Elimination Reaction Experiment on Alcohols and Alkyl Bromides

Elimination Reaction

 The purpose of the elimination reaction experiment is to perform elimination reactions on alcohols and alkyl bromides. The two alcohols used in the laboratory were 1-butanol and 2-butanol. These alcohols undergo dehydration reactions which follow the E1 mechanism pathway. The two alkyl bromides used in the laboratory were 1-bromobutane and 2-bromobutane which undergo dehydrobromination reaction that follows E2 mechanism pathway.

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 The unimolecular elimination reaction, also known as E1 elimination is a two-step process that involves the removal of a substituent. Upon removal of the HX substituent will result in the formation of a double bond to give the carbon cation intermediate. Achieving this carbon cation intermediate is the rate-determining step, which determines whether the reaction happens or not. This part of the mechanism occurs at a slow pace. The second step is the deprotonation of carbocation creating the alkene product (Laboratory Techniques 4e). The bimolecular elimination reaction follows the E2 mechanistic pathway that involves a single transition state. Different from the unimolecular elimination, this reaction involves one step mechanism in which a double bond is formed once the bond holding the halogen and carbon-hydrogen break together forming the alkene product. E2 reactions are heavily dependent on steric hindrance and the strength of the base used, that is why alkyl halides are treated as E2 elimination reactions rather than E1 elimination reactions. Attached below are the different possible types of products formed.

The possible E1 elimination reaction products for 1-butanol are but-1-ene, cis 2-butene and trans 2-butene. The possible E1 elimination reaction products for 2-butanol are but-1-ene, cis 2-butene and trans 2-butene. The possible E2 elimination reaction products for 1-bromobutane are but-1-ene, cis 2-butene and trans 2-butene. The possible E2 elimination reaction for 2-bromobutane are but-1-ene, cis 2-butene and trans 2-butene. The physical state of the following products is in gaseous forms. Each of the products formed is a type of hydrocarbon known as alkene, they are unsaturated hydrocarbons that contain at least one carbon-to-carbon double bond (LibreTexts). The following products can be collected by fractional distillation technique. Depending on the compound used, the reactants was heated to the appropriate temperatures and solvent was added in order to collect gaseous product from glass tube. In my case, the 2-bromobutane was heated with sandbathe at approximately 80oC and the reagent, ethanolic potassium hydroxide solution was added.

Based on the gas chromatogram, the manually integrated peaks can help us identify and analyze the elimination products by knowing the theoretical boiling points for the products and order of elution. Reading the gas chromatogram, compounds with lower boiling point will appear to the left because they will be the first ones to elute out of the GC column when heated (Laboratory Techniques 4e). This indicates that compounds with higher boiling points will have a higher retention time and vice versa. Since the chromatogram is sorted by retention time (minutes), peaks towards the right will have a higher boiling point whereas peaks to the left will have a lower boiling point. The three major products formed after elimination reaction were 1-butene, trans-2-butene, and cis-2-butene. The theoretical boiling point of 1-butene is -6.3oC which will appear as the first peak on the gas chromatograph (1-butene PubChem). The theoretical boiling point for trans-2-butene is 0.8oC and will be the next peak after 1-butene. (trans-2-butene PubChem). The theoretical boiling point for cis-2-butene is 3.5oC and will appear as the last peak (Cis-2-butene PubChem).

Relative Concentrations

1-Butanol

2-Butanol

1-Bromobutane

2-Bromobutane

1-Butene = 24.25 %

1-Butene = 5.70 %

1-Butene = 64.00 %

1-Butene = 57.13 %

Cis-2-Butene = 37.40 %

Cis-2-Butene = 41.40 %

Cis-2-Butene = 9.00 %

Cis-2-Butene = 17.33 %

Trans-2-Butene = 38.35 %

Trans-2-Butene = 52.90 %

Trans-2-Butene = 27.00 %

Trans-2-Butene = 25.53 %

 Based on the class data obtained and boiling points for the elimination products, chromatographic data can be interpreted, giving the identification of major and minor products. For my chemical used, 2-bromobutane had products of but-2-ene (Major Product), cis-2-butene (Minor Product), and trans-2-butene. The products for 1-bromobutane were 1-butene (Major Product), cis-2-butene (Minor Product), and trans-2-butene (Minor Product). The products for 2-butanol were trans-2-butene (Major Product), cis-butene, and 1-butene (Major Product). The products for 1-butanol were trans-2-butene (Major Product), cis-2-butene, and 1-butene (Minor Product). For the elimination of 1-butanol, the class data indicates that trans-2-butene had the highest relative concentration of 38.35%. There was more trans-2-butene than cis-2-butene due to higher stability. Since cis-2-butene has less regioselectivity than 1-butene, this yielded a higher relative concentration (Socratic). For 2-butanol, the class data indicates that trans-2-butene was the major product with a relative concentration of 52.90%. Due to steric hindrance and less exothermic heat, there was more amount of trans than cis and 1-butene. For 1-bromobutane, the highest relative concertation product was 1-butene with a percentage of 64.00 %. Since the reaction undergoes dehydrogenation reaction, following E2 mechanistic pathway, the potassium hydroxide will attack the alkyl bromide, causing the leaving group of the electrophile to depart at the same time. Since 1-butene is highly regioselective and no carbocation was formed during the reaction will yield exceedingly large amount. There was also more trans-2-butene than cis due to steric effects. The same scenario follows for the 2-bromobutane. There was a large amount of 1-butene when compared to cis-2-butene and trans-2-butene.

 For 1-bromobutane, the following reaction underwent E2 elimination reaction, meaning that the no carbocation intermediate is formed and that the hydrogen, along with the bromide ion departs simultaneously from the electrophile, indicating why there was almost a 100 % yield for the 1-butene and not for other two isomeric butenes. However, the minor products are also formed through solvolysis, which explains the small percentage of cis-2-butene and trans-2-butene. Similarly, with 1-bromobutane, the dehydrohalogenation of 2-bromobutane follows E2 mechanistic pathway having a major product of 1-butene. For the dehydration of 1-butanol, the alcohol containing group must be protonated to create the water molecule. Detaching the water will lead to the least stable carbocation and must rearrange by hydride shift to from secondary carbocation since more stability. The addition of water and rearrangement of the hydrogen ions will lead to the three possible products.

References