*6.1.2 carbonyls

Question 1

carbonyls

Answer 1

aldehyde (al ending, at the end)

and ketones (one ending, in middle of chain)

Question 2

solubility in water

Answer 2

smaller carbonyls are soluble as they can form hydrogen bonds with water

pure carbonyls cannot hydrogen bond but bond with permanent dipole forces

Question 3

reactions of carbonyls

Answer 3

C=O bond polarised as O is more electronegative than carbon

positive carbon atom attracts nucleophiles

in comparison to C=C bond in alkenes, C=O is stronger and doesn’t undergo addition reactions easily (electrophiles attracted to C=C)

Question 4

oxidation reactions of carbonyls

Answer 4

K2Cr2O7 oxidising agent oxidising alcohols and aldehydes

primary alc > aldehydes > carboxylic acid

secondary alc > ketones

tertiary alc > don’t oxidise

Question 5

oxidation of aldehydes

Answer 5

aldehyde > carboxylic acid

reagent: potassium dichromate (VI) solution and dilute H2SO4

3CH3CHO + (Cr2O7)2- + 8H+ > 3 CH3CO2H + 4H2O + 2Cr3+

orange dichromate ion (Cr2O7)2- reduced to green Cr3+

Question 6

test for presence of aldehyde

Answer 6

tollens reagent (aqueous ammonia and silver nitrate) - [Ag(NH3)2]+

heat gently

aldehydes oxidised by tollens reagent into carboxylic acid
- silver (I) ions reduced to silver atoms

silver mirror forms coating inside of test tube
- ketones have no change

CH3CHO + 2Ag+ + H2O > CH3COOH + 2Ag + 2H+

Question 7

reduction of carbonyls

Answer 7

NaBH4 in aqueous ethanol

room temperature and pressure

reducing agents e.g. NaBH4 and LiAlH4 reduce carbonyls to alcohols

aldehydes > primary alcohols
ketones > secondary alcohols

Question 8

NaBH4

Answer 8

contains a source of nucleophilic hydride ions (H-) which are attracted to positive carbon in C=O bond

Question 9

alternative way to reduce carbonyls

Answer 9

catalytic hydrogenation

reagent: hydrogen and nickel catalyst

high pressure

Question 10

Carbonyl + hydrogen cyanide

Answer 10

Carbonyl > hydroxynitrile

NaCN and dilute H2SO4

Room temp

Nucleophilic substitution

Question 11

Tollens reagent

Answer 11

Aqueous ammonia + silver nitrate [Ag(NH3)2]

Heat gently

Aldehydes oxidised into carboxylic acid
- silver (I) ions reduced to silver atoms

Aldehydes - silver mirror forms , ketones no change

CH3CHO + 2Ag+ + H2O > CH3COOH + 2Ag + 2H+