Alkenes from Alcohols

An elimination ration involves the loss of two atoms or groups from the substrate, resulting in a formation of a pi bond. Elimination may be first-order (E1) or second-order (E2). E1 is a unimolecular because the rate-determining transition state involves one molecule instead of a collision between two molecules, which is seen in E2. In the presence of a strong acid, alcohols protonate to form a good leaving, for example, water. After the loss of a proton next to the good leaving group, unsaturation occurs resulting in a double bond. There are several promoting factors involved in these two reactions, favoring one process over another. E1 reaction usually involves a weak base, while in E2 a strong base is required. E1 also requires a good ionizing solvent. A substrate also has effects on the reactions: for both E1 and E2, the order of reactivity is tertiary>secondary>primary. (Wade 2006) One example of a reaction that will favor E1 would be the reaction of t-butyl bromide in ethanol and heat to give 2-methylpropene. An example of E2 can involve a reaction between isopropylbromide with potassium ethoxide in ethanol to give isobutylene, ethanol and potassium bromide. Zaitev's rule is related to elimination reactions it states: if more than one alkene can be formed by an elimination reaction, the more stable alkene is the major product. The compound that has the most substituted C=C double bond is more stable due to the electron donating properties of the alkyl group. In this experiment the reaction yielded a trisubstituted alkene (2-methyl-2-butene) and a disubstituted alkene (2-methyl-1-butene). According to Zaitev 2-methyl-2-butene would predominate. A gas chromatograph is a chemical analysis instrument used in separating chemicals in a mixed sample. A gas chromatograph uses a flow-through narrow tube known as the column, through which different chemical constituents of a sample pass in a gas stream (carrier gas-mobile phase) at different rates depending on their various chemical and physical properties such as polarity, and their interaction with a specific column filling, called the stationary phase, which is usually a non-volatile liquid. A chromatogram is printout of the different peaks which the chemicals involved give. GC is a good way to determine purity and the identity of the chemicals in the mixture. (Salvatore 2001)

Data and Results:

Temperature of first drop: 34 oC

Temperature of first drop: 38 oC

Weight of product: 18.443-17.715= 0.778g

Percent yield calculations:

Density of alcohol: 0.8096 g/mL

Molecular weight: 88.15g/mol

Molecular weight of alkene: 70.13 g/mol

Theoretical yield: (0.8096 g/mL x 2.00ml) / 88.15 x 1/1 x 70.13= 1.288 mol

Percent

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