carbonyl compounds

Question 1

physical properties of aldehydes and ketones
( bp, solubility)

Answer 1

1. carbonyls are moderately polar and has pd-pd interactions due to difference in polarity between C and O hence it has higher bp than alkanes but lower than hydrogen bonding
2. ketones and aldehydes are soluble in water as they are polar and form H bonds with water molecules. As carbon chain increases, strength of id-id increases so more E needed to separate carbonyl molecules.

Question 2

how to form aldehydes and ketones

Answer 2

for aldehydes - from primary alcohols - K2Cr2O7(aq), H2SO4(aq) with immediate distillation

for ketones - from secondary alcohols - K2Cr2O7, H2SO4(aq) with heat

Question 3

what type of reactions do carbonyl groups undergo

Answer 3

Nucleophilic addition - C double bond O is unsaturated so addition takes place
- CO bond is polar so delta plus on the carbonyl C atom makes it susceptible to nucleophilic attack

Question 4

what are the reactions that carbonyl groups undergo

Answer 4

1. Step up reaction
   - HCN with trace NaOH(aq) or with trace NaCN(aq) as catalyst
   note that the product can undergo acid hydrolysis to form carboxylic or reduction

• React with 2,4 dinitrophenylhydrazine
  Forms an orange ppt also the test for carbonyl compounds

Reduction to form alc
LiAlH4 in dry ether or …

Oxidation of aldehydes to carboxylic
K2Cr2O7 or KMnO4 both with H2SO4(aq) and heat under reflux

Tollens’ and Fehling’s reagent with heat forming silver mirror and brick-red ppt
- Aldehyde + 2(Ag(NH3)2)+ + 3OH- —> RCOO- + 4NH3 + 2Ag + 2H2O
- Aldehyde + 2Cu2+ +5OH- —> RCOO- + Cu2O + 3H2O

tri-iodomethane formation (only for RCOCH3)
I2(aq) with NaOH(aq) and heat
- RCO(CH3) + 3I2 + 4NaOH —> RCOO-Na+ + 3NaI + 3H2O + CHI3

Question 5

how to test for presence of carbonyl compounds

Answer 5

Bradys reagent or 2-4 DNPH which forms orange ppt

Question 6

how to distinguish aldehydes from ketones

Answer 6

Oxidation of aldehydes to carboxylic acid with K2Cr2O7 or KMnO4 in H2SO4 and heat

Question 7

describe the nucleophilic addition mechanism

Answer 7

formation of nucleophile - HCN is a weak acid that dissociates readily to form H+ and CN-
- Base neutralises HCN to give CN-
- CN- is regarded as a catalyst and regenerated

Step 1. Nucleophilic CN- attacks the e- deficient carbon forming tetrahedral intermediate
( show partial positive and negative for CO bond and lp on CN arrowed to e- deficient C and lp on O in the pdt)
Step 2. Intermediate is protonated and CN- is regenerated
( show lp of O arrowed to the partial positive H and bond between HC arrowed to partial negative C and lp on C of CN that is regenerated)

Question 8

what is the rate of nu addition given by

Answer 8

rate = k(ketone/aldehyde)(CN-)

Question 9

are ketones or aldehydes more reactive

Answer 9

aldehydes for 2 reasons

1. Carbonyl carbon of ketones is less e- deficient as it is attached to 2 e- donating grps
2. Steric hindrance as alkyl grp bulkier than H

Question 10

are the products formed optically active or not

Answer 10

not because carbonyl carbon is planar meaning there is an equal chance for CN- to attack it form above or below the plane giving rise to a racemic mixture