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Module 1: Practical chemistry for exams›PAG 5.1 - Synthesis of a haloalkane

PAG 5.1 - Synthesis of a haloalkane

An overview of PAG 5.1 - Synthesis of a haloalkane

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In OCR Chemistry Required Practical 5 (PAG 5.1), a haloalkane is prepared.
2-methylpropan-2-ol reacts with concentrated hydrochloric acid, in a nucleophilic substitution reaction.

The alcohol group is substituted with a chloride group to produce 2-chloro-2-methylpropane.

The reaction is summarised by the equation:

The experiment involves procedures for the purification of an organic liquid.

In OCR PAG 5.1: Synthesis of a haloalkane, there are several important safety requirements to be aware of (beyond normal lab practice such as wearing goggles).

2-methyl-propan-2-ol is highly flammable – keep away from naked flames.
Silver nitrate can stain skin and surfaces – take care when handling.
Measuring and addition of concentrated hydrochloric acid should be carried out in a fume cupboard.

Experimental method for OCR Chemistry Required Practical 5 (PAG 5.1)

Part 1 – The synthesis of 2-chloro-2-methylpropane.

EXPERIMENTAL METHOD OCR PAG 4.2. 1 Prepare a tests and observations table for the exercise. Give detail of the tests carried out and space for observations. Test A B C D. Dilute nitric acid. Barium chloride (aq). Warm dilute NaOH. Test gas with pH paper. 2 Test for carbonates with dilute nitric acid. Pour 1 cm³ of solutions A, B, C, and D into separate test tubes. Label test tubes. Add a few drops of dilute nitric acid to each test tube in turn and observe for effervescence. Record observations in a results table, and confirm the identity of the carbonate. 3 Test remaining solutions for sulfates with barium chloride solution. Add a few drops of barium chloride solution to each of the remaining test tubes after test 1. Look for a white precipitate. Record observations and confirm the identity of the sulfate. 4 Test the remaining two solutions for ammonium ions with warm NaOH(aq). Pour a fresh 1 cm³ of the remaining two solutions into separate test tubes. Add 0.5 cm³ of NaOH(aq) and gently warm the tubes over a low blue Bunsen flame. Place a piece of damp indicator paper in the mouth of the tube and look for a colour change. Record the observations and confirm the ammonium compound. 5 Confirm the identity of NaCl. Add 1 cm³ of the suspected NaCl solution to a test tube and add an equal volume of dilute nitric acid and a few drops of silver nitrate solution. A white precipitate of silver chloride confirms the identity. This will dissolve in dilute ammonia.

Experimental method for OCR Chemistry Required Practical 5 (PAG 5.1)

Part 2 – The purification of 2-chloro-2-methylpropan

EXPERIMENTAL METHOD OCR PAG 5.1 - PART 2. 1. Remove the lower aqueous layer. Remove the tap funnel stopper and place a small beaker under the funnel. Carefully open the tap and slowly allow the lower layer of unreacted acid and water to run into the beaker. Stop the flow of liquid when the interface is just in the tap. Discard the lower layer, which is mainly unreacted acid. 2. Neutralise and remove water-soluble impurities. Wash with two 5 cm³ portions of 5% sodium hydrogen carbonate solution (to remove unreacted acid) and two 5 cm³ portions of distilled water (to remove any unreacted alcohol). Add the carbonate/water, shake the funnel and vent to allow the removal of any gas produced. Allow the two layers to separate and run off the lower layer to waste each time. Finally, run off the product into a small conical flask. 3. Dry the liquid product. Add a spatula of anhydrous sodium sulfate powder, and swirl the flask. Continue adding until no clumping is apparent. The product liquid will appear cloudy due to dampness. Swirling with the drying agent will remove this moisture, and the liquid will become transparent. 4. Collect the pure product by distillation. Add the dried liquid product to the distillation flask and arrange the water connections and clamps for distillation. Set a receiving flask to collect the distillate. Heat gently using a water bath and collect the pure, dry product. If a pure product has been obtained, the 2-chloro-2-methylpropane should distil over a narrow range at 51 °C. Weigh the product collected to confirm yield.

Vapour can be produced when volatile substances are mixed in a separating funnel.

A laboratory setup showing a stopcock that is opened to release the pressure, with steam coming out and a flask containing a blue liquid.

Do

Vent the separating funnel periodically. This allows the release of any gas produced.

A laboratory apparatus with a blue liquid and steam coming out of a flask, held by a clamp.

Don't

Shake the separating funnel with the stopper in place without venting – pressure can build up and cause the stopper to fly off.

The distillation process requires careful setup to avoid loss of product or accidents.

An illustration of a distillation apparatus showing a thermometer, water out, receiver, water in, anti-bumping granules, and distillate. The flask is labeled to heat gently.

Do

Use anti-bumping granules and heat the mixture gently during distillation.

A diagram illustrating a distillation setup. It includes a thermometer, a flask labeled 'Heat vigorously', a receiver, and arrows indicating 'Water in' and 'Water out'. The flask contains blue liquid, and the receiver is labeled 'Distillate'.

Don't

Allow the mixture to boil violently or heat it without anti-bumping granules – this can cause dangerous splashing or bumping.

Use the balanced equation for the preparation to identify the limiting reagent and calculate the theoretical amount and mass of haloalkane, using:

Calculate percentage yield using:

A yield below 100% is expected because product is lost during transfers, separation, and purification.