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Chemistry A - Level > Topic 17B - Carbonyl Compounds > Flashcards

Topic 17B - Carbonyl Compounds Flashcards

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1
Q

What is the functional group that aldehydes and ketones both contain?

A

C=O, Carbonyl functional group

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2
Q

What is the structural formula of the carbonyl functional group?

A

-CHO

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3
Q

What is the structural formula of propanal?

A

CH3CH2CHO

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4
Q

What is the structural formula of propanone?

A

CH3COCH3

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5
Q

Why is there no need to use numbers to locate the carbonyl group on an aldehyde?

A

As the carbonyl group on an aldehyde is always at the end o f the carbon chain

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6
Q

How would you prepare a sample of propanal?

A

REACTANT:
- Aldehydes are formed from the oxidation of primary alcohols, in this case propan-1-ol

REAGANT
- Potassium dichromate (VI), dilute sulphuric acid

CONDITIONS:
Heat in distilling apparatus so you are able to remove the aldehyde as it forms (if you heat under reflux the primary alcohol will be fully oxidised to a carboxylic acid, you could also use an excess of alcohol to ensure the reaction doesn’t go further than making an aldehyde)

APPARATUS:
- Reflux condenser with the cold water going into the condenser furthest away from the heat
- Anti-bumping granules in the glass container where you primary alcohol is
- Thermometer at the top to measure the temperature that the aldehyde is being formed at
- The collecting place of your aldehyde must be open

EQUATION:
CH3CH2CH2OH + [O] → CH3CH2CHO + H2O

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7
Q

How would you prepare a sample of Propanone?

A

REACTANT:
- Ketones are formed from the oxidation of secondary alcohols, in this case propan-2-ol

REAGANT
- Excess potassium dichromate (VI), dilute sulphuric acid

CONDITIONS:
Heat under reflux

APPARATUS:
- Reflux apparatus with the cold water going into the tube closest to the heat
- Anti-bumping granules in the glass container where you secondary alcohol is
- Thermometer at the top to measure the temperature that the ketone is being formed at

EQUATION:
CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O

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8
Q

What is the general characteristic tests for differentiating aldehydes from ketones use?

A

The fact that aldehydes are easily oxidised and ketones are not

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9
Q

What is the main difference between the preparation of aldehydes and ketones?

A

With ketones you heat under reflux as no further oxidation can occur. With aldehydes you distill your product off immediately to prevent further oxidation.

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10
Q

How can you tell when an aldehyde has been oxidised to a carboxylic acid?

A

There would be a colour change form orange to green due to the reduction of Cr2O7(2-) ions to Cr(3+) ions

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11
Q

How do the boiling points of aldehydes compare with those of alcohols and alkanes of the same carbon chain length?

A

Aldehydes have a higher boiling point than alkanes but a lower boiling point than alcohols

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12
Q

Why is the boiling point of an aldehyde/ketone generally higher than that of an alkane?

A

Alkanes have a lower amount of electrons in comparison to aldehydes which means they form weaker LF’s . Moreover, aldehydes form permanent dipole-dipole forces as they are a polar molecule due to the C=O bond.

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13
Q

Why is the boiling point of an aldehyde/ketone generally lower than that of an alcohol?

A

Aldehydes cannot form hydrogen bonds, the strongest type of intermolecular force, as they do not have a H atom bonded to an O, F or N atom whereas alcohols can as they have an O-H bond

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14
Q

Are short chain Aldehydes and Ketones water soluble? Why?

A

Short chain Aldehydes and Ketones are water soluble as relatively most of the molecule is able to interact with the water via a hydrogen bond (2 per molecule as the O atom has two lone pairs)

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15
Q

Are long chain Aldehydes and Ketones water soluble? Why?

A

No they are not, as ,relatively, more of the molecule is unable to interact with the water via a hydrogen bond

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16
Q

Why does a polar bond arise in a carbonyl bond?

A

The polar bond arises due to the difference in electronegativities of the C and O atom in the C=O bond

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17
Q

In what way does the double bond between the C=O differ from a usual C=C bond?

A

When drawing the sigma and pi electron densities between the two atoms in the C=O bond they are shifted towards the O as it is the more electronegative atom. The electron density is unsymmetrical

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18
Q

What does the difference between the way the electron density is spread between the C=C bond in alkenes and C=O bond in aldehydes and ketones mean?

A

They react differently. Carbonyls are not susceptible for attack by an electrophile as the electronegative O in the bond controls the electrons.

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19
Q

What type of reactants are attracted to carbonyls?

A

Nucleophiles

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20
Q

What happens when the Nucleophile attacks the carbonyl compound?

A

As the nucleophile, that is attracted to the partially positive charge on the carbon atom, approaches the carbon atom, the electrons that are in the pi bond are repelled away from the carbon towards the electronegative oxygen which becomes negatively charged

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21
Q

What type of reaction is aldehyde/ketone to alcohol?

A

Reduction/ nucleophilic addition

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22
Q

What are the 2 reasons why aldehydes are generally more reactive than ketones?

A
  • The 2 alkyl groups bonded to the carbonyl group in ketones hinders that approach of attacking reagents
  • The alkyl groups donate electrons and thus reduce the positive partial charge on the carbonyl carbon atom. An aldehyde only has one alkyl group attached to the carbonyl carbon whereas a ketone has two. Therefore the Partial positive charge will be larger on the aldehyde and so is more susceptible for attack by a Nucleophile.
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23
Q

What are the 5 reactions of aldehydes and ketones we need to consider?

A
  • Oxidation
  • Reduction
  • Reaction with hydrogen cyanide
  • Condensation reaction with 2,4-dinitrophenylhydrazine
  • Triiodomethane reaction with iodine and alkali
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24
Q

What is a positive result for ketones and aldehydes when testing with 2,4-DNPH?

A

A bright orange precipitate (for a negative result no precipitate is formed)

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25
Q

How does the reaction of a carbonyl compound and 2,4-DNPH work?

A

2,4-DNPH is a benzene ring with and NH2 group coming off of it . The N of the NH2 acts as the nucleophile and attacks the carbonyl carbon on the compound

26
Q

What type of reaction is carbonyl compound + 2,4-DNPH?

A

Condensation/Addition-elimination

27
Q

What is so useful about the reaction of carbonyl compounds with 2,4-DNPH?

A

The products are all bright, orange crystalline solids with well defined melting points, which make them very useful for identifying carbonyl compounds. This reaction is often used prior to any other widespread instrumental techniques for identifying carbonyl compounds as the melting point of the compound deduced from the method is so accurate

28
Q

What does 2,4-DNPH test for?

A

Aldehydes and ketones

29
Q

What does Fehlings solution test for?

A

Aldehydes

30
Q

What is the positive test for Fehlings?

A

Red/brown precipitate

31
Q

What is Fehlings solution?

A

Cu (+) ions in alkaline solution

32
Q

Why does Fehlings produce a positive test for aldehydes but not for ketones?

A

As aldehydes can be oxidised thereby reducing the Cu2+ ion, whereas ketones cannot

33
Q

Write a half equation to show what causes the colour change when Fehlings is added to an aldehyde

A

2Cu (2+) + 2e- + H2O → Cu2O (this is the red/brown precipitate) + 2H+ + 2e-

34
Q

Write a half equation for the oxidation of propanal

A

CH3CH2CHO + H2O → CH3CH2COOH + 2H (+) + 2e-

35
Q

Write an overall equation for the oxidation of propanal by Cu (2+) ions

A

Cu(2+) ions are in an alkaline solution

2Cu(2+) + 2e- + 2OH- -> Cu2O (s) + H2O

CH3CH2CHO + 3OH- -> CH3CH2COO- + 2e- + 2H2O

FULL EQUATION:
CH3CH2CHO + 5OH- + 2Cu(2+) -> CH3CH2COO- + Cu2O(s) + 3H2O

36
Q

What does the Tollens Reagant test for?

A

Aldehydes

37
Q

What is the positive Tollens Reagant test in Aldehyde result?

A

Silver Mirror (If no aldehyde is present nothing happens)

38
Q

Why does Tollens produce a positive test for aldehydes but not for ketones?

A

As aldehydes can be oxidised , thereby reducing the Ag(+) ions present in Tollens , whereas ketones cannot be oxidised

39
Q

Write a half equation to show the colour change when Tollens Reagant is added to an Aldehyde

A

Ag(NH3)2 (+) + e(-) → Ag + NH3

40
Q

Write an overall equation for the oxidation of propanal with Ag (+) ions

A

2Ag(NH3)2 (+) + CH3CH3CHO + H2O → CH3CH2COOH + 2Ag + 4NH + 2H+

41
Q

What does potassium dichromate test for?

A

Primary and secondary alcohols, aldehydes

42
Q

What is a positive test for potassium dichromate?

A

Orange to green colour change

43
Q

What is the organic product equation for the reaction between propanal and acidified potassium dichromate?

A

CH3CH3O + [O] → CH3CH2COOH

44
Q

What causes the colour change when you add potassium dichromate to a solution that would cause a positive result?

A

The reduction of the dichromate ion, Cr2O7 (2-), to the chromium ion Cr (3+)

45
Q

Is potassium dichromate harmful?

A

Yes, it is toxic

46
Q

What does the triiodomethane reaction test for?

A
  • A carbonyl group bonded to a methyl group (CH3CO)
  • A hydroxyl group bonded to a methyl group (CH3CH(OH))
47
Q

What is the result for a positive triiodomethane test?

A

A yellow precipitate forms (triiodomethane, CHI3)

48
Q

Describe the reaction of an aldehyde such as ethanol with the triiodomethane Reagant

A
  • In the first step I2 is substituted for the hydrogens on the methyl group (only one if there are two) to form CI3CHO
  • Then the bond between the two carbon atoms is broken by water (OH- ions)
  • This forms the products CHI3 (triiodomethane) and methanoic acid
49
Q

What is the only aldehyde that will for a yellow precipitate when triiodomethane is added to it?

A

Ethanal

50
Q

Describe the reduction of aldehydes and ketones?

A

REAGANTS:
- LiAlH4 in dry ether

CONDITIONS:
- An absence of water

EQUATION:
Propanal - CH3CH2CHO + 2[H] → CH3CH2CH2OH
Propanone - CH3CHCH3 + 2[H] → CH3CHOHCH3

MECHANISM
- The H- ions in the LiAlH4 act as nucleophiles, attacking the partially positive carbonyl carbon atoms

51
Q

Why does the reduction of aldehydes and ketones need to be carried out in the absence of water?

A

Due to LiAlH4 being a powerful reducing agent, releasing H- (hydride ions) if water was present the hydride ions would react

52
Q

What
Is special about when you react an aldehyde/ketone with HCN?

A

The carbon chain length is increased

53
Q

What is the first reaction that takes place before the HCN reacts with the aldehyde/ketone?

A

KCN + H2SO4 → HCN + K2SO4

54
Q

What are the; reagants, conditions and type of mechanism for the reaction of HCN with an aldehyde/ketone?

A

REAGANTS
- KCN
- dilute H2SO4

CONDITIONS
- Room temperature
- PH 8 (a specific ph)

TYPE OF MECHANISM
- Nucleophilic addition (due to there only being one product and the first step occur in due to nucleophilic attack)

55
Q

Describe the mechanism for the reaction of HCN with an aldehyde/ketone e.g ethanal

A

1st step:
There is a partially positive charge on the carbonyl carbon and a partially negative charge on the O atom bonded to it. The Carbonyl carbon is therefore attacked by the CN- ions, acting as a nucleophile.
Consequently this breaks one of the bond between the C=O

2nd step
In the second step you have now formed CH3CHOCN with a negative charge on the O atom due to it now having a lone pair of electrons that were previously present in the C=O bond. The negative O now acts as a base and gains a proton from H-CN. Consequently, the H-CN bond is broken

3rd step
You have produced you hydroxynitrile, CH3CH(OH)CN alongside another cyanide ion

56
Q

What is the second reaction that takes place when HCN reacts with an aldehyde/ketone e.g ethanal?

A

HCN + CH3CHO → CH3CH(OH)CN

You form a hydroxynitrile, 2-hydroxypropanenitrile

57
Q

Why must the
KCN be in excess?

A

To ensure there is a supply of CN- ions to start the reaction

58
Q

In the reaction of aldehyde/ketone with HCN why must there be the specific ph of 8?

A

As if the ph is too low, acidic, the H+ protons will react with all the CN- ions. Therefore there will be not any CN- ions to act as a nucleophile in the first step

As if the ph is too high, alkaline, the OH- ions will react with the H+ from the H-CN and so there won’t be a sufficient amount of HCN to protonate oxygen in the second step

59
Q

Is the product produced from the reaction of HCN with an aldehyde/ketone, chiral?

A

Yes, as the 2nd carbon is bonded to 4 different groups so the molecule is assymetrical

60
Q

Will the product produced from the reaction of HCN with an aldehyde/ketone be optically active? Why?

A

No, the product won’t be optically active.

REASON
This is because the shape around the carbonyl carbon is Trigonal planar due to the 3 areas of electron densities and no lone pairs. Therefore there is an equal chance for the nucleophile to attack from above or below

61
Q

Describe how to draw the mechanism to show the production of both enantiomers in the reaction of HCN with an aldehyde/ketone?

A

1st step
Nucleophilic addition of CN- Nucleophile to the Trigonal planar molecule (ensure to draw this showing the bonds all in one plane)

2nd step
When you add the CN to the molecule the shape changes to tetrahedral around the carbonyl carbon why the C-CN and C-O bond are in the same plane. The oxygen acts as a base an accepts a proton from H-CN

3rd step
Both enantiomers are produced, one from above attack of nucleophile and one form the below attack

Chemistry A - Level (26 decks)

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