Alkanal and Alkanone
Introduction
- contain carbonyl group
- alkanals (aldehyde)
- alkanone (ketone)
Physical Properties of carbonyl group
- appearance, odour, taste
- melting and boiling point
(1) higher b.p. than alkane (since polar)
(2) lower b.p. than alcohol (since no intermolecular hydrogen bond)
- solubility: short hydrocarbon chain, miscible in water
Preparation of alkanal and alkanone
- chemical oxidation
- catalytic dehydrogenation
Oxidation reactions of alkanal and alkanone
- Alkanone is unable to take part in the reaction below because it does not possess a hydrogen atom
on carbonyl carbon atom.
(1) by H+/KMnO4 or H+/K2Cr2O7
CH3CHO + [O] --------> CH3COOH
(2) Tollen's reagent (ammoniacal silver nitrate)
Positive result: silver is deposited on the wall of a test-tube.
The chemical equation.
mechanism.
should be freshly prepared because silver nitrate Ag3N will be formed on standing which is highly
explosive.
(3) Fehling solution
The chemical equation.
Benzalkanal does not reduce Fehling solution because benzene ring donate electrons to C atoms to
reduce the partial positive charge.
Glucose
- Oxidation of Glucose
(1) Fehling's Reaction (copper sulphate, Rochelle salt and caustic soda)
greenish ppt., to bright red and then to reddish brown.
Glucose reduce cupric salt to cuprous oxide, it is oxidized to gluconic acid.
(2) Silver mirror test
- Detection of glucose in urine by clinistrix paper: pink to purple coloration indicates the presence of
glucose.
Uses of carbonyl compound
- as raw materials for making plastic e.g. urea-methanal, perspex
- as solvent: propanone is able to dissolve both polar and non-polar compound
- as a preservative: formalin contains 40% methanal aqueous solution
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