Chemistry PAG 5

What is filtration under reduced pressure?

A technique to separate solid product from solvent/liquid reaction mixture using Buchner flask, Buchner funnel, filter paper, pressure tubing and vacuum pump. Vacuum pulls liquid through filter paper leaving solid crystals.

What equipment is needed for filtration under reduced pressure?

Buchner flask (side-arm flask), Buchner funnel, filter paper, pressure tubing, vacuum pump or water pump. Funnel fits onto flask with rubber seal. Pressure tubing connects flask side-arm to vacuum.

How do you set up filtration under reduced pressure?

Place filter paper in Buchner funnel and wet with little solvent (creates seal). Connect Buchner flask to vacuum pump with tubing. Turn on vacuum and check suction. Slowly pour mixture into funnel. Liquid removed, solid crystals remain on filter paper.

[DIAGRAM] Filtration under reduced pressure apparatus - Buchner flask, Buchner funnel, filter paper, vacuum connection

How do you collect crystals from vacuum filtration?

After filtering, rinse crystals with cold solvent to remove impurities. Leave under suction for few minutes to start drying. Carefully remove filter paper with crystals. Transfer to watch glass or filter paper to complete drying.

What is recrystallisation?

Purification technique for solids based on different solubilities of desired product and impurities in chosen solvent. Product more soluble in hot solvent, less soluble in cold. Impurities remain dissolved when solution cools. Pure crystals form on cooling.

What is recrystallisation method?

Dissolve impure solid in minimum volume of hot solvent (warming with Bunsen or water bath). Allow solution to cool slowly (may use ice bath). Pure crystals form as solubility decreases on cooling. Impurities remain in solution. Filter under reduced pressure. Wash with cold solvent and dry.

Why is minimum volume of hot solvent used in recrystallisation?

To ensure solution is saturated so maximum crystals form on cooling. Too much solvent = crystals may not form. Minimises loss of product. Minimum volume = smallest amount needed to just dissolve solid when hot.

Why is solution cooled slowly in recrystallisation?

Slow cooling allows pure crystals to form gradually. Fast cooling can trap impurities in crystal lattice. Slow cooling gives larger, purer crystals. Crystals have time to form ordered lattice excluding impurities.

How does recrystallisation work in terms of solubility?

Solubility of most compounds increases with temperature. At high temperature, both compound and impurities dissolve. On cooling, desired compound becomes less soluble and crystallises out. Impurities remain dissolved because present in smaller amounts.

[DIAGRAM] Recrystallisation steps - dissolving in hot solvent, cooling, crystals forming, filtration

What is a melting point tube?

Glass capillary tube sealed at one end. Small sample of solid forced into open end by tapping/dipping. Used for melting point determination. Tube placed in apparatus with sample at bottom.

How do you determine melting point using electric apparatus?

Place melting point tube containing sample into sample hole. Heat whilst observing through magnifying window. Record temperature range when solid starts to melt and when melting complete. Repeat heating more slowly near melting point for greater accuracy.

[DIAGRAM] Electric melting point apparatus

How do you determine melting point using a Thiele tube?

Fill Thiele tube with oil. Attach melting point tube to thermometer with rubber ring. Insert into oil so sample in line with thermometer bulb. Heat side arm with microburner slowly. Observe sample and record temperature range when melting occurs.

[DIAGRAM] Thiele tube apparatus for melting point determination

What does a pure compound show in melting point determination?

Pure compound melts at expected literature temperature. Sharp melting range of 1-2°C. Impure compounds have lower melting points and melt over wider ranges. Sharp melting point confirms purity.

What does an impure compound show in melting point determination?

Impure compound has lower melting point than pure compound. Melts over wider range (>2°C). Presence of impurities disrupts crystal lattice. Melting point depression proportional to amount of impurity.

Describe method to obtain pure sample of organic solid from reaction mixture.

Filter off impure solid under reduced pressure. Recrystallise by dissolving in minimum volume of hot solvent. Allow to cool in ice bath (pure crystals recrystallise). Filter off pure solid under reduced pressure. Wash with cold solvent and dry. Test purity by melting point and/or TLC.

What is test for a carboxylic acid?

Add sodium carbonate (or sodium hydrogencarbonate). Effervescence/fizzing/bubbling of CO₂ gas observed. Equation: 2RCOOH + Na₂CO₃ → 2RCOONa + CO₂ + H₂O. Confirms carboxylic acid. Phenol does not react (weaker acid).

What is test for an aldehyde?

Add Tollens' reagent (ammoniacal silver nitrate). Silver mirror forms (or grey/black precipitate). Equation: RCHO + [O] → RCOOH. Aldehydes oxidised to carboxylic acids. Ketones do not react.

What is test for a ketone?

Add 2,4-dinitrophenylhydrazine (Brady's reagent). Orange/yellow/red precipitate forms. Confirms carbonyl group (C=O). Precipitate can be recrystallised and melting point compared to known values to identify specific ketone.

[DIAGRAM] 2,4-DNP test for carbonyl compounds

What is test for unsaturation (C=C)?

Add bromine water (orange solution). Orange colour decolourised/disappears. Indicates C=C double bond. Alkenes decolourise bromine water; alkanes do not. Equation: C=C + Br₂ → Br-C-C-Br (colourless).

What is test for a phenol?

Phenols give purple/blue/violet colour with neutral iron(III) chloride (FeCl₃). Characteristic test for phenols. Phenols weakly acidic (pH ~5). Phenols do not react with sodium carbonate (weaker acid than carbonic acid).

Why do phenols not react with sodium carbonate?

Phenols are weaker acids than carbonic acid (H₂CO₃). Do not react with Na₂CO₃ to produce CO₂. Carboxylic acids (stronger than carbonic acid) react producing CO₂. Distinguishes phenols from carboxylic acids.

What is 2,4-DNP (2,4-dinitrophenylhydrazine)?

Reagent used to test for carbonyl compounds (aldehydes and ketones). Forms orange/yellow/red precipitates (hydrazones) with C=O groups. Also called Brady's reagent. Derivative can be recrystallised for melting point identification.

How can 2,4-DNP derivatives be used to identify carbonyl compounds?

Orange/yellow precipitate filtered and recrystallised to purify. Melting point measured and compared to known values in database. Different carbonyl compounds give derivatives with different melting points. Confirms identity of carbonyl compound.

What is Tollens' reagent?

Ammoniacal silver nitrate solution. Contains [Ag(NH₃)₂]⁺ complex. Oxidises aldehydes to carboxylic acids. Silver ions reduced to metallic silver forming silver mirror. Mild oxidising agent that does not oxidise ketones.

What is equation for Tollens' test?

RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag + 4NH₃ + H₂O. Aldehyde oxidised to carboxylate; Ag⁺ reduced to Ag (silver mirror). Ketones do not react.

What is Fehling's solution?

Test for aldehydes (and reducing sugars). Blue solution containing Cu²⁺ in alkaline solution. Aldehydes reduce Cu²⁺ to Cu⁺ forming brick-red precipitate of Cu₂O. Ketones do not react. Equation: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O.

What is bromine water test for alkenes?

Bromine water is orange/brown. When added to alkene, decolourised quickly (turns colourless). Addition across C=C forms colourless dibromoalkane. Equation: C₂H₄ + Br₂ → C₂H₄Br₂ (colourless). Confirms unsaturation.

What is test for an alcohol?

Add acidified potassium dichromate(VI). Primary and secondary alcohols cause orange to green colour change (oxidation). Tertiary alcohols do not oxidise (no colour change - remains orange). Distinguishes alcohols and identifies primary/secondary vs tertiary.

What is test for a primary alcohol?

Add acidified potassium dichromate(VI) and heat. Orange to green colour change. Further test: aldehyde produced can be confirmed with Tollens' (silver mirror) or Fehling's (brick-red precipitate). Primary alcohols oxidise to aldehydes then carboxylic acids.

What is test for a secondary alcohol?

Add acidified potassium dichromate(VI) and heat. Orange to green colour change. Ketone produced does not react with Tollens' (no silver mirror). Secondary alcohols oxidise to ketones. Ketones do not react with Tollens' or Fehling's.

What is test for a tertiary alcohol?

Add acidified potassium dichromate(VI) and heat. No colour change (remains orange). Tertiary alcohols cannot be oxidised because no C-H bond on carbon bearing OH. No reaction occurs.

What is purpose of recrystallisation?

To purify a solid organic compound. Removes impurities based on different solubilities in chosen solvent. Desired product crystallises while impurities stay dissolved. Gives pure crystals with sharp melting point.

What solvent is chosen for recrystallisation?

Solvent should dissolve compound when hot but not when cold. Impurities should be soluble in cold solvent (stay dissolved). Solvent should not react with compound. Suitable solvent determined by trial and error.

What is purpose of melting point determination after recrystallisation?

To check purity of recrystallised product. Compare experimental melting point to literature value. Sharp melting point close to literature indicates pure product. Wide or depressed melting point indicates impurities remain.