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ASChemistryCIEExam Questions18. Carboxylic Acids & Derivatives18.1 Carboxylic Acids

Carboxylic Acids (CIE AS Chemistry)

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18.1 Carboxylic Acids

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18.1 Carboxylic Acids

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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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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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2a2 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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2b6 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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2c3 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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2d3 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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3a1 mark

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

Name the intermediate formed during this oxidation.

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3b3 marks
i)
State the colour of the chromium species after the potassium dichromate(VI) has reacted.
 
[1]
 
ii)
Identify the conditions to ensure that propanoic acid is obtained in a high yield.
 
[2]

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3c2 marks

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

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1a1 mark

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.

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1b4 marks

Two students try to prepare 2‑hydroxybutanoic acid in the laboratory as shown in Fig. 1.1

Both students oxidise butane‑1,2‑diol to form P in reaction 1.
One student then reduces P using LiAlH4. Q is formed.
The other student reduces P using NaBH4. R is formed.

3-5-3b-h-reaction-pathway
Fig. 1.1

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

Only one of the students successfully prepares 2‑hydroxybutanoic acid.
[2]
ii)
Identify which of Q or R is 2‑hydroxybutanoic acid and explain the difference between reactions 2 and 3.
[2]

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1c4 marks

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

3-5-3c-h-reaction-pathway-a
Fig. 1.2


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

  • Identify the ion that reacts with propanal.
  • Draw the structure of the intermediate of the reaction.
  • Include all charges, partial charges, lone pairs and curly arrows.

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1d1 mark

Complete the equation for the reaction in step 2, when S is heated under reflux with HCl (aq).


C2H5CH(OH)CN + ..................... → C2H5CH(OH)COOH + ......................

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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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