17- Common 4/6 mark questions

Explain why chloroethanoic acid is more acidic than ethanoic acid.

Chlorine is electronegative and withdraws electron density from the COO⁻ ion. This makes the ion less negative and more stable. A more stable conjugate base makes dissociation more likely, so chloroethanoic acid is the stronger acid.

Explain why propanoic acid is less acidic than ethanoic acid.

The alkyl group is electron releasing and pushes electron density towards the COO⁻ ion. This makes the ion more negative and less stable, so dissociation is less likely and the acid is weaker.

Explain why esterification gives a relatively low yield.

The reaction is reversible and establishes an equilibrium. Products can react to reform reactants, so not all reactants are converted into products and the yield is limited.

Explain why alkaline hydrolysis of an ester is not reversible.

The reaction forms a carboxylate ion. The carboxylate ion is resistant to attack by weak nucleophiles such as alcohols, so the reverse reaction does not occur significantly. Therefore the reaction goes to completion.

Explain why acyl chlorides and acid anhydrides are more reactive than carboxylic acids and esters.

They contain good leaving groups and have less effective delocalisation. The good leaving group can be eliminated readily, so nucleophilic addition–elimination occurs more readily.

Explain why an acyl chloride is a better reagent for making an ester than a carboxylic acid.

The reaction is much quicker and is not reversible, so a higher yield can be obtained.

Explain how soap removes grease.

The COO⁻ end is hydrophilic and mixes with water. The long non-polar hydrocarbon chain is hydrophobic and mixes with grease. This allows grease and water to mix so the grease can be washed away.

Explain why biodiesel can be considered carbon neutral.

CO₂ released when biodiesel is burnt would have been extracted from the air by photosynthesis when the plant grew.

Explain why biodiesel may not actually be carbon neutral.

The argument does not account for energy used to irrigate plants, extract oil, heat the methanol mixture or process the fuel. If fossil fuels provide this energy, additional CO₂ is released. It also does not account for land used for food production.

Describe the recrystallisation process.

Dissolve the impure compound in a minimum volume of hot solvent. Hot-filter quickly to remove insoluble impurities. Cool the filtrate so crystals form. Use suction filtration to collect the crystals. Wash with distilled water and dry between absorbent paper.

Explain why the minimum volume of hot solvent is used during recrystallisation.

It produces a saturated solution and allows the maximum amount of compound to crystallise when the solution is cooled.

Explain why the solution is hot during filtration in recrystallisation.

Heat prevents crystals reforming during filtration while insoluble impurities are removed.

Explain how the purity of aspirin can be assessed.

Measure its melting point/range and compare it with the quoted value. A very pure sample has a sharp melting point at the quoted value. Impurities lower the melting point and cause melting over a range.

Describe the mechanism for acyl chloride + alcohol.

The alcohol acts as a nucleophile and attacks the δ⁺ carbonyl carbon. The C=O π electrons move onto the oxygen. The C=O bond reforms and the leaving group is eliminated. This is nucleophilic addition–elimination.

Describe the mechanism for an acyl chloride + primary amine.

The nitrogen lone pair of the amine attacks the δ⁺ carbonyl carbon. The C=O π electrons move onto the oxygen. The C=O bond reforms and the leaving group is eliminated. This is nucleophilic addition–elimination.

Compare acyl chloride and acid anhydride reactions with primary amines.

Both undergo nucleophilic addition–elimination and form a secondary amide. Acyl chloride produces an alkylammonium chloride salt, whereas acid anhydride produces an alkylammonium carboxylate salt.