PREPARATION OF 1-BROMOBUTANE
VIA A NUCLEOPHILIC SUBSTITUTION (7/20/2004)
Alcohols do not undergo nucleophilic substitution reactions because hydroxide ion is strongly
basic and a poor leaving group. Thus, alkyl bromides cannot be prepared by either an SN2 or an
SN1 reaction upon heating alcohols with bromide salts. However, alcohols readily undergo
nucleophilic substitutions (either SN2 or SN1) if the hydroxyl group is first activated to produce
a better leaving group. This is the basis for nearly all of the reactions of alcohols.
X
R OH
Br
X
R OH
R
RBr
+
+
Br
OH
SN2
OH
S N1
RBr
One of the easiest methods for activating an alcohol for a nucleophilic substitution reaction is to
protonate the hydroxyl oxygen with a strong acid in the presence of a nucleophile. The
H
R OH
R OH 2
Nu
+ H2 O
R Nu
protonated hydroxyl group can now leave as a weakly basic water molecule instead of as a
strongly basic hydroxide ion. If the alcohol is primary, the protonated hydroxyl is readily
displaced by a nucleophile in an SN2 reaction, whereas, if the alcohol is tertiary, the protonated
hydroxyl-carbon bond fragments to form a carbocation by an SN1 process which then reacts with
a nucleophile to form a substitution product. The protonated hydroxyl of a secondary alcohol
may undergo replacement by a nucleophile either an SN2 or an SN1 mechanism, depending upon
the exact structure of the alkyl group.
X
R3 C
RCH 2
OH 2
RCH 2
OH 2
R3 C
X
X
R3 C
+
H2 O
X
+
H2 O
Primary and tertiary alcohols are readily converted into the corresponding alkyl bromides or
iodides upon treatment with concentrated hydrobromic or hydriodic acids.
Tertiary alcohols also react with concentrated hydrochloric acid to give alkyl chlorides, whereas
primary alcohols are inert to concentrated hydrochloric acid.
R OH
R X
+ HX
+ H2 O
R = primary, X = Br or I
R = tertiary, X = Cl, Br, or I
Primary alcohols are commonly converted into alkyl bromides by treatment with hydrobromic
acid in the presence of sulfuric acid. The sulfuric acid serves two purposes: (1) it increases the
amount of protonated alcohol present in the reaction mixture and (2) it helps tie up the water
molecules generated in the reaction shifting the equilibrium in favor of the alkyl bromide. An
alternative and more convenient method involves the in situ generation of hydrobromic acid by
the addition of concentrated sulfuric acid to an aqueous solution of sodium bromide.
There are some drawbacks to using sulfuric acid. It can cause side reactions such as the
dehydration of alcohols and/or ether formation to occur which lower the yields of the alkyl
bromides.
CH3 CH 2 CH
CH 2
OH 2
CH3 CH2 CH
CH2
H
Nu
CH3 CH2 CH 2 CH2
OH 2
(CH 3 CH2 CH 2 CH2 )2 O + H2 O
CH 3 CH2 CH2 CH 2 OH
In this experiment, 1-butanol will be converted into 1-bromobutane via the in situ generation of
HBr from sodium bromide and sulfuric acid. Analysis of the reaction product by gas
chromatography will allow for the determination of whether the reaction occurs via an SN2 or an
SN1 reaction. If the reaction occurs via an SN2 mechanism, the product should be almost
entirely 1-bromobutane, but if by an SN1 mechanism, the product should consist of mostly 2bromobutane derived from the sec-butyl cation formed via a 1,2-hydride shift.
Br
CH3 CH 2 CHCH 2
OH 2
CH3 CH2 CHCH 3
Br
CH3 CH2 CHCH 3
H
Such an interpretation of the product distribution must be made with caution. The presence of
small amounts of 2-bromobutane in the reaction product does not necessarily mean that it was
produced from 1-butanol via an SN1 mechanism because an alternative sequence of an E2
elimination of the protonated 1-butanol to give 1-butene followed by HBr addition to the 1butene would also produce 2-bromobutane.
CH3 CH2 CH 2 CH2 OH 2
CH 3 CH2 CH
CH2
-H2 O
HBr
CH 3 CH2 CH
CH2
Br
CH 3 CH2 CHCH 3
Experimental
Procedure:
Dispense 9.25 mL (7.49 g, 0.101 mol) of 1-butanol directly into a 100 mL round-bottomed flask
and add 0.100 mol of sodium bromide and 10 mL of water. Thoroughly mix the contents by
swirling, and then cool the mixture in an ice-water bath. Very slowly add 10 mL of concentrated
sulfuric acid (caution: concentrated sulfuric acid is very caustic – see handling cautions that
follow!) to the cold mixture with swirling and cooling. Remove the flask from the bath, add two
or three boiling stones, and equip it with a reflux condenser and a thermowell. Gently warm this
mixture. When most of the salts have dissolved, heat the mixture under a gentle reflux. A
noticeable change will occur along with the separation of two layers. Continue heating at reflux
for 45 minutes.
Convert the reflux setup to one for simple distillation and rapidly distill the mixture, collecting
the distillate in an ice-cooled receiver. The initial distillate will appear cloudy because it
contains both water and 1-bromobutane. Continue the distillation until the distillate comes over
clear, at which time the temperature of the distilling liquid should reach about 115oC (The
increased boiling point results from the codistillation of sulfuric and hydrobromic acids with
water).
Transfer the distillate to a separatory funnel, add about 10mL of water, and gently shake the
funnel with venting. Separate the layers. Wash the organic layer with 4 mL of 2 M sodium
hydroxide followed by 10 mL of saturated sodium chloride solution. Transfer the organic layer
to a 25 mL Erlenmeyer flask and dry it with approximately 2g of anhydrous sodium sulfate.
Occasionally swirl the flask until the crude 1-bromobutane is clear. Addition of small amounts
of anhydrous sodium sulfate may be necessary if the crude product does not become clear. To
minimize loss of product, carefully transfer the crude 1-bromobutane with a Pasteur pipet, being
certain not to carry along any of the drying agent, to a clean, dry, tared flask. Weigh the product
and calculate a percent yield. Analyze this product by gas chromatography and obtain an IR
spectrum of the product. Based upon your gas chromatogram what is the actual percent yield of
1-bromobutane?
Sulfuric Acid Handling Cautions
™ BURN PREVENTIONS: Wear gloves when handling sulfuric acid or pour very carefully!
Be wary of drips on the outside of containers!
™ DISPENSING: Sulfuric acid is too viscous for a pasteur pipet, so pour ~10mL into a small
beaker. A graduated cylinder isn’t necessary since it’s used in excess (which you already
know from your reagent table, right?).
™ SPILL HANDLING
¾ SKIN: Go to the sink and run water on the spill area immediately. Next, pour some
baking soda onto the spill (in box above the sink) and rub into the affected area. Report
the spill to your instructor.
¾ EQUIPMENT, CLOTHING, OR LAB BENCH: Pour baking soda onto the spill (make a
wet paste with it if clothing is involved). Next, use a wet sponge or paper towels to
remove the baking soda residue.
Name:___________________________________________ Section:_______ Date:_________
POSTLAB EXERCISE:
PREPARATION OF 1-BROMOBUTANE
VIA A NUCLEOPHILIC SUBSTITUTION (6/17/2009)
 Due the lab following the completion of the experimental portion of the notebook (30pts
TOTAL). Please answer questions on this form. Attach a copy of the GC trace and the
IR spectra which you interpret.
1. PRODUCT INFORMATION (SHOW CALCULATIONS!) (4pts)
%Yield – Crude (2pts):
%Yield – 1-Bromobutane
(determined w/ GC) (2pts):
A. SUMMARY QUESTIONS (26pts)
1. (3pts) How many compounds were present in the product mixture? What is the most likely
identity of the compounds?
2. (3pts) Give the order of elution (from GC) for the compounds listed above (largest to
smallest retention times):
3. (3pts) According to the GC analysis, which compound was the major product of the reaction?
4. (3pts) What does the GC analysis say about the mechanism of the reaction?
5. (3pts) What was the purpose of washing the organic layer with 2M sodium hydroxide and
saturated sodium chloride in the purification process?
6. (3pts) Why does one always add concentrated acids to water and not vice versa?
7. (3pts) A chemist wanted to prepare 1,3-dibromopropane according to the procedure in this
experiment. Thus, he substituted 0.101 mol of 1,3-propanediol in place of 1-butanol and
kept the same quantities of all the other reagents. Would he expect 1,3-dibromo-propane to
be formed in good yield? Explain.
8. (3pts) Explain how the contents of a vial that contains either 1-butanol or 1-bromobutane
could be quickly and accurately identified.
9. (2pts) Compare your IR spectrum of the product with the furnished IR spectrum of 1-butanol.
Attach the spectra.