topic 17B

Question 1

functional group of aldehyde

Answer 1

name ends in -al
-CHO at the end of the chain

Question 2

functional group of ketone

Answer 2

name ends in -one
-C=O in the middle of the chain

Question 3

solubility of aldehydes and ketones

Answer 3

• smaller carbonyls are soluble in water because they form hydrogen bonds with water

Question 4

solubility of pure carbonyls

Answer 4

• cannot hydrogen bond but instead bond by permanent dipole bonding

Question 5

physical properties of aldehydes and ketones

Answer 5

• higher melting & boiling points than ethers but lower than alcohols

Question 6

bonding in aldehydes and ketones

Answer 6

• C=O double bond hence polar molecules
• no hydrogen bonding hence weaker

Question 7

oxidation reaction of aldehydes
- reaction
- reagent
- conditions
- observation

Answer 7

• aldehyde to carboxylic acid
• potassium dichromate solution and dilute sulfuric acid
• heat under reflux
• orange to green

Question 8

reaction of carbonyl compounds with Tollen’s reagent
- reagent
- conditions
- reaction
- observation

Answer 8

• tollen’s reagent formed by mixing aqueous NH3 and silver nitrate
• heat gently
• aldehydes only are oxidised by Tollen’s into a carboxylic acid
• silver mirror forms. Ketones result in no change

Question 9

reaction of carbonyl compounds with Fehling’s solution
- reagent
- conditions
- reaction
- observation

Answer 9

• Fehling’s solution containing blue Cu2+ ions
• heat gently
• only aldehydes are oxidised by Fehling’s solution & Cu2+ ions are reduced to copper oxide
• aldehydes : blue to red. Ketones: no reaction/ stays blue cos it cant be oxidised

Question 10

reduction of carbonyls
- reagent
- conditions
- reaction type
- role of reagent

Answer 10

• LiAlH4 in dry ether
• room temp & pressure
• reduction to alcohol
• reducing agent

Question 11

reduction of aldehydes and ketones - what will they become? -

Answer 11

• aldehydes: reduced to primary alcohols
• ketones: reduced to secondary alcohols

Question 12

addition reaction of carbonyls
- reaction
- reagent
- conditions
- mechanism

Answer 12

• carbonyl to hydroxynitrile
• HCN in the presence of KCN
• room temp & pressure
• nucleophilic addition

Question 13

reaction of carbonyls with iodine in presence of alkali
- reagent
- conditions

Answer 13

• iodine and NaOH
• warm very gently

Question 14

Reaction with 2,4-dinitro phenylhydrazine

Answer 14

• reacts with both aldehydes and ketones
• product is an orange precipiatate
• can be used as a test for a carbonyl group in a compound

Question 15

what test gives a positive result for aldehydes but a negative result for ketones

Answer 15

• Fehling’s/Benedict’s and Tollen’s

Question 16

how do you identify an aldehyde?

Answer 16

• 1cm3 of Fehling’s 1 and 1cm3 of Fehling’s 2 in to a test tube
• add 3cm3 of the unknown substance to the test tube
• swirl the test tube and place it in the water bath

Question 17

how is the test for iodines carried out

Answer 17

• mix iodine and NaOH
• warm gently with a methyl carbonyl to form a pale yellow precipitate

Question 18

what does 2,4-DNPH test for

Answer 18

• carbonyl group (aldehyde or ketone)
• not carboxylic acids or esters

Question 19

how can 2,4-DNPH be used to distinguish between 2 carbonyl compounds

Answer 19

• 5cm3 of 2,4-DNPH to 3 drops of the reagent
• orange precipitate will form
• carry out recrystallise the products
• carry out melting point determination of the pure crystals
• compare database values of the m.p.

Question 20

further purifying a substance

Answer 20

• recrystallization
  1. dissolve impure solid in minimum amount of warm solvent
  2. if any solid remains filter under suction, discarding the solid
  3. cool filtrate in ice bath - crystals should form
  4. filter under suction retaining crystals and discarding filtrate
  5. rinse crystals with cold solvent to stop redissolving
  6. scrape crystals onto a watch glass and allow to dry

Question 21

why do crystals form

Answer 21

• the cooler the temperature the lower the solubility hence the greater the mass of crystals formed

Question 22

what is the method of filtration under pressure

Answer 22

1. place filter paper on diameter of the funnel
2. wet the paper so it sticks to the funnel
3. turn on vaccum and pour in mixture
4. solid will remain, as liquid is sucked through

Question 23

comparing melting points to determine impurity

Answer 23

• larger the difference between melting point found and data book value, the more impure