Dehydration of Alcohol, Chemistry tutorial


There are 4 basic kinds of chemical reactions in organic chemistry: combination, elimination, substitution, and rearrangement. The dehydration of alcohols to provide alkenes is a significant conversion and is an instance of elimination reaction. Strong mineral acids thee as sulfuric and phosphoric acid catalyze the reaction. 

Dehydration of an alcohol can chase either the E2 or the E1 mechanism. Though, in each case, acid is needed as a catalyst, since OH- is a strong base, it is a reduced leaving group, but HOH is a weaker base, and a better leaving group. Adding a strong acid, such as H2SO4, to the mixture allows the protonation of the -OH group to give water as a leaving group. Once this protonation occurs, the mechanism that is followed based on the nature of the R group. As mentioned above, 1-pentanol (a 1° alcohol), dissociation of water would produce the very unstable 1° carbocation, so we would project that elimination via the E1 mechanism (with carbocation intermediate) will not take place. As a consequence, reaction would be supposed to proceed via the E2 elimination mechanism. Nevertheless, for 2-pentanol, dissociation of water generates the more stable 2° carbocation. Because water isn't an extremely strong base, the competing E2 mechanism will be slow, which will permit the E1 mechanism to proceed faster for 2-pentanol. 

The mechanism below depicts reaction by E2 mechanism to product, in a single, concerted step, elimination, producing an alkene. The only product, via an E2 reaction mechanism, would be 1-pentene. 

E1 mechanism for 2- Propanol 

Step 1: An acid/base reaction. Protonation of the alcoholic oxygen to build a better leaving group.  This step is extremely fast and reversible. The lone pairs on the oxygen make it a Lewis base.  

Step 2: Cleavage of the C-O bond permits the loss of the good leaving group, a neutral water molecule, to provide a carbocation intermediate. This is the rate determining step (bond breaking is endothermic).

Step 3: An acid/base reaction. Deprotonation via a base (a water molecule) from a C atom adjacent to the carbocation center leads to the creation of the C=C 

Reagents and Chemicals

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The dehydration reaction will be illustrated by the conversion of cyclohexanol to cyclohexene. The choice of cyclohexanol as starting material is based on the subsequent considerations:

a)  Because of its structure, cyclohexene can give only one alkene upon dehydration, normally cyclohexene.

b)  The rate of dehydration of cyclohexanol using 85% phosphoric acid is conveniently fast.

c)  The product is easily purified through distillation at an eagerly accessible temperature, (83oC).

d)  When heated by strong acids catalysts (most commonly H2SO4, H3PO4), alcohols typically undergo a 1, 2-elimination reactions to generate an alkene and water. Also known as dehydration since it involves the elimination of a molecule of water. Alcohol relative reactivity order: 3o > 2o > 1o

e) Regioselectivity: major product is generally the more highly substituted alkene (alkene stability)

Zaitsev's Rule.

f) Stereoselectivity : trans Æ cis- again controlled by stability

f) Reaction usually proceeds via an E1mechanism that proceeds via a carbocation intermediate that can often undergo rearrangement.

g) Primary alcohols will proceed via an E2 mechanism since the primary carbocation is extremely unfavorable.

h) Other ordinary strong acids such as HCl, HBr or HI are less appropriate catalysts as nucleophilic substitution reactions will probably interfere.

Experimental procedure

Safety note 

Caution: Cyclohexanol is a volatile and flammable liquid and is an irritant. No flames will be permitted in the lab. Wear gloves while handling such chemicals. Concentrated phosphoric acid (or sulphuric acid) is muscularly corrosive and toxic wear gloves while handling it, and be sure to wash our gloves and our hands instantly after handling it. Sodium sulfate is irritant gloves are advised. 

Procedure: Set up an easy distillation as given below. Add 8.00 ml (D = 0.96 g/ml) of cyclohexanol and 2 ml of concentrated sulfuric acid (or 5 ml of concentrated phosphoric acid) to a 50 -ml round-bottomed flask. Mix the content thoroughly via swirling before joining the flask to the distillation setup. Add 2 boiling stones, and heat the flask gently so that the temperature of the distilling vapor doesn't exceed 1000C. Continue the distillation until only a few milliliters (< 2 ml) of high-boiling remainder remain in the flask. If white fumes show near the end of the distillation, stop heating a once via lowering the heating mantle. (As we know that these fumes are oxides of sulphur, SO2, if sulphuric acid is being utilized).

802_simple distillation.jpg

Fig: Simple distillation

As we know that the distillate in the receiver consists of 2 layers. Transfer the distillate to a small separatory funnel and add 2 ml of saturated sodium chloride solution (to diminish the solubility of cyclohexene in the water layer), then add drop-by-drop 2 ml of 10% sodium bicarbonate solution (to neutralize the traces of any remaining unreacted acid). Swirl or shake the mixture gently. Permit the layers to divide, and then draw off and remove the lower layer (aqueous layer). Pour the upper layer (organic layer - crude cyclohexene) out the top of the separatory funnel into a small, dry 50- ml Erlenmeyer flask. Adjoin half a teaspoon of anhydrous calcium chloride (utilized to dry, eliminate, traces of water) to the cyclohexene and permit it to stand for 10-15 min, swirling it rarely. The product should be clear, not cloudy.

The product analysis

I) Baeyer (cold kmno4) test - To make sure the product is alkene, test our product by potassium permanganate solution that is a test for the existence of double bond in compound Potassium permanganate, a purple solution loses colour with alkenes and forms manganese dioxide, a brown precipitate. Situate 5-6 drops of our alkene product in a tiny test tube and add 1-2 drops of KMnO4 solution. Swirl the tube to combine the reagents and depart it for observations. Record our observations. 

II) Bromination test - Set 5-10 drops of our alkene product in a tiny test tubes and test through drop- wise bromine (decolouration) for observations. Record our observations.

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