Carboxylic Acids (Cambridge (CIE) AS Chemistry): Exam Questions

A series of reactions based on propanoic acid is shown.

Construct an equation for reaction 1, using [H] to represent one atom of hydrogen from the reducing agent.

i) Name the type of reaction for reaction 2.

[1]

ii) State a suitable reagent and conditions for reaction 2.

[2]

Construct an equation for the reaction of propanoic acid with calcium carbonate, CaCO₃.

i) State a suitable reagent and conditions for reaction 3.

[2]

ii) Identify the other product of reaction 3.

[1]

2-ethyl-3-methylbutanoic acid is present in fermented products such as rum.

i) Deduce the molecular formula of 2-ethyl-3-methylbutanoic acid.

[1]

ii) State the number of chiral carbon atoms in a molecule of 2-ethyl-3-methylbutanoic acid.

[1]

A sample of 2-ethyl-3-methylbutanoic acid may be prepared in a school or college laboratory by the oxidation of:

2-ethyl-3-methylbutan-1-ol (CH₃)₂CHCH(C₂H₅)CH₂OH

i) State the reagent(s) that would be used for this oxidation.

reagent(s) .................................................................................................

colour change from .............................. to ..............................

[2]

ii) This reaction is carried out by heating the reacting chemicals together. What could be the main organic impurity present in the sample of the acid? Explain your answer.

[2]

iii) State whether a distillation apparatus or a reflux apparatus should be used.

Explain your answer.

[2]

A structural isomer of 2-ethyl-3-methylbutan-1-ol is 2-ethyl-3-methylbutan-2-ol, (CH₃)₂CHC(OH)(C₂H₅)CH₃.

What colour change would be seen if this were heated with the reagents you have given in part (b)(i)?

Explain your answer.

An isomer of 2-ethyl-3-methylbutanoic acid which is an ethyl ester is a very strong smelling compound which is found in some wines. This ethyl ester contains a branched hydrocarbon chain and is chiral.

Draw the displayed formula of this ethyl ester.

Identify the chiral carbon atom with an asterisk (*).

Propan-1-ol can be oxidised to propanoic acid using acidified potassium dichromate(VI).

State the systematic name of the intermediate formed during this oxidation.

i) State the final colour of the reaction mixture after the potassium dichromate(VI) has reacted.

[1]

ii) State the conditions to ensure that propanoic acid is obtained in a high yield.

[2]

Describe a chemical test and observation which confirms the presence of a carboxyl functional group.

LiAlH₄ is a reducing agent.

LiAlH₄ cannot be used in aqueous solution because it reacts with water to produce LiOH(aq), H₂(g) and a white precipitate which is soluble in excess sodium hydroxide. Identify the white precipitate.

Butane-1,2-diol is oxidised to form compound P in reaction 1. Compound P is then reduced using LiAlH₄ in reaction 2 to give Q, and using NaBH₄ in reaction 3 to give R, as shown in Fig. 1.1.

Fig. 1.1

i) State the reagents and conditions required for reaction 1.

[2]

ii) Deduce whether Q or R is 2-hydroxybutanoic acid and explain the difference between reactions 2 and 3.

[2]

A third student prepares 2-hydroxybutanoic acid using propanal as the starting material as shown in Fig. 1.2. In step 1, propanal reacts with a mixture of NaCN and HCN.

Fig. 1.2

Draw the mechanism for the reaction of propanal with the mixture of NaCN and HCN to form S.

In your mechanism, include:

• the identity of the ion that reacts with propanal

• the structure of the intermediate

• all charges, partial charges, lone pairs and curly arrows

Construct a balanced equation for step 2, in which S is heated under reflux with dilute hydrochloric acid.

Lactic acid, 2-hydroxypropanoic acid, CH₃CH(OH)COOH, occurs naturally in sour milk.

Lactic acid is chiral and shows stereoisomerism.

Draw the three-dimensional structures of the two optical isomers of lactic acid.

Use an asterisk (*) to identify the chiral carbon atom in one of the structures.

Lactic acid may be synthesised from ethanol by the following route.

CH₂CH₂OH CH₃CHO CH₃CH(OH)CN CH₃CH(OH)COOH

State the reagent(s) and essential condition(s) for each step.

step 1

reagent(s) .......................................................................................

conditions .......................................................................................

step 2

reagent(s) .......................................................................................

condition(s) .......................................................................................

step 3

reagent(s) .......................................................................................

condition(s) .......................................................................................

Lactic acid, CH₃CH(OH)COOH can be reduced by LiAlH₄.

i) Construct an equation to show this reaction, using [H] to represent one atom of hydrogen from the reducing agent.

[1]

ii) State the systematic name of the organic product formed in this reaction.

[1]

Glycolic acid is commonly used in skin care products.

The structure is shown in Fig. 3.1.

Fig. 3.1

Glycolic acid is added separately to each of the three reagents below.

Complete the table to show what you would observe. If a reaction occurs, state the functional group of glycolic acid responsible for the reaction.

Two reaction sequences to produce glycolic acid are shown.

Sequence A:

HCHO X CH₂(OH)COOH

Sequence B:

CH₃COOH CH₂BrCOOH CH₂(OH)COOH

i) Draw the structure of X.

[1]

ii) Identify the reagent used for reaction 2.

[1]

iii) Name the mechanism for reaction 3.

[1]

iv) State the essential condition for reaction 3.

[1]

v) Reaction 4 occurs via an S_N2 mechanism. Complete the diagram for the mechanism for reaction 4.

In your mechanism, include all relevant charges, partial charges, curly arrows and lone pairs.

[3]

Glycolic acid can also be made by reacting glyoxylic acid with NaBH₄.

i) State the role of NaBH₄ in this reaction.

[1]

ii) Construct an equation for this reaction using molecular formulae. Use [H] to represent one atom of hydrogen from NaBH₄.

[1]