CIE AS Chemistry

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ASChemistryCIETopic Questions3. Organic Chemistry3.6 Carboxylic Acids & Derivatives

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First teaching 2020

Last exams 2024

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3.6 Carboxylic Acids & Derivatives

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1a1 mark

This question is about carboxylic acids. 

State the general formula of a carboxylic acid

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1b1 mark

Name the carboxylic acid shown in Fig. 1.1.



3-methylbutanoic-acid
Fig. 1.1

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1c3 marks
i)
Write a balanced symbol equation to show the dissociation of the acid from part (b).
[1]

ii)
State where the position of the equilibrium lies and what this says about the strength of the acid.
[2]

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1d3 marks

Write a balanced symbol equation, including state symbols, for the reaction of propanoic acid with sodium hydrogen carbonate powder to form the soluble sodium propanoate salt and two other products.

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2a3 marks

This question is about esters. 

Complete the table to show how some common esters are formed. 

 Carboxylic Acid  Alcohol   Ester 
ethanoic acid    methyl ethanoate
  ethanol ethyl propanoate
butanoic acid propan-1-ol  

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2b2 marks

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

State the conditions needed for this reaction.

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2c2 marks

Each of the esters in part (a) can be hydrolysed to reform the carboxylic acid and alcohol. 

Give the reagents and conditions needed for this reaction. 

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2d1 mark

Suggest why esters are used in the manufacture of foods.

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3a2 marks

This question is about carboxylic acids and esters.

Draw the structures of:

i)
3-methylbutanoic acid
[1]
ii)
methyl ethanoate 

[1]

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3b1 mark

2-methylbutanoic acid can be produced by the oxidation of 2-methylbutan-1-ol using acidified K2Cr2O7. 

Give the colour change that would be observed during this reaction. 

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3c1 mark

Name the products formed when methyl ethanoate is hydrolysed under acidic conditions.

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3d4 marks

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

i)
Draw the displayed formula of the salt and alcohol formed in this reaction. 

ii)
How is the salt converted into carboxylic acid? 

iii)
Apart from the conditions, and products made, give one other difference between acid and alkaline hydrolysis.

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1a6 marks

Esters have important commercial uses, such as artificial flavourings in food. 

They can be prepared in several ways, including through the reactions of alcohols with carboxylic acids, as well as the carboxylic acid derivatives, acid anhydrides and acyl chlorides.

Ester S, CH3CH2COOCH2CH(CH3)2 is used in rum flavouring. 

Outline how you could obtain a sample of ester S, beginning with a named alcohol and carboxylic acid.
 

Include any essential reaction conditions and write an equation for the reaction. 

You do not need to include details of the separation or purification methods involved. 

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1b4 marks

Ester S can undergo acid or alkaline hydrolysis. 

The products formed depends on the type of hydrolysis that has taken place.

Compare the two types of hydrolysis.

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1c4 marks

A second ester, Ester T is responsible for a raspberry scent and has the molecular formula C5H10O2.

Ester T can be produced by the reaction of an acid with a branched primary alcohol.

Identify the acid and alcohol used to prepare ester T.


Draw and name ester T.

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2a1 mark

Lactic acid, 2-hydroxypropanoic acid, CH3CH(OH)COOH occurs naturally in sour milk and in our muscles when we take hard exercise.

Lactic acid is chiral and shows stereoisomerism. 

Draw fully displayed structures of the two optical isomers of lactic acid. 

Circle the chiral carbon atom in the lactic acid molecule.

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2b6 marks

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

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

3-6-2b-h--q2breaction-pathway

step 1

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

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

step 2

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

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

step 3

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

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

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2c2 marks

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

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

[1]

ii)
Name the organic produced formed in this reaction. 

[1]

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3a4 marks

Glycolic acid is commonly used in skin care products. 

The structure is shown in Fig. 3.1. 

 
3-6-3a-h-3a-glycolic-acid-structure
 
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 that is responsible for the reaction.

 
reagent observation with glycolic acid does a reaction occur? functional group 
Na2CO3      
2,4-DNPH      
acidified Cr2O72-      
 

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3b6 marks

Two reaction sequences to produce glycolic acid are shown. 

glycolic-acid-reaction-sequece

i)
Draw the structure of X.
[1]
ii)
Name the reagent used for reaction 2. 
[1]
iii)
Name the mechanism for reaction 3. 
[1]
iv)
Suggest the essential condition for reaction 3. 
[1]
v)
Reaction 4 occurs via an SN2 mechanism.
Complete the diagram for the mechanism for reaction 4.
Include all relevant charges, partial charges, curly arrows and lone pairs. 
sn2-mechanis
[2]

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3c3 marks

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

i)
State the role of NaBH4 in this reaction.

[1]

ii)
Write an equation for this reaction using molecular formulae.
Use [H] to represent NaBH4.
[2]

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1a2 marks

A series of reactions based on propanoic acid is shown. 

3-6-1a-m-3-6-reaction-of-carboxylic-acid-flow-chart

Write an equation for reaction 1, using [H] to represent the reducing agent.

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1b3 marks
i)
What type of reaction is reaction 2?

[1]

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

[2]

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1c2 marks

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

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1d3 marks
i)
Suggest a suitable reagent and conditions for reaction 3.
[2]

ii)
Identify the other product of reaction 3.
[1]

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2a
Sme Calculator1 mark

An organic ester, B, has the empirical formula C2H4O. An experiment by a student in a college gave a value of 87.5 for Mr of B.

What is the molecular formula of B?

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2b4 marks

In the boxes below, draw the structural formulae of four isomers of B that are esters.

SDFSDF

W

SDFSDF

X

SDFSDF

Y

SDFSDF

Z

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2c4 marks

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 an orange precipitate with 2,4-dinitrophenylhydrazine reagent. A second sample of D gave no reaction with Tollens’ reagent. 

i)
What group does the reaction with 2,4-dinitrophenylhydrazine reagent show to be present in D?

[1]

ii)
What does the result of the test with Tollens’ reagent show about D?

[1]

iii)
What is the structural formula of the alcohol C?

[1]

iv)
Which of your esters, W, X, Y, or Z has the same structure as that of the ester B?

[1]

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2d1 mark

Which, if any of your esters, W, X, Y, or Z is chiral? Explain your answer.

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3a4 marks

Esters are compounds which provide the flavour of many fruits and the perfumes of many flowers.

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

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

[2]
ii)
Write the equation for the hydrolysis of this ester.

[1]
iii)
Apart from their use as perfumes and food flavourings, state one major commercial use of esters.
[1]

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3b3 marks

Leaf alcohol is a stereoisomer that can form when insects such as caterpillars eat green leaves.

leaf-alcohol-structure

i)
Draw the other stereo-isomer of leaf alcohol.
[1]
ii)
Draw the structure for the ester formed when leaf alcohol reacts with ethanoic acid.

Show all the bonds in the ester group
[2]

How did you do?

3c3 marks
i)
Deduce the relative molecular mass, Mr, for leaf alcohol.

[1]

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

structure of product


type of reaction .....................................................
[2]

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3d2 marks

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

test ...................................................................................................................................


observation ......................................................................................................................

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4a2 marks

Fermentation of sugars by bacteria or moulds produces many different organic compounds.

One compound present in fermented molasses is 2-ethyl-3-methylbutanoic acid which gives a distinctive aroma to rum.

(CH3)2CHCH(C2H5)CO2H

2-ethyl-3-methylbutanoic acid

i)
What is the molecular formula of 2-ethyl-3-methylbutanoic acid?
[1]
ii)
How many chiral carbon atoms are present in a molecule of 2-ethyl-3-methylbutanoic acid? If none write ‘none’.
[1]

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4b6 marks

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.

Describe what colour change would be seen.

reagent(s) .................................................................................................................
colour change from ............................................... to ...............................................

[3]
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.

[1]

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4c3 marks

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 as clearly as you can.

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4d3 marks

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 (*).

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