17B: Carbonyls

Question 1

Carbonyl Chemistry

Answer 1

1) dipole dipole intermolecular forces, no hydrogen bonding, between molecules, lower mp/bp compared to alcohols and carboxylic acids of similar size
2)water soluble: can H bond with water molecules, solubility decreases with increased chain length due to more london forces and greater strength (delocalised electrons)
3) open to nucleophilic attack, delta +ve carbon targeted by nucleophile
H- in reduction, CN- in nucleophillic addition
4) Oxidation: primary alcohols –> aldehydes + water (distill with H+/Cr2O72- –> carboxylic acids (refulux with H+/cr2O72-), secondary alcohols (reflux H+/Cr2O72-) –> ketone +water

Question 2

Reducing agent for Carbonyl reduction

Answer 2

LiAlH4 in dry ether

Question 3

Aldehyde + 2(H)

Answer 3

primary alcohol, nucleophilic addition

Question 4

Ketone + 2(H)

Answer 4

secondary alcohol, nucleophillic addition

Question 5

aldehydes + HCN

Answer 5

1) 2-hydroxy______nitriles
2) nucleophillic addition
3) HCN —> NaCN/KCN in acid (H2SO4), HCN produced in situ

Question 6

Aldehydes + HCN mechanism

Answer 6

1) CN- attacks delta C
2) C=O pi bond breaks by heterolytic fission
3) O- attacks delta H in HCN
4) HCN breaks by heterolytic fission
5) chain length increased by 1

Question 7

ketone + HCN

Answer 7

1) 2-hydroxy-2-alkyl_________nitrile
2) reagent: HCN: NaCN/KCN in H2sO4 (HCN produced in situ)
3) Shortest R group now considered alkyl side chain and CN is carbon 1

Question 8

Nucleophilic addition - chiral products

Answer 8

1) reaction of HCN with all aldehydes and asymmetrical ketons give optical isomers
2) CN- attacks delta C from either side on trigonal planar
3) 50/50 - produced 2 different enantiomers in equimolar quantities
4) racemix mixture formed
5) resulting solution will have no overall effect on plane polarised light

Question 9

Testing for carbonyls

Answer 9

1)reagent: 2,4 DNPH at room temp
2)orange sol. gives orange ppte in prescene of any carbonyl (aldehyde/ketone)
3)colour varies depending on carbonyl compound
4) specific carbonly present can be found by
filtering out ppte
drying in oven
obtaining mp
compare mp to data booklet to identify specific carbonyl
5)I2(aq)/NAOH(aq) - the iodoform test
6) gives a pale yellow crystalline ppte with only COCH3
7) produces iodoform CHI3 using CH3 group from molecule