Synthesis of an Alkyl Halide

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Synthesis of an Alkyl Halide

Pablo Miguel Barranco Fernando
Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101 Philippines

Date Performed: December 11 , 2012; Date Submitted: January 8, 2013

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Abstract: Alkyl halides can be synthesized from alcohols through its reaction with strong acids acting as hydrogen halides, HX (X = Cl, Br, I). The mechanisms of acid-catalyzed substitution of alcohols can be classified as either SN1 or SN2, where SN stands for nucleophilic subsitution and 1 or 2 designating its molecularity. The main objective of the experiment was to synthesize tert-butyl chloride with the utilization of an SN1 mechanism and purifying it with a simple distillation set-up, wherein a mixture of different substances separate by taking advantage of the different volatility and boiling points of the individual components. Tertiary alcohols are the most conducive alcohols whenever those must be synthesized into alkyl halides. In the experiment, hydrochloric acid was used in order to react with tert-butyl chloride to produce tert-butyl alcohol through an SN1 reaction. This reation includes three steps. The first part is the rapid, reversible (through hydrolysis) protonation of the alcohol followed by a slower rate-determining step which is the loss of water, eventually producing a more stable tertiary carbocation. To conclude the process, the carbocation is swiftly attack by the halogen ion (chloride in this experiment) in order to form the alkyl halide. Alkyl halide is insoluble in water, making it easy for it to separate from the aqueous layer. Hydrochloric acid was then mixed with tert-butyl alcohol producing water and tert-butyl chloride and was purified through distillation. Eventually, a clear and colorless solution of alkyl halide was synthesized, 0.8 milliliters in volume, with only a 7.38% percent yield. It can be concluded that the synthesis of alkyl halides with the reaction between an alcohol and a strong acid is an accurate and effective purification procedure. [pic]

Introduction

Alkyl halides are compounds which have the general formula R – X, where R is usually an alkyl group with a halogen, such as chlorine or bromine, substitutes in place of one of the hydrogens. It can also be referred to as haloalkanes or halogenoalkanes. Multiple substitutions of halogens for hydrogens can take place, and in the process, may produce certain variations wherein not only alkanes are involved but alkenes and alkynes as well. The presence of the highly electronegative halogen in alkyl halides tend to contribute to its increased polarity. Alkyl halides can be classified depending on the number of alkyl substituents attached to the carbocation. It can be primary, secondary or tertiary. Presently, the applications of the concept of alkyl halides can still be seen. This includes the use of carbon tetrachloride for fire extinguishers and fabric cleaners and chlorofluorocarbons (CFCs) which are usually a part in the process of making air conditioners. Because of the prevalent industrial and commercial benefits which arise from the use of alkyl halides, it is crucial that techniques must be developed in order to be able to prepare, isolate and acquire alkyl halides in the purest possible form. Two processes can be used in order to prepare or synthesize alkyl halides. One of the two methods which are used is the reaction of alcohols with sulfur and phosphorous halides. During the reaction between alcohols and sulfur or phosphorous halides such as thionyl chloride (SOCl2-), phosphorous trichloride (PCl3-), phosphorous pentachloride (PCl5-) or phosphorous tribromide (PBr3), in the process, alkyl halides can be derived from alcohols. An example of that is the synthesis of ethyl chloride or ethyl bromide from the reaction of ethyl alcohol with either sulfur or phosphorous halides. However, the more commonly used among the two is the one which involves the reaction of alcohols...
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