Download DEHYDRATION OF 2-METHYLCYCLOHEXANOL

Chem 323
B. Terem
DEHYDRATION OF 2-METHYLCYCLOHEXANOL

CH3
CH3
OH
bp. 165-168oC
H3PO4
CH3
+
+
bp. 110oC
CH2
bp. 104oC
bp. 102oC
trace
Alkenes can be easily prepared from alcohols by acid-catalyzed dehydration (i.e. elimination of
water, H+ and OH-, to form a double bond).1 When the alcohol is unsymmetrically substituted, as
in the present case, a mixture of isomeric alkenes usually results, in which the most stable one
usually predominates2. For this reason other (generally more difficult and expensive) synthetic
methods are used, whenever a single, pure isomer is desired.
Almost any strong Bronsted acid will catalyze elimination of water from secondary and tertiary
alcohols, as will many Lewis acids, such as BF3 and I2. Sulfuric acid is too strong of an acid and
will likely isomerizes the alkene products (via re-protonation of the C=C bond); for this reason a
weaker proton donor such as phosphoric acid is used.
In the present experiment 2-methylcyclohexanol is dehydrated to a mixture of alkenes. After the
removal of co-distilled water and traces of acid, the overall yield is calculated. It is possible to
determine the ratios of the isomeric products through further analysis, such as gas
chromatography.
Experimental Procedure:
To a 50 or 100 mL round bottomed flask add 8.0 mL of 2-methylcyclohexanol, 3 mL of
phosphoric acid, and a few boiling chips. Clamp the flask in a heating mantle and assemble the
appropriate glassware for fractional distillation (see PLK Fig. 7.2, p 567). Don’t forget to
lubricate the joints. Use “Keck” clips between the condenser and the distillation head, as well as
between the condenser and the bent adaptor. Heat the flask slowly and evenly in the heating
mantle until the product distills out. The boiling point of the distillate should be kept below 96oC
by regulating the rate of heating3. The efficiency of fractional distillation cannot be maintained if
the reaction flask is heated intermittently or unevenly. Continue distilling until 6-8 mL of liquid
have been collected, but stop before the reaction vessel is dry.
Check the collecting flask to see if you have a two-phase distillate. (Alkenes have lower densities
than water; therefore, the lower layer is water). If the amount of distillate is very small, remove
the water with a Pasteur pipette. Transfer the (two-phase) distillate to a small (125 mL) separatory
funnel and remove the water layer (if you have not already done so). Wash the upper organic
layer with about 3 mL of 10% sodium carbonate. Drain the aqueous sodium carbonate (lower
layer). Wash the remaining organic layer with 5 mL of saturated brine (NaCl solution). Drain the
brine (lower layer). Transfer the organic liquid in the separatory funnel into a preweighed vial
Chem 323
B. Terem
(make sure that you record the actual weight of the vial) using a Pasteur pipette (Transferring a
liquid from a separatory funnel using a pipette is somewhat unusual; however, it works better in
this case with the small quantities being handled). Determine the weight of the product in the vial.
Add the appropriate drying agent into the vial. Label your vial and keep it in your locker. It is
best to place the vial in a small beaker.
Notes:
1
See Smith, Sect 9.8; 2See Smith, Sect.8.5 p. 291-3
If the head temperature exceeds 96oC, the distillate may contain measurable amounts of
2-methylcyclohexanol, which will invalidate the yield calculations.
3
Questions:
If possible, answer these questions before leaving the lab (you might start thinking about them
while the experiment is going); discuss your answers with the instructor.
1.
2.
3.
The 2-methylcyclohexanol used in this experiment is actually a mixture of two
stereoisomers. Draw their structures (in a way they can be distinguished from each
other) and name them.
Write a step-wise mechanism for the phosphoric acid catalyzed dehydration of
2-methylcyclohexanol, showing how both major and minor products can be formed.
Show how a tertiary carbocation can be formed by the reprotonation of the major
reaction product, and show how this can lead to methylenecyclohexane. How might
this acid-catalyzed isomerization be minimized experimentally (already mentioned in
the hand-out)