Alkene Synthesis from Alcohol: Preparation of Cyclohexene From Cyclohexanol

Topics: Nucleophilic substitution, Organic chemistry, Infrared Pages: 13 (2552 words) Published: June 4, 2014

Alkene Synthesis from Alcohol:
Preparation of Cyclohexene From Cyclohexanol

A 42.89% yield cyclohexene was successfully synthesized from 10.0 mL cyclohexanol by unimolecular elimination (E1) through the dehydration of cyclohexanol and confirmed via a bromine test and the IR spectra. Introduction:

Alkenes are hydrocarbons that have carbon–carbon double bonds and are one of the many functional groups in organic molecules. Alkenes are sp2 hybridized and are unsaturated because two of their hydrogen’s are missing from the saturated alkane formula (CnH2n+2). Typically alkenes are synthesized by elimination reactions, however, organic molecules can also undergo substitution reactions amongst many other reactions like oxidation and reduction based on the structure of the organic molecule and the conditions under which the reaction is performed. These reactions are possible because the carbon atom in organic molecules is electron deficient due to induced dipoles created by the presence of functional groups or more electronegative atoms/groups. The organic molecule as a whole is referred to as the substrate. The most likely to be attacked carbon atom in the organic molecule is referred to as the electrophile and is and it is usually bonded a more electronegative atom/group such as a halide, oxygen or pseudo-halide, called the leaving group. In a reaction, a leaving group leaves the substrate by heterolysis (a reaction in which the breaking of bonds leads to the formation of ion pairs) and it is replaced by a nucleophile (usually a base) which is attracted to the partial positive carbon atom because the nucleophile has excess electron pairs or a negative charge.

The relative strength of the nucleophile determines its nucleophilicity. Nucleophilicity depends on many factors, including charge, basicity, solvent, polarizability, and the nature of the substituents present on the organic molecule. In general, a nucleophile containing a negatively charged reactive atom is better than a nucleophile containing a reactive atom that is neutral and basicity parallels nucleophilicity meaning a strong base is a good nucleophile and a week base is a weak nucleophile. Also, there are several factors that contribute to the ability of a group/ atom to function as a good leaving group, which includes the property of the carbon-leaving group bond (polarizability and strength) and the stability of the leaving group. Weak based are good leaving groups and strong based are bad leaving groups. The substrate organic molecule containing the electrophile is characterized in terms of steric effect that entails the bulkiest of the substrate and it's shape/size in 3D and whether the electrophilic carbon is a primary, secondary or tertiary carbon. The nature of the substrate, nucleophile, electrophile and solvent (polar protic or polar aprotic) used will determine whether an organic molecule will undergo a substitution or elimination reaction.

An elimination reaction is one in which an alpha- hydrogen and a LG are removed from an inorganic molecule in either a one or two step mechanism to form an alkene. The one and two-step mechanisms are bimolecular (E2) and unimolecular (E1) elimination reactions respectively, defined based on the rates of the reactions. The rates of the reactions are based on the kinematics of each reaction and not on the number of steps in the reaction. As implied by the names, E2 reactions have a rate factor of two (second-order) because the rate of the reaction is based on the concentration of the substrate and the nucleophile while E1 reactions have a rate factor of one (first-order) because the rate of the reaction is based solely on the concentration of the substrate. E2 is a one-step concerted process with individual transition states typically undergone by primary and secondary substrates, while E1 is a two-step process of elimination (carbocation formation and...

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