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chemistry > topic 17B > Flashcards

topic 17B Flashcards

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1
Q

functional group of aldehyde

A

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

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2
Q

functional group of ketone

A

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

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3
Q

solubility of aldehydes and ketones

A
  • smaller carbonyls are soluble in water because they form hydrogen bonds with water
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4
Q

solubility of pure carbonyls

A
  • cannot hydrogen bond but instead bond by permanent dipole bonding
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5
Q

physical properties of aldehydes and ketones

A
  • higher melting & boiling points than ethers but lower than alcohols
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6
Q

bonding in aldehydes and ketones

A
  • C=O double bond hence polar molecules
  • no hydrogen bonding hence weaker
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7
Q

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

A
  • aldehyde to carboxylic acid
  • potassium dichromate solution and dilute sulfuric acid
  • heat under reflux
  • orange to green
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8
Q

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

A
  • 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
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9
Q

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

A
  • 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
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10
Q

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

A
  • LiAlH4 in dry ether
  • room temp & pressure
  • reduction to alcohol
  • reducing agent
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11
Q

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

A
  • aldehydes: reduced to primary alcohols
  • ketones: reduced to secondary alcohols
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12
Q

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

A
  • carbonyl to hydroxynitrile
  • HCN in the presence of KCN
  • room temp & pressure
  • nucleophilic addition
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13
Q

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

A
  • iodine and NaOH
  • warm very gently
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14
Q

Reaction with 2,4-dinitro phenylhydrazine

A
  • reacts with both aldehydes and ketones
  • product is an orange precipiatate
  • can be used as a test for a carbonyl group in a compound
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15
Q

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

A
  • Fehling’s/Benedict’s and Tollen’s
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16
Q

how do you identify an aldehyde?

A
  • 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
17
Q

how is the test for iodines carried out

A
  • mix iodine and NaOH
  • warm gently with a methyl carbonyl to form a pale yellow precipitate
18
Q

what does 2,4-DNPH test for

A
  • carbonyl group (aldehyde or ketone)
  • not carboxylic acids or esters
19
Q

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

A
  • 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.
20
Q

further purifying a substance

A
  • 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
21
Q

why do crystals form

A
  • the cooler the temperature the lower the solubility hence the greater the mass of crystals formed
22
Q

what is the method of filtration under pressure

A
  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
23
Q

comparing melting points to determine impurity

A
  • larger the difference between melting point found and data book value, the more impure
24
Q
A

chemistry (28 decks)

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