aldehydes and ketones Flashcards
what is the functional group of aldehydes and ketones?
carbonyl group : C=O (carbon hybridised sp2 , trigonal planar plane, 120 angles)
carbonyl group properties
- stronger s character in orbitals
- bond is more polarised since pi orbital can be attracter to electronegative oxygen
- resonance forms can underscore polarisation to create complete positive and negative charges (in minor quantity)
- strong dipole -> low boiling points
- carbonyl can act as acceptor but not donor in H bonds
- aldehydes and ketones are sightly soluble in water (less than corresponding alcohol)
- increasing chain length reduces solubility
what is the difference between aldehyde and ketone?
- if carbonyl is linked to -H it is an aldehyde
- if carbonyl is attached to 2 C compounds it is a ketone
what is a keto-enol tautomerism?
if 1 H is present on the α-carbon (carbon atom adjacent to carbonyl) aldehydes and ketones undergo rearrangement to an enol (alkene+alcohol)
- ketone is electrophilic due to polarised bond (hydrogen bond acceptor)
- enol is nucleophilic (can doth donate and accept H)
what are the properties of a keto-enol tautomerism?
- equilibrium is shifted towards ketone is water
- apolar protic solvents favour the enol form
- H shifts from C2 and bonds with oxygen while the carbon becomes sp2 hybridised to form the enol compound
what is the acid-base equilibria of keto-enol tautomerism?
- α-carbon can dissociate to leave carbanion that is stabilised by resonance
- very weak acids (pKa - 20)
- weaker than alcohols (16)
- enol is less stable than ketone -> structure is more favourable for reaction
- keto-enol dissociation produces stabilised enolate ion through resonance
nomenclature of carbonyl groups
- aldehydes are named after their acid -al
- ketones -one and specify position of carbonyl in chain
- benzene + aldehyde = benzaldehyde
- when more groups are present, priority is given according to oxidisation of the group (more oxidised takes name)
describe the oxidation of aldehydes
oxidised to the corresponding organic acid:
- Tollens’ solution (Ag+ ions in solution of ammonia) is used to distinguish between ketones and aldehydes
- ketones can’t be oxidised (to distinguish)
- breaking C-C bond in ketones requires too much energy
- α-hydroxyl aldehydes can be selectively oxidised
- benedict’s reagent (Cu2+ ions in basic environment)
describe the reduction of carbonyl groups
- reduced by H2 or sodium borohydride (NaBH4)
- catalyst such as nickel, platinum, palladium (reduced cofactor NADH)
- reduced to alcohols by decreasing the number of C-O bonds
describe the nucleophilic addition to carbonyl
- aldehydes more reactive as the carbonyl is freer -less steric hindrance-
- alcohol + aldehyde/ketone -> hemiacetal
- alcohol + hemiacetal -> acetal (usually reacts again with same alcohol in excess)
- causes change in hybridisation of carbon (flat planar sp2 to tetrahedral sp3)
- catalysed by protons to polarise the bond further and favour nucleophilic reaction
how do hemiacetals react?
present in very small amounts and very difficult to isolate as single specie (less stable than acetal)
- CYCLIChemiacetals are very stable and arise from intramolecular reactions
- single C-C bonds are able to rotate and twist to form cycle
- 5/6 atoms in chain -> hemiacetal is more stable in cyclic than open chain form (GLUCOSE)
- hemiacetals react further with alcohol to form cyclic acteal (stable) -> formation of glycosidic bond in disaccarides (using protons)
REMEMBER: more stable in a longer chain of C because the bonds are around 109 therefore less strained, less prone to break (most probable 5-6 rings)
describe the reaction of carbonyl with ammonia
R-NH2 react with carbonyl group under acidic conditions
- addition followed by water elimination
- amino acid metabolism -> alkyl amines (R=alkyl) give imines or Schiff’s bases
what are aldol reactions?
aldehydes/ketone (much less) undergoing condensation under basic conditions (C-C bond is formed/broken)
describe the mechanism of aldol reactions
- base shifts equilibrium towards the enolate
- carbanion performs nucleophilic attack on carbonyl
- very heavily equilibrium based reaction
- product: 3-hydroxyl aldehyde/ketone
example of aldol reactions
metabolism of glucose (breakdown= retro-aldol reaction)
