Carboxylic Acids (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
1/360Still learning
Know0
Define benzoic acid.
Was this flashcard helpful?
Cards in this collection (36)
Define benzoic acid.
Benzoic acid is the simplest aromatic carboxylic acid, with the molecular formula C6H5COOH. It can be produced by oxidising alkylbenzenes.
What reagent is used to oxidise methylbenzene to benzoic acid, and what conditions are required?
Hot alkaline KMnO4 is used as the oxidising agent. The mixture is heated under reflux, then acidified with dilute acid to protonate the product and form benzoic acid.
True or False?
During the oxidation of methylbenzene with alkaline KMnO4, the purple colour disappears as Mn7+ is reduced to Mn4+.
True.
The purple colour of permanganate fades as Mn7+ is reduced to Mn4+, and a brown precipitate of MnO2 forms.
The alkyl side-chain of an alkylbenzene is oxidised to a .......... group by hot alkaline KMnO4. The mixture is then acidified to produce a .......... acid.
The alkyl side-chain of an alkylbenzene is oxidised to a carboxylic acid group by hot alkaline KMnO4. The mixture is then acidified to produce a benzoic acid.
Why is the reaction mixture acidified after oxidising methylbenzene with alkaline KMnO4?
Acidification protonates the organic product to form the final benzoic acid. Under alkaline conditions, the product exists as a carboxylate salt rather than the free acid.
True or False?
Benzoic acid and its derivatives can be used as reagents in the synthesis of esters.
True.
The revision note states that benzoic acids and their derivatives are often used as reagents in ester synthesis.
What visible change is observed in the KMnO4 solution and what precipitate forms during the oxidation of an alkylbenzene?
The purple solution decolourises as Mn7+ is reduced, and a brown precipitate of MnO2 is formed.
Benzoic acid is produced from methylbenzene by heating under .......... with hot alkaline KMnO4, then acidifying with .......... acid.
Benzoic acid is produced from methylbenzene by heating under reflux with hot alkaline KMnO4, then acidifying with dilute acid.
Define alkylbenzene.
An alkylbenzene is a compound consisting of a benzene ring with one or more alkyl side-chains. The side-chain can be oxidised to a carboxylic acid group using hot alkaline KMnO4.
Define acyl chloride.
An acyl chloride is a compound with a -COCl functional group. It is more reactive than the corresponding carboxylic acid and is used as a starting material in the synthesis of esters and amides.
Name three reagents that can be used to convert a carboxylic acid into an acyl chloride.
PCl5 (solid), PCl3 (liquid) with heat, and SOCl2 (liquid).
True or False?
Methanoic acid can be oxidised further to carbon dioxide using Tollens' reagent.
True.
Methanoic acid is a strong reducing agent. It can be oxidised to CO2 by mild oxidising agents such as Tollens' reagent or Fehling's solution.
When methanoic acid is warmed with Fehling's solution, Cu2+ is reduced to Cu+, which precipitates as .......... Cu2O.
When methanoic acid is warmed with Fehling's solution, Cu2+ is reduced to Cu+, which precipitates as red Cu2O.
What is observed when ethanedioic acid is oxidised by warm acidified KMnO4?
The purple KMnO4 solution turns colourless as Mn7+ is reduced to Mn2+, and carbon dioxide is produced.
True or False?
All carboxylic acids can be further oxidised by mild oxidising agents such as Fehling's solution.
False.
Only methanoic acid (and ethanedioic acid, which requires strong oxidising agents) can be further oxidised. Most carboxylic acids are not oxidised by mild reagents like Fehling's solution.
Why are acyl chlorides more useful than carboxylic acids in ester synthesis?
Acyl chlorides are more reactive so the reaction is faster, and the reaction goes to completion so the yield of ester is higher. Reactions with carboxylic acids are slower and reach an equilibrium.
Acyl chlorides can be prepared from carboxylic acids using solid .......... or liquid .......... or liquid SOCl2.
Acyl chlorides can be prepared from carboxylic acids using solid PCl5 or liquid PCl3 or liquid SOCl2.
Define ethanedioic acid.
Ethanedioic acid is a dicarboxylic acid that can be further oxidised to carbon dioxide by warm acidified KMnO4.
Define carboxylate ion.
A carboxylate ion is the conjugate base of a carboxylic acid, formed when the O-H bond breaks. The negative charge is delocalised over both oxygen atoms of the -COO- group, stabilising the ion.
Place ethanol, phenol and ethanoic acid in order of increasing acid strength and give their approximate pKa values.
Ethanol (pKa 16) < phenol (pKa 10) < ethanoic acid (pKa 4.8). Ethanoic acid is the strongest acid of the three.
True or False?
Carboxylic acids are stronger acids than phenols because their carbonyl group weakens the O-H bond.
True.
The carbonyl (C=O) group in carboxylic acids withdraws electron density from the O-H bond, weakening it so that the proton is lost more easily than in phenols or alcohols.
In alcohols, the alkyl group is .......... and donates electron density to the oxygen, making the .......... ion less stable than the phenoxide ion.
In alcohols, the alkyl group is electron-donating and donates electron density to the oxygen, making the alkoxide ion less stable than the phenoxide ion.
Why is the carboxylate ion more stable than the phenoxide ion?
In the carboxylate ion, the charge is delocalised onto an electronegative carbonyl oxygen atom, making the electrons less available for bonding with H+. In the phenoxide ion, the charge is spread over carbon atoms in the ring, which are less electronegative.
True or False?
Alkoxide ions can delocalise their negative charge over a ring system.
False.
Alkoxide ions lack an aromatic ring and cannot delocalise charge. The alkyl group actually donates electron density to the oxygen, concentrating charge rather than spreading it.
How does the carbonyl group in carboxylic acids affect the O-H bond?
The carbonyl group is electron-withdrawing and pulls electron density away from the O-H bond through the C-O bond, weakening the O-H bond so that the proton can be lost more easily.
Benzoic acid (pKa 4.2) is a .......... acid than ethanoic acid (pKa 4.8) because the benzene ring is .......... and stabilises the carboxylate ion.
Benzoic acid (pKa 4.2) is a stronger acid than ethanoic acid (pKa 4.8) because the benzene ring is electron-withdrawing and stabilises the carboxylate ion.
Define alkoxide ion.
An alkoxide ion is the conjugate base of an alcohol. The electron-donating alkyl group concentrates charge on the oxygen, making the alkoxide a strong base and the parent alcohol a very weak acid.
Define electron-withdrawing group.
An electron-withdrawing group is a substituent that pulls electron density away from the rest of the molecule, weakening the O-H bond in carboxylic acids and increasing their acid strength.
Why does chloroethanoic acid have a lower pKa than ethanoic acid?
The chlorine atom is electron-withdrawing and pulls electron density away from the -COOH group, weakening the O-H bond and further stabilising the carboxylate ion formed. This makes chloroethanoic acid a stronger acid.
True or False?
Trichloroethanoic acid (pKa 0.6) is a stronger acid than chloroethanoic acid (pKa 2.9).
True.
The three chlorine atoms in trichloroethanoic acid withdraw more electron density than one chlorine atom in chloroethanoic acid, weakening the O-H bond further and giving a lower pKa.
As the number of chlorine atoms on the carbon attached to -COOH increases, the O-H bond becomes .......... and the acid becomes .......... .
As the number of chlorine atoms on the carbon attached to -COOH increases, the O-H bond becomes weaker and the acid becomes stronger.
Why is ethanoic acid the weakest acid in the series of chlorine-substituted ethanoic acids?
Ethanoic acid contains an electron-donating methyl group that strengthens the O-H bond and donates negative charge to the -COO- group, making it more likely to accept a proton and reform the acid.
True or False?
Electron-withdrawing groups increase acid strength by destabilising the carboxylate ion.
False.
Electron-withdrawing groups increase acid strength by stabilising the carboxylate ion. They spread the negative charge further, making the ion less likely to accept a proton.
Place ethanoic acid, chloroethanoic acid, dichloroethanoic acid and trichloroethanoic acid in order of increasing acid strength.
Ethanoic acid (pKa 4.8) < chloroethanoic acid (pKa 2.9) < dichloroethanoic acid (pKa 1.3) < trichloroethanoic acid (pKa 0.6). More chlorine substituents give stronger acids.
In trichloroethanoic acid, three electronegative .......... atoms withdraw electron density from the -COOH group, making the .......... ion very stable.
In trichloroethanoic acid, three electronegative Cl atoms withdraw electron density from the -COOH group, making the carboxylate ion very stable.
Define chloroethanoic acid.
Chloroethanoic acid is a chlorine-substituted derivative of ethanoic acid (CH2ClCOOH) with a pKa of 2.9. It is a stronger acid than ethanoic acid due to the electron-withdrawing effect of the chlorine atom.
Sign up to unlock flashcards
or