Organic Chemistry Practicals (AQA A Level Chemistry): Flashcards
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What is recrystallisation used for in organic chemistry practicals?
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What is recrystallisation used for in organic chemistry practicals?
A homologous series is a group of organic compounds that share the same general formula and functional group, differ by a CH2 unit, show a gradual change in physical properties, and have similar chemical properties.
Impure substances have a .......... melting point than the pure substance and a .......... melting point range. Pure substances melt sharply over a range of about .......... °C.
Impure substances have a lower melting point than the pure substance and a broader melting point range. Pure substances melt sharply over a range of about 1 °C.
True or False?
In the preparation of aspirin, the reaction mixture is heated in a water bath at around 100 °C for 20 minutes.
False.
The mixture is held in a warm water bath at around 60 °C for about 20 minutes. Too high a temperature could cause side reactions or decomposition of the product.
In the purification of benzoic acid, why is hot filtration carried out before the solution is allowed to cool?
Hot filtration removes insoluble impurities while the benzoic acid remains dissolved in the hot solvent. If the solution were cooled first, the product would crystallise before the insoluble impurities could be removed.
In the nitration of methyl benzoate, the temperature must be kept below .......... °C using an .......... bath. The product, methyl 3-nitrobenzoate, has a melting point of .......... °C.
In the nitration of methyl benzoate, the temperature must be kept below 6 °C using an ice bath. The product, methyl 3-nitrobenzoate, has a melting point of 78 °C.
True or False?
Buchner filtration is faster than normal gravity filtration because it operates under reduced pressure.
True.
In Buchner filtration, a vacuum (reduced pressure) is applied through a side-arm flask, pulling the liquid through the filter paper more rapidly than gravity alone.
State two reasons why benzoic acid is a particularly suitable substance for melting point testing.
Benzoic acid does not decompose on heating, has a melting point in the accessible range (100–200 °C), and has a sharp well-defined melting point, making it ideal for testing the accuracy of melting point apparatus.
True or False?
The melting point of pure aspirin is 135 °C.
True.
Pure aspirin (acetylsalicylic acid) melts at 135 °C. A lower or broader melting range indicates the presence of impurities in the recrystallised product.
What is the purpose of using saturated sodium carbonate solution in the preparation of ethyl ethanoate?
Saturated sodium carbonate neutralises any unreacted ethanoic acid or sulfuric acid catalyst, and helps separate the ester product as an upper oily layer from the aqueous layer in the separating funnel.
In the preparation of ethyl ethanoate, 10 cm3 of .........., 12 cm3 of glacial ethanoic acid, and 15 drops of concentrated .......... acid are heated under .......... for about 15 minutes.
In the preparation of ethyl ethanoate, 10 cm3 of ethanol, 12 cm3 of glacial ethanoic acid, and 15 drops of concentrated sulfuric acid are heated under reflux for about 15 minutes.
True or False?
Ethyl ethanoate forms the lower layer in the separating funnel after the reaction mixture is shaken with sodium carbonate solution.
False.
Ethyl ethanoate is less dense than the aqueous layer, so it forms the upper layer. The aqueous layer (containing sodium carbonate, salts, and dissolved impurities) is the lower layer and is drained off first.
How is the identity of an ester confirmed after hydrolysis?
The ester is hydrolysed under reflux with dilute sulfuric acid to give the carboxylic acid. The acid is purified by recrystallisation and its melting point is measured and compared with data book values.
After separation, solid .......... is added to the ethyl ethanoate to remove any remaining water. The product is then purified by .......... which exploits the ester's lower .......... point compared to the reactants.
After separation, solid sodium sulfate is added to the ethyl ethanoate to remove any remaining water. The product is then purified by distillation which exploits the ester's lower boiling point compared to the reactants.
True or False?
Esters have lower boiling points than alcohols and carboxylic acids of similar molecular mass because they cannot form hydrogen bonds with each other.
True.
Esters lack O-H groups, so they cannot act as hydrogen bond donors. Without hydrogen bonding between ester molecules, intermolecular forces are weaker and the boiling point is lower.
State the expected boiling point of methyl benzoate and the expected melting point of benzoic acid obtained from its hydrolysis.
Boiling point of methyl benzoate: 199 °C
Melting point of benzoic acid: 122 °C
The melting point of the recrystallised acid is compared to 122 °C to confirm its identity.
Reflux
A technique in which a reaction mixture is heated to boiling and the vapour is continuously condensed and returned to the flask by a condenser, allowing prolonged heating without loss of volatile reactants or products.
What is Tollens' reagent and how is it prepared?
Tollens' reagent is ammoniacal silver nitrate, containing the silver(I) complex ion [Ag(NH3)2]+. It is prepared by adding NaOH to silver nitrate solution to form a brown precipitate, then adding concentrated ammonia dropwise until the precipitate just dissolves.
A positive Tollens' test gives a .......... on the inside of the test tube, formed when the [Ag(NH3)2]+ ion is .......... to solid metallic silver by the .......... .
A positive Tollens' test gives a silver mirror on the inside of the test tube, formed when the [Ag(NH3)2]+ ion is reduced to solid metallic silver by the aldehyde.
True or False?
Ketones give a positive Tollens' test because they can be oxidised to carboxylic acids.
False.
Ketones give a negative result: the solution remains colourless. Ketones cannot be oxidised by Tollens' reagent because the carbonyl carbon has no hydrogen attached to it.
What is the correct observation to record for a negative result in a Tollens' test?
'No visible change' or 'solution stays colourless'. Writing 'no observation' is penalised by AQA examiners.
In the Tollens' test, the aldehyde is .......... to a .......... acid. The [Ag(NH3)2]+ ions are the .......... agents in this reaction.
In the Tollens' test, the aldehyde is oxidised to a carboxylic acid. The [Ag(NH3)2]+ ions are the oxidising agents in this reaction.
True or False?
Tollens' test can be used to distinguish an aldehyde from a primary alcohol, since primary alcohols give a negative result.
True.
Primary alcohols give a negative result with Tollens' reagent (no silver mirror). Only aldehydes reduce [Ag(NH3)2]+ to silver metal. Tollens' reagent therefore distinguishes aldehydes from both primary alcohols and ketones.
Why is it important to use very clean glassware when carrying out a Tollens' test?
Contamination on the glass surface can produce a dull grey silver deposit rather than a clear silver mirror, making the positive result less convincing or harder to interpret.
Aldehyde
An organic compound containing a carbonyl group (C=O) at the end of a carbon chain, with the general formula RCHO. Aldehydes can be oxidised to carboxylic acids and give a positive Tollens' test.
What is the stationary phase in thin-layer chromatography?
The stationary phase is the fixed solid coating on the TLC plate (typically silica or alumina) that interacts with the components of the mixture. More polar components are retained more strongly by the polar stationary phase.
In TLC, the baseline and sample spots must be drawn in .........., not pen, because ink would .......... up the plate. The solvent level must be kept .......... the spots.
In TLC, the baseline and sample spots must be drawn in pencil, not pen, because ink would travel up the plate. The solvent level must be kept below the spots.
True or False?
A lid or cover is placed over the TLC tank to prevent evaporation of the solvent and keep the atmosphere saturated with solvent vapour.
True.
A saturated vapour atmosphere prevents uneven evaporation from the plate during development, which would distort the separation and give unreliable Rf values.
In TLC analysis of common analgesics, tablets are dissolved in ethanol before spotting. What volume of ethanol is used for aspirin and why might a different volume be used for caffeine?
Aspirin is dissolved in 0.5 cm3 of ethanol (0.1 g of powder). Caffeine (as Anadin Extra) requires 7 cm3 of ethanol because the tablet contains a lower proportion of caffeine, so more solvent is needed to produce a sufficiently concentrated solution.
After running the TLC plate, spots are located under .......... light and marked in pencil.
Rf = distance travelled by .......... ÷ distance travelled by .......... front.
After running the TLC plate, spots are located under UV light and marked in pencil.
Rf = distance travelled by component ÷ distance travelled by solvent front.
True or False?
The solvent must reach the top of the TLC plate for the Rf values to be valid.
False.
The solvent does not need to reach the top of the plate. A valid Rf value requires only that the solvent front has travelled a sufficient distance to separate the components.
In the TLC analysis of common analgesics, name the solvent used as the mobile phase.
Ethyl ethanoate (ethyl acetate) is used as the mobile phase solvent in the TLC analysis of common analgesics such as aspirin, ibuprofen, paracetamol, and caffeine.
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