EXPERIMENT 5 NUCLEOPHILIC SUBSTITUTION REACTIONS SN1 and SN2 Background By the time you do this experiment we will have covered nucleophilic substitution reactions in class Please review your class notes and chapter seven of your text before coming to lab You should already be able to predict the results of these experiments based on your knowledge of the concepts Many laboratory manuals include experiments designed to illustrate nucleophilic substitution reactions This experiment was taken primarily from Introduction to Organic Laboratory Techniques 3rd edition by Donald L Pavia Gary M Lampman and George S Kriz In this experiment you will determine the importance of the nucleophilie under SN1 and SN2 conditions You will allow an equimolar mixture of competing nucleophiles Br and Cl to react under SN1 conditions with 2 methyl 2 propanol tert butyl alcohol and under SN2 conditions with 1 butanol The alkyl halide products will be analyzed by gas chromatography to determine the relative amounts of the components in the product mixture Gas Chromatography GC You are already familiar with thin layer and column chromatography from Experiment 1 review your notes if you need to refresh yourself on the concept of chromatography Then please read section 15 2 p 749 752 in your textbook Your text shows a picture of a typical chromatogram The time it takes for a compound to elute from the column and then for the signal to be detected as a peak on the chromatogram is called the retention time and can be used to qualitatively identify a compound The chromatogram also gives quantitative information since the area under a peak is proportional to the amount of the compound in the sample You can therefore calculate the molar ratio of the components produced in the nucleophilic substitution reactions by comparing the peak areas that are tabulated by the instrument computer Important Note Quantitative comparison of chromatogram integrations assumes that the GC detector responds equally to each component of the mixture This is not necessarily true A rigorous quantitative estimation would require us to calibrate the instrument by estimating the response factor Experimental You will be running the following reaction where X is either Br or Cl H ROH H R O H H R X O RX H2 O H You will need to prepare the equimolar mixture of Br and Cl in concentrated sulfuric acid H2SO4 CAUTION Sulfuric acid is a very strong corrosive acid Be very careful handling it Carefully add 38 mL of concentrated H2SO4 to 50 g of ice in a 250 mL Erlenmeyer flask Set aside Weigh 9 5 g of ammonium chloride and 17 5 g of ammonium bromide Thoroughly crush any lumps then transfer these salts to a 500 mL Erlenmeyer flask Slowly add the sulfuric acid a little at a time and swirl to dissolve the salts You may have to warm the mixture slightly and add as much as 5 mL of water to get the salts into solution Allow the solution to cool slightly as you prepare the apparatus for the SN1 and SN2 reactions SN2 Assemble an apparatus for reflux using a 500 mL round bottomed three necked flask a condenser and a gas trap A demonstration set up will be available for you to see Sketch the apparatus in your notebook SN1 You will just need to set up a 125 mL separatory funnel Pour 35 mL of your acidic nucleophile mixture into the separatory funnel and replace the stopper Pour the rest of the mixture into the 500 mL round bottomed flask add a boiling stone and replace its stopper Begin the SN2 reaction first as directed below SN2 reaction Add 5 mL of 1 butanol n butyl alcohol to the reflux apparatus by pouring it down the condenser Replace the gas trap and heat at reflux gently for 75 minutes making sure that the vapor ring does not rise more than a quarter of the way up the condenser While your reaction is under reflux complete the SN1 reaction as directed in the next section At the end of the reflux period discontinue heating and lower the heating mantle to allow the flask to cool undisturbed at this point shaking the reaction flask may cause violent boiling and loss of product Allow the flask to cool about 5 minutes in the air before putting it into an ice water bath Cool in an ice water bath for a few minutes and then begin to swirl the mixture to facilitate more rapid cooling Transfer the cooled solution to a 125 mL separatory funnel leaving any solid material behind and separate the layers Be sure you know which is the organic layer and which is the aqueous layer Wash the organic layer with 10 mL of water and then with 10 mL of saturated sodium bicarbonate solution Again make sure you know which layer is which and remember to release the pressure build up in the separatory funnel Dry the organic layer with anhydrous Na2SO4 and decant the clear solution into a small dry vial Cap the vial immediately to avoid loss of product Analyze your products by gas chromatography SN1 reaction Add 5 mL of 2 methyl 2 propanol to the separatory funnel containing the nucleophile mixture Since the melting point of 2 methyl 2 propanol is 25oC use a warm graduated cylinder for measuring Swirl the mixture gently then release the pressure build up by venting the funnel Keep swirling and venting the funnel until the pressures are equalized and then shake the funnel vigorously with occasional venting for 2 minutes Allow the layers to separate for about a minute then drain the lower layer Wait 10 15 seconds longer then drain another small portion this time including a bit of the upper organic layer just to be sure that the remaining organic layer is not contaminated with water Pour the organic layer out of the top of the separatory funnel into a beaker containing 1 g of solid sodium bicarbonate Stir and as soon as the bubbling stops decant the clear solution into a small dry vial Cap the vial immediately to avoid loss of product Analyze your products by gas chromatography 2 Prelab 1 2 3 4 5 6 7 8 9 Which is a better nucleophile in aqueous solution Br or Cl Why What products form in the SN1 reaction in this experiment In what ratio approximately do you predict they will be formed What are the products of the SN2 reaction in this experiment Do you expect the same ratio of products as in the SN1 reaction Explain why or why not Look up the boiling points and densities of all of the organic reactants and products Densities will help you to determine which layer is the organic layer and which is the aqueous layer in your separatory funnel and boiling points can help you to