Nucleophilic Substitution Reaction Lab | Lab Reports Chemistry

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-:

ROH + H+RO

RX + H2O

X-

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

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EXPERIMENT

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 SN 1 conditions with 2 - methyl– 2 - propanol ( tert - butyl alcohol) and under SN 2 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-: ROH + H+^ R O H H R (^) O^ RX^ +^ H 2 O H H

X- You will need to prepare the equimolar mixture of Br

and Cl - in concentrated sulfuric acid, H 2 SO 4. CAUTION: Sulfuric acid is a very strong, corrosive acid. Be very careful

handling it. Carefully add 38 mL of concentrated H 2 SO 4 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 Na 2 SO 4 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.

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EXPERIMENT