CHM 235LL-Monday, 3/19/2012 & 3/26/2012
Part A: Dehydration of 1-butanol & 2-Butanol/Part B: Dehydrobromination of 1-Bromobutane & 2-Bromobutane
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 π-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.
See lab notes. No significant changes were made to the procedure. Results and Discussion
Part A: Dehydration of 1-butanol & 2-butanol
Compound| Temperature (⁰C)| 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...