Topic 17.2 Carbonyl compounds Flashcards
Carbonyl group
- A carbon atom joined by a double bond to an oxygen atom.
- -C=O
- Carbonyl compounds can be classed as either aldehydes or ketones.
Aldehydes
- A hydrogen atom is bonded to the carbonyl group.
- General formula: RCHO
- =O at the end of carbon chain
- Aledehydes are oxidised further to produce carboxylic acids.
Ketone
- There is only hydrocarbon groups joined to the carbonyl group.
- Cannot be further oxidised as they do not have hydrogen atom attached to their carbonyl.
Bonding in carbonyl compounds
- Contain -C=O group.
- Polar bond.
- Electron density is greater near the δ- O atom.
Physical properties in carbonyl compounds
- Distinctive smells.
- Boiling temperature are between those of alkanes (low) and alcohol (high).
- Boiling points increase with chain length.
- No hydrogen bonding but permanent dipole dipole attraction.
Physical properties in ketones and aldehydes
- Smaller aldehydes and ketones are soluble in water - they can form hydrogen bonds with water.
- Solubility decreases as chain length increases- hydrocarbon part of the molecules become more significant.
Primary alcohol
Oxidised:
Primary alcohol —> aldehyde —>
carboxylic acid
Secondary alcohol
Oxidised:
Secondary alcohol —> ketone —> X
Tertiary alcohol
Tertiary alcohol —> X
Reagent observations: Felling’s reagent
Aldehyde: brick red, precipitate solid
Ketone: stays blue, no visible change
Reagent observations: Tollen’s reagent
Aldehyde: silver mirror, black precipitate
Ketone: stays colourless, no visible change
Reagent observations: sodium dichromate
Aldehydes: decolourises
Ketones: stays purple, no visible change
Reagent observations: K2Cr2O7
Aldehyde: colour change, orange to green
Ketone: stays orange, no visible change
Reaction with proponal/anone: Sodium dicromate
Propanal: green
Propanone: orange (no change)
Reaction with proponal/anone: Felling’s solution
Propanal: light blue
Propanone: dark blue
Reaction with proponal/anone: Tolling’s reagent
Propanal: white precipitate
Propanone: colourless
Reduction reactions (eg. 2° alcohol –> 1° alcohol)
Aldehydes and ketones can be reduced to alcohols using lithium tetrahydridoaluminate (LiAlH4) dissolved in dry ether.
-Reducing agent represented by [H]
Oxidation reactions
-Aldehydes can be oxidised to form carboxylic acids
-Ketones are not easily oxidised (assume they aren’t)
-Oxidising agents shown as [O]
Reactions with iodine
- Triidomethane reaction.
- Carbonyl compound added to an alkaline solution of iodine.
- Mixture warmed then cooled.
- Yellow precipitate forms - this is a positive result.
- Test for CH3CO group (found in ethanal and all methyl ketones).
Reaction with HCN
-Aldehydes and ketones react with HCN in an alkaline solution of KCN
-These are addition reactions where a hydrogen atom joins to the oxygen in the carbonyl group and the cyanide group attaches to the carbon of the carbonyl group
Naming products
OH group- hydroxy
CN group- nitrile
The products are known as hydroxynitriles
Nucleophilic addition mechanism (Step 1)
Step 1: nucleophilic attack by a cyanide ion on the δ+ carbon atom of the carbonyl group.
Nucleophilic addition mechanism (Step 2)
Step 2: the intermediate reacts with a hydrogen cyanide molecule.
Optical activity
- As C=O is planar there is an equal chance of the CN- ion attacking from each side of the plane.
- Even though the product contains a chiral centre, the product is not optically active as there are equal amounts of each enantiomer - a racemic mixture.
Reaction with 2,4-dinitrophenylhydrazine (Brady’s reagent)
A bright orange precipitate indicates the presence if a carbonyl compound.
How to find the melting temperature of derivatives
- The compounds formed, known as derivatives, can be used to identify individual compounds.
- The derivatives are purified and dried and their melting point measured.
Reagent observations: 2,4 DNPH
Aldehyde: orange precipitate
Recyrstalise and find melting point to find which compound it is.
Recrystalisation
1) Dissolve min hot solvent
2) Filter
3) Cool & crystalise
4) Filter with Buchner funnel
5) Wash with minimum cold solvent
6) Dry in warm oven
Acyl chlorides
- RCOCl.
- Contain carbonyl group.
- Replaced the oh in cooh with Cl.
Iodoform test (triodomethane)
CH3CO in a molecule reacts with Iodine/OH- to form a yellow precipitate.
Carboxylic acids reactions
- Reduction - to primary alcohols.
- Neutralisation - completely with aqueous alkali forms carboxylate salts.
- Halogenation - OH is replaced by halogen forms aceyl halogenides.
- Esterfication- alcohol and acid catalyst forms este.
Esters
- Naming: -thyl -oate
- RCOOR
- =O
- Colourless liquids
- Low melting and boiling
- Insoluble in water
- Smells of fruit
Hydrolysis of esters
- Opposite of esterification.
- Acidic- acid speed up as a catalyst.
- Alkaline - goes onto completion rather than equilibrium.
- Produces carboxylate salt.
Differences in esterfacation with oyl and carboxylic acid
- Irreversable - reversable
- HCl formed - H2O formed
- Faster
- No acid catalyst needed - needed
Esther and acyl chloride reactivity
- Acyl chlorides more reactive.
- As more susceptible to attack from nucleophiles.
Polyesters
Polymers that have monomers bonded with an ester link.
