Aldehydes And Ketones Flashcards
What makes a carbonyl an aldehyde
If the C=O is on the end of the
chain with an H attached it is an aldehyde. The name will end in –al
What makes a carbonyl a ketone
If the C=O is in the middle of the chain it is a ketone
The name will end in -one
What is a carbonyl
Carbonyls are compounds with a C=O bond. They can be either aldehydes or ketone
Intermolecular forces in carbonyls
Pure carbonyls cannot hydrogen bond to themselves, but are attracted instead by permanent dipole forces
Carbonyls solubility in water
The smaller carbonyls are soluble
in water because they can form hydrogen bonds with water.
Reactions of carbonyls
The C=O bond is polarised because
O is more electronegative than carbon. The positive carbon atom attracts nucleophile
In comparison to the C=C bond in
alkenes, the C=O is stronger and does
not undergo addition reactions easily.
This is in contrast to the electrophiles
that are attracted to the C=C .
Oxidation reactions in carbonyls
Potassium dichromate K2Cr2O7 is
an oxidising agent that causes alcohols and aldehydes to oxidise
Aldehydes can be oxidised to carboxylic acids, but ketones cannot be oxidised.
Products of oxidation reactions with alcohols
Primary alcohol —> aldehydes —> carboxylic acid
Secondary alcohol —> ketones
Tertiary alcohols do not oxidis
Oxidation of aldehydes
Reaction: aldehyde —> carboxylic acid
Reagent: potassium dichromate (VI) solution and dilute sulfuric acid.
Conditions: heat under reflux
RCHO + [O] —> RCOOH
Full equation for oxidation
3CH3CHO + Cr2O7 2- + 8H+ —> 3 CH3CO2H + 4H2O + 2Cr3+
Observation: the orange dichromate ion
(Cr2O7 2-) reduces to the green Cr 3+ ion
Reduction of carbonyls reagents and conditions
Reagents: NaBH4 In aqueous ethanol
Conditions: Room temperature and pressure
Reducing agents such as NaBH4 (sodium tetrahydridoborate)
or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls
to alcohols.
What are aldehydes reduced to
Aldehydes will be reduced to primary alcohols
Propanal + [H] —> propan1ol
What will ketones be reduced to
Secondary alcohols
Propanone —> propan2ol
What reducing agents will reduce carbonyls to alcohols
Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols.
Why does nucleophilic addition happen when reducing carbonyls
NaBH4 contain a source of
nucleophilic hydride ions (:H-)
which are attracted to the positive
carbon in the C=O bond.
What is catalytic hydrogenation?
Carbonyls can also be reduced using catalytic hydrogenation
Reagent: hydrogen and nickel catalyst
Conditions: high pressure
Example equations for catalytic hydrogenation
CH3CHO + H2 —> CH3CH2OH
CH3COCH3 + H2 —> CH3CH(OH)CH3
Addition of hydrogen cyanide to carbonyls to form hydroxynitriles
Reaction: carbonyl —> hydroxynitrile
Reagent: potassium cyanide (KCN) and dilute sulfuric acid.
Conditions: Room temperature and pressure
Mechanism: nucleophilic addition
Why is the addition of hydrogen cyanide to carbonyls to form hydroxynitriles nucelophilic ddition
The KCN supplies the nucleophilic CN- ions. The H2SO4 acid supplies H+ ions
needed in second step of the mechanism
What happens when naming hydroxynitriles
When naming hydroxy nitriles
the CN becomes part of the
main chain and carbon n1
What is the advantage of using KCN or NaCN over HCN
We could use HCN for this reaction but it is a toxic gas that is difficult to contain. KCN/NaCN are still, however, toxic, because of the cyanide ion.
Another advantage of using KCN or NaCN is that there will be a higher concentration of the CN- ion as these
compounds will completely
ionise. HCN is a weak acid an
will only partially ionise
Example equations of addition of hydrogen cyanide to carbonyls to form hydroxynitriles
CH3COCH3 + HCN —> CH3C(OH)(CN)CH3
2-hydroxy-2-methylpropanenitrile
CH3CHO + HCN —> CH3CH(OH)CN
2-hydroxypropanenitrile
Shape of the nucleophilic addition of hcn to aldehydes and ketones
Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the planar carbonyl group is approached equally from both sides by the HCN attacking species: results in the formation of a racemate
