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A2 Chemistry > Organic |3.3.8 Aldehydes and ketones > Flashcards

Organic |3.3.8 Aldehydes and ketones Flashcards

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1
Q

What is the carbonyl group?

A
  • C=O
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2
Q

What is the functional group and general formula for an aldehyde?

A
  • RCHO (C double bonded to O, single bonded to H and R)
  • Ethanal
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3
Q

What is the functional group for a ketone?

A
  • RCOR’ (C double bonded to O)
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4
Q

How do you name aldehydes?

Give an example using 2 carbon

A
  • -al suffix.
  • C=O is on the end of the chain.
  • CH3CHO
  • Ethanal
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5
Q

How do you name ketones?

Give an example using 3 carbon

A
  • -one suffix
  • C=O is in the middle of the chain
  • CH3COCH3
  • Propanone
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6
Q

What type of intermolecular forces do molecules with carbonyls group have and why?

A
  • Permanent dipole-dipole forces due to the polar C=O bond (O is delta-)
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7
Q

How soluble are carbonyls in water? What influences their solubility?

A
  • Smaller carbonyls are soluble in water as they can form hydrogen bonds with water.
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8
Q

What bond in carbonyl compounds is usually involved in reactions and why?

A
  • C=O bond
  • O is more electronegative than carbon.
  • The positive carbon atom attracts nucleophiles.
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9
Q

Which bond between C=C in alkenes or C=O is stronger and why?

A
  • C=O is stronger than the C=C bond in alkenes.
  • doesn’t undergo addition reactions easily.

Contrast to the electrophiles that are attracted to the C=C.

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10
Q

What is an aldehyde oxidised into?

A
  • Carboxylic acid.
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11
Q

Outline the reaction, reagent and conditions for the oxidation of aldehydes into carboxylic acids.

A

REACTION
Aldehyde -> carboxylic acid.

REAGENT
Potassium dichromate (VI) solution and sulfuric acid.

CONDITIONS
Heat under reflux.

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12
Q

What is the full oxidation equation for aldehydes?

A

3CH3CHO + Cr2O72- + 8H+ -> 3 CH3CO2H + 4H2O + 2Cr3+

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13
Q

What is the observation for the oxidation of aldehydes with potassium dichromate solution?

A
  • Orange dichromate ion (Cr2O72-) reduces to green Cr3+ ion.
  • Turns from orange to green.
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14
Q

What other chemical tests can be used to distinguish between aldehydes and ketones?

A
  • Fehlings solution
  • Tollen’s reagent.
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15
Q

Outline the reaction, reagent, conditions and observation for the functional group test for aldehydes using tollen’s reagent.

A

REACTION
Aldehyes only oxidised by tollen’s reagent into a carboxylic acid.
Silver (I) ions reduced to silver atoms.

REAGENT
Tollen’s reagent } mix aqueous ammonia and silver nitrate -> [Ag(NH3)2]+.

CONDITIONS
heat gently

OBSERVATION
Aldehydes = silver mirror forms
Ketones = no change

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16
Q

Outline an equation for tollen’s reagent with an aldehyde.

A

CH3CHO + 2Ag+ + H2O -> CH3COOH + 2Ag + 2H+

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17
Q

Outline the reaction, reagent, conditions and observation for the functional group test for aldehydes using fehling’s solution.

A

REACTION
Aldehydes only oxidised by fehlings solution into a carboxylic acid.
Coppper (II) ions are reduced to copper (I) oxide.

REAGENT
Fehling’s solution containing blue Cu2+ ions.

CONDITIONS
Heat gently.

OBSERVATION
Aldehydes = Blue Cu2+ ions = red preciptate of Cu2O.
Ketones = do not react.

18
Q

Outline an equation for fehling’s solution with an aldehyde.

A

CH3CHO + 2Cu2+ + 2H2O -> CH3COOH + Cu2O + 4H+

19
Q

What are aldehydes reduced into?

A
  • Primary alcohols.
20
Q

What are ketones reduced into?

A
  • Secondary alcohols.
21
Q

What reagent is used to reduce carbonyls (aldehydes and ketones)?

A
  • NaBH4 in aqueous solution.
    (sodium tetrahydridoborate)

OR

  • LiAlH4 (lithium tetrahydridoaluminate)
22
Q

The reduction reactions of carbonyls is an example of what mechanism?

A
  • Nucleophillic addition.
23
Q

Outline a nucelophillic addition mechanism with propanone. (CH3C=OCH3)

A
  1. :H- (from NaBH4 contains source of nucleophillic hydride ions (:H-) which are attracted to the + carbon in the C=O bond.) attacks C on C=O bond, breaks the bond between C and O in double bond.
  2. O becomes O:- and gains H+ from a weak acid or water.
  3. CH3COHHCH3.
24
Q

Outline the reagents and conditions of the reduction of carbonyls.

A

REAGENT
NaBH4 in aqueous ethanol

CONDITIONS
Room temperature and pressure

25
Q

Outline an equation of the reduction of aldehydes using NaBH4.

Propanal

A
  • CH3CH3C=OH + 2[H] -> CH3CH2CH2OH
  • Propanal -> Propan-1-ol
26
Q

Outline an equation of the reduction of ketones using NaBH4.

Propanone

A
  • CH3C=OCH3 + 2[H] -> CH3CHOHCH3
  • Propanone -> propan-2-ol
27
Q

What other way can carbonyls be reduced aside from using NaBH4?

A
  • Catalytic Hydrogenation.
28
Q

Outline the reagent and conditions of the reduction of carbonyls using catalytic hydrogenation.

A

REAGENT
Hydrogen and nickel catalyst.

CONDITIONS
High pressure.

29
Q

Outline an equation for the reduction of an aldehyde using catalytic hydrogenation.

CH3CHO

A

CH3CHO + H2 -> CH3CH2OH

30
Q

Outline an equation for the reduction of a ketone using catalytic hydrogenation.

CH3COCH3

A

CH3COCH3 + H2 -> CH3CH(OH)CH3

31
Q

Nucleophillic addition reactions of carbonyl compounds with …

A
  • KCN, followed by dilute acid, to produce hydroxynitriles.
32
Q

Outline the reaction, reagent, conditions and mechanism for carbonyls to form hydroxynitriles.

A

REACTION
Carbonyl -> hydroxynitriles

REAGENT
NaCN / KCN / HCN and dilute sulfuric acid.

CONDITIONS
Room temperature and pressure.

MECHANISM
Nucleophillic addition

33
Q

What is provided by the NaCN and H2SO4 acid when carbonyls form hydroxynitriles?

A

NaCN
nucleophillic CN- ions.
H2SO4 acid
H+ ions in the second step of the mechanism.

34
Q

How do you name hydroxynitriles?

A
  • CN becomes part of the main chain and carbon n.o 1.
35
Q

Outline an equation for ketones when HCN is added.

A
  • CH3COCH3+ HCN -> CH3C(OH)(CN)CH3
  • 2-hydroxy-2-methylpropanenitrile
36
Q

Outline an equation for aldehydes when HCN is added.

A
  • CH3CHO + HCN -> CH3CH(OH)CN
  • 2-hydroxypropanenitrile
37
Q

What is the advantage of using KCN / NaCN?

A
  • there is a higher concentration of CN- ions as these compounds will completely ionise.
  • HCN is a weak acid -> partially ionise.
38
Q

Hazards of using KCN.

and how to reduce risks.

A
  • Irritant
    Dangerous if ingested or inhaled.
    React with moisture to produce hydrogen cyanide, a highly toxic gas.
  • To reduce risk lab coats, goggles and gloves should be worn and the experiment carried out in a fume cupboard.
39
Q

What do aldehydes and unsymmetrical ketones form?

A
  • mixtures of enantiomers when they react with KCN followed by a dilute acid.
40
Q

Explain why nucleophilic addition reactions of carbonyls with KCN, followed by dilute acid, can produce a mixture of enantiomers.

A
  • The C=O bond is planar, so nucleophillic attack of the delta + carbon can occur from above or below the C=O bond.
  • This means 2 different optical isomers/enantiomers can form

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