4.5 Carboxylic Acids And Their Derivatives Flashcards
What is an acid
A compound that is a proton donor
What is a strong acid
An acid that dissociates fully when dissolved in water
What is a weak acid
An acid that partially dissociates when dissolved in water
Are carboxylic acids strong or weak acids?
Weak
Explain the relative acidity of a carboxylic acid
• 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
Rate the acidity of the following
Water, carboxylic acid, phenol, ethanol
- Carboxylic acid
- Phenol
- Water
- Ethanol
Why are the ions formed from carboxylic acids more stable than those formed from ethanol and water
The negative charge is delocalised across 2 oxygen atoms
Why is the phenoxide ion formed from phenol more stable than the ions formed from ethanol and water
The negative charge is delocalised around the pi-electron ring system of benzene
Test to distinguish between carboxylic acids and phenols
• 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
Why dont ethanol molecules ionise
The ethoxide ion isn’t stabilised by the delocalisation of electrons
How to form a carboxylic acid from an aldehyde or primary alcohol
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
Why is heated under reflux important when oxidising to carboxylic acid
As distillation will form the aldehyde - must have further warming for further oxidation to occur
Why might it be difficult to see oxidation has occurred using excess acidified potassium manganate
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
Can NaBH4 reduce a carboxylic acid? Explain
No
Carboxylic acid is a stable compound and this is not a strong enough reducing agent
Which reducing agent to use when trying to reduce carboxylic acid
Lithium tetrahydridoaluminate (III) dissolved jn ethoxyethane
What is formed when a carboxylic acid is reduced?
A primary alcohol and water is produced
What is an aromatic carboxylic acid?
-COOH group is attached to benzene ring
How to form an aromatic carboxylic acid
• 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
What is decarboxylation
Loss of a carboxyl group
• descends the homologous series
When will a carboxylic acid be decarboxylated
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
What is produced when a carboxylic acid is decarboxylated
An alkane and a carbonate
How is sodalime prepared
Heating together sodium hydroxide and calcium oxide
Is it represented by NaOh
Can decarboxylation occur without the use of sodalime
Yes
CO2 will be removed if calcium salt of an acid is heated
A ketone will be produced
This will increase the chain length
How is a carboxylic acid converted to an ester
• 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
What are two ways of hydrolysing esters
Using acid or using a base
How to hydrolyse an ester using acid
Reagent : ester, dilute sulphuric acid (catalyst)
Conditions : heat under reflux
How to hydrolyse an aster using a base
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
How many ways to convert carboxylic acids to acid chlorides? What are they
3
• phosphorus (V) chloride - PCl5
• phosphorus (III) chloride- PCl3
• sulfur dichloride oxide - SOCl2
Which way is best to form acid chlorides from carboxylic acids and why
Sulfur dichloride oxide
Only gaseous co-products are formed
Easily lost
Hydrolysis of acid chlorides
• react vigorously with water
• forms corresponding carboxylic acid and hydrogen chloride
Which is quicker the hydrolysis os esters or acid chlorides
Acid chlorides
Functional group of a amide
O
//
- C
\
N
What are two ways to form amides from carboxylic acids
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
How to form nitrile from amide
Heat the amide with phosphorus (V) oxide (P4O10) which is a dehydrating agent
Amide is dehydrated to corresponding nitrile
How to form nitriles from aliphatic halogenoalkanes
Reagent : potassium cyanide dissolved in ethanol, alcohol-water mixture solvent
Conditions : heat under reflux
Mechanism : nucleophillic substitution
• ascends homologous series
Equation to form nitriles from aliphatic halogenoalkanes
CH3CH2CH2Br + KCN —-> CH3CH2CH2CN + KBr
Hydrolysis of nitriles
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
Hydrolysis of amides
• 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
What reducing agent is needed to reduce a nitrile
LiAlH4 dissolved in eyhoxyethane
Reduction of nitriles
Forms amine
Addition reaction as hydrogen is added across the C to N triple bond
