aldehydes + ketones Flashcards
what are carbonyls
C=O
what r the IMF in carbonyls
Pure carbonyls cannot hydrogen bond to themselves, but are attracted instead by permanent dipole forces.
solubility in water of carbonyls
The smaller carbonyls are soluble in water because they can form hydrogen bonds with water.
why do carbonyls attract nucleopjoles
The C=O bond is polarised because O is more electronegative than C
The positive carbon atom attracts nucleophiles
Oxidation of aldehydes -> CA
Reaction: aldehyde carboxylic acid
Reagent: potassium dichromate (VI) solution and dilute sulfuric acid.
Conditions: heat under reflux
Full equation for oxidation of ethanal
3CH3CHO + Cr2O72- + 8H+ 3 CH3CO2H + 4H2O + 2Cr3+
observation for oxidation of aldehyde -> CA
the orange dichromate ion (Cr2O72-) reduces to the green Cr 3+ ion
how does pollens work, give equation, substances used to make it
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+
how does fehlings soln work
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+
what reducing agents can be used to reduce cabronls
Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols.
Reduction of carbonyls
Reagents: NaBH4 In aqueous ethanol
Conditions: Room temperature and pressure
NUCLEOPHILIC ADDITION
NA mechanism:
why use NaBH4
NaBH4 contain a source of nucleophilic hydride ions (:H-) which are attracted to the positive carbon in the C=O bond.
how can carbonyls be reduced. 2 ways
- nucleophilic addition
- catalytic hydrogenation
catalytic hydrogenation to reduce carbonyls
Reagent: hydrogen and nickel catalyst Conditions: high pressure
Example Equations
CH3CHO + H2 CH3CH2OH CH3COCH3 + H2 CH3CH(OH)CH3
carbonyls -> hydroxynitriles
Reagent: potassium cyanide (KCN) and dilute sulfuric acid.
Conditions: Room temperature and pressure Mechanism: nucleophilic addition
why not use HCN 2
We could use HCN for this reaction but it is a toxic gas that is difficult to contain. KCN/NaCN are still, however, toxic, because of the cyanide ion.
Another advantage of using KCN or NaCN is that there will be a higher concentration of the CN- ion as these compounds will completely ionise. HCN is a weak acid an will only partially ionise
why does a racemate form w NA of HCN
Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the planar carbonyl group is approached equally from both sides by the HCN attacking species: results in the formation of a racemate.
