Ketones and aldehydes contain the carbonyl functional group CO Formation of the CO carbonyl π bond π bond formed by sideways overlap of p orbitals above and below the plane of the molecule The bonding electrons are ID: 234227
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Slide1
Carbonyl Compounds
Ketones and aldehydes contain the carbonyl functional group, C=O.
Formation of the C=O carbonyl
π bondπ bond formed by sideways overlap of p orbitals above and below the plane of the molecule. The bonding electrons are drawn towards the more electronegative oxygen atom.
O
p orbitals overlap
C
O
C
δ
+
δ
-
Electrons drawn towards the more electronegative oxygen atom, creating a
dipole
in the C=O bond. Slide2
Physical Properties of Carbonyl Compounds
Compound
Intermolecular forces present
Boiling Point
Aldehyde
Dipole-Dipole
21
Alcohol
Dipole-Dipole
Hydrogen bonds
79
Carboxylic Acid
Dipole-Dipole
Hydrogen Bonds
118
Miscibility with Water:
The
polarity of carbonyl compounds is sufficient to enable the lower members of the homologous series to be completely miscible with water. Water will form hydrogen bonds to the carbonyl group. Aldehydes and ketones with more than 4 carbon atoms become increasingly immiscible in water because the hydrocarbon chain is hydrophobic – it hinders the formation of hydrogen bonds between the lone pair of electrons on the oxygen atom of carbonyl group and water molecules.
Aldehydes have a lower boiling point than the comparable alcohol or carboxylic acid. They have dipole dipole forces, which are weaker than the hydrogen bonds present in alcohols and carboxylic acids.
. .
Uses of aldehydes (
methanal
):
manufacturing plastic coating such as
formica
; preserving and embalming; feedstock in pharmaceuticals, perfumes and flavouring agents.
Uses of ketones (
propanone
): solvents – like nail varnish removed, in paints and varnishes. Slide3
Reduction reaction of carbonyl compounds
Reduction reactions of carbonyl compounds form alcohols.
+ 2[H]
Propanal
+ 2[H]
Propan-1-ol
This reaction occurs by a
nucleophilic addition mechanism
NaBH
4 (sodium
borohydride) in the presence of water
The lone pair of electrons from the :H- nucleophile attack the electron deficient carbon atom of the carbonyl functional group forming a dative covalent bond.
At the same time, the π bond in the C=O bond breaks to produce a negatively charged intermediate. Both π
electrons are on the oxygen atomThe intermediate ion rapidly reacts with a hydrogen atom of a water moleculeThe organic product formed is an alcohol. Slide4
Characteristic Tests for Carbonyl Compounds
Identifying a carbonyl compound
Add 2,4-dinitrophenylhydrazine (2,4-DNPH)Bright orange-yellow crystals will form if the compound in an aldehyde or ketone
Filter the precipitate and purify by recrystallisation Measure the melting point of dry crystals Compare the known melting point from data bases to identify the actual carbonyl compound 2. Aldehyde or Ketone?Add warm Tollen’s Reagent
Only aldehydes will give a positive test result: if an aldehyde is present, a ‘
silver mirror’ will form
Tollen’s Reagent is ammoniacal silver nitrate. It is a mild oxidising agent containing the complex ion [Ag(NH
3)
2]+ . If an aldehyde is present, it will be oxidised to a carboxylic acid, and the Ag+ in the Tollen’s reagent will be reduced to Ag. This is what forms the ‘silver mirror’.
Ag
+
(
aq
) + e- Ag (s)