Synthesis of Alkyl Halide

Performed: DecemberSubmitted: January

AbstractTert-butyl Chloride was formed by mixing tert-butyl alcohol and HCl. Purification techniques, such as adding NaHCO3 and CaCl2 were done to remove unwanted byproducts of 2-methylpropene. Distillation was done to separate the alkyl halide, yielding 48% purity. I. Introduction

The synthesis of tert-butyl chloride from tert-butyl alcohol in this procedure is a simple SN1 reaction; all of the succeeding steps in this methodology are for purification purposes only. This synthesis reaction has been reproduced many times and is done in this case for the purpose of acquainting the researchers with performing synthesis reactions and purification techniques.

II. Methodology

10mL of tert-butyl alcohol mixed with 20mL cold conc. HCl are swirled gently in a 30mL separatory funnel. While mixing, the funnel is open to the air to prevent pressure from building. After sufficient swirling, the solution is left undisturbed for 20 minutes, to allow the layers to separate. The layers are composed of water and tert-butyl chloride. The aqueous layer can be removed by dropping out the crude tert-butyl chloride (in this case, a dropper was used to remove the water layer instead).

The separated crude tert-butyl chloride is placed into a flask containing small amounts of solid NaHCO3. The remaining liquid is decanted and mixed with dry CaCl2. The mixture is decanted again and moved to a 25mL round bottom flask containing boiling chips.

The collected sample is to be purified by distillation. The round bottom flask containing the sample is to be placed in a hot bath of adjustable temperature. The distillation set-up connects sample flask to a collecting flask in ice bath through the condenser. Distilling adapters connect the condenser to the flasks. External tubes carrying water is necessary to prevent the buildup of pressure through heat. A thermometer is inserted into the adapter near the flask in hot bath, to monitor the temperature. The temperature is increased to begin boiling the tert-butyl chloride.

The gaseous tert-butyl chloride travels to the cooler collecting flask. After condensing around 1mL of the tert-butyl chloride, this 1mL is discarded. The following collected sample should have boiled around 49-52ᵒ. This is the purified tert-butyl chloride.

III. Results and Discussion

Tert-butyl chloride is synthesized by substitution of the alcohol group in tert-butyl alcohol by cold excess HCl, this reaction occurs via SN1, which undergoes a carbocation intermediate, leaving the tertiary carbon needing a nucleophile to stabilize. In instances where Cl- does not react with the carbocation, the electrophillic site is stabilited by one of the methyl groups by losing a H+ proton and double bonding with the tertiary carbon.

Tert-butyl Chloride has a low boiling point, HCl must be cold to prevent explosive volatilization. The tert-butyl chloride is mixed with solid NaHCO3 to remove excess acid. It is necessarily solid because an aqueous NaHCO3 could cause some reversal of the reaction by replacing the chloride with OH-. The sample is decanted and mixed with dry CaCl2 to complex out the excess water and unreacted alcohol. CaCl2 is dry for the same reason stated above.

Distillation must be performed at the appropriate temperature, 51ᵒC, where only tert-butyl chloride vaporizes while the contaminants do not. The initial 1mL of distilled sample would contain impurities with boiling point lower than tert-butyl chloride, which is why it is discarded. Boiling chips which cause bubble formation at the precise boiling point through its additional surface area are added to prevent volatilization of higher boiling point impurities.

7.8g of the initial tert-butyl alcohol yielded roughly 48% of the theoretical 9.71g of tert-butyl chloride in a 100% yield scenario. This shows the procedure is moderately effective in synthesizing pure tert-butyl chloride. The loss of product is due to the purification steps involved. After the reaction step, the product is moved around to react with NaHCO3 and CaCl2, and distilled. Many of the liquid is lost just from transferring containers,

as evidenced from the volume change from 10mL to 5.6mL, almost half is lost. Furthermore, tert-butyl Chloride is highly volatile, during the reaction step, the separatory funnel was open to the air, an amount of vapor is lost too during transfers.

IV. Conclusion

Synthesis reactions give low yields when many purification steps are necessary and when the reaction itself is not done in a closed environment. This experiment produced quite a decent amount of yield. The short reaction mechanism makes this a very good method of synthesizing alkyl halides.

V. References

Brown, T., LeMay, H, E & et al (2009). Chemistry: The Central Science 11th Ed. Pearson Education South Asia PTE, LTD. Philippines

Alexeyef, P. (1906). General Principles of Chemistry 1st Ed., John Wiley and Sons. New York

McMurry, J., (2004) Organic Chemistry 7th Ed. Thomson Higher Education, Belmont, California

MSDS of tert-butyl chloride , MATHESON TRI-GAS, INC.

VI. Appendix

Answers to Questions1. Why is it necessary to use cold concentrated HCl? Why is it added in excess?

Tert-butyl chloride is very volatile; during formation, explosive volatilization may occur. Cold HCl can be mixed instead to prevent this.

2. Why is solid NaHCO3 used instead of aqueous NaHCO3?

NaHCO3 is used to remove excess HCl, in aqueous solution, OH- increases in concentration and can revert the tert-butyl chloride back to tert-butyl alcohol.

3. Why must crude alkyl halide product be dried carefully with anhydrous CaCl2 before distillation?Same reason as #3

4. What is the purpose boiling chips?Boiling chips add additional surfaces for heat transfer, which at these surfaces the water boils at the accurate boiling point. Boiling chips ensures boiling at an accurate temperature.

5. Discuss the importance of the continuous flow of water in the condenser during distillation?Water flow allows the temperature gradient in the condenser to flow more freely, which prevents pressure buildup.

6. Explain why some 2-methylpropene can be formed in the reaction as a byproduct. Give the mechanism for its production. How can it be removed during purification?

This can result if there is no Cl- to react with, this is why HCl must be in excess.