7. Carbonyl compounds

Question 1

are carbonyls, aka ketones and aldehyde, soluble in water?

Answer 1

yes. lone pair of electrons on carbonyl oxygen allows carbonyl to form H bonds with H of H2O. BUT as length of carbon chain increases, solubility decreases due to increasing length of hydrophobic alkyl chain.

Question 2

reagent and condition for oxidising primary alcohol to aldehyde.

Answer 2

K2CrO7(aq), H2SO4(aq), heat w immediate distillation.
- cannot use KMnO4 bc will strongly oxidise alcohol to COOH
- need to immediately distillate to prevent further [O] to COOH

Question 3

reagent and condition for oxidising secondary alcohol to ketone.

Answer 3

dichromate or KMnO4 (aq), heat w reflux
- dont need to distillate as ketone wont be further oxidised

Question 4

why can carbonyl compounds undergo both substitution and addition rxns?

Answer 4

substitution: bc C=O bonds causes C to bear partial positive charge, which can attract electron rich nucleophiles.
addition: bc C=O bond is unsaturated

Question 5

state the factors that can impact reactivity of carbonyls to nucleophilic attack [2]

Answer 5

1. electronic factor: electron donating alkyl/aryl groups reduce magnitude of partial positive charge on C of C=O –> decrease attraction for nucleophiles
2. steric factor: steric hindrance by number or size of functional groups bonded to C=O

Question 6

reagent and condition for formation of cyanohydrin from carbonyls.

recall: cyanohydrin has CN and OH grp

Answer 6

HCN, w trace amt of KCN as catalyst

Question 7

cyanohydrins can undergo acidic hydrolysis to form ____

Answer 7

2-hydroxycarboxylic acids

Question 8

cyanohydrins can undergo alkaline hydrolysis to form ____

Answer 8

CANNOT undergo alkaline hydrolysis bc pdt will convert back to carbonyl

Question 9

cyanohydrins can undergo reduction. T/F? product is?

Answer 9

true. amines

Question 10

state the diff pdts formed when aldehyde and ketones are reduced

Answer 10

aldehyde-> primary alcohol
ketone-> secondary alcohol

Question 11

which of the following can reduce carbonyls to alcohol?
1. H2, Ni
2. LiAlH4 in dry ether
3. NaBH4

Answer 11

all 3 can. but NABH4 is specific for carbonyls
NOTE: choice of reducing agent depends on presence of other functional grps and intended product eg. have alkene but want to retain alkene in final pdt

Question 12

describe the rxn between carbonyls and 2,4-DNPH.

Answer 12

• an addition-elimination reaction
• forms 2,4-dinitrophenylhydrazone [orange ppt] with the elimination of water
• can be used to detect carbonyls

Question 13

tollen’s reagent is used to detect ___

Answer 13

aldehydes ONLY [both aliphatic and aromatic]
- can be used to distinguish aldehyde form ketones

Question 14

state the chemical eqn for tollens test

Answer 14

Question 15

fehling’s solution is used to detect __

Answer 15

ALIPHATIC aldehydes, that are not directly bonded to benzene

Question 16

positive test for Fehling’s test would show __

Answer 16

formation of reddish brown ppt [Cu2O]

Question 17

positive test of tollen’s test would show

Answer 17

formation of silver mirror [Ag solid]

Question 18

what kind of carbonyl compounds can undergo +ve iodoform test?

Answer 18

C=O must be bonded to at least one -CH3 group. R can be H or another C atom

R can be H or another C atom

recall: 1-methylalcohols can also undergo iodoform

Question 19

how would you distinguish carbonyls from other functional groups? [1]

Answer 19

2,4-DNPH

Question 20

how would you distinguish b/w aldehydes and ketones [4]

Answer 20

1. KMnO4/K2Cr2O7 + …. [ ketone cannot be further oxidised
2. tollens: aldehyde gives silver mirror
3. fehlings: aliphatic aldehydes give orange brown ppt

Question 21

how would you distinguish b/w aliphatic and aromatic aldehyde?

Answer 21

fehlings

Question 22

how would you distinguish methyl carbonyl compounds from other carbonyl compounds?

Answer 22

iodoform test
(alkaline aqueous I2, heat)