EXPERIMENT 5
PREPARATION OF BROMOCYCLOPENTANE
REFERENCES: Introduction to Organic Laboratory Techniques, 3rd Ed., Pavia, Lampman and Kriz,
"Instruction Manual for the Perkin-Elmer Spectrophotometer", Schoening
OBJECTIVES: The student should be able to:
1. Set up a simple reflux apparatus.
2. Write a pre write-up for a preparative organic experiment.
3. Synthesize an alkyl halide from an alcohol.
4. Extract the product of the reaction by use of water and an alkaline solution.
5. Dry a liquid solution with a drying agent.
6. Run an infrared spectrum of a neat liquid in a sealed cell.
INTRODUCTION: Reactions and side reactions: The bromocyclopentane is to be prepared by the reaction of hydrobromic acid with cyclopentanol. The hydrobromic acid is in turn prepared in situ (i.e., in place). The reactions thus are:
In this SN2 reaction the cyclopentanol is protonated first and then the bromide ion attacks the protonated alcohol, the leaving group is water and the nucleophile is the bromide ion:
The sulfuric acid is used to generate the hydrobromic acid, as a dehydrating agent to shift the equilibrium in equation (2b) to the right and as an added source of hydrogen ions to increase the concentration of the protonated alcohol. In addition to these desired reactions, the following side reactions occur:
The cyclopentyl hydrogen sulfate ester can reform the alcohol, reversing the equilibrium of equation 3 as the bromocyclopentane is produced in the main reaction. The cyclopentyl hydrogen sulfate ester can also upon heating undergo an elimination reaction to yield cyclopentene, equation 4, or undergo a substitution reaction with another molecule of alcohol to form dicyclopentyl ether, equation 5. Purification: At the end of the reaction period, the reaction mixture contains, in addition to the desired product, the other products of the main reaction, the products of the side reactions, and unreacted starting materials. The water insoluble substances can be separated from the water soluble substances using a separatory funnel. The trace amounts of acidic substances can be extracted from the organic layer by washing it with an alkaline solution to neutralize the acids. Traces of water can be removed from the mixture by the addition of a drying agent - a solid which absorbs water, usually by forming a new crystal which contains complex ions in which water is a ligand. Calcium chloride is a good choice of drying agent here, for it absorbs most oxygen containing substances, including alcohols and ethers. Final purification is accomplished by distillation.
DIRECTIONS: Quantities: Sodium bromide 30.9 gm Water 34 ml Cyclopentanol 21.8 gm; convert to mls conc. Sulfuric acid 26 ml ($) indicates points at which the experiment should be stopped. ($$) indicates points at which the experiment may be stopped if time is short. Reaction: In a 250 ml standard taper flask provided with a reflux condenser is placed the sodium bromide, water and cyclopentanol. The flask is cooled in an ice bath. The concentrated sulfuric acid in a small erlenmeyer flask is also cooled in the ice bath. When both are cool, the sulfuric acid is added slowly with continuous swirling to the flask which is kept in the ice bath. After the sulfuric acid has been added, two to three boiling chips are added to the flask, the reflux condenser is attached and the flask heated gently. The mixture is then refluxed for 30 minutes after all of the salts dissolve.($$)
Separation and Purification: The reaction mixture is allowed to cool and then the liquid decanted into a 250 ml separatory funnel. The reaction flask is rinsed once with 50 ml of water and the rinse is added to the separatory funnel. The mixture is shaken and the layers allowed to separate. Once two clear layers are obtained the layers are separated, bromcyclopentane is identified by volume and density. The product is washed first with a 10 % sodium hydroxide solution, then a saturated sodium chloride solution and finally with water1. The bromocyclopentane is dried by swirling occasionally in an Erlenmeyer flask for 15-20 minutes with 2-3 g of anhydrous calcium chloride2. The liquid is filtered through a fluted filter paper in a small, dry stemless funnel into a 50 ml standard taper flask ($) and distilled3. Two (2) fractions may be collected, a lower boiling point fraction impurity and the product collected at 130 - 140 oC, in a weighed sample bottle. Product weight is determined and percent yield calculated.
Before breaking down the distillation apparatus, place a standard taper round bottom flask on the vacuum adaptor and attach the vacuum adaptor to a water aspirator and aspirate for five minutes.
Characterization: The refractive index of the product should be taken and an IR spectrum of the product should also be run4. In CHE 235 results, the component(s) of the product must be identified based on their IR spectrum, boiling points and refractive index.
POST WRITE-UP - ANALYSIS: Comment on the percent yield of bromocyclopentane. From the experimental IR spectrum and standard IR spectrum, and from experimental and literature boiling points, comment on the identity of the product. From the experimental refractive indices and literature refractive indices, and from the experimental IR spectrum and standard IR spectrum, comment on the purity of the product, and identify the possible impurity if present. From the experimental infrared spectrum, comment on the structure of the product.
NOTES: 1. With each wash it is possible that an emulsion may form; if so, add a slight amount of solid sodium chloride and shake again until clear separation is obtained.
2. Additional calcium chloride may need to be added so that some of the calcium chloride is free flowing in the flask.
3. If final distillation is not done the same day as the purification, separate the product from the drying agent and store in the 50 ml standard taper flask.
DO NOT LEAVE the bromocyclopentane sitting over the drying agent over night.
4. The infrared spectrum of this product is run neat on salt plates. Acetone is used to clean the salt plates.
tps rev 5/23/03