Abstract The objective of this experiment is to successfully perform a dehydration of 1-butanol and 2-butanol, also dehydrobromination of 1-bromobutane and 2-bromobutane to form the alkene products 1-butene, trans-2-butene, and cis-2-butene. The dehydration reactions react under and acid-catalysis which follows an E1 mechanism. It was found that dehydration of 1-butanol yielded 3. 84% cis-2-butene, 81. 83% trans-2-butene, and 14. 33% 1-butene, while 2-butanol is unknown due to mechanical issues with the GC machine.
For the dehydrobromination, with the addition of a strong base that can abstract a proton, which then pushes off the leaving group and a new sigma bond makes a new p-bond all at one time, this is follows E2 mechanism.
It was found that the dehydrobromination of 1-bromobutane yielded 100% 1-butene, while 2-bromobutane yielded 13. 09% cis-2-butene, 49. 95% trans-2-butene, and 36. 97% 1-butene. Introduction For E1 (1st order) reaction mechanisms, under acid-catalysis an alcohol may be dehydrated to form an alkene. The most common acids employed for the reaction are sulfuric or phosphoric acids.
The reaction proceeds via initial protonation of the hydroxyl group (a typical acid-base reaction). This converts the hydroxyl unit from a poor leaving group (-OH) into a much better one (H2O). Loss of water generates a carbocation, which can stabilize itself by elimination of a proton from an adjacent carbon to produce the alkene. The elimination of the proton will predominately occur in the direction that results in the production of the more highly substituted carbon-carbon double bond. The carbocation has other fates depending upon substrate, reaction conditions, and acid employed.
The carbocation can undergo rearrangement to a more stable species for example, 1°to a 2°, or 3°, via a shift of a hydride or a Me, from an adjacent carbon, followed by elimination. If a hydrogen halide is used as the acid, it produces the substitution product rather than the elimination product. The reason is that the conjugate bases of these acids are more nucleophilic than the HSO4- or H2PO4- produced from sulfuric or phosphoric acids. This nucleophilic conjugate base then adds to the carbocation rather than abstracting a proton from the adjacent carbon, thus substitution occurs.
For an E2 (2nd order) reaction mechanism, bromide is a good leaving group and in the presence of a good nucleophile, the nucleophile can push off the leaving group. However, if the nucleophile is also a strong base, an alternate reaction can occur. Instead of pushing the leaving group off, the base can abstract a proton. The electrons that once held the proton in place can in turn push the leaving group off. As a result, an alkene is produced. Gas Chromatography (GC) which is used to separate and measure vaporized compounds. In some cases it can be used to prepare pure compounds or identify them.
Gaseous compounds being analyzed react with the columns, which is coated in different stationary phases. The comparison retention time is what gives GC its analytical usefulness. Procedure See lab notes. No significant changes were made to the procedure. Results and Discussion Part A: Dehydration of 1-butanol & 2-butanol Compound| Temperature (°C)| Products| Peak Area(mm2)| %
Composition| 1-Butanol| 140| Trans-2-butene| 1113| 81. 83| | | Cis-2-butene| 6354| 3. 84| | | 1-butene| 298| 14. 33| 2-Butanol| 80| Trans-2-butene| ? | ? | | | Cis-2-butene| ? | ? | | | 1-butene| ? | ? -(GC) Calculations for relative amount of products: 1-butanol: % Composition Total peak area=7765 mm2 1113mm2/7765 mm2x100%= 14. 33%, 1-butene 6354 mm2/7765 mm2x100%= 81. 83%, tans-2-butene 298 mm2/7765 mm2x100%=3 . 84%, cis-2-butene In the GC results, it is clear that 3 alkenes were produced with the dehydration of 1-butanol via E1 mechanism, based on the three peaks in the print out. That the more stable alkene trans-2-butene, is the major product at approximately 82%. While, the two minor products would be 1-butene at 14%, and cis-2-butene at only approximately 4%.
Although, dehydration was not performed on 2-butanol, it would also produce the 3 isomeric alkenes like 1-butanol via an E2 mechanism. The percentages would probably be very similar as well. With trans-2-butene, being the major product, and cis-2-butene and 1-butene being minor products as well. The GC results would appear again very similar to 1-butanol with 3 peaks representing the 3 alkenes produced, however, 2-butanol has a lower BP, so it takes longer to react and produce gaseous mixture because it is a secondary alcohol.
Part B: Dehydrobromination of 1-bromobutane & 2-bromobutane Compound| Temperature (°C)| Products| Peak Area(mm2)| % Composition| 1-bromobutane| 90| Trans-2-butene| —–| —-| | | Cis-2-butene| —–| —-| | | 1-butene| 17500| 100| 2-bromobutane| 80| Trans-2-butene| 2469| 49. 95| | | Cis-2-butene| 3336| 13. 09| | | 1-butene| 8741| 36. 97| -(GC) Calculations for relative amount of products: 1-bromobutane: Total peak area=17500 mm2 17500 mm2/17500 mm2x100%=100%, 1-butene 2-Bromobutane: Total peak area=6679mm2 2469 mm2/6679 mm2x100%=36. 97%, 1-butene 336 mm2/6679 mm2x100%-=49. 95% trans-2-butene 8741 mm2/6679 mm2x100%=13. 09% cis-2-butene 1-bromobutane with a strong base reacts via an E2 mechanism. Rearrangement occurs to produce a more stable carbocation species and it is a primary alkyl halide, also, there are only two ß Hydrogen on the adjacent to the Carbon with the leaving group. Therefore, there is only one major product. As well 2-bromobutane reacts via E2 mechanism, but a secondary alkyl halide like 2-bromobutane, on the other hand, have five ß-hydrogens that can be removed.
Removing a proton (a ß-hydrogen) from the primary carbon produces 1-butene. However, removing a proton (a ß-hydrogen) from the secondary carbon produces 2-butene, with two possible stereoisomers, either cis-, or trans-. Trans, being the major product because it is the most stable. Areas for improvement would be to make sure the equipment is properly clean and dry to avoid potential contaminants which could potentially affect rate of reaction and show up in GC. Also, to maintain the appropriate temperature range so that the reactant does not heat too quickly.
Lastly, taking care that the o-ring is tightly secure to keep gas in the column so that it can collect in the tube with septum. Conclusion Overall, the dehydration of 1-butanol, and the dehydrobromination of 1-bromobutane, and 2-bromobutane was successful, in producing consistent and accurate GC results following E1 (1st order) and E2 (2nd order) reaction mechanisms. All but one compound produced three isomeric alkenes, which are based on the mechanism of the reactions, as well as the similarities of the compositions.
The compositions were similar in their ability to produce the alkenes, however varied slightly in percentage based on which type of mechanism they followed. For E2, it is clear why 1-bromobutane, was only able to produce one regioselective product, being that is was a primary alkyl halide also rearrangement happened to make a more stable carbocation, therefore only producing one major product. Also E1, is much like Sn1 substitution with its carbocation being the rate limiting first step in the reaction to produce the most stable compound. Extra Questions
Draw the balanced equation for the formation of the most stable alkene from 1-butanol, 2-butanol, 1-bromobutane, and 2-bromobutane 1-butanol 2-butanol 1-bromobutane 2-bromobutane Pg. 210 Questions 1-6 1. The dehydration of 2-butanol follows an E1 mechanism which will produce a 2-butyl carbocation, which will from the 3 isomeric alkenes. The dehydration of 1-butanol initially leads to a very unstable 1° carbocation.
This cation rearranges by hydride shift to the same 2-butyl cation which is formed in 2-butanol. This 2-butyl should yield approximately the same percentages of 3 isomeric alkenes as in 2-butanol. . Dehydrobromination of 1-bromobutane follows an E2 mechanism in which a hydrogen ion is removed by a strong base at the same time as the leaving group bromide ion leaves yielding 1-butene. Some 2-butene is formed by solvolysis of the halide or isomerization of 1-butene. 3. The concerted E2 reactions do not involve a carbocation intermediate so that 1-bromobutane gives rise to nearly 100% 1-butene (highly regioselective). Whereas, 1-butanol reacts by E1 mechanism giving rise to a carbocation intermediate, which will rearrange to a more stable 2° carbocation.
Therefore, the result is a mixture of alkenes, which is less regioselective. 4. The elution order is according to boiling point. The lowest boiling point compound elutes first: 1-butene, trans-2-butane, then cis-2-butane. 5. a. trans-2-pentene (major product), cis-2-pentene, and 1-pentene. b. 2-methyl-2-butene (major product), and 2-methyl-1-butene. c. trans-2-butene (major product), cis-2-butene, and 1-butene d. Same is in part B 6. a. 2-methyl-2-butene (major product), and 2-methyl-1-butene. b. 1-butene c. 2,3-dimethyl-2-butene (major product), and 2,3-dimethyl-1-butene. d. trans-2-pentene (major product) and cis-2-pentene.
Cite this Dehydration of 1-Butanol & 2-Butanol
Dehydration of 1-Butanol & 2-Butanol. (2016, Oct 10). Retrieved from https://graduateway.com/dehydration-of-1-butanol-2-butanol/