Dehydration reactions are a subset of elimination reactions that yield an alkene and water. The hydroxyl group (–OH) is not a poor leaving group, having an acid catalyst may help by protonating the hydroxyl group to give the better leaving group, –OH2+. The reverse of a dehydration reaction is a hydration reaction. Common dehydrating agents are used in organic synthesis includes concentrated sulfuric acid, concentrated phosphoric acid, hot aluminium oxide and hot ceramic. The reaction is used to produce ether from alcohol.
The rate of dehydration of different alcohols is below:
The rate of carbocation formation is
determined by the relative stability of the carbocation. Because the order of
carbocation stability is 3o>2o>1o . So
the carbocation forms faster the rate of dehydration would raise.
Mechanism
Chemical
1. Anhydrous calcium chloride:~0.5g
2. Cyclohexanol: 5g
A bottle of cyclohexanol
3. 85% phosphoric acid: 1.25mL
4. Saturated NaCl solution: 2.5mL
5. Concentrated sulfuric acid: 0.5mL
Procedure
1.
Place 5.0g of cyclohexanol in a
round-bottom flask. Add 1.25mL 85% phosphoric acid and concentrated sulfuric
acid mixture into the flask.
The mixture is colorless and sticky at the beginning
2.
Put a stir bar into the flask,
and distillate the mixture. And put the receiver in ice bath to prevent the
produce loses.
Cyclohexene starts evaporating
Put the receiver in ice bath
3.
Distillate the mixture slowly
so that it takes about 45 minutes.
Crude cyclohexene is produced
The crude product containing water so make it turbite
Some Phospholipid formed which makes the mixture looks green
4.
Transfer the distillate in to a
separatory funnel and use 2.5mL saturated NaCl solution to extract the mixture
and discard the aqueous layer.
Upper layer is crude cyclohexene
5.
Drain the cyclohexene into a
flask and add some anhydrous calcium chloride to absorb water and some residual
cyclohexanol from cyclohexene.
Place anhydrous calcium chloride into cyclohexene than it turns clean
6.
Distillate the dried
cyclohexene and determine the refraction rate of the product.
Experimental Record
|
Theory Weight of Cyclohexene
|
3.910g
|
|
Weight of Cyclohexene
|
2.910g
|
|
Yield
|
74.4%
|
|
Theory nD20
|
1.4462
|
|
nD20 of product
|
1.4458
|
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