Part 8: Core Practical 6: preparation, purification and yield

Follow 2-chloro-2-methylpropane through its preparation and work-up, explain what each purification step removes, and calculate a defensible isolated yield.

A tertiary alcohol gives a tertiary chloroalkane

Core Practical 6 reacts 2-methylpropan-2-ol with concentrated hydrochloric acid to prepare 2-chloro-2-methylpropane. The OH group is replaced by Cl, leaving the four-carbon skeleton unchanged. Water is the other product. This is substitution, even though the practical title uses the word chlorination.

The tertiary alcohol reacts readily because the pathway can involve a relatively stable tertiary carbocation after acid assists departure of water. This explanation connects to carbocation stability, but detailed alcohol-substitution mechanisms are not required by 6.38. The practical priority is preparing, separating, drying and testing the intended product.

Work in a fume cupboard with eye protection and suitable gloves: concentrated HCl is corrosive and gives harmful fumes, and the organic reactant and product are flammable. Use the laboratory’s supervised quantities and risk assessment. The following account explains the purpose and order of the operations.

Mix, allow reaction and release pressure

The Pearson worksheet combines the alcohol and concentrated HCl in a conical flask, gently mixes over the specified reaction time and repeatedly releases pressure. Two liquid layers form; in this particular preparation the crude organic product is the upper layer. Do not heat a sealed vessel or allow pressure to accumulate.

After about 20 minutes of controlled mixing, the worksheet adds calcium chloride before transfer to the separating funnel to help retain unreacted alcohol in the lower aqueous phase. This is distinct from the later addition of anhydrous sodium sulfate to the isolated organic phase for drying. Simply calling every salt addition “drying” misses what it is doing at that stage.

Alcohol can hydrogen-bond with water; the chloroalkane cannot make equivalent strong hydrogen bonds and has much lower water solubility. The product also lacks the O–H intermolecular hydrogen bonding of the alcohol, helping explain its lower boiling temperature. Use the known density/layer information for this preparation; an organic layer is not always the upper layer in every extraction.

Use partition between two immiscible phases

Transfer to a separating funnel, allow the phases to settle and remove the lower aqueous layer. A separating funnel separates immiscible liquid phases; it does not separate two liquids that dissolve completely in one another. Where layer identity is uncertain, use reliable density information or a small water-addition test: the aqueous layer increases in volume.

For washing, gently mix the organic product with the chosen aqueous solution so water-soluble impurities can transfer into the aqueous phase. Vent as directed, pointing the outlet away from people. Let the phases separate again. Remove the stopper when draining through the tap so air can enter; otherwise flow may be irregular or stop.

Drain the lower phase into a labelled container and retain both fractions until the product-containing layer has been identified confidently. Avoid carrying an aqueous droplet trapped in the tap into the organic collection. Dry glassware for the subsequent organic stage prevents adding back the water you are trying to remove.

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Neutralisation and drying remove different impurities

Wash the retained organic layer with aqueous sodium hydrogencarbonate to remove residual HCl. Acid–hydrogencarbonate reaction produces CO₂, explaining the effervescence and pressure build-up. Add and mix in controlled portions, vent frequently and repeat the wash as directed. A drying agent is not a replacement for neutralising residual acid.

After separating off the wash water, transfer the organic product into a clean dry flask and add anhydrous sodium sulfate. It binds residual water as hydrated salt. Swirl and allow enough time; the liquid becomes clear and fresh drying agent remains free-moving when sufficient water has been removed. Decant the liquid away from the solid, or filter appropriately, before distillation.

A suitable drying agent must remove water without reacting with the wanted product, and it must be separable. Adding water to dissolve the drying agent defeats the drying step. Excessive solid can also retain some product, contributing to a lower isolated yield.

Collect a boiling range, not every liquid that arrives

Assemble distillation apparatus with anti-bumping granules added before heating. Use an appropriate water bath or controlled electric heat for the volatile, flammable product. Keep an open path to the receiver and place the thermometer bulb in the vapour at the entrance to the condenser sidearm; this measures the temperature of vapour being collected, not the flask liquid or cooling water.

Water enters the lower end of the condenser jacket and leaves at the upper end. Vapour condenses and runs into the receiver. Pearson’s CP6 sheet specifies collection of the fraction boiling between 50 and 52 °C; a centre method may specify a slightly broader range. Record the actual range and use the pressure/reference information supplied, rather than treating a memorised value as universal.

A narrow boiling range close to a reliable value supports purity, but does not prove identity by itself. Atmospheric pressure affects boiling temperature; some different substances have similar values and mixtures can behave unexpectedly. Combine the boiling range with a suitable chemical test and/or spectroscopy. Keep the sample stoppered appropriately once cool to reduce evaporation losses.

Show that chlorine is covalently bonded

To test a chloroalkane chemically, a separate small sample can be hydrolysed with aqueous hydroxide, then acidified with dilute nitric acid before silver nitrate is added. Hydrolysis releases Cl⁻; Ag⁺ gives white AgCl. The acid removes excess hydroxide that could otherwise produce a silver-containing precipitate. Nitric acid avoids introducing chloride from HCl.

Residual hydrochloric acid would itself give chloride, causing a misleading positive result. That is why correct washing and use of a separate sample matter. The test, suitable boiling range and absence of an alcohol O–H absorption together provide stronger evidence than any one observation.

Worked limiting-reagent calculation and atom economy

Illustrative data: 7.40 g of pure 2-methylpropan-2-ol, M = 74.0 g mol⁻¹, reacts with excess HCl. n(alcohol) = 7.40/74.0 = 0.100 mol. The 1:1 equation gives 0.100 mol 2-chloro-2-methylpropane. With M = 92.5 g mol⁻¹, theoretical product mass = 0.100 × 92.5 = 9.25 g.

If the dry purified product mass is 6.66 g, percentage isolated yield = 6.66/9.25 × 100 = 72.0%. Use the starting alcohol amount only because HCl was stated to be in excess; if both amounts are supplied, compare moles divided by their equation coefficients to find the limiting reagent.

Atom economy for the intended equation = 92.5/(74.0 + 36.5) × 100 = 83.7%. The remaining atoms form water. This theoretical value is independent of the measured 72.0% yield, and does not count extra solvent, washing agents or excess HCl. Full resource use needs additional process measures.

When obtaining a transferred mass by weighing before and after, there are two balance readings. For ±0.01 g per reading, a conservative absolute uncertainty of ±0.02 g gives 0.02/7.40 × 100 = 0.270% in the starting mass. This is a measurement contribution, separate from chemical or transfer losses. Do not describe a low yield as “within uncertainty” without comparing its magnitude.

Explain how an error changes the result

Incomplete conversion and competing reactions lower chemical yield. Product left on glassware, retained by drying agent, dissolved in a discarded wash, lost by evaporation or excluded by an overly narrow collection range lowers isolated yield. Reduce unnecessary transfers, use appropriate closed storage after collection and optimise collection consistently with purity.

Residual water, HCl or unreacted alcohol can increase the apparent product mass, even producing an apparent yield above 100%. A wide collection range may increase collected mass while lowering purity. More material collected is not automatically a better preparation: explain the trade-off between recovery and contamination.

Reading the method and answering written questions establish practical understanding, not observed competence for the A-Level practical endorsement. Actual safe assembly, manipulation, recording and evaluation must be demonstrated in supervised practical work.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources