Aldehydes and Ketones Flashcards
Aldehydes oxidised
readily, to carboxylic acids
Reagent: potassium dichromate (VI) solution and dilute sulfuric acid.
Conditions: heat under reflux
Observation: the orange dichromate ion (Cr2O72-) reduces to the green Cr 3+ ion
Fehling’s
Reagent: Fehling’s solution containing blue Cu 2+ ions.
Conditions: heat gently
Reaction: aldehydes only are oxidised by Fehling’s Solution into a carboxylic acid. The copper (II) ions are reduced to copper(I) oxide . .
Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react.
CH3CHO + 2Cu2+ + 2H2O->CH3COOH + Cu2O + 4H+
Tollen’s
Reagent: Tollens’ reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3)2]+ .
Conditions: heat gently
Reaction: aldehydes only are oxidised by Tollens’ reagent into a carboxylic acid. The silver(I) ions are reduced to silver atoms
Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no change.
CH3CHO + 2Ag+ + H2O -> CH3COOH + 2Ag + 2H+
Aldehydes
If the C=O is on the end of the chain with an H attached it is an aldehyde.
The name will end in –al
Ketones
If the C=O is in the middle of the chain it is a ketone
The name will end in -one
Intermolecular forces
Pure carbonyls cannot hydrogen bond to themselves, but are attracted instead by permanent dipole forces
Solubility
The smaller carbonyls are soluble in water because they can form hydrogen bonds with water.
Reactions of Carbonyls
The C=O bond is polarised because O is more electronegative than
carbon. The positive carbon atom attracts nucleophiles.
In comparison to the C=C bond in alkenes, the C=O is stronger and does not undergo addition reactions easily.
Reduction
Aldehydes-> primary alcohol
Ketones-> secondary alcohol
NaBH4
Nucleophilic addition
Room temp and pressure
Nucleophilic Addition Mechanism
:H- to C
C=O to O
then
O:- to H+
Catalytic Hydrogenation
Reduction
Reagent: hydrogen and nickel catalyst
Conditions: high pressure
Nucleophilic addition HCN
carbonyl -> hydroxynitrile
Reagent: sodium cyanide (NaCN) and dilute sulfuric acid.
Conditions: Room temperature and pressure
nucleophile: CN-
KCN advantage
higher concentration of the CN- ion as these compounds will completely ionise. HCN is a weak acid an will only partially ionise
Enantiomers
Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the trigonal planar carbonyl is approached from both sides by the HCN attacking species: results in the formation of a racemate.
dangers of KCN
irritant
dangerous if ingested or inhaled
react with moisture to produce hydrogen cyanide, toxic gas
