Synthesis of 1-bromobutane

Adapted from H.D. Durst and G.W. Gokel, Experimental Organic Chemistry, McGraw-Hill, N. Y., 1980.

Alcohols are easily converted into the corresponding alkyl halides by treating them with concentrated aqueous hydrogen halides. Tertiary alcohols react almost instantly at cool temperatures via an SN1 mechanism since 3° carbocations are relatively easy to form. Primary alcohols, on the other hand, react via the SN2 mechanism because 1° cations are difficult to form. Addition of strong acid forms an oxonium ion from the 1° alcohol OH group, changing a poor leaving group into a good one (HO־ is poor, but HOH is good because it is electrically neutral). In spite of this, because a 1° cation is too hard to form, water does not actually leave until it is pushed out by an incoming nucleophile (the bromide ion). Note that this is an equilibrium process, so it cannot be expected to go to completion. The reaction is shown below.

+

R-CH2-OH + H+ ⇄ R-CH2-OH2 ⇄ R-CH2-Br + HOH + Br- The purposes of this experiment are (a) to do an SN2 reaction and (b) to learn various techniques for synthesis, isolation, purification, and characterization of a compound. Some techniques have multiple uses: in this experiment, distillation is useful for both isolation/purification and characterization. EXPERIMENTAL PROCEDURE Caution: Sulfuric acid is extremely corrosive. Keep it away from your person and your clothing. Wash copiously with water in case of contact. Bromobutane is a lachrymator. Avoid breathing vapors and skin contact . Note: several extractions are done using a separatory funnel. The position of the product layer may change

unpredictably. Do not discard anything until you know what it is!!! Recommended: use Erlenmeyer flasks to hold layers you want to keep, and beakers to hold potential discards.

Synthesis: Put 25 mL of water in a 250 mL boiling flask and add 27 g of solid NaBr; partially dissolve the salt by swirling for about a minute. Tare this flask in a beaker on a high-capacity balance and add 16 g of 1-butanol. The weight does not have to be exact, but record the actual mass used. Get a 50 mL graduated beaker and pour approximately 25 mL of concentrated H2SO4 into a it. Take the beaker and flask to a sink, and as you swirl the flask under running water to cool the contents, cautiously and in small amounts add the H2SO4 until all is in. This will generate a lot of heat. Cool enough to keep it from boiling. If you are careless some acid may run down the outside of the beaker or flask and get on your hand. Be sure that no water enters the neck of the flask. After mixing is complete, clamp the flask near the front of your hood, mount a heating mantle, add a boiling chip, and fit with a vertically-oriented condenser. Leave the upper end of the condenser open. Begin heating the flask. While it is heating, lift the entire assembly off the desk top and agitate so that the contents of the flask are thoroughly mixed (show the instructor). Also do this mixing at least once after boiling starts. Boil the contents of this reflux apparatus slowly for 30 minutes. [During the refluxing, obtain 10 mL of conc. H2SO4 in your 50 mL beaker and put it inside your hood.] Two layers gradually will become visible. When time is up, turn off the power and lower the heating mantle. Allow the mixture to cool slightly until boiling ceases. Remove the condenser (leaving hoses connected) and lay it temporarily on your desk. To the flask add two new boiling chips. Then mount a distilling head, attach your condenser, and complete the set-up for simple distillation into a 100 mL round flask. Insulate the distilling head and flask, then raise and turn on the heating mantle. Distill rapidly (as fast as condenser can handle it; Variac about 90) until the distillate no longer contains two phases (check by capturing a few drops of distillate in a test tube). This process is called "steam distillation" and serves to separate the desired product (along with some water) from most of the unwanted nonvolatile reactants. Removal of contaminants: Pour the contents of the receiving flask into a separatory funnel. [You need to identify the organic layer. To do this, carefully draw off about 0.25 mL of the lower layer into a test tube containing about 1 mL of water and mix vigorously. If the liquid from the funnel dissolves in the water as you mix it, the lower layer is aqueous. If two immiscible layers form in the test tube, the lower layer in the funnel is organic (in this case return the contents of the test tube to the funnel).] Separate the layers, and save the organic layer (return it to the cleaned separatory funnel if necessary). If there is any color in the organic layer (possibly due to elemental bromine), check with the instructor before proceeding. Put the aqueous layer from the funnel into a 600 mL beaker kept in your hood.

[As you wait for layers to separate in the following steps, disassemble your distillation apparatus as convenient. Pour the pot residue carefully with stirring into the 600 mL waste beaker -- it will get hot. Keep this beaker inside in your hood. Rinse all the glassware, making sure all the rinse water goes directly down the drain opening. Thoroughly dry the distilling head, thermometer adapter, vac. adapter and condenser. Ask instructor about proper glassware drying procedures.]

The product (in the separatory funnel) may be contaminated with unreacted alcohol. After getting rid of as much water layer as possible, cautiously add about half of the concentrated H2SO4 from the 50 mL beaker in your hood (the acid will react with and dissolve any residual alcohol and water). Swirl without inverting, venting if necessary and noting whether heat is generated or any other changes occur. Finally, shake vigorously, venting occasionally, for at least 30 sec., allow the layers to separate, and identify the organic layer. (The H2SO4 layer is probably the lower one. If so, when a few drops of it are mixed with water in a test tube, you will see Schlieren lines and the test tube will get warm. Minor cloudiness in the test tube is due to 1-butanol coming out of H2SO4 solution. Check with the instructor if in doubt.) Stirring as you do so, carefully discard the sulfuric acid layer into the 600 mL beaker. Save the organic layer and shake it vigorously with the remaining unused H2SO4 to assure complete removal of unreacted 1-butanol. Discard acid as above . Purification: Weigh a 50 mL boiling flask, eventually to use as your receiving flask. Put the acid-washed organic layer into a DRY 25 mL boiling flask, add a boiling chip, and distill through DRY glassware. Be sure that your thermometer is properly positioned!! Insulate the apparatus (see instructor). Collect any “forerun” (b.p. lower than 95°) in an Erlenmeyer flask and discard down the sink. If the temperature is rapidly rising as it passes 95°, do not discard anything. Change to the weighed receiving flask and collect only that material which boils 95 - 104° C. Record the temperature at which you started collecting your product, the temperature at which most of it boiled, and the temperature at which you stopped collecting (three entries). After weighing, pour the product into the flask marked “1-bromobutane - student prep”. Discard any pot residue into the container marked halogenated organic waste. Clean-up: Take the 600 mL beaker containing acid wastes to a hood. Get a plastic dish basin and put a layer of sodium bicarbonate all over the bottom. Pour your waste acid over the bicarbonate in small portions with good stirring. Beware of strong foaming. Continue adding small portions of waste while stirring. You are finished when all waste has been added and no foaming occurs if you add new solid bicarbonate. Solid Na2SO4 will form; this is expected. Discard everything down the drain with lots of water. Rinse glassware cautiously, pouring all rinse water directly down the drain (don’t let it splash in the sink). Then wash with Alconox, and again pour the wash water directly down the drain. Instructors will show you how to clean "tarry" glassware. Report: follow the usual guidelines. Don't forget these items in their proper places: - Show the complete equation for this reaction, including a curved arrow mechanism. - Report your yield in both grams and as % of theory. Show calculations in an Appendix- you will need to use

information from the Table in your lab notebook. - Compare the boiling point of your product (the temperature at which the bulk of it boiled) with the published value

(source?). - Yields are always less than 100% due to conditions that did not favor complete reaction or to losses during work-up.

Recalling theory as well as observations you made during the experiment, discuss specific causes of low yield. (Do not use the words "Human error". Instead, consider the concept of equilibrium, possible competing reactions, and identify any specific mistakes you know of.) Can you think of any improvements to this procedure?

Questions (answer in an appendix): 1. a. Liquid reagents are often measured by volume rather than mass. Show how to compute the volume of 1-butanol that

would provide 16 g. b. Why is it more accurate to weigh liquids than to use a graduated cylinder to measure them? 2. 1-butanol is a likely contaminant of your product. Its boiling point is higher than that of 1-bromobutane. Compare

their molecular structures to explain this fact.