Esters — Structured Questions (Cambridge International AS Chemistry): Exam Questions

Complete Table 1.1 to show the carboxylic acid and alcohol required to form each ester.

Table 1.1

All of the esters in part (a) are formed via a condensation reaction.

State the catalyst and conditions required to form an ester from a carboxylic acid and an alcohol.

State the reagents and conditions required for the acidic hydrolysis of an ester.

State one commercial use of esters.

Draw the fully displayed formulae of:

i) 3-methylbutanoic acid

ii) methyl ethanoate

2-methylbutanoic acid can be produced by the oxidation of 2-methylbutan-1-ol when heated under reflux with acidified potassium dichromate(VI).

State the colour change observed during this reaction.

State the systematic names of the organic products formed when methyl ethanoate is hydrolysed by heating with dilute sulfuric acid.

Extract:

ii) methyl ethanoate

State the systematic names of the organic products formed when methyl ethanoate is hydrolysed by heating with dilute sulfuric acid.

Methyl ethanoate can also be hydrolysed under alkaline conditions using sodium hydroxide.

i) Draw the fully displayed formulae of the two products formed in this reaction.

ii) State the reagent required to convert the organic salt formed in (i) into a carboxylic acid.

iii) State one difference between the acid hydrolysis and the alkaline hydrolysis of esters, other than the reagents and products.

Two isomeric compounds are shown below in Fig. 1.1.

Fig. 1.1

i) State the systematic name of each isomer.

ii) Suggest a chemical reagent to distinguish between these isomers and deduce the type of reaction taking place.

iii) State the observations made in each case.

i) Compound B, CH₃CH(CH₃)CH(OH)COOH, can be oxidised into compound C.

Construct an equation for this oxidation, using [O] to represent the oxidising agent.

ii) The half-equation for the reduction of acidified potassium dichromate(VI) is:

Cr₂O₇^2- (aq) + 14H⁺ (aq) + 6e^– → 2Cr³⁺ (aq) + 7H₂O (l)

Deduce the overall ionic equation for the oxidation of compound B to compound C using acidified dichromate(VI).

Acidified dichromate(VI) can be used to oxidise ethanol into ethanal or ethanoic acid, depending on the reaction conditions.

Outline how the reaction conditions can be changed to produce ethanal or ethanoic acid.

Extract:

Two isomeric compounds are shown below in Fig. 1.1.

Fig. 1.1

i) State the systematic name of each isomer.

A = 3-hydroxy-3-methylbutanoic acid

AND

B = 2-hydroxy-3-methylbutanoic acid

Under the right conditions, two molecules of compound A can react together to produce a dimer.

i) Name the type of reaction taking place and state the reaction conditions.

ii) Draw the structure of the product, showing clearly how the two molecules are joined.

An organic ester, B, has the empirical formula C₂H₄O. The relative molecular mass, M_r, of B is 88.0.

Deduce the molecular formula of B.

Extract:

An organic ester, B, has the empirical formula C2H4O. The relative molecular mass, Mr, of B is 88.0.

Draw the skeletal formulae of four isomers of B that are esters.

Extract:

An organic ester, B, has the empirical formula C2H4O. The relative molecular mass, Mr, of B is 88.0.

The student hydrolysed his sample of B by heating with aqueous mineral acid and then separating the alcohol, C, that was formed. He heated the alcohol C under reflux with acidified dichromate(VI) ions and collected the product D.

A sample of D gave a yellow/orange precipitate with 2,4-dinitrophenylhydrazine (2,4-DNP) reagent. A second sample of D gave no reaction with Tollens' reagent.

i) State the functional group shown to be present in D by the reaction with 2,4-DNP reagent.

ii) State what the result of the test with Tollens' reagent shows about D.

iii) Deduce the structural formula of the alcohol C.

iv) Which ester has the same structure as that of the ester B?

Extract:

An organic ester, B, has the empirical formula C2H4O. The relative molecular mass, Mr, of B is 88.0.

Draw the skeletal formulae of four isomers of B that are esters.

The skeletal formulae of four isomers of B that are esters are:

Which ester, if any, is chiral? Explain your answer.

The ester methyl butanoate, CH₃(CH₂)₂CO₂CH₃, contributes to the aroma of apples.

i) State the reagents and conditions needed for the hydrolysis of this ester.

ii) Construct an equation for the hydrolysis of this ester.

iii) Apart from their use as perfumes and food flavourings, state one major commercial use of esters.

Leaf alcohol has the structure shown below.

i) Draw the other stereoisomer of leaf alcohol.

ii) Draw the structure of the ester formed when leaf alcohol reacts with ethanoic acid. Show all bonds in the ester linkage.

Extract:

Leaf alcohol has the structure shown below.

(Z)-hex-3-en-1-ol with the hydroxyl group and a cis double bond

i) Calculate the relative molecular mass, M_r, for leaf alcohol.

ii) Leaf alcohol was reacted to form a product with an M_r value 18 units less.

Suggest a structure for this product and deduce the type of reaction that took place.

Extract:

Leaf alcohol has the structure shown below.

ii) Leaf alcohol was reacted to form a product with an Mr value 18 units less.

Suggest a structure for this product and deduce the type of reaction that took place.

The structure of the dehydration product and type of reaction:

• CH2 = CH – CH = CH – CH2CH3
• Dehydration / elimination

A product with Mr 18 less than leaf alcohol has lost water (Mr of H2O = 18). The loss of H2O from an alcohol is dehydration / elimination and forms an alkene.

Describe a simple chemical test to distinguish between leaf alcohol and your product in part (c)(ii).

State the reagent and the observation for each compound.

Ester S is 2-methylpropyl propanoate.

State the systematic names of the carboxylic acid and the alcohol used to form ester S.

State the catalyst and one condition for this reaction.

Construct an equation for the formation of ester S.

Extract:

Ester S is 2-methylpropyl propanoate.

Draw the skeletal formulae of the organic products formed when ester S is:

i) heated under reflux with dilute aqueous sulfuric acid

ii) heated under reflux with aqueous sodium hydroxide

Ester T has the molecular formula C₅H₁₀O₂. When ester T is hydrolysed, one of the organic products is 2-methylpropan-1-ol.

Deduce the systematic names of the carboxylic acid and the alcohol used to prepare ester T.

Draw the displayed formula of ester T and state its systematic name.