Aldehydes and ketones
 

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Aldehydes RCHO and ketones RCOR'.

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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.

2,4DNP_ethanal.gif (5082 bytes)


 

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


 

JRG Beavon 2002