Aldehyde: C= O or carbonyl group at the end. Written as CHO. Suffix –al.
Ketones: C= O or carbonyl group in middle. Written as CO. Suffix –one. Carbonyl group needs to
be numbered, unlike aldehyde- which always in position 1.
Octanal- suffix starts with vowel, so ending shortened to –an.
1) Oxidation of Aldehydes: Aldehydes oxidised to carboxylic acids when
refluxed with potassium dichromate and dilute sulfuric acid. Solutions turns orange to green.
Ketones do not undergo oxidation reactions- can distinguish between ketones and aldehydes.
Going to need 3[O] if an alcohol and aldehyde.
Nucleophilic Addition Reactions of Carbonyl Group: 2 examples below….
Nucleophile: A nucleophile is an electron pair donor.
Double Bonds: Carbonyl and alkenes react differently because C= C double bond in alkenes is
non- polar.
C =O Bond: Oxygen is more electronegative than carbon, so carbon slightly positive and oxygen
slightly negative. Aldehyde and ketones react by nucleophilic addition because C=O electron
deficient and nucleophile attracted to slightly positive carbon.
Carbonyl C=O is polar bond as has permanent dipole across bond. Double O bond provides a
region of high electron density, like C=C.
Alkenes react by electrophilic addition instead.
The C= O double bond in aldehyde and ketone made of σ and π bonds.
Formation of pi bond in carbonyl C = O Bond: Sideways double overlap of P orbitals above and
below plane of molecule forms pi bond. Electrophiles will be repelled/nucleophiles attracted to
delta positive carbon.
2) Reaction of Carbonyl Compounds with NaBH4: Sodium
tetrahydridoborate (III) is used as reducing agent to reduce aldehydes to primary alcohols and
ketones to secondary alcohols. Both reactants warmed. Reduction of organic compounds uses
[H] to represent reducing agent. Example of nucleophilic addition, reduction and redox reaction.
Remember 2[H].
Only Aqueous Sodium Borohydride Acts as Reducing Agent: It dissolves in water so the ions
dissociate NaBH4 -> Na+ + BH4. Dative bond breaks releasing Hydride ions (H-) in solution
Mechanism for Reaction with NaBH4: NaBH4 gives hydride ion :H- the nucleophile- donates pair
of electrons. Nucleophile attracted to delta positive carbon in C= O bond. The lone pair of
electrons from hydride ion donated to the δ+ carbon atom to form dative covalent bond.
Pi electron pair goes to oxygen atom, so the π bond in C= O breaks by hetrolytic fission forming
negatively charged intermediate. Oxygen of the intermediate donates lone pair of electrons to H
atom in water molecule. Intermediate has then been protonated to form an alcohol.
Hydride ion is H-, NOT H+.
3) Reaction of Carbonyl Compounds with HCN: Hydrogen
cyanide adds across C= O bond. The organic product formed contains –OH hydroxyl group and C
≡ N nitrile group- hydroxynitriles. Example of nucleophilic addition. This reaction useful as
increases length of carbon chain.
HCN is poisonous so not used in labs but cyanide ions increase reaction rate. Sodium cyanide
and sulfuric acid produce HCN in reaction.
Mechanism for Reaction with NaCN/ H+
: NaCNgives cyanide ion :CN- the nucleophile.
Both mechanisms can be drawn showing protonating by water or H+.
Make sure to show negative charge on C of CN above.