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Chemistry (Organic) > Aldehydes and Ketones > Flashcards

Aldehydes and Ketones Flashcards

1
Q

What is a carbonyl group

A

C=O group

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2
Q

In terms of the position of the carbonyl group, what is the difference between aldehydes and ketones

A

Aldehydes: Carbonyl group on an end of carbon of the chain chain
Ketones: Carbonyl group on an inner carbon of the chain

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3
Q

What is the suffix for ketones

A

-one

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4
Q

What is the suffix for aldehydes

A

-al

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5
Q

What are aldehydes oxidised into and what are the conditions and reagent for this reaction

A

Carboxylic acids
Reagent: potassium dichromate (VI) solution and
dilute sulphuric acid.
Conditions: heat under reflux

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6
Q

What are ketones oxidised into

A

Ketones cant be oxidised easily

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7
Q

Write an equation showing the oxidation of propanal

A
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8
Q

How do we make tollens

A

Add a few drops of NaOH to a solution of silver nitrate (forms a pale brown precipitate)
Add a few drops of dilute ammonia until the precipitate dissolves
This is tollens reagent

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9
Q

What are the practical steps in using tollens to distinguish between an aldehyde and ketone

A
  • Add aldehyde/ketone to tollens in a test tube
  • Place in a hot water bath
    If an aldehyde is present, silver mirror appears
    If ketone is present no silver mirror appears
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10
Q

Why do we not heat the test tube with a Bunsen burner when testing for aldehydes and ketones

A

Aldehydes and ketones are flammable

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11
Q

Write an equation for the reaction between tollens and ethanal
State what occurs

A

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

Aldehydes only are oxidised by
Tollen’s reagent into a carboxylic acid and
the silver(I) ions are reduced to silver atoms

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12
Q

What is the observation when aldehydes are oxidised into a ketone

A

Orange dichromate ion (Cr2O72-) is reduced to the green Cr3+ ion

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13
Q

Write the full equation for the oxidation of ethanal (using dichromate) into ethanol

A

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

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14
Q

What are the conditions and positive test result for distinguishing between aldehydes and ketones using Fehling’s

A

Conditions: heat gently
Observation: Aldehydes :Blue Cu 2+ ions in solution
change to a red precipitate of Cu2O. Ketones do
not react.

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15
Q

Write an equation for the reaction between Fehling’s solution and ethanal
State what occurs

A

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

Aldehydes :Blue Cu 2+ ions in solution
change to a red precipitate of Cu2O. Ketones do
not react.

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16
Q

What do we use as a reducing agent when reducing aldehydes and ketones

A

NaBH4
Represented as [H]
Source of hydride (H-) ions

17
Q

What are aldehydes reduced to

A

Primary alcohols

18
Q

How many [H]’s are required for the reduction aldehydes and ketones and where do these [H]’s come from

A

2 are required
H- from reducing agent
H+ from water

19
Q

What are ketones reduced to

A

Secondary alcohols

20
Q

What is the name of the mechanism by which aldehydes and ketones are reduced using NaBH4

A

Nucleophilic addition

21
Q

Outline the general mechanism for the reduction of aldehydes and ketones, by representing R as either H for an aldehyde or a carbonyl group for a ketone
State the name of this mechanism

A

Nucleophilic addition

Partial charges on first C&O
22
Q

What is an alkyl group

A

e.g. CH3

23
Q

What are carbonyl compounds reduced to by CN- ions

A

Hydroxynitriles

24
Q

What is the name of the mechanism by which carbonyl compounds are reduced to hydroxynitriles

A

Nucleophilic addition

25
Q

What is a hydroxynitrile

A

A molecule which contains an OH and CN group

26
Q

Outline the general mechanism by which CN- ions reduce carbonyl compounds to a hydroxynitrile

A
27
Q

What are the 2 methods of reducing a carbonyl compound to a hydroxynitrile. State the reagent and conditions for both

A

Method 1:
Reagent: HCN
Conditions: None

Method 2:
Reagent KCN
Conditions: Dilute H2SO4(aq) (Source of H+)

28
Q

What is the general equation for the reduction of carbonyl compounds to hydroxynitriles using CN- ions

A
29
Q

Evaluate the different safety risks of using KCN or HCN in the reduction of carbonyl compounds to form hydroxynitriles

A

Both release toxic CN-
But KCN is a solid so is only toxic if ingested
HCN is a gas so easier to inhale especially is there’s a leak
Therefore for safety reasons KCN and dilute H2SO4 is often used over HCN

30
Q

Evaluate the different rates of reaction when using KCN or HCN in the reduction of carbonyl compounds to form hydroxynitriles

A

HCN: SLOW RATE. HCN is a weak acid and only partially dissociates and so lower [CN-]
KCN: FASTER RATE. KCN completely dissociates therefore a higher [CN-]

31
Q

What is a hazard of using KCN

A

FASTER RATE. KCN completely dissociates therefore a higher [CN-]
Toxic if ingested

32
Q

When do we form a mix of enantiomers in the nucleophilic addition reactions of KCN, followed by dilute acid

A

If we use an unsymmetric ketone or aldehyde (apart from methanal)

Chemistry (Organic) (8 decks)

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