone via an acid-catalyzed dehydration mechanism. Specific importance of the mechanism is that of the concentration of the acid and the migration of the group which more effectively stabilizes the intermediate carbocation.
The reaction of 2,3-dimethyl-2,3-butanediol (1) with sulfuric acid produced a carbonylpinacolone (2) product. IR and 1H NMR spectroscopy served as effective methods for analysis, while background knowledge and research lead to deduction of a mechanistic pathway of the 2 product.
Table 1. Product Structure, IR Data and 1H NMR Data.
structure IR data 1H NMR data
According to Table 1, the assessment of the IR spectrum contributed to the identification of functional groups present in 2. The strong peak present at 1705.56 cm-1 indicates a sat acyclic ketone functional group. The absorption peaks at 2969.45 cm-1 and 2878.23 cm-1 reveal sp3 C-H stretches. There are two peaks at 1354.44 cm-1 and 1365.01 cm-1 which are uneven, meaning the peak at 1354.44 cm-1 is smaller than the peak at 1365.01 cm-1. The presence of these two peaks are evidence of a tertiary butyl group. The absence of the characteristic O-H stretch between 3400-3600 cm-1 indicates the absence of starting material in our obtained IR spectrum.
The data illustrated in Table 1 from the 1H NMR spectrum of the product reveal that there are two distinct hydrogens (A and B). As indicated by the 1H NMR spectrum, there is a singlet for 9H present at δ= 1.12 ppm due to hydrogen atoms at carbons 4, 5, and 6 (A). At δ= 2.12 ppm there is a singlet for 3H due to the hydrogen atom at carbon 1 (B). Typically, CH3 groups reveal around 0.9 ppm, but considering the presence of a C=O bond, the inductive effect causes the protons of the methyl group to become deshielded, and shift downfield. Through analysis of the IR and 1H NMR spectra the structure of the product was determined to be 2 as detailed in Table 1. Unfortunately, only a 64% yield of 2 was obtained.
To determine a possible mechanistic pathway for the reaction, first the structure of the product was determined, and based on background knowledge and research we deduced that the mechanism proceeded via an acid-catalyzed dehydration. The key concept of the reaction was acid-catalyzed dehydration afforded by the sulfuric acid. A proposed mechanism must always be consistent with the stated conditions, as seen here with the use of H2O in acidic conditions rather than hydroxide for the last step of the mechanism (Scheme 1). In this case, the attacking nucleophile is H2O rather than an anion, and therefore, a charged intermediate is generated as a result of nucleophilic attack. This intermediate is the oxonium ion. It is not possible to protonate an alcohol and then react the resulting oxonium ion (ROH2+) with a strong base to, for example, give an E2 reaction. The mechanism is more specifically an E1 process called acid-catalyzed dehydration (Scheme 1).6
A study by Mundy and Otzenberger postulated that the mechanistic formation of 2 is sensitive to isomerization and reaction conditions.7 For acid-catalyzed dehydration, the equilibrium is sensitive to the concentration of water that is present in the system. A strong acid, such as sulfuric acid, produces a butanone product while a more dilute acid could produce a diene product (Scheme 2).7 This may be attributed to the involvement of a larger quantity of water acting as a base to remove adjacent protons as opposed to the removal of the adjacent alkyl group as seen in our mechanism (Scheme 1).8 The strength of sulfuric acid is a likely contributor to our 2 product. Furthermore, according to the literature, in the acidic conditions necessary for the rearrangement, initially formed products can be isomerized to more stable isomers.7 Based on the conclusions of these researchers, a mechanism (Scheme 1) was produced.
Reaction of Pinacol to produce 2,3-Dimethyl-1,3-Butadiene
Furthermore, our proposed mechanism agrees with a study by Sattar and coworkers which found that the predominant formation of 2 indicates that 1,2-migration is the most favorable process leading to a stable intermediate.8 The research further confirmed our notion that concentration of acid impacts the mechanism as well as the stability of the intermediate. Their conclusive data indicated that generally, as the concentration of acids decreased, a decreasing trend in the formation of 2 and an increase in 2,3-dimethyl-1,3-butadiene was observed (Scheme 2).8
Having a facile methodology for the reaction of 1 and sulfuric acid was useful for synthesizing 2.9 IR spectroscopy and 1H NMR spectroscopy proved to be effective methods for evaluating our product and determining a possible mechanistic approach. Unfortunately, a lack of boiling point could be a source of error. A corrected boiling point was unattainable as an initial temperature was obtained, but a temperature drop was not observed. The detailed mechanistic investigations are currently underway and will be reported in due course.
General Information. The following chemicals were obtained from the instructor and used without further purification: sat aqueous sodium chloride and sulfuric acid (3 M). Pinacol (>98%) was obtained from TCI Chemicals (Tokyo, Japan), chloroform-d (99.8%) was obtained from Aldrich Chemistry (St. Louis, MO), and anhydrous calcium chloride (97%) was obtained from BeanTown Chemicals. All aforementioned chemicals were used without further purification. 1H NMR spectral data was obtained via the Ultrashield 300 Bruker Sample Xpress (Billerica, MA), and IR spectral data was obtained via the ThermoNicolet IR100 FT/IR Spectrometer (Madison, WI).
Reaction. 2,3-dimethyl-2,3-butanediol (>98%, 12.0 mmol, 1.4 g) was added to sulfuric acid (3 M, 4.0 mL) and distilled at 100 °C.
Separation. The distillate was washed with sat aqueous sodium chloride (1.0 mL), x2 and dried with anhydrous calcium chloride (97%).
Purification and Analysis. The crude product was purified via simple distillation (0.9 g, 64%). IR: 1365.01, 1354.44, 1705.56, 2873.23, 2969.45 cm-1. 1H NMR ( 250 MHz, CDCl3, δ): 1.12 (s, 9H, CH3), 2.12 (s, 3H, -CH3-C(=O).
Synthesis and Mechanistic Analysis of Pinacolone via Acid-Catalyzed Dehydration of Pinacol. (2024, Feb 22). Retrieved from https://studymoose.com/document/synthesis-and-mechanistic-analysis-of-pinacolone-via-acid-catalyzed-dehydration-of-pinacol
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