4.5 Carboxylic Acids And Their Derivatives

Question 1

What is an acid

Answer 1

A compound that is a proton donor

Question 2

What is a strong acid

Answer 2

An acid that dissociates fully when dissolved in water

Question 3

What is a weak acid

Answer 3

An acid that partially dissociates when dissolved in water

Question 4

Are carboxylic acids strong or weak acids?

Answer 4

Weak

Question 5

Explain the relative acidity of a carboxylic acid

Answer 5

• the hydrogen from the carboxyl can be donated
• the negative charge can be shared over the carbon and oxygen of the carbonyl group
• this delocalisation of charge stabilises the carboxylate ion

Question 6

Rate the acidity of the following
Water, carboxylic acid, phenol, ethanol

Answer 6

1. Carboxylic acid
2. Phenol
3. Water
4. Ethanol

Question 7

Why are the ions formed from carboxylic acids more stable than those formed from ethanol and water

Answer 7

The negative charge is delocalised across 2 oxygen atoms

Question 8

Why is the phenoxide ion formed from phenol more stable than the ions formed from ethanol and water

Answer 8

The negative charge is delocalised around the pi-electron ring system of benzene

Question 9

Test to distinguish between carboxylic acids and phenols

Answer 9

• carboxylic acids react with carbonate compounds and form bubbles of CO2
• phenol doesn’t react with carbonates as it is insufficiently acidic and so no effervescence is seen

Question 10

Why dont ethanol molecules ionise

Answer 10

The ethoxide ion isn’t stabilised by the delocalisation of electrons

Question 11

How to form a carboxylic acid from an aldehyde or primary alcohol

Answer 11

Oxidation
• use excess oxidising agent such as acidified potassium dichromate or acidified potassium manganate
• heat under reflux
• distil carboxylic acid from reaction mixture and purfiy

Question 12

Why is heated under reflux important when oxidising to carboxylic acid

Answer 12

As distillation will form the aldehyde - must have further warming for further oxidation to occur

Question 13

Why might it be difficult to see oxidation has occurred using excess acidified potassium manganate

Answer 13

The solution turns from purple to colourless but if theres excess when the solution is purple it will be hard to see when the reaction would be colourless

Question 14

Can NaBH4 reduce a carboxylic acid? Explain

Answer 14

No
Carboxylic acid is a stable compound and this is not a strong enough reducing agent

Question 15

Which reducing agent to use when trying to reduce carboxylic acid

Answer 15

Lithium tetrahydridoaluminate (III) dissolved jn ethoxyethane

Question 16

What is formed when a carboxylic acid is reduced?

Answer 16

A primary alcohol and water is produced

Question 17

What is an aromatic carboxylic acid?

Answer 17

-COOH group is attached to benzene ring

Question 18

How to form an aromatic carboxylic acid

Answer 18

• oxidation of methyl side-chain
• reflux an alkylbenzene with an alkaline potassium manganate (VII) solution
• purple solution of potassium manganate (VII) is reduced to brown manganase (IV) oxide
• the potassium or sodium salt of the benzenecarboxylic acid is formed
• mixture is acidified with a dilute acid (HCl (aq)) to reprotonate the salt to produce white crystals of benzenecarboxylic acid

Question 19

What is decarboxylation

Answer 19

Loss of a carboxyl group
• descends the homologous series

Question 20

When will a carboxylic acid be decarboxylated

Answer 20

The carboxylic acid (or its sodium salt which is generally more effective) is strongly heated with an alkali such as solid NaOH or preferably sodalime

Question 21

What is produced when a carboxylic acid is decarboxylated

Answer 21

An alkane and a carbonate

Question 22

How is sodalime prepared

Answer 22

Heating together sodium hydroxide and calcium oxide
Is it represented by NaOh

Question 23

Can decarboxylation occur without the use of sodalime

Answer 23

Yes
CO2 will be removed if calcium salt of an acid is heated
A ketone will be produced
This will increase the chain length

Question 24

How is a carboxylic acid converted to an ester

Answer 24

• react carboxylic acid with 1• or 2• alcohol
• conc sulfuric acid used as catalyst
• mixture is refluxed using a water bath or electrical heater
• the OH bond is broken
• the remaining acid is neutralised by NaHCO3(aq)
• the 2 layers are separated using a separating funnel
• the ester-containing layer is dried and distilled
• the ester is collected at boiling temperature
• esterification reaction
• condensation reaction

Question 25

What are two ways of hydrolysing esters

Answer 25

Using acid or using a base

Question 26

How to hydrolyse an ester using acid

Answer 26

Reagent : ester, dilute sulphuric acid (catalyst)
Conditions : heat under reflux

Question 27

How to hydrolyse an aster using a base

Answer 27

Reagent : ester, aqueous sodium hydroxide (catalyst)
Conditions : heat under reflux
Produces the salt of the carboxylic acid and would need to reprotonate with dilute acid to give carboxylic acid

Question 28

How many ways to convert carboxylic acids to acid chlorides? What are they

Answer 28

3
• phosphorus (V) chloride - PCl5
• phosphorus (III) chloride- PCl3
• sulfur dichloride oxide - SOCl2

Question 29

Which way is best to form acid chlorides from carboxylic acids and why

Answer 29

Sulfur dichloride oxide
Only gaseous co-products are formed
Easily lost

Question 30

Hydrolysis of acid chlorides

Answer 30

• react vigorously with water
• forms corresponding carboxylic acid and hydrogen chloride

Question 31

Which is quicker the hydrolysis os esters or acid chlorides

Answer 31

Acid chlorides

Question 32

Functional group of a amide

Answer 32

O
//
- C
\
N

Question 33

What are two ways to form amides from carboxylic acids

Answer 33

1) react acid with ammonia to give ammonium carboxylate salt. Heat this and so water is lost and amide produced
2) heat acid or its ammonium salt with urea at 120 degrees celsius

Question 34

How to form nitrile from amide

Answer 34

Heat the amide with phosphorus (V) oxide (P4O10) which is a dehydrating agent
Amide is dehydrated to corresponding nitrile

Question 35

How to form nitriles from aliphatic halogenoalkanes

Answer 35

Reagent : potassium cyanide dissolved in ethanol, alcohol-water mixture solvent
Conditions : heat under reflux
Mechanism : nucleophillic substitution
• ascends homologous series

Question 36

Equation to form nitriles from aliphatic halogenoalkanes

Answer 36

CH3CH2CH2Br + KCN —-> CH3CH2CH2CN + KBr

Question 37

Hydrolysis of nitriles

Answer 37

Use of a dilute acid
Heat under reflux with a dilute acid
Gives a carboxylic acid
• nitrogen is converted into ammonium group and the dilute acid will react with it

Use of alkali
Heat under reflux with alkali (NaOH) to give the carboxylate ion or its sodium salt
The mixture is then acidified to form the carboxylic acid

Question 38

Hydrolysis of amides

Answer 38

• heat under reflux with aqueous sodium hydroxide
• carbon to nitrogen bond is broken which gives a sodium carboxylate ion and a nitrogen containing compound
• nitrogen compound depends on whether the nitrogen was substituted or not
• carboxylic acid is obtained by acidification using dilute acid

Question 39

What reducing agent is needed to reduce a nitrile

Answer 39

LiAlH4 dissolved in eyhoxyethane

Question 40

Reduction of nitriles

Answer 40

Forms amine
Addition reaction as hydrogen is added across the C to N triple bond