The Synthesis of tert-Butyl Chloride Handout adapted from Dr. Allison Dick, Wheaton College, Wheaton, IL Lecture concepts illustrated: SN1 reactions Lab-specific concepts: Purification by distillation Section 1: Introduction 1.1 Nucleophilic Substitution Alkyl halides are an important class of compounds in synthetic organic chemistry because they are readily prepared from other families of organic compounds, such as alcohols, alkenes, and other alkyl halides, and they undergo a large number of reactions to prepare other families of organic compounds. Because of the large number of nucleophiles capable of displacing the halide ion, nucleophilic substitution reactions are particularly useful in synthetic organic chemistry. For this reason, the effect of many variables on nucleophilic substitution reactions has been extensively studied. The ease of substitution depends to a large degree on the nature of the alkyl halide. Alkyl halides that favor an SN2 reaction follow the order of reactivity of methyl, primary, secondary, tertiary as shown below. On the other hand, the reverse order is followed by alkyl halides in the SN1 mechanism. SN2 mechanisms are bimolecular, meaning that both nucleophile and alkyl halide come together at the same time. The bond formation with the nucleophile and departure of the leaving group are simultaneous events. Thus the SN2 reaction is sensitive to steric factors; these account for the fast rate of methyl halides, with little steric hindrance, and the slow rate for tertiary alkyl halides, with much steric hindrance. With a good leaving group, the alkyl halide can ionize to form a carbocation. Since tertiary carbocations are lower in potential energy compared to methyl or primary carbocations, the reaction of tertiary alkyl halides tends to follow the SN1 mechanism. However, primary carbocations do not follow the SN1 mechanism because primary carbocation intermediates are too high in energy. Since the rate-determining step in the SN1 reaction is the slow ionization of the leaving group, they follow unimolecular kinetics where the reaction rate depends only on the concentration of the alkyl halide. 1.2 Synthesis of tert-Butyl Chloride The synthesis of 2-chloro-2-methylpropane, also known as tert-butyl chloride, is a classic example of the distinction between the difficulty of formation of primary alkyl carbocations and the ease of formation of tertiary alkyl carbocations. Since primary alkyl carbocations have no substantial means of stabilization, molecules such as 1-butanol, even with the good leaving group -OH2+ do not form a primary carbocation as in the SN1 mechanism, but instead follow a bimolecular pathway (SN2 mechanism). Primary Alcohol Reactions In the scheme above, concentrated hydrochloric acid protonates the hydroxyl group converting it to a better leaving group. Theoretically, the reaction can then follow one of two courses. In the SN1 reaction (top), the leaving group leaves to form a primary carbocation, followed by the reaction with chloride ion to form butyl chloride. In the bottom reaction, SN2, chloride ion displaces the protonated hydroxy group in a bimolecular substitution reaction to yield butyl chloride. Though the mechanism is valid mechanistically, this reaction is not synthetically useful since chloride ion is a poor nucleophile giving low yields. Other methods to make primary alkyl chlorides such as the reaction with thionyl chloride give higher yields. tert-Butyl alcohol, however, does not follow the SN2 mechanism when reacted with HCl. The SN2 mechanism requires backside attack of the nucleophile with the * antibonding orbital of the leaving group, shown on the next page. The backside attack therefore leads to steric hindrance at the transition state where five groups surround the central carbon atom. This greatly increases the activation energy and decreases the rate for the reaction when this mechanism is followed. Tertiary Alcohol Reactions To avoid this unfavorable steric interaction, protonated tert-butyl alcohol loses water in a reversible ionization step to form the more stabilized tertiary carbocation intermediate, stabilized by the inductive effect and hyperconjugation. After formation of the carbocation, the nucleophile, in this case the chloride ion, reacts from either side of the molecule to afford the substitution product, tert-butyl chloride. In this case no stereocenter is generated so only one product is formed. In this experiment the SN1 mechanism will be followed and the reaction will take place in a separatory funnel. Since tert-butyl chloride is relatively water-insoluble, and therefore also insoluble in concentrated HCl as the reaction takes place, the product, tert-butyl chloride, separates as a top layer in the separatory funnel since its density (d = 0.842 g/mL) is less than that of concentrated HCl (d = 1.20). This facilitates washing with water and base to remove residual hydrochloric acid before drying and distilling. Since the boiling point of tert-butyl chloride is relatively low (51 °C) compared to water (100 °C) or tert-butyl alcohol (83 °C) it can easily be purified by simple distillation. Section 2: The Experiment 2.1 Goal In this experiment, tert-butyl chloride will be synthesized by an SN1 reaction and purified by distillation. 2.2 Materials • • • • • • • • Concentrated (12M) HCl Ice bath tert-Butyl alcohol 125 mL separatory funnel and stopper 5% sodium bicarbonate solution (premade for you) Glassware, etc. for a simple distillation without Claisen adapter (see intro material section 5.8) Erlenmeyer flask, cork (can be obtained from drawer near UV light boxes) CaCl2 drying agent 2.3 Hazardous Waste/Safety CAUTION: Do all work in the hood; Hydrochloric acid is concentrated and extremely hazardous. Wear gloves and SAFETY GLASSES Waste Disposal: Place all organic waste in the container in the Waste Collection Hood. Aqueous waste from the extraction may go down the drain with lots of water. 2.4 Tips • • • • • If the room is cold, the tert-butyl alcohol may solidify (freeze) in your graduated cylinder. Measure it out as soon as you are ready to use it, or gently heat the cylinder in a warm water bath to melt it. Make sure to dry your material thoroughly before distillation. When setting up the distillation, double check to make sure all joints are tightly connected. Otherwise you will lose material to evaporation. The distillation goes quickly! Do not set the Variac too high. It is possible the reading on the thermometer will not reach the values given in the procedure; record the reading you observe in your notebook. Remember to put the standard blue sample tube back into the NMRs and put them on standby mode when you are finished (unless another student is waiting to use the instrument). 2.5 Procedure Place 28 mL of concentrated HCl (CAUTION) in a 125 mL Erlenmeyer flask and cool it to 5 °C. Check your 125 mL separatory funnel for leaks (add water, then drain, rinse with acetone and dry with compressed air). Pour the 28 mL of cooled, concentrated HCl into the separatory funnel (stopcock closed) using a funnel. Then add 10 mL of tert-butyl alcohol (CAUTION). Swirl the separatory funnel occasionally with the stopper off for about a minute. Then add the stopper and carefully invert it. Release any pressure immediately by opening the stopcock on the inverted funnel. Do not shake the funnel until the pressure has been equalized. Then invert and shake the funnel for several minutes with occasional venting. Allow the mixture to stand, undisturbed, until two layers separate distinctly. Drain off and discard the bottom aqueous layer (residual HCl). Wash the remaining liquid with 4 mL of water (this removes most of the HCl from the organic layer). Discard this bottom layer. To remove the rest of the HCl, wash the liquid with 10 mL of 5% NaHCO3 solution (CAUTION: CO2 will build up pressure) and discard the bottom layer. Wash again with 4 mL of water and discard the bottom layer. Transfer the liquid to a 25 mL Erlenmeyer flask. Add 1.5 g of anhydrous CaCl2 to the crude product with swirling. Cork it and let it dry for 10 minutes. Decant (carefully pour off) the liquid into a 25 mL or 50 mL round-bottom boiling flask, add a magnetic stirring bar, and distill the product using a simple distillation apparatus. Collect the fraction boiling in the range 45-53 °C (your values may be slightly lower; that’s ok!) in a tared vial; discard only the first few drops. Weigh the product and calculate the percent yield. After the spectroscopic analysis below, submit your product in a labeled vial sealed with Parafilm. Take IR spectra of the starting alcohol and the isolated product. Use the circular “foot” for liquid samples on the IR spectrometer to minimize evaporation, particularly of the product. In your conclusions, comment on the key functional group signals in the two samples. Take an NMR spectrum of your product. In your conclusions, comment on the purity of the sample. (A reference spectrum of the alcohol may be provided for you.) 
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