lab 9 - Dehydration of 4-Methyl-2-Pentanol by Acid...

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Unformatted text preview: Dehydration of 4-Methyl-2-Pentanol by Acid Catalyzed Elimination Reaction/ Bromine and Baeyer Tests for Unsaturation Mayank Kumar April 20, 2007 Methods and Background The purpose of the experiment was to dehydrate 4-methyl-2-pentanol by an acid catalyzed elimination reaction to form a mixture of alkenes and verify the product identity through the bromine and Baeyer tests for unsaturation. Alkenes are organic compounds possessing a polarizable carbon-carbon π-bond (double bond); they can be prepared through dehydrohalogenation or dehydration of alcohols, both of which are elimination reactions. In dehydrohalogenation, an alkyl halide undergoes bimolecular elimination (E2) to form an alkene. The electronegative halogen atom causes the carbon-halogen bond to become polarized, which can be inductively transmitted through the σ-bond network to increase the acidity of hydrogen atoms on the carbon atom (β-carbon atom) adjacent to the carbon atom bonded to the halogen atom (α-carbon atom). A concerted reaction can ensue, in which a proton from the β-carbon atom is abstracted by a strong base while the halide ion, the leaving group, departs from the α-carbon atom, and a carbon-carbon double bond is formed. In E2 reactions, the preferred orientation of the β-hydrogen being removed and the departing halogen atom is anti-periplanar, meaning the angle between the two atoms is 180°. Since E2 reactions are concerted, the rate of reaction depends on the concentrations of the substrate and the base, thus both reactants are involved in the transition state of the rate-determining step: Rate = k 2 [alkyl halide][B:¯], where k 2 is the second-order rate constant and B:¯ is the base. If the halogen is unsymmetrically located on the carbon skeleton, dehydrohalogenation may give a mixture of products, and since these types of reactions are typically irreversible under the given conditions, the product alkenes do not equilibrate, so the ratio of products is determined by the relative rates of formation, and thus the relative free energies of the transition states. According to Zaitsev’s rule, the major product in an elimination reaction is the more highly substituted alkene, as an increase in the number of alkyl substituents on the double bond usually increases the stability (lowers the free energy) of an alkene as well as that of the transition state. However, steric factors may also influence the relative free energies of transition states by increasing the energies of some transition states. If substituents on the β-carbon atom sterically provide hindrance to the approaching base, removal from a less substituted carbon atom to yield a less substituted alkene will become favorable, as the energy of the transition state leading to the more highly substituted alkene becomes higher than that of the less highly substituted alkene....
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This lab report was uploaded on 04/07/2008 for the course CHEM 233 taught by Professor Landrie during the Spring '08 term at Ill. Chicago.

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lab 9 - Dehydration of 4-Methyl-2-Pentanol by Acid...

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