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a2 chem - organic > aldehydes and ketones > Flashcards

aldehydes and ketones Flashcards

1
Q

what is the carbonyl group?

A

C=O

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2
Q

where is the carbonyl carbon found in aldehydes?

A

at the end of a carbon chain (func. group = COH)

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3
Q

what suffix is used to name aldehydes?

A

-anal (e.g CH3CH2CH2COH = butanal)

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4
Q

aldehydes are…

A

easily oxidised to form carboxylic acids

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5
Q

what is functional group for carboxylic acids?

A

-COOH

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6
Q

what suffix is used for carboxylic acids?

A

-ioc acid (e.g. CH3CH2COOH = propanoic acid)

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7
Q

where is the carbonyl carbon found in ketones?

A

in the middle of a carbon chain

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8
Q

what suffix is used for ketones?

A

-one (e.g CH3C(O)CH3 = propanone)

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9
Q

out of aldehydes and ketones, which is easier to oxidise?

A

aldehydes

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10
Q

what two chemical tests are used to distinguish between aldehydes and ketones?

A
  • Fehling’s solution
  • Tollens reagent (silver mirror)
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11
Q

both Tollen’s and Fehling’s are…

A

weak oxidising agents - therefore change is observed when there is an aldehyde, NOT A KETONE

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12
Q

what is Fehling’s solution?

A

an alkaline solution of copper(II) sulfate - complex Cu2+ ions give it its blue colour

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13
Q

what happens to the solution in the presence of Fehling’s solution?

A

Cu2+ is reduced (Cu2+ + e- → Cu ) to form a brick red precipitate of
copper(I) oxide which has the formula Cu2O

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14
Q

what happens to the ALDEHYDE in the Fehling’s solution reaction?

A

it is oxidised to form a carboxylic acid

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15
Q

what is the process of using Fehling’s solution?

A
  • a few drops of the organic solution added to a test tube containing Fehling’s
  • gently warmed in a hot water bath
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16
Q

how do you prepare Tollens reagent?

A
  • add a drop of sodium hydroxide to silver(I) nitrate solution to make a precipitate of silver(I) oxide
  • dissolve this precipitate by adding dilute ammonia
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17
Q

what complex ion does Tollen’s reagent contain?

A

[Ag(NH3)2]+

18
Q

what happens to the complex ion in Tollen’s reagent in the presence of an aldehyde?

A

[Ag(NH3)2]+ is reduced to form a precipitate of metallic silver (Ag+(aq) + e- → Ag(s)

19
Q

what happens to the ALDEHYDE in the presence of Tollen’s reagent?

A

it is oxidised to form a carboxylic acidw

20
Q

what forces exist in aldehydes and ketones?

A
  • van der waals forces
  • permanent dipole-dipole forces
21
Q

what is the effect of the forces on BP of aldehydes and ketones?

A

it means that the b.p. of aldehydes and ketones are higher than those of alkanes with a similar Mr

22
Q

bp of aldehydes/ketones: as the length of the carbon chain increases…

A

the b.p. of the aldehydes/ ketones increases too. This is due to a greater number of electrons,
and therefore stronger van der Waals’ forces

23
Q

as branching increases…

A

the b.p. of the aldehydes/ketones decreases.
* This is due to less effective permanent dipole-dipole forces.
* Think of it as though the branches are getting in the way of the Intermolecular forces, while they’re trying to pack the molecules
together.

24
Q

how soluble are aldehydes and ketones in water?

A

all short chain aldehydes and ketones are soluble in water because the
polar carbonyl groups are able to form H-bonds with water molecules.

25
Q

what is the effect of increasing chain length on solubility?

A
  • As the chain length increases, the aldehydes and ketones become less
    soluble.
  • This is because the hydrocarbon chain that doesn’t have the carbonyl group on it is non-polar.
  • The longer this chain is, the less polar the molecule is so therefore the less soluble
26
Q

what kind of reaction do carbonyls undergo?

A

nucleophilic addition reactions

27
Q

why do carbonyls undergo nucleophilic addition reactions?

A

oxygen is more electronegative therefore attracts the electrons from the covalent bond - leaves the carbon with a partial positive charge

28
Q

what are some examples of nucleophilic addition reactions which aldehydes and ketones undergo?

A
  • The reduction of aldehydes to primary alcohols, and ketones to secondary alcohols, using the reducing agent NaBH4 in aqueous
    solution.
  • The reaction of the -:CN ion with aldehydes and ketones to produce
    hydroxynitriles (KCN, followed by dilute acid)
29
Q

what is NaBH4?

A

sodium borohydride
- it is a complex anion but behaves like a hydride ion (H-) So we draw it as a H- in the mechanism; that’s the species acting as the
nucleophile

30
Q

what happens in the intermediate of the addition reaction with NaBH4?

A

gains a H+ from the acid

31
Q

what are the conditions for the reduction of aldehydes by NaBH4?

A

Reagent: Aqueous NaBH4 followed by H2SO4

Conditions: Room temp.

Mechanism: Nucleophilic - H- = nucleophile

Reaction type: Addition/Reduction

Product: Primary alcohol

32
Q

what are the conditions for the reduction of ketones by NaBH4?

A

Reagent: Aqueous NaBH4 followed by H2SO4

Conditions: Room temp.

Mechanism: Nucleophilic - H- = nucleophile

Reaction type: Addition/Reduction

Product: Secondary alcohol

33
Q

what is the overall equation for the reaction of aldehydes with sodium borohydride?

A

R-CO-H + 2[H] –> R-CH2(OH)

34
Q

what is the overall equation for the reaction of ketones with sodium borohydride?

A

R-CO-R + 2[H] –> R-CH(OH)-R

35
Q

what are the conditions for the reaction of carbonyls with hydrogen cyanide?

A

Reagent: HCN - formed by reaction with KCN followed by dilute H2SO4

Conditions: Room temp.

Mechanism: Nucleophilic SO CN- is the nucleophile

Reaction type: Addition

Product: Hydroxynitrile

36
Q

what is the overall equation for the reaction of aldehydes with hydrogen cyanide?

A

R-CO-H + HCN –> RCH(OH)CN

37
Q

what is the overall equation for the reaction of aldehydes with hydrogen cyanide?

A

R-CO-R’ + HCN → RC(OH)R’CN

38
Q

what are the hazards of using KCN?

A

HCN gas is not used directly in the lab as it is an extremely toxic + flammable gas. Instead it is formed in situ in the reaction mixture by
adding a dilute acid (H2SO4) to an aqueous solution of KCN.
* KCN must be handled carefully as it is toxic when ingested + forms HCN when in contact w/ acid.

39
Q

what do aldehydes and unsymmetrical ketones form?

A

mixtures of enantiomers (when they react with KCN followed by dilute acid)
- this is because the products of the reactions contain chiral carbon atoms

40
Q

nucleophillic addition reactions of KCN followed by dilute acid can produce…

A

a racemic mixture of enantiomers which is optically inactive
because:
1. The C atom in the planar carbonyl group can be attacked by a
nucleophile from above or below.
2. There is an equal chance of attack from above or below.

a2 chem - organic (6 decks)

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