Synthesis of 7.7-Dichlorobicyclo [4.1.0]Heptane – Phase Transfer Catalysis

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The preparation of 7.7-dichlorobicyclo [4.1.0]heptane which is also known as 7,7-dichloronorcarane was done by reacting cyclohexene , chloroform and a base( 50% aqueous sodium hydroxide) with benzyl triethylammonium chloride. The latter being a water soluble phase transfer catalyst (PTC).reaction was performed at room temperature and was distilled at atmospheric pressure. The mechanism of action of the above mentioned PTC is described in the text along with the mechanism for the addition of a dichlorocarbene. The percentage yield obtained was 38.80%. This is considerably low due to an unfortunate spillage of the product at the stage of semi- micro distillation.

Phase transfer catalysts (PCT) are used to catalyze reactions involving chemical species which are present in different phases. These types of reactions are known as a homogeneous two phase reactions and are usually very slow because the two primary reactants (in this case CHCl3 and NaOH) are in different phases. The reaction catalyzed in this experiment is the addition of a dichlorocarbene to cyclohexene. The dichlorocarbene must first be generated in solution and this is done using the strong base, 50% aqueous sodium hydroxide, with the aid of the PTC. The benzyl triethylammonium chloride (PTC in this experiment) serves as a transporter of OH ions which dissociate from sodium, from the aqueous phase to the organic phase where chloroform is present. The reaction between OH ions and chloroform can then proceed to generate the dichlorocarbene which then reacts with cyclohexene to produce the desired product. The dissociation of NaOH and the subsequent transporting reaction between the PTC and OH ion is illustrated below.
Previously, literature reports describing the generation of a dichlorocarbene often have low yields due to the reaction been conducted under strict anhydrous conditions. The reason for these conditions is because once a dichlorocarbene is generated in an aqueous solution, it readily undergoes hydrolysis to yield undesired products (see reactions a and b below).2 These side reactions are avoided when the reaction is carried out in a biphase system in the presence of concentrated NaOH and a quartenary ammonium PTC. The effectiveness of this procedure was first demonstrated by Makosza and resulted in high yields of 7.7-dichlorobicyclo [4.1.0]heptane.2

Preparation of 7.7-dichlorobicyclo [4.1.0]heptane

Cyclohexene Chloroform Mass: 2 .05 grams Volume: 10 ml Molecular Weight: 82.08 g/mol Density: 1.49g/ml

Melting point: 104 oC Molecular Weight:
Amount: 0.0249 mol Amount: 0.13 mol

Cyclohexene and Cloroform react in a 1: 1 stochiometric ratio therefore cyclohexene is clearly the limiting reagent in this reaction. Therefore the maximum amount of 7.7-dichlorobicyclo [4.1.0]heptane that can be formed is 0.0249 mol.

7.7-dichlorobicyclo [4.1.0]heptane
Mass: 1.58 g
Molecular Weight: 163.04 g/mol
Boiling point: 180-190 degrees Celsius
Amount: 0.0096 mol
Percentage Yield = actual amount of product / theoretical amount of product = 1.58 g/ 4.072g x 100 = 38.80%

The synthesis of the desired product (7.7-dichlorobicyclo [4.1.0]heptane)was obtained by following the experimental procedure very carefully but due to unfortunate circumstances during the actual practical, some of the product was lost by spillage at the stage of semi-micro distillation. Therefore the actual yield of product recovered was quiet low which resulted in a final percentage yield of only 38.80%. Nevertheless the rest of the experiment was carried out with as much precision as possible. After the addition of all the reactants, the reaction flask was placed under...
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