Unimolecular Elimination Reactions

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  • Topic: Elimination reaction, Alcohol, Alkene
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CHEM 206- ORGANIC CHEMISTRY

EXP 10-UNIMOLECULAR ELIMINATION REACTIONS

Cansın Etli

Lab Instructor: Emel Yılgör

Experiment date:06.12.2010

Report Date: 20.12.2010

ABSTRACT

In this experiment, the purpose was to investigate E1 elimination mechanism with the reaction of cyclohexanol to cyclohexene. The reaction was acid catalyzed dehydration of alcohol and with catalyst sulfuric acid cyclohexene was obtained from cyclohexanol. And the product was obtained from the simple distillation and later it was reacted with bromine and potassium permanganate. The observations and results were all recorded. Simple distillation was used for distillation technique, and the distillate was seperated into test tubes, and to the one of them, bromine was added, and to the another potassium permanganate was added. Permanganate’s color changed from purple to brown, and bromium’s color changed brown to colorless, it became color solution which was light yellow.

INTRODUCTION

One of the most useful and general methods of preparing alkenes or olefins is based on the dehydration of alcohols with acids. Strong acids such as sulphuric and phosphoric acids are required to form the oxonium ion from which the hydronium ion is eliminated.

“This cyclohexene from cyclohexanol reaction belongs to a broad class of organic transformations called elimination reactions, and within that class, this E1 elimination reaction constitutes another of the cornerstone reactions of organic chemistry. Mechanistically related to the SN1 reaction, the E1 often competes with attempted substitution.” (5) Through proper choice of reaction conditions, it can usually be made the predominant or exclusive pathway. For example, one of the principal ways to prepare alkenes is via the acid catalyzed dehydration of an alcohol

Dehydration of Alcohols - Zaitsev
• When an alcohol has two or three ( carbons, dehydration is regioselective and follows the Zaitsev rule. • The more substituted alkene is the major product when a mixture of constitutional isomers is possible. [pic]

• “2º and 3º alcohols react by an E1 mechanism (of the protonated alcohol) • 1º alcohols react by an E2 mechanism (of the protonated alcohol) • The E1 Mechanisms
• The dehydrohalogenation of (CH3)3CI with H2O to form (CH3)C=CH2 can be used to illustrate the second general mechanism of elimination, the E1 mechanism.”(2) E1 mechanism
• It is a two-step process of elimination ionization and deprotonation. o Ionization, Carbon-halogen breaks to give a carbocation intermediate. o Deprotonation of the carbocation.

• Typical of tertiary and some secondary substituted alkyl halides. • The reaction rate is influenced only by the alkyl halide. • Reaction mostly present in absence of base or weak ones. (4)

E1 eliminations happen with highly substituted alkyl halides due to 2 main reasons. • Highly substituted alkyl halides are bulky, limiting the room for the E2 one-step mechanism; therefore, the two-step E1 mechanism is favored. • Highly substituted carbocations are more stable than methyl or primary substituted. Such stability gives time for the two-step E1 mechanism to occur. (4)

An E1 reaction exhibits first-order kinetics
The E1 reaction proceed via a two-step mechanism: the bond to the leaving group breaks first before the ( bond is formed. The slow step is unimolecular, involving only the alkyl halide. The E1 and E2 mechanisms both involve the same number of bonds broken and formed. The only difference is timing. In an E1, the leaving group comes off before the ( proton is removed, and the reaction occurs in two steps. In an E2 reaction, the leaving group comes off as the ( proton is removed, and the reaction occurs in one step.

[pic]

Table 1. Characteristics of the E1...
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