Experiment 2 Preparation of N-Butyl Bromide (1 ... - PCC

Introduction:
Experiment 2
Preparation of N-Butyl Bromide (1-Bromobutane)
Exp. 2 butyl bromide 1
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!).
Procedure:
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.
Use a bit of grease on the ground glass joints to prevent
them from sticking together permanently.
Chem 266L
Steven Forsey
Reflux Assembly
Reflux Condenser
Water Out to sink
Connect condenser with ruber hose
Water In
Connect to tap carefully
and adjust flow to give
a gentle flow.
250-mL Conical vial
Boiling chips

Exp. 2 butyl bromide 2
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.
Screw cap
Water
Out
Water In
to sink
Connect to tap
carefully
and adjust flow to give
a gentle flow.
Boiling chips
Thermometer
(note position of
the bulb)
Screw cap with teflon ring washer
Clamp
Distillation Assembly
Reflux Condenser
Ice water mixture
to reduce
evaporation of
the distillate
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.
Chem 266L
Steven Forsey
Discard the residue from distillation flask into the waste container at the front sink. Do not

throw the aqueous waste into the organic acetone waste containers!!!!!!!!!!!!!!
Exp. 2 butyl bromide 3
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.
Chem 266L
Steven Forsey
CH3CH2ONa + CH3CH2CH2I » CH3CH2CH2OCH2CH3 + NaI
CH3CH2NH3 + + I - » NH3 + CH3CH2-I
NaCN + CH3Br » CH3CN + NaBr
CH3CO2CH3 + NaBr » CH3CO2Na + CH3Br
CH3CH2OH + NaBr » CH3CH2Br + NaOH

Exp. 2 butyl bromide 4
2. Predict the product by using arrows to show the flow of electrons for the following SN2 reactions.
a) + C H 3 O - K +
C H 3 C H 2 B r
b)
c)
H C C N a +
( C H 3 ) 2 C H C H 2 B r
d) C H 3 C H 2 I
+
+
e) 1-chloropentane + NaI
f)
Chem 266L
Steven Forsey
N a C N
CH3CH2S - Na + O
+ H3C O S
O
C H 3 CH 2 CH 2 CH 2 Cl
+
e xcess NH 3
CH 3

Chem 266L
Steven Forsey
Prelab
Exp. 2 butyl bromide
Fill in the chart
n-butanol NaBr H2SO4 n-butyl bromide
Mol Wt (g/mol) 74.12 102.90 98.08 137.03
Amount used ---
density (g/mL) 0.810 --- 1.84 1.276
Conc. (M) --- --- 18.0 ---
moles
What is the limiting reagent? What is the theoretical yield?
Flow Chart.
TA Signature : ________________________________________
Date: ___________
5

Marking Scheme
Section Mark
Introduction and Theory 4.5
Results and Observations 3.0
Discussion and Conclusions 3.0
Quality of Product 1.0
References 0.5
Questions 6.0
Prelab 2.0
20.0
Chem 266L
Steven Forsey
Exp. 2 butyl bromide
6