3.8 Aldehydes and Ketones Flashcards
what intermolecular forces are present in carbonyls?
- pure carbonyls cannot H-bond to themselves
- attracted by permanent dipole forces
why are only small chain carbonyls soluble in water?
- they can form hydrogen bonds with water
- longer chain carbonyls effectively become non polar, as vdw forces arise and dominate
explain why carbonyls can undergo reactions
- C=O bond polarised as O is more electronegative than C, attracting nucleophiles
- C=O bond is stronger than C=C, ddoes not undergo addition reactions easily
give the reaction, reagent and conditions for oxidation of aldehydes
reaction - aldehyde into carboxylic acid
reagents - potassium dichromate solution and dilute H2SO4
conditions - heat under reflux
what is the observations for oxidation of aldehyde!
- orange dichromate ion reduces to green Cr3+ ion
give the reaction and observation when using TOLLENS REAGENT to test for aldehydes
conditions - heat gently
reaction - silver ions reduced to silver atoms
observation - silver mirror forms inside coating of test tube
give the reaction and observation when using FEHLINGS SOLUTION to test for aldehydes
conditions - heat gently
reaction - copper ions reduced to copper oxide
observation - blue ions change into a red precipitate
give the reagents and conditions for reduction of carbonyls with NABH4
reagents - NaBH4 in aqueous ethanol
conditions - room temp and pressure
mechanism - nucleophilic addition
why can a carbonyl undergo reduction with NaBH4?
NaBH4 contains a source of nucleophilic hydride ions (:H-), which are attracted to the C
give the reagent and conditions and equation for CATALYTIC HYDROGENATION
reagent - hydrogen and nickel catalyst
conditions - high pressure
CH3CHO + H2 > CH3CH2OH
give the reagent conditions and mechanism for addition of hydrogen cyanide to a carbonyl
reagent - potassium cyanide, and dilute H2SO4
conditions - room temp and pressure
mechanism - nucleophilic addition
why is using NaCN / KCN better than HCN?
- HCN is a toxic gas, hard to contain
- higher concentration of CN nucleophile as complete dissociation.
why is a racemate formed from addition of HCN
- planar carbonyl group
- approached equally from both sides
- by attacking species
