Investigating the Regioselectivity of Unsymmetrical Alkene Hydration:
Hydration of 1-Hexene
A fundamental and industrially important reaction of alkenes is the addition of the Scheme 1. Commercial synthesis of ethanol
elements of water to the alkene C=C in the presence of an acid catalyst to afford an
alcohol; e.g., most of the ethanol produced for industry is not made by alcoholic
fermentation, but by an addition reaction between ethylene and water (Scheme 1).
Ethylene is a symmetrical alkene, and only a single isomer of Scheme 2. Two-step hydration of an unsymmetrical alkene
an alcohol could result upon hydration; however, hydration
of unsymmetrical alkenes (regardless of their configuration;
E/Z) could afford two possible alcohol isomers (Scheme 2).
In this experiment you will use an older industrial process to
hydrate l-hexene (a terminal, and thus unsymmetrical alkene)
by treating it first with 85% (w/w) H2SO4 followed by
heating the intermediate product (a sulfuric acid ester) in the
presence of water, thereby hydrolyzing the ester group to
liberate an alcohol (Scheme 2). You will then purify the
alcohol product by simple distillation and determine which
alcohol isomer was formed by boiling point determination in
comparison with the reported boiling points for the possible
isomeric alcohol products.
Before coming to lab remember to consult the MSDS for all chemical substances. CAUTION: All concentrated (>50%) aqueous
solutions of sulfuric acid, which includes 85% (w/w) sulfuric acid, are highly corrosive; avoid skin/eye contact by wearing gloves, eye
protection and a buttoned up lab coat. Avoid direct contact with all other reagents/reactants/products in this experiment and work in a
fume hood as much as possible. NO FLAMES will be allowed in the lab as ether and 1-hexene are highly flammable.
Fig. 1
reaction
flask
Steps in procedure
Fig. 2
reflux
apparatus
1) In a clean and dry 50-mL
Erlenmeyer flask place 10 mL of
85% (w/w) H2SO4 followed by 5
mL of 1-hexene.
2) Gently slide a 1-inch magnetic
stirring bar into the flask, place
the flask on top (and in the
center) of a magnetic stirrer,
loosely cork the flask, and
securely clamp it to a stand.
Vigorously stir the mixture using
a magnetic stirrer for 15 minutes
(Fig. 1).
Note: 1-hexene and 85% H2SO4
are nearly immiscible and the
mixture must be stirred rapidly
for the reaction to take place!
The reaction is mildly exothermic
and the mixture gradually
becomes homogeneous (i.e., one
layer).
-
3) Add another 5 mL of 1-hexene,
loosely cork the flask, and rapidly
stir the mixture for 15 minutes.
4) Securely clamp a 100-mL round-bottomed (RB) flask to a ring stand and place 20 mL of tap water in it. Then, using a liquid
funnel, add the contents of the 50-mL Erlenmeyer flask to the RB flask containing the tap water.
5) Place 10 mL of tap water in the 50-mL Erlenmeyer flask, swirl the flask and add the rinse to the RB flask.
6) Gently swirl the RB flask, attach a condenser to the RB flask and drop a boiling stick (instead of a stone) into the flask directly
through the opening at the top of the condenser (see Fig. 2). The mixture in the RB flask should consist of two layers after you
add the contents of the Erlenmeyer flask.
7) Connect thin-walled tubing to the water inlet (lower nozzle) and water outlet (upper nozzle) of the condenser (see Fig. 2), and
place the end of the outlet hose into the sink. Carefully open the faucet to afford a gentle stream of water, and then connect the
inlet hose to the faucet.
8) Using a heating mantle attached to a voltage regulator, heat the contents of the RB flask to reflux for 5 min. Note: Set the voltage
regulator to about 3.5 and start timing the reflux only after the liquid in the flask has come to a boil and you observe some
condensate dripping back into the flask!
9) Lower and completely remove the heating mantle, and allow the flask to cool for a minute or so in air. Then cool the flask in an
ice-water bath for a further 5 min (you can use the porcelain bowl in your lab drawer for the ice-water bath). The condenser
must remain attached to the flask and water must be flowing through the condenser during ice-cooling.
10) Mount a separatory funnel on an iron ring attached to a ring stand. Then, remove the condenser and, using a liquid funnel, pour
the contents of the flask into the separatory funnel. Rinse the used boiling stick with water and discard in the trash.
11) Add 10 mL of ether into the separatory funnel, then stopper and shake the separatory funnel for about 1 min with frequent
venting. Mount the separatory funnel back on the ring, remove the stopper and allow the layers to separate. Then drain the
lower aqueous layer into a 50-mL Erlenmeyer flask, but leave the upper organic layer in the separatory funnel. At the END of
the lab period, discard the aqueous layer down a fume hood sink drain with the water running.
12) Add 10 mL ether followed by 3 mL of 5% aqueous NaOH to the separatory funnel. Stopper and shake the separatory funnel for
about 2 min with frequent venting. Remove the stopper and allow the layers to separate. Drain the lower aqueous layer into a
small Erlenmeyer flask and test it for acidity/basicity using litmus paper (red litmus paper will turn blue if basic but will
remain red if acidic!). If acidic, add 3 mL of 5% NaOH to the organic layer in the separatory funnel and shake the funnel for
about 2 min. Remove the stopper, allow the layers to separate, drain the lower layer into an empty Erlenmeyer flask and test it
for acidity/basicity using litmus paper. If acidic, repeat washing as before with another 3 mL of 5% NaOH and check the
acidity/basicity of the aqueous layer; repeat this until the aqueous layer is basic! At the END of the lab period, discard all
aqueous NaOH layers down a fume hood sink drain with the water running.
13) Pour the organic layer from the top opening of the separatory funnel into a dry 50-mL Erlenmeyer flask and dry the organic
layer with anhydrous Na2SO4 (about 4 g). Cork the flask, swirl briefly and allow the flask to sit for 10 min.
14) Carefully decant the liquid away from the spent drying agent into a dry 50-mL RB flask for distillation.
15) Rinse the spent drying agent with approx. 5 mL of ether and add the rinse to the RB flask. After the ether rinse, attempt to
dissolve the spent drying agent in water with periodic stirring over a period of time as you proceed with the rest of the
experiment; the water solution can be discarded down a fume hood sink drain and any undissolved drying agent can be
discarded in the trash (don’t discard SOLIDS down the sink drain).
16) Add a boiling stick, assemble a simple distillation apparatus (Fig. 3), and begin to distill the solution. Use a 50-mL Erlenmeyer
flask as the receiving flask and set the voltage regulator to 3.5. As the temperature increases, the lower boiling components in
the mixture (diethyl ether and any unreacted 1-hexene, etc.) will begin to distill. Allow the distillation to proceed until the
thermometer reaches 3 degrees below the boiling point of 2-hexanol. At this point remove the 50-mL Erlenmeyer flask
(containing ether and unreacted 1-hexene, etc.) and place a clean and dry pre-weighed 25-mL Erlenmeyer flask. Allow the
distillation to proceed until the temperature on thermometer reaches 3 degrees past the boiling point of 2-hexanol. At this point
remove the 25-mL Erlenmeyer flask and place another 25-mL Erlenmeyer flask to distill the rest of the solution but do NOT go
to dryness!
REVIEW HOW TO PERFORM A SIMPLE DISTILLATION (including how to assemble the apparatus, how and when to
collect fractions, etc.)! Do some thinking and planning before you arrive in the lab.
Look up and record the bp ranges of diethyl ether, 1-hexene, trans- and cis-2-hexene, trans- and cis-3-hexene, & 1- and 2hexanols before coming to the lab!
Use clean and dry glassware to set up your distillation apparatus.
As always, record bp ranges, etc. of all distillation fractions collected, and record the weight and appearance of the alcohol
product obtained.
PLEASE wash the magnetic stir bar and remember to return it as directed by your instructor. WHEN WASHING THE
STIR BAR PLEASE BE CAREFUL THAT IT DOES NOT FALL DOWN THE SINK!
Figure 3. Fully assembled simple distillation
apparatus
Vapors will collect in
distillation adapter
& begin to condense
Remaining vapors will liquefy in
water-cooled condenser & run
down inner tube, through the
vacuum (bent) adapter
into a receiving vessel
Expt. #1
Investigating the Regioselectivity of Unsymmetrical Alkene Hydration: Hydration of 1Hexene
Introductory Background
Alkenes (cyclic and acyclic) react with H2SO4 and produce alkyl hydrogen sulfates (see generic chemical equation below).
When the alkene molecule is structurally unsymmetrical and one of the carbons of C=C is directly bonded to a greater
number of hydrogens, the reaction is regioselective (follows Markovnikov’s rule) and leads to a major alkyl hydrogen
sulfate accompanied by little or no minor product.
e.g.
Mechanistically, this reaction involves carbocations as intermediates and carbocation rearrangement (via 1,2-hydride shift
or 1,2-methyl shift) may occur during the reaction.
e.g.
Since the positively charged carbon atoms of carbocations are trigonal planar (sp2 hybridization), syn and anti addition of
H and OSO3H across C=C is possible and may lead to stereoisomes. In previous example, A is produced as a racemic
mixture of the R and S enantiomers.
The OSO3H is a good leaving group and upon reaction with H2O (a weak nucleophile), alkyl hydrogen sulfates are
converted to alcohols (SN1 mechanism).
e.g.
The Experiment – Its Goal
You will first react 1-hexene with H2SO4 to form an alkyl hydrogen sulfate. You will subsequently treat the alkyl
hydrogen sulfate with H2O and isolate the alcohol product by distillation. You will identify your alcohol product as (R,S)2-hexanol, 1-hexanol or a mixture of both based on the boiling point of your alcohol product.
Note: If Markovnikov regioselectivity is followed in your reaction, (R,S)-2-hexanol should be the major product and if
not, 1-hexanol should be the product.
Note: bp 2-hexanol 136 oC; bp 1-hexanol 156-157 oC
What you should write in your lab notebook before coming to the lab
1. Title of experiment
2. Goal of experiment
3. Equations 1 and 2
4. Prepare a table as shown below & write in the appropriate figures for each reactant used and possible organic
product(s)
FW (g/mol)
Density (g/mL)
Mass or Volume (g or mL)
Number of moles
1-hexene
H2SO4 (85%)
H2O
2-hexanol/1-hexanol
84.16
98.07
18.02
102.17
0.678
1.78
1.00
0.81/0.814
10 mL
10 mL
30 mL
8.23 g*
0.0806
0.154
1.66
0.0806*
___________________________________________________________
* Theoretical quantities (calculated)
5. Provide the references from which you obtained the information in the table
6. Determine the limiting reagents (LR) for Eq. 1 and Eq. 2
7. Determine the theoretical yield for final organic product(s) from Eq. 2
Note: For this experiment, the information in the table is provided for you but for future experiments you must
provide the information!
Additional Notes
To calculate # of moles of H2SO4 used in this experiment (10 mL 85%w/w) you need to know the density of
85% (w/w) which is 1.78 g/mL (CRC Handbook of Chemistry & Physics)
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