Aldehydes RCHO and ketones RCOR'.
Links
Aldehyde or ketone
" cyanohydrin.
Reagents: sodium cyanide, dilute sulphuric acid
and sodium hydrogen sulphite at pH5; or potassium cyanide in ethanol at
pH8.
Conditions: r.t.
CH3CHO + HCN " CH3CH(OH)CN
The reaction works with all aldehydes, but the only ketones to give cyanohydrins are propanone, butanone and pentan-3-one.
Alkanes
& alkenes
Arenes
Halogenoalkanes
Alcohols
Aldehydes and
ketones
Carboxylic acids
Acyl chlorides
Amines
Nitro compounds
Nitriles
Amides
Aldehyde
or ketone "
2,4-dinitrophenylhydrazone
Reagents: 2,4-dinitrophenylhydrazine in conc HCl or conc
H3PO4
Conditions: r.t.
Aldehyde only with [Ag(NH3)2]+
ions " silver mirror.
Reagents: silver nitrate solution with ammonia added
until the precipitate of silver oxide just re-dissolves; or silver nitrate solution with
sodium hydroxide solution to give a precipitate, which is then dissolved in ammonia. Only
the latter is Tollens' Reagent.
Conditions: warm gently (ammoniacal silver nitrate); r.t.
(Tollens').
CH3CHO + H2O + 2[Ag(NH3)2]+ " CH3COOH + 2Ag + 2NH4+ + 2NH3
Tollens
Compounds containing CH3CO or CH3CHOH
"
iodoform, CHI3
Reagents: iodine in potassium iodide
solution and sodium hydroxide; or potassium iodide solution and sodium
chlorate(I).
Conditions: warm gently. Iodoform is given as a pale yellow
precipitate.
CH3COCH3 + 4OH - + 3I2 " CHI3 + CH3COO- + 3I - + 3H2O
Note that the iodoform reaction is NOT a general test for ketones or for secondary alcohols.
Aldehyde or ketone " primary or
secondary alcohol.
Reagents: sodium borohydride in aqueous solution
Conditions: r.t. or with cooling.
CH3CHO + 2[H] " CH3CH2OH
CH3COCH3 + 2[H] " CH3CH(OH)CH3