Topic 17B+C Carbonyl Compounds and Carboxylic Acids Flashcards
What is the carbonyl functional group?
- C=O
Draw an aldehyde and a ketone
- Draw pls
Can aldehydes and ketones form hydrogen bonds and how does this affect their properties?
- no
- they don’t have a polar O-H bond
- solutions of aldehydes and ketones have lower boiling points than their equivalent alcohols (which have H bonding)
- however, they still bond through London forces and permanent dipole-permanent dipole bonds
Why can aldehydes and ketones hydrogen bond with water?
- they have a lone pair of electrons on the O atom of C=O group
- oxygen uses lone pairs to for H-bonds with H atoms
- small aldehydes and ketones will dissolve in water
- larger ones have longer carbon chains with aren’t able to form H bonds with water
- these disrupt that H bonding between water molecules, but don’t form H bonding themselves
- if aldehyde or ketone is large enough, the London forces between aldehyde or ketones molecules and the H bonding between water molecules will be stronger than H bonds between aldehyde/ketone and water
- so compound won’t dissolve
Why are there different ways to test for aldehydes?
- an aldehyde can be easily oxidised to a carboxylic acid, but ketone can’t
- when aldehyde is oxidised, another compound is reduced, so a reagent is used that changes colour as it’s reduced
Describe the three ways of testing for aldehydes
Tollens’ Reagent:
- a colourless solution of silver nitrate dissolved in aqueous ammonia
- if heated with aldehyde, a silver mirror forms
- Ag(NH3)2(+) (aq) + e- = Ag(s) + 2NH3(aq)
- 2Ag(NH3)2(+) (aq) + RCHO(aq) + 3OH-(aq) = 2Ag(s) + RCOO-(aq) + 4NH3(aq) + 2H2O
Fehling’s or Benedict’s solution:
- fehling’s is a blue solution of complexed copper(II) ions dissolved in sodium hydroxide (or sodium carbonate for benedicts)
- if heated with aldehyde, copper(II) ions are reduced to a brick-red precipitate of copper(I) oxide
- Cu2+(aq) + e- = Cu+(s)
- RCHO(aq) + 2Cu2+ + 5OH- = RCOO-(aq) + Cu2O(s) + 3H2O(l)
Acidified dichromate(VI) ions:
- if you heat aldehyde with acidified dichromate(VI) ions, you get a carboxylic acid
- dichromate(VI) ions are oxidising agent [O]
- often used is potassium dichromate(VI) with dill sulfuric acid
- solution turns orange to green as dichromate is reduced
- ketone won’t be oxidised
- Cr2O7(2-) + 14H+ + 6e- = 2Cr3+ + 7H2O
What type of alcohol does reducing an aldehyde make?
- a primary alcohol
What type of alcohol does reducing a ketone make?
- a secondary alcohol
What reducing agent can be used to reduce aldehydes and ketones back to alcohols?
- LiAlH4 in dry ether
- very powerful and reacts violently with water
Describe and draw the nucleophilic addition reaction when hydrogen cyanide reacts with carbonyls to produce hydroxynitriles
- hydrogen cyanide is a weak acid: partially dissociates to form H+ + CN- ions
- the CN- ion attacks the slightly positive carbon atom and donates a pair if electrons to it
- both electrons from the double bond transfer to oxygen
- H+ (from either HCN or H2O) bonds to the oxygen to form the hydroxyl group (OH)
Why is the hydroxynitrile product of hydrogen cyanide and carbonyls optically inactive?
- groups surrounding the carbonyl carbon are planar so nucleophile can attack from either side
- a racemic mixture is made
How to test for a carbonyl group and how can you identify the carbonyl compound?
- 2,4-dinitrophenylhydrazine is dissolved in methanol and conc. sulphuric acid
- it reacts to form a bright orange precipitate if a carbonyl
group is present (only aldehydes and ketones) - the orange precipitate is a derivative of the carbonyl compound which can be purified by recrystallisation
- each carbonyl compounds gives a crystalline derivative with a different melting point
- melt and compare against known melting points to identify
How do some carbonyls react with iodine?
- carbonyls that contain a methyl carbonyl group react when heated with iodine in presence of an alkali
- yellow precipitate of triiodomethane (CHI3)
- an antiseptic smell
- RCOCH3 + 3I2 + 4OH- = RCOO- + CHI3 + 3I- + 3H2O
Draw a carboxylic acid forming hydrogen bonds with water
- go go
Why do carboxylic bonds have relatively high boiling points?
- they can form H bonds with each other
How soluble are carboxylic acids in water?
- very soluble due to H bonding
How and why do solubility of carboxylic acids changes as carbon chain length increases?
- as length of carbon chain increases, solubility decreases
- the hydrocarbon chains can’t form H bonds but disrupt H bonds between water molecules
- long carboxylic acids don’t dissolve in water
Describe the ways carboxylic acids can be formed
oxidation of primary alcohol and aldehydes:
- often potassium dichromate (K2Cr2O7/H2SO4) is used
- draw
hydrolysis of nitriles:
- reflux the nitrile with dilute HCl and then distil off the carboxylic acid
- draw
- nitrile + 2H2O + HCl = carboxylic acid + NH4Cl
What do carboxylic acids react with alkalis to form?
- salt and water
- neutralisation
What do carboxylic acids react with carbonates or hydrogencarbonates to form?
- salt
- CO2
- water
What do you make when reducing a carboxylic acid and how do you?
- hard to reduce it
- uses LiAlH4 in dry ether
- reduces to a primary alcohol
How do you make an acyl chloride from carboxylic acid?
- mix with phosphorus(V) chloride
- Carboxylic acid + PCl5 = acyl chloride + POCl3 + HCl
How do you name an ester?
- alkyl group that came from alcohol is the first part
- second part is from carboxylic acid
How do you make esters?
- heat a carboxylic acid under reflux with an alcohol in the presence of a strong acid catalyst e.g. H2SO4 or HCl
- called esterification
e. g. ethanoic acid + ethanol = ethyl ethanoate + water
How can you collect the ester?
- reaction is reversible, so you need to separate out the product as it’s formed
- do by distillation
- then react with sodium carbonate to react with any carboxylic acid
- separate with separating funnel
Describe acid hydrolysis of ester
- splits the ester into an acid and an alcohol
- reverse of condensation reaction
- add ester into dilute acid, such as HCl or H2SO4
- heat under reflux
- reversible reaction, so needs lots of water to push eqm to right
Describe base hydrolysis of ester
- reflux the ester with dilute alkali e..g NaOH
- get a carboxylate ion and an alcohol
- irreversible
e. g.
- ethyl ethanoate + OH- = ethanoate§ + ethanol
What are diols?
- contain two OH functional groups
What are dicarboxylic acids?
- contain 2 COOH functional groups
What is the reaction to form polyesters (long ester chains) called?
- condensation polymerisation
Draw a polyester chain
- drawwwwwwwww
Draw Terylene and describe
- formed of benzene-1,4-dicarboxylic acid and ethane-1,2-diol
What are the properties of Terylene, PET
- fibres are strong, flexible and abrasion resistant
- terylene used in clothes to keep crease free and long lasting
- used in carpets
- treated terylene is used to make drink bottles and containers
What is the functional group and general formula of acyl chlorides
- COCl
- CnH(2n-1)OCl
What do acyl chlorides react with?
Water:
- a vigorous reactions with cold water, producing carboxylic acid
- ethanoyl chloride + H2O = ethanoic acid + HCl
- white steamy fumes of HCl
Alcohols:
- a vigorous reaction at room temp, producing ester
- ethanoyl chloride + methanol = methyl ethanoate + HCl
- white steamy fumes of HCl
Conc ammonia:
- a violent reaction at room temperature, producing an amide
- ethanoyl chloride + NH3 = ethanamide + HCl
- white steamy fumes of HCl
Amines:
- a violent reaction at room temp, producing an N-substituted amide
- ethanoyl chloride + CH3NH2 = H3CCO(NHCH3)
N-methylethanamide + HCl
