Preparation of a Halogenoalkane

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Experiment 2
Title: preparation of a halogenoalkane

Objective: To synthesis 2-cholo-2-methylpropane by using corresponding alcohol and HCl(aq). The reaction is via SN1 of tertiary alcohol.

Techniques: i) Distillation of the reactant and the product
ii) Purification by separating funnel

Theory: In the experiment, SN1 reaction take place, HCl is added to initiate the reaction. The R-OH attack the H+ with the lone pair on O. Then the R-O+H2 bond breaks the release a water molecule and a carbonium ion.

The Cl- will then attack the carbonium ion forming a 2-cholo-2-methylpropane. Since Cl lack ability of forming hydrogen bond, this compound cannot dissolve in water, hence forming a 2 layer liquid.

Then add NaHCO3, it is used to clean the excess HCl in organic layer. Later, NaSO4 is added to remove the small amount of water in organic layer. Distillation is then carried to purify the product.

Separating funnel
Electronic balance
Test tube
Distillation quick fit
Filter funnel
Filter paper
Bunsen burner
Insulating mat
Wire gauze

Sodium hydrogencarbonate
Sodium sulphate
Anti-bumping granules

Set up

Procedure: 1. 10 cm3 of 2-methylpropane-2-ol was added into a measuring cylinder. The total mass was measured and recorded 2. The alcohol was poured in a separating funnel the mass of measuring cylinder was recorded. 25cm3 of concentrated HCl was added into the funnel in 10 portions. After each addition, the funnel was closed and inverted several times. The switch was opened for releasing pressure. 3. The reaction mixture was left for half an hour.

4. A sample distillation set-up was set up and a small flask was weighed for collecting the distillate. 5. The organic layer was collected from the separating funnel. 6. Some sodium hydrogencarbonate was added 2cm3 at a time until no pressure can be released. 7. Again, the organic layer was collected. The organic layer was dried by using anhydrous sodium sulphate. All the aqueous layer was discarded. 8. The dried organic liquid were filtered into the pear-shaped flask set up for distillation. 9. Anti-bumping granules was added into the pear-shaped flask. The distillation fraction was collected(45oC-55oC) into the pre-weighed conical flask. It was heated gently with water bath and was kept coming out at rate of 1-2 drops per second. 10. The mass of the conical flask was measured with the produce 2-chloro-2methylpropane.

mass of cylinder + 2-methylpropan-2-ol39.343g
mass of empty cylinder31.609g
mass of 2-methylpropan-2-ol7.734g
mass of collecting flask96.016g
mass of collecting flask + 2-chloro-2-methylpropane97.312g mass of 2-chloro-2-methylpropane1.296g

1.No. of mole of 2-methylpropan-2-ol used

No. of mole of 2-chloro-2-methylpropane yield
percentage yield of 2-chloro-2-methylpropane
=(actual yield/theoretical yield)*100%
2.The addition of NaHCO3 was to remove the HCl molecule dissolved in the organic layer. The HCl react with NaHCO3 and give NaCl, CO2 and H2O as product. The NaCl and H2O will migrate to the aqueous layer and CO2 gas will leave mixture. The reaction is as follow: HCl(aq)+NaHCO3(aq)NaCl(ag)+H2O(l)+CO2(g)

3.The upper layer is collected in both step 5, 7. Water molecule has a density of 1g/cm3 and 2-chloro-2-methylpropane has a density of 0.84g/cm3. Since the organic layer is of lower density, so the upper part is collected in both steps. 4.Addition reaction of 2-methylpropene with HCl:

halogenation of PCl3 with 2-methylpropan-2-ol:


6.No, it is not possible. Since Cl- is a weak base, it will hence favor SN1 reaction. But in order to form the 1-chloro-2-methylpropane, we need to use a 2-methylpropan-1-ol, which is a primary alcohol. A primary alcohol does not...
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