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A Level Chemistry > Organic Chemistry 2 - Carbonyl Compounds to Carboxylic acids and Their Derivatives > Flashcards

Organic Chemistry 2 - Carbonyl Compounds to Carboxylic acids and Their Derivatives Flashcards

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1
Q

What is the general formula of an aldehyde?

A 
RCHO, where R is a hydrogen atom or alkyl group
..........O    
........// 
R - C
.........\ 
..........H
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2
Q

What is the boiling point of aldehydes in relation to other organic homologous series?

A

Boiling point lies between alkanes and alcohols and methanal is the only gas. This is because aldehydes have a permanent dipole which means there are stronger intermolecular forces than in alkanes, but weaker than the hydrogen bonds that exist between alcohol molecules

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3
Q

Solubility of aldehydes

A

Soluble in organic solvents and low Mr aldehydes (methanal and ethanal) are soluble in water

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4
Q

Explain the pattern of boiling points in aldehydes and ketones

A

Boiling point increases as number of carbons increases because the strength of London forces increases while the dipole forces stay constant

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5
Q

Explain why methylpropanal has a lower boiling point than butanal

A

Methylpropanal is branched when butanal is straight chain, and the branched methylpropanal has lower surface area of contact so a lower boiling point

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6
Q

What is the general formula of a ketone?

A 
RCOR', where R and R' are alkyl groups
...........O
.........//
R - C 
.........\
...........R'
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7
Q

What is the geometry of an aldehyde?

A

Trigonal planar carbonyl group with a bond angle of 120°

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8
Q

Which aldehydes and ketones are soluble and why?

A

Lower members (up to 3 carbons) are soluble in water because of hydrogen bonding between the lone pair of electrons in the 𝛿- oxygen in the carbonyl compound and the 𝛿+ hydrogen in the water molecule

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9
Q

Why are most carbonyl compounds insoluble in water?

A

Larger molecules have large R groups which are in no way attracted to the water molecules, outweighing the slight attraction of the C = O group

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10
Q

Why is propanone a very good solvent for organic molecules?

A

C = O part can have dipole attractions to polar molecules

Two CH3 groups have London forces

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11
Q

Oxidation of a primary alcohol

A

Primary alcohol -> aldehyde -> carboxylic acid

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12
Q

Oxidation of a secondary alcohol

A

Secondary alcohol -> ketone

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13
Q

Oxidation of a tertiary alcohol

A

No oxidation

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14
Q

What is the general preparation of an aldehyde?

A

Partial oxidation of primary alcohols using a saturated solution of sodium or potassium dichromate (VI), acidified with sulfuric acid. Temperature must be below the boiling point of the alcohol and above that of the alcohol and a distillation apparatus should be used FIX

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15
Q

Method for the preparation of ethanal

A

Heat ethanol in a flask in a distillation apparatus to 60°C using an electric heater. Add a solution of potassium dichromate in dilute sulfuric acid slowly from a tap funnel. As it distils off, collect the ethanal in a flask surrounded by iced water.

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16
Q

Why is iced water used when collecting ethanal?

A

The boiling point of ethanal is 21°C, so this prevents it from evaporating

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17
Q

What are the reagents, conditions and observations of the preparation of ethanal?

A

Reagents: Excess alcohol and potassium dichromate (VI) dissolved in dilute sulfuric acid
Conditions: Heat in a distillation apparatus at 60°C. Collect the aldehyde as it distils off
Observations: Orange potassium dichromate changes to green as chromium ions are formed

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18
Q

How can the ethanal produced through distillation be purified?

A

Ethanal can be purified by re-distillation. A water bath is used as the heat source and the fraction collected boils between 20 - 23°C

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19
Q

Equations in the preparation of ethanal

A

CH3CH2OH + [O] -> CH3CHO + H2O

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20
Q

Method for the preparation of propanone

A

Heat propan-2-ol and excess potassium dichromate (VI) dissolved in dilute sulfuric acid. Heat under reflux for about 15 minutes. Change to a distillation apparatus to distil off the propanone

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21
Q

What are the reagents, conditions and observations of the preparation of propanone?

A

Reagents: Secondary alcohol and excess potassium dichromate dissolved in dilute sulfuric acid
Conditions: Heat under reflux and distil off the ketone
Observations: Orange potassium dichromate changes to green as chromium ions are formed

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22
Q

Equations in the preparation of propanone

A

……………………………………………….O
………………………………………………..||
CH3CH(OH)CH3 + [O] -> CH3CCH3 + H2O

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23
Q

Carbonyl compounds undergo oxidation reactions with which solutions/compounds?

A

Potassium dichromate (VI)
Fehling’s or Benedict’s solution
Ammoniacal silver nitrate (Tollens’ reagent)
Iodine in the presence of alkali (iodoform)

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24
Q

Carbonyl compounds undergo reduction reactions with solutions/compounds?

A

Lithium tetrahydridoaluminate (III) (lithium aluminium hydride)

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25
Q

Carbonyl compounds undergo heterolytic nucleophilic addition reactions with which solutions/compounds?

A

Hydrogen cyanide/potassium cyanide

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26
Q

Carbonyl compounds undergo addition-elimination (condensation) reactions with which solutions/compounds?

A

Ammonia
Primary amines
Hydrazine
2,4 - dinitrophenylhydrazine

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27
Q

What is the general equation for oxidation of aldehydes in acidic conditions?

A

RCHO + [O] -> RCOOH

Produces a carboxylic acid

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28
Q

What is the general equation for oxidation of aldehydes in alkaline solution?

A

………………………………………..O
……………………………………….//
RCHO + OH^- + [O] -> RC + H2O
………………………………………..\
………………………………………….O^-

29
Q

What is Fehling’s solution and what is its test result with an aldehyde?

A

A mixture of copper (II) sulfate (which makes it blue) and sodium potassium tartrate in alkali. It is reduced from a deep-blue solution to a red precipitate of copper (I) oxide, Cu2O when warmed with an aldehyde

30
Q

What is Tollens’ reagent and what is its test result with an aldehyde?

A

Tollens’ reagent is made by adding a few drops of sodium hydroxide to silver nitrate solution and then dissolving the precipitate in dilute ammonia. The silver (I) is reduced to silver metal on warming with the aldehyde to give a silver mirror on the inside of the test tube

31
Q

What result does potassium dichromate (VI) in dilute sulfuric acid have with an aldehyde?

A

Orange acidified potassium dichromate (VI) is reduced to green chromium (III) on heating with an aldehyde

32
Q

What result does potassium manganate (VII) have with an aldehyde in acidic solution?

A

KMnO4 is reduced to colourless MN^2+

MnO4^- + 8H^+ 5e^- -> Mn^2+ + 4H2O

33
Q

What result does potassium manganate (VII) have with an aldehyde in alkaline solution?

A

KMnO4 is reduced to a brown precipitate of MnO2

MnO4^- + 2H2O + 3e^- -> MnO2 + 4OH^-

34
Q

What tests can be used to distinguish an aldehyde from a ketone?

A

A red precipitate of Cu2O on warming with Fehling’s solution
A silver mirror on warming with Tollens’ reagent.

With ketones, oxidation will not occur, so Fehling’s solution will remain blue and Tollens’ reagent would remain colourless

35
Q

Which compounds will undergo the iodoform reaction?

A

Ethanal and methyl ketones

2° methyl alcohols or ethanol also will

36
Q

What is the equation for the iodoform reaction?

A

……..O
………||
CH3CR + 3I2 +4NaOH -> CHI3 + RCOONa + 3NaI + 3H2O

Triiodomethane, CHI3, is responsible for the yellow precipitate and it has a distinctive antiseptic smell

37
Q

Process for reduction using lithium aluminium hydride

A

The dry carbonyl compound is treated with dry ether at a low temperature (from 0°C down to -78°C; dry ice temperature). There much be no naked flames in the laboratory as ether is extremely flammable. The LiAlH4 is a source of H^+ ions and produces a complex
The complex is boiled with dilute HCl to release the free alcohol - it is “worked up with dilute hydrochloric acid”

38
Q

General equation for the reduction of aldehydes using LiAlH4

A

RCHO + 2[H] -> RCH2OH

Reduced to 1° alcohols

39
Q

General equation for the reduction of ketones usng LiAlH4

A

RCOR’ + 2[H] -> RCH(OH)R’

40
Q

Why does LiAlH4 reduce compounds containing C=O but not compounds containing C=C?

A

LiAlH4 is a reducing agent that reacts specifically with polar pi bonds, so it reduces the C=O group in aldehydes, ketones, carboxylic acids and acid chlorides to alcohols, but does not reduce the non polar pi bond in C=C

41
Q

How can both C=C and C=O bonds be reduced?

A

With hydrogen and a platinum catalyst, but this is a very unusual method for reducing C=O bonds

42
Q

What type of mechanism is the addition of hydrogen cyanide and potassium cyanide?

A

Heterolytic nucleophilic addition

43
Q

What is the role of OH^- ions in the addition of hydrogen cyanide and potassium cyanide?

A

They catalyse the reaction

44
Q

What is the role of H^+ ions in the addition of hydrogen cyanide and potassium cyanide?

A

They slow the reaction

45
Q

What is the equation for the addition of hydrogen cyanide and potassium cyanide?

A

HCN + OH^- ⇌ H2O + CN^-

46
Q

What are the reagents, conditions and observations of the addition of hydrogen cyanide and potassium cyanide?

A

Reagents: HCN / KCN
Conditions: Carefully buffered at pH 9, room temperature, in a fume cupboard (toxic HCN gas)

47
Q

What is the product of the addition of hydrogen cyanide and potassium cyanide?

A

A hydroxynitrile

48
Q

What is a buffered solution?

A

A solution that resits small changes in pH -> pH does not change much when small quantities of acid and alkali are added

49
Q

Why must the pH of the solution not be too high? (addition of hydrogen cyanide and potassium cyanide)

A

H^+ ions react with OH^- ions which moves equilibrium to the LHS, meaning there are not enough CN^- ions for the 1st step

50
Q

Why must the pH of the solution not be too low? (addition of hydrogen cyanide and potassium cyanide)

A

If concentration of OH^- ions is high then equilibrium will move to the RHS, meaning there is not enough HCN for the second step

51
Q

Why does the cyanide ion react with carbonyl compounds but not with alkenes?

A

The C = O is polar while C = C is non-polar, and the 𝛿+ carbon in the carbonyl attracts the cyanide ion, but C = C has no dipole so it cannot attract the ion.
In the intermediate step, the molecule has a negatively charged oxygen, and the electronegative oxygen is more able to hold this charge than the carbon (that it would have to in C = C)

52
Q

How does reaction of hydrogen cyanide and potassium cyanide with carbonyls compare to its reaction with halogenoalkanes?

A

Reagents and conditions for halogenoalkanes: KCN in aqueous alcohol, heat under reflux for 24 hours
Reagents and conditions for carbonyls: HCN / KCN, buffered at pH 9, room temperature

Less vigorous conditions needed for carbonyls
Intermediate for carbonyls is tetrahedral (with angle 109.5°) whereas the SN2 intermediate for halogenoalkanes has the 5-bonded carbon atom which has a smaller bond angle so it experiences steric hindrance

53
Q

How can you convert ethanol into 2-hydroxypropanoic acid?

A

Add K2Cr2O7 and dilute H2SO4, heating in a distillation apparatus to form ethanal, then react with HCN / KCN, buffered at pH9 to form 2-hydroxypropanenitrile, then heat under reflux with dilute HCl to form the 2-hydroxypropanoic acid

54
Q

Isomerism of hydroxynitriles after addition of hydrogen cyanide and potassium cyanide to a carbonyl

A

Four different groups are attached to the central C atom, so the molecule can exist as 2 non-superimposable mirror images.
When HCN adds on to an aldehyde or asymmetric ketone the product is a racemic mixture. This is because the carbonyl compound is planar around the C = O group so the cyanide ion (nucleophile) can attack from above or below the molecule, meaning equal amounts of both optical isomers can form so the product is not optically active

55
Q

What are the reagents and conditions for hydrolysis of nitriles to hydroxycarboxylic acids?

A

Reagents: Dilute HCl
Conditions: Heat under reflux

56
Q

What is the equation for hydrolysis of nitriles to a hydroxycarboxylic acid?

A

RCHOHC≡N + 2H2O + H^+ -> RCHOHCOOH + NH4^+

57
Q

What are the reagents and conditions for the reduction of nitriles to a hydroxyamine?

A

Reagents: Reducing agent, LiAlH4 in dry ether at 0°C, then work up with dilute HCl
Conditions: Dry ether at 0°C, step 2 at room temperature

58
Q

Addition-elimination reactions of carbonyl compounds

A

Aldehydes and ketones react with compounds containing the H2N- group (amine). The lone pair of electrons on the nitrogen atom acts as a nucleophile and forms a bond with the 𝛿+ carbon atom in the C=O group. Instead of an H^+ ion adding onto the O^- formed, the substance loses a water molecule to form a C=N bond

59
Q

What is the derivative of a carbonyl compound’s reaction with 2,4-dinitrophenylhydrazine?

A 
2,4-dinitrophenylhydrazone
R
..\
...C = N.........NO2
../...........\........|\_\_\_ 
R'.............N - /  O  \  - NO2 (pretend that's a ring)
.............../.......\\_\_\_/   
............H
Where R and R' are groups attached to the C=O of a carbonyl compound
60
Q

What are the key characteristics of 2,4-dinitrophenylhydrazone, the derivative of the carbonyl and 2,4-DNPH reaction?

A

It appears as a yellow or orange precipitate and it is insoluble in water. It has a much greater Mr than the original compound and therefore a higher melting point

61
Q

How can the identity of a carbonyl compound be found from the derivative of the 2,4-DNPH reaction?

A

React the carbonyl compound with 2,4-dinitrophenylhydrazine then filter off the precipitate. Recrystallise the precipitate using hot ethanol, dry the purified product and measure its melting point. Refer to a data book and compare this melting point with those of 2,4-dinitrophenylhydrazine derivatives of different aldehydes and ketones to establish the identity of the original carbonyl

62
Q

What is a problem with using the derivative of the 2,4-DNPH reaction to identify a carbonyl compound?

A

It destroys the original compound as it cannot be released from the derivative

63
Q

Isomerism in 2,4-dinitrophenylhydrazones

A

The C=N allows for EZ isomerism to appear, as there are 2 different groups attached to the C which are equivalent to EZ isomers, and the lone pair of electrons on the N takes up space

64
Q

What are the 4 factors that can affect the susceptibility of a carbonyl carbon atom to nucleophilic attack?

A
  • Inductive effect
  • Mesomeric effect
  • Steric effect
  • Stability of the leaving group
65
Q

What is the inductive effect and how does it affect the susceptibility of a carbonyl carbon atom to nucleophilic attack?

A

The more electronegative group X is, the more electrons in the C-X bond are pulled towards X and the more 𝛿+ the carbonyl carbon atom is. Hence the more readily is the carbonyl carbon atom attacked by nucleophiles

66
Q

What is the mesomeric effect and how does it affect the susceptibility of a carbonyl carbon atom to nucleophilic attack?

A

If X has a lone pair of electrons, these can be donated back towards the carbonyl carbon atom to some extent making the carbonyl carbon atom less 𝛿+ and less readily attacked by nucleophiles. This is called a +M mesomeric effect

67
Q

What is the steric effect and how does it affect the susceptibility of a carbonyl carbon atom to nucleophilic attack?

A

If the group X is large and bulky, attack by nucleophiles on the carbonyl carbon atom is sterically hindered.

68
Q

How does the stability of the leaving group affect the susceptibility of a carbonyl carbon atom to nucleophilic attack?

A

Once the nucleophile has attacked, the C=O bond can reform with X being lost as a X^-, making it a nucleophilic substitution. For this to happen, X has to be stable, which is the case for acid derivatives as X contains an electronegative atom.
Aldehydes and ketones will not undergo nucleophilic substitution as H^- and R^- are unstable, so they either undergo nucleophilic addition or addition-elimination reactions (where it is the O from C=O which is lost)

A Level Chemistry (25 decks)

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