Download Dehydration of Cyclohexanol – Preparation of an Alkene

Dehydration of Cyclohexanol – Preparation of an Alkene
CHM 220
INTRODUCTION
Earlier this semester you reacted an alcohol (n-butanol) with a Hydro-halide acid (Hydro-bromic Acid) to
generate an alkyl halide. This reaction occurred through the SN2 pathway. In this experiment you will
react an alcohol (cyclohexanol) utilizing an acid catalyst (sulfuric acid) to “dehydrate” the alcohol and
form an alkene (cyclohexene).
Alcohols are frequently converted into the desired alkene using an acid catalyzed Elimination reaction.
The term “dehydrate” means to remove water and is used to identify the nature of the atoms/molecules
eliminated to form the double bond of the alkene in our product. Primary (1O) alcohols require strong
acids and significant amounts of heat (up to 180O C or more). Secondary alcohols (2O) are somewhat
easier to dehydrate requiring slightly lower temperatures, where Tertiary (3O) alcohols will undergo
dehydration at room temperature or only slightly above that.
The dehydration of cyclohexanol follows the E1 mechanistic pathway. The dehydration reaction
involved three steps. First to occur is the protonation of the alcohol by the acid, in the process the
Hydroxyl- group is converted from a poor leaving group to a good leaving group. This then result is the
formation of a 2O carbocation intermediate in the second step. Finally, we generate an alkene following
the loss of a proton adjacent to the carbocation.
OH2
OH
+
+ H2SO4
HSO4
OH2
+
+
HSO4
H2O
+
H2SO4
H
In each case these reactions are reversible and are under conditions of equilibrium. Since each step is in
equilibrium we must make use of Le Chatelier’s principle to drive the reaction in the desired direction.
The simplest method of doing this is to remove the products of the reaction as they are formed. In this
experiment you will be distilling the product from the reaction mixture as it is formed.
Dehydration of Cyclohexanol – Preparation of an Alkene
CHM 220
PROCEDURE: (take care to record any necessary data for your lab report)
1) To a 50mL round bottom flask add 10mL of water and then slowly add 10mL of concentrated
sulfuric acid. Cool the mixture to room temperature, and add a few boiling chips to the cooled
solution.
2) Prepare a distillation apparatus using a claisen adapter, three-way adapter, distillation
condenser, thermometer adapter, thermometer(as shown below). Arrange the glassware on
the wire grid in the hood so that you can distill the product into a 25mL graduated cylinder.
Water out
Water in
100 mL
Ice Water
3) Open the system, via the stopper, and add 10 mL of cyclohexanol using a funnel. Close the
system and begin heating with a heating mantle. Heat the solution until it begins to boil and
then proceed to distill approximately 10mL of the crude cyclohexene product into the graduated
cylinder.
4) In a separatory funnel, wash the crude cyclohexene with two 10mL portions of a 10% sodium
bicarbonate solution to remove any excess acid. Remember to vent the funnel often. Discard
the aqueous washes.
5) Wash the remaining organic layer (cyclohexene) with one portion of saturated salt solution, and
then one portion of R.O. water. Again discarding the aqueous layers.
6) Transfer the cyclohexene to a beaker or flask and add a small amount of calcium chloride to dry
(remove any excess water) the solution.
7) If the product is NOT clear, centrifuge to complete the drying process.
8) Decant the clear product to a tared weighing bottle to determine your yield (and percent yield).
Dehydration of Cyclohexanol – Preparation of an Alkene
CHM 220
Procedure: continued
9) Determine the refractive index of your product.
10) Take a small amount of your product and fill a sample vial and speak with your instructor on
how to determine the purity of your product via Gas Chromatography (see G.C. Introduction on
the CHM 220 lab website).
11) Obtain an IR spectrum of your product.
12) Test the remaining amount of your product using Br2 in hexane, and record your observations
(refer to the Addition lab for more information).
Questions to consider in your lab report:
1) When a straight chain 2O alcohol is dehydrated you will often obtain a mixture of products, see
example below. Why do we only obtain a single product from cyclohexanol instead of a
mixture?
OH
H2SO4
major
heat
+
minor
minor
2) What would happen if you allowed the cyclohexene to stand in contact with an aqueous sulfuric
acid solution?
3) Which compound has a higher boiling point, cyclohexanol or cyclohexene?