March 8 & 15, 2012
Alkyl halides can be synthesized when alcohols react with hydrogen halides. An alkyl halide is a halogen-substituted alkane, and a hydrogen halide is a compound consisting of a hydrogen bonded to a halogen (H-X). Alkyl halides are classified as primary, secondary, or tertiary depending on the number of alkyl substituents directly attached to the carbon bearing the halogen atom. The purpose of this laboratory experiment was to prepare t-butyl-chloride, an alkyl halide, by dissolving t-butyl alcohol in concentrated hydrochloric acid. The reaction occurs via nucleophilic substitution, in which a nucleophile replaces the leaving group in the substrate. In this case, the hydroxyl group of t-butyl alcohol is replaced by a chlorine atom. The reaction proceeds via Sn1mechanism. The second part of the experiment consisted of purification of t-butyl chloride using the distillation process. A nucleophile is any neutral or uncharged molecule with an unshared pair of electrons. In the substitution reaction, the nucleophile donates an electron pair to the substrate, leading to the formation of a new bond to the nucleophile, while breaking the existing bond to the leaving group. The two types of nucleophilic substitution reactions, Sn1 and Sn2, are identified based on whether these events occur simultaneously or in two separate steps. To synthesize t-butyl chloride, the t-butyl alcohol undergoes first order nucleophilic substitution, also known as SN1. To understand why t-butyl reacts via Sn1 pathway, the kinetics of the reaction mechanisms must be observed. The steps of the nucleophilic substitution involved in the preparation of t-butyl chloride can be identified in the experiment. When concentrated hydrochloric acid is added to the t-butyl-alcohol and mixed, t-butyl chloride forms. This product is not soluble in aqueous hydrochloric acid and forms a distinct separate layer. The t-butyl chloride is less dense then water and therefore is the top layer. An excess of hydrochloric acid was used in this step to drive reaction towards equilibrium to ensure an adequate amount of t-butyl-chloride was formed. The bottom aqueous layer is discarded from the funnel, separating t-butyl-chloride from aqueous hydrochloric acid. At this point the crude product contains hydrochloric acid, unreacted alcohol, and traces of water. To remove excess acid, water is added is t-butyl-chloride. This is a physical method, and purification is based on solubility. Next, the last traces of acid are removed by repeating the wash process with aqueous sodium carbonate, which acts to neutralize the solution. As a result, sodium chloride salt, water, and carbon dioxide gas are formed. Being highly soluble in water, sodium chloride is discarded with aqueous layer. The wash process is then repeated with water to remove sodium carbonate. Lastly, anhydrous calcium chloride is added as a drying agent to remove the trace amounts of water droplets. This addition creates dry crude t-butyl-chloride.
Crude t-butyl-chloride is purified by distillation where substances of different volatilities and boiling points are separate from each other. Purification is achieved by collecting distillate product at forty-eight and fifty-two degrees Celsius, which is within boiling point range of t-butyl-chloride. Because the product was collected between these specific temperatures, all impurities lower and higher than the range were removed, leaving pure t-butyl-chloride.
Main and Side Reactions:
T-butyl-chloride is prepared from t-butyl-alcohol and concentrated hydrochloric acid. The main overall reaction is shown below:
There are side reactions that take place during the preparation of t-butyl chloride. One side product formed is isobutylene. This product forms when the tert-butyl carbocation intermediate undergoes a first order elimination reaction. The formation of isobutylene is shown in the...