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Experiment 2 Preparation of N-Butyl Bromide (1-Bromobutane) Flipbook PDF
Heat the flask strongly so that liquid distils steadily. ... Distil the n-butyl bromide using a Bunsen burner and record
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Exp. 2 butyl bromide
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Experiment 2 Preparation of N-Butyl Bromide (1-Bromobutane) Introduction: Alkyl halides are very useful intermediates in organic syntheses. The most common synthetic preparation of alky halides is the replacement of the OH group of an alcohol by a halogen. Displacement of a hydroxyl group by halide ion is successful only in the presence of a strong acid. The acid protonates the alcohol to create a suitable leaving group, water, for the SN2 reaction. ROH + HX » R+OH2 + X XG + R+OH2 → X...R...OH2 → XR + H2O If this displacement reaction is attempted in the absence of an acid it is unsuccessful because leaving group would be a hydroxide ion which is a poor leaving group and a strong base. XG + ROH → [X...R...OH]G → X–R + OHG (no reaction) The reaction carried out with n-Butanol and hydrogen bromide is relatively slow. Instead sulfuric acid is used to supply the protons more effectively and NaBr is used to supply the bromide ions. In your report, show the mechanism of reaction of n-butanol with sulfuric acid and NaBr. (Do not use the above general reaction!).
Reflux Condenser
Procedure: Water Out to sink Connect condenser with ruber hose
In a 250 ml round bottom flask place 18.0 g of sodium bromide and 20 ml of water and add 12.5 ml of n-butanol (density = 0.81). While cooling the mixture in an ice bath, carefully add 15 ml of conc. sulphuric acid (18 M) in small portions (~2 ml each) swirling the solution after each addition and cooling in ice. Add two boileezers. The flask containing the reaction mixture is now incorporated into a reflux assembly. (see diagram) After setting it up, ask a TA to check it out before you begin heating. Note concentrated H2SO4 is very corrosive.
Water In Connect to tap carefully and adjust flow to give a gentle flow.
250-mL Conical vial
Boiling chips
Use a bit of grease on the ground glass joints to prevent them from sticking together permanently. Reflux Assembly
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Boil the mixture well by heating the flask with a Bunsen flame for 30 minutes. (The reaction mixture will turn yellowish, brownish or black!). Allow the reaction mixture to cool down for about 3 minutes. Remove the condenser and arrange the flask for distillation. To the end of the condenser is attached a receiver adapter which leads into a 50 ml Erlenmeyer flask containing 10 ml of water. The receiving flask itself is cooled with a bath of ice and water.
Thermometer (note position of the bulb)
Screw cap Screw cap with teflon ring washer Clamp Reflux Condenser Water Out to sink
Water In Connect to tap carefully and adjust flow to give a gentle flow.
Boiling chips
Ice water mixture to reduce evaporation of the distillate
Distillation Assembly
Heat the flask strongly so that liquid distils steadily. Oily drops should appear in the bottom of the receiver. Continue the distillation until no more oil drops distil into the water. In order to determine when to stop, make the following test. Remove the Erlenmeyer and place a test tube containing 2-3 mL of water over the end of the adapter. If no oily drops collect at the bottom of the test tube within a few minutes, the distillation is complete. Also, foaming of the liquid in the distillation flask is a sign that only water is distilling now and your product is already distilled over. Discard the residue from distillation flask into the waste container at the front sink. Do not Chem 266L Steven Forsey
Exp. 2 butyl bromide
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throw the aqueous waste into the organic acetone waste containers!!!!!!!!!!!!!! Pour the distillation from the Erlenmeyer flask into a separatory funnel and draw off the lower nbutyl bromide layer into a clean flask. Keep the aqueous (upper) layer aside and at the end of the experiment through the waste into the aqueous waste container in the front sink). Return the n-butyl bromide layer into the separatory funnel and shake well with a roughly equal volume of 10% aqueous solution of sodium carbonate. (Why do you do this?) Carefully withdraw the n-butyl bromide into a clean dry, 50 ml size Erlenmeyer anhydrous flask. Add pellets of CaCl2 until the liquid is clear and CaCl2 no longer clumps together. (Usually, about half a teaspoon of CaCl2 is sufficient). Allow to stand for 5 minutes with occasional swirling to help absorption of any residual H2O by CaCl2. Clean and dry (using a heat gun) the Claisen head, condenser, receiver adapter, 50 ml size roundbottom flask and a 50 ml size Erlenmeyer flask. Wipe the thermometer dry with a paper towel. Decant the n-butyl bromide into the clean dry 50 ml round-bottomed flask; clamp the flask onto a rod-stand in the fume hood. Set up a distillation assembly using all the dried equipment and use the dry 50 ml Erlenmeyer flask as the receiver. Distil the n-butyl bromide using a Bunsen burner and record the temperature of the thermometer when its reading has stabilized (usually halfway through the distillation). This temperature is the boiling point of the n-butyl bromide and it must be reported in your lab write-up. When the distillation is complete, transfer the product, using a Pasteur pipette, into a pre-weighed vial and weigh it again. Report the mass of your product and the percentage yield. Keep the product vial tightly capped and labelled with both partner’s names. (Do not move it out of the laboratory). Submit the whole product vial with your report.
Questions: 1.
In the following reactions indicate which compounds are Lewis acids and bases on both sides of the arrows. Also state which way the equilibrium will be shifted i.e. to the right or left. CH3CH2ONa + CH3CH2CH2I CH3CH2NH3+ + I
-
» CH3CH2CH2OCH2CH3 + NaI
» NH3 + CH3CH2-I
NaCN + CH3Br » CH3CN + NaBr CH3CO2CH3 + NaBr » CH3CO2Na + CH3Br CH3CH2OH + NaBr » CH3CH2Br + NaOH
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Exp. 2 butyl bromide
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Predict the product by using arrows to show the flow of electrons for the following SN2 reactions. CH3 O- K+
a) CH3CH2Br
+
b) H C C
Na+ +
c) (CH3)2CH CH2 Br d) CH CH I 3 2
+
CH3CH2CH2CH2 Cl + e xcess NH3
NaCN
e) 1-chloropentane + NaI f)
O CH3CH2S- Na+ + H3C O S CH3 O
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Exp. 2 butyl bromide
Prelab
Mol Wt (g/mol) Amount used density (g/mL) Conc. (M) moles
n-butanol 74.12 0.810 ---
What is the limiting reagent?
Fill in the chart NaBr 102.90 -----
H2SO4 98.08 1.84 18.0
What is the theoretical yield? Flow Chart.
TA Signature :
________________________________________
Date:
___________
Chem 266L Steven Forsey
n-butyl bromide 137.03 --1.276 ---
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Exp. 2 butyl bromide
Marking Scheme Section
Mark
Introduction and Theory Results and Observations Discussion and Conclusions Quality of Product References Questions Prelab
4.5 3.0 3.0 1.0 0.5 6.0 2.0 20.0
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