Outline a mechanism for the dehydration of 2-methylcyclohexanol catalyzed by phosphoric acid. Predict and rationalize the expected product distribution
Outline a mechanism for the dehydration of 2-methylcyclohexanol catalyzed by phosphoric acid. Predict and rationalize the expected product distribution
Outline a mechanism for the dehydration of 2-methylcyclohexanol catalyzed by phosphoric acid. Predict and rationalize the expected product distribution
Outline a mechanism for the dehydration of 2-methylcyclohexanol catalyzed by phosphoric acid.
Predict and rationalize the expected product distribution based on thermodynamic aspect
can it be written in summary
Transcribed Image Text:Introduction
Alcohol dehydration is an acid-catalyzed reaction performed by strong, concentrated mineral acids
such as sulfuric acid and phosphoric acid. The acids protonate the alcoholic hydroxyl group,
permitting it to dissociate as water. Loss of a proton from the intermediate (elimination) yields an
alkene. Because concentrated sulfuric acid often causes extensive charring in this reaction,
concentrated phosphoric acid, which is comparatively free of this problem, is a better choice.
To make the reaction proceed faster, however, you will also use a minimal amount of concentrated
sulfuric acid.
ОН
H;PO4
A
Figure 1: Dehydration of 2-Methylcyclohexanol
The equilibrium constant for this reaction is not very high (~5-10). However, the equilibrium that
attends this reaction will be shifted in favor of the product, the cyclohexenes, by distilling it from
the reaction mixture as it is formed. The cyclohexenes will co-distill with the water that is also
formed. By continuously removing the product, one can obtain a high yield of the cyclohexenes
(>85 %). Since the starting material, 2-methylcyclohexanol is also a relatively moderate boiling
point (b.p.: 160 °C), the distillation must be done carefully, not allowing the temperature to rise
much above 100 °C. Unavoidably, some phosphoric acid co-distills with the products. It is
removed by washing the distillate mixture with aqueous bicarbonate. To remove the water that
co-distills with cyclohexene, and any traces of water introduced in the base extraction, the product
will be dried over anhydrous sodium sulfate. The typical yield for the reaction is 85 %.
Compounds containing double bonds react with a bromine solution (brown) to decolorize it.
Similarly, they react with a solution of potassium permanganate (purple) to discharge its color and
produce a brown precipitate (MnO2).
Br
Br2
KMNO4
+ MnO2
brown
purple
Br
НО
ОН
brown
colorless
colorless
Figure 2: Reaction of Alkene with Bromine and Permanganate
These reactions are often used as qualitative tests to determine the presence of a double bond in
an organic molecule. Both tests will be performed on the cyclohexene formed in this experiment.
1
Transcribed Image Text:Procedure
Place 1.50 mL of 2-methylcyclohexanol (d=0.93 g/mL) in a 5-mL conical vial, which is equipped
with a magnetic spin vane. Some students will use 85 % (m/m) phosphoric acid as a catalyst, and
some students will use sulfonic resin, in place of phosphoric acid. If you are using phosphoric acid
as a catalyst, add 0.50 mL of the 85 % phosphoric acid to the conical vial with the use of
(d = 1.685 g/mL). If you are using a sulfonic resin, add 0.200 g (about 30 beads) of Nafion (trade
name of Du Pont) NR50 resin (1.6 * 103 equivalent of sulfonic acid group/g of resin).
Set up a fractional distillation apparatus (as shown below) Replace the round-bottom flask with a
5 mL conical vial and make sure that the thermometer is placed in the correct position of the still
head. Use a 3-mL conical vial as a receiver vial immersed to its neck in an ice-water bath to
minimize the possibility of product vapors escaping into the laboratory. Start heating slowly, heat
the mixture until the product begins to distill. Rapid distillation leads to incomplete reaction and
isolation of the starting material. Continue the distillation until few drops of liquid remain in the
distilling flask. The distillate contains I-methylcyclohexene, 3-methylcyclohexene, methylene-
cyclohexane as well as water, and traces of phosphoric acid (if you used phosphoric acid).
distilling
head-air
.condensor
receiver
vial
fractionating
column packed
with copper metal
ice-water
bath
sponge
sand,
electric flask
heater
boiling chip
Figure 3: Fractionated Distillation Microscale
Saturate the distillate with solid sodium chloride. Add salt, little by little, and shake the flask
gently. When no more salt will dissolve, add enough 10 % aqueous sodium carbonate solution to
make the distillate basic to litmus. Pour the neutralized mixture into a clean conical vial and
separate the two layers. Add enough anhydrous sodium sulfate to the flask and swirl occasionally
until the solution is dry and clear.
Collect the liquid into a clean, dry pre-weight vial. Weigh the product and calculate its percentage
yield. After performing unsaturation tests, analyze it by GC, and IR spectroscopy (see data set).
Science that deals with the amount of energy transferred from one equilibrium state to another equilibrium state.
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