PAG6: Organic Solids, Purity and Chromatography

A preparation first forms an organic product using the teacher-selected reaction, then separates and purifies it. The particular approved solid is an example of the required skill, rather than a universal requirement to use a single synthesis. Use the provided stoichiometry to identify the limiting reagent and theoretical product amount. Impurities may include starting material, by-products and solvent.

Recrystallisation relies on the product being appreciably soluble in a suitable hot solvent and much less soluble when cold. A good solvent also has a useful behaviour for impurities and does not react with the product. Add the minimum sufficient hot solvent specified by the method. Excess solvent leaves more product dissolved at the final temperature, lowering recovery.

If insoluble impurity is present, an appropriate hot filtration removes it before crystals form; the apparatus and method should limit premature crystallisation. Allow the filtrate to cool so the product crystallises, then use the specified further cooling. Collect crystals by suction filtration using sound apparatus designed for reduced pressure. Wash with a small amount of cold solvent to remove surface solution while limiting product dissolution.

Dry the product by the approved method before weighing or measuring a melting range. A wet crystal mass is not a valid isolated product yield. Handle hot solvent, flammable vapours and vacuum glassware according to the laboratory procedure. A higher crude mass can mean more retained impurity rather than better synthesis.

Report a melting range, from first visible melting to complete melting, with a controlled heating rate near the expected value. An impure solid often melts over a broader range at a depressed temperature, but agreement with a literature value is supporting evidence rather than absolute proof. Sample packing, residual solvent, thermometer calibration and fast heating can distort the range.

Calculate percentage yield as dry recovered product divided by theoretical product, using matching units. Distinguish reaction yield from recrystallisation recovery: recovery compares purified mass with the crude mass processed, which itself may contain impurities. Evaluate whether a lower recovered mass reflects product left in solution or successful removal of unwanted material.

TLC uses a thin stationary phase, commonly silica, on a plate and a moving solvent. Different components move different distances because their interactions with both phases differ. Mark a pencil baseline above the solvent level and apply small concentrated sample and reference spots with a capillary. Let applications dry; repeated small applications can concentrate a sample without making a broad initial spot.

Place the plate in the development chamber without submerging the baseline. A cover reduces solvent evaporation and helps provide more consistent chamber conditions. Avoid disturbing the plate. Follow the approved solvent and ventilation arrangements; many mobile phases are volatile and flammable. Handle a plate by its edges to avoid contaminating the stationary surface.

Remove the plate before the front reaches the top and mark the solvent front immediately in pencil, before it evaporates. Measure from the baseline to the centre of each spot and from that same baseline to the front. Rf = spot distance/front distance, so it has no unit. Using the plate edge for one distance and the baseline for the other gives an invalid ratio.

Colourless compounds need a suitable visualisation method. Use UV only with the specified shielded viewing equipment and protection; do not look directly into an unshielded source. Some approved visualising reagents alter the sample, so record the developed pattern as instructed. A streak rather than a discrete spot may indicate overloading or an unsuitable solvent system.

Scroll the diagram to see the full view.

Run unknown and reference on the same plate with the same solvent. Rf depends on the mobile phase, stationary phase and conditions, so a tabulated number from a different system is not a reliable direct comparison. A matching spot supports a possible identity; different compounds can travel together. A second solvent system or independent spectral evidence provides a stronger test.

More than one spot supports a mixture if the spots arise from sample components. One spot does not prove complete purity: components may co-migrate or escape the visualisation method. Improve resolution with a suitable solvent composition, smaller starting spots and controlled development. Do not merely develop longer if the front is already near the plate edge.

Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.