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A level Chemistry > Unit 4.4 - Aldehydes and ketones > Flashcards

Unit 4.4 - Aldehydes and ketones Flashcards

1
Q

What type of compounds are aldehydes and ketones?

A

Carbonyl compounds

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2
Q

What is a carbonyl compound?

A

Contains the carbonyl group C=O

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3
Q

Carbonyl group

A

C=O

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4
Q

C=O group

A

Carbonyl group

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5
Q

Where is the carbonyl group in aldehydes?

A

Found at the end of the aliphatic chain

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6
Q

Where is the carbonyl group in ketones?

A

Is found at positions in the aliphatic chain other than the end

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7
Q

Why is there a dipole found in the carbonyl group?

A

Because of the different electronegativities of the carbon and oxygen atoms (O2 is more electronegative than C)

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8
Q

What is formed in the carbonyl group due to the different electronegativities of the carbon and oxygen atoms?

A

A dipole

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9
Q

What type of bond is the C=O bond (carbonyl group) and why?

A

Polar covalent bond
A dipole is present

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10
Q

What do aldehydes and ketones react with?

A

Nucleophiles

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11
Q

How are aldehydes named?

A

By adding the suffix -al to the alkane stem

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12
Q

How are ketones named?

A

By adding the suffix -one to the alkane stem

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13
Q

Methanal shortened structural formula

A

HCHO

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14
Q

Ethanal shortened structural formula

A

CH3CH2O

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15
Q

Propanal shortened structural formula

A

CH3CH2CHO

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16
Q

Simplest ketone

A

Propanone

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17
Q

Propanone shortened structural formula

A

CH3COCH3

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18
Q

Butanone shortened structural formula

A

CH3COCH2CH3

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19
Q

Pentan-2-one shortened structural formula

A

CH3COCH2CH2CH3

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20
Q

What do we need to do after butanone?

A

Specify the position of the C=O bond

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21
Q

What type of forces are found between molecules of aldehydes and ketones?

A

Dipole-dipole forces

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22
Q

What are dipole-dipole forces?

A

Strong Van der Waal forces

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23
Q

Why are dipole-dipole forces found between aldehyde and ketone molecules?

A

Because of the polar nature of the carbonyl group

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24
Q

What do aldehydes and ketones have higher boiling points than?

A

Alkanes of similar molar mass

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25
Q

What do aldehydes and ketones have compared to alkanes of similar molar mass? Why?

A

Higher boiling points
Form dipole-dipole forces

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26
Q

Why do aldehydes and ketones have higher boiling points than alkanes with a similar molar mass?

A

Aldehydes and ketones = dipole-dipole forces
Alkanes = only induced dipole-induced dipole forces in alkanes

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27
Q

Compare the boiling point of ethanal to that of propane (both with Mr 44) and explain

A

Ehtanal’s is higher since it’s an aldehyde and forms dipole-dipole forces between molecules, whereas propane only has induced dipole-induced dipole forces

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28
Q

What are the names of the forces between alkanes?

A

Induced dipole-induced dipole

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29
Q

What are lower homologous of aldehydes and ketones soluble in?

A

Not only in polar solvents but also in water

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30
Q

Which aldehydes and ketones are soluble in polar solvents and water?

A

The lower homologues

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31
Q

Name an extremely useful solvent

A

Propanone

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32
Q

Why is propanone an extremely useful solvent?

A

It’s soluble not only in polar solvents but also in water

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33
Q

How are aldehydes and ketones prepared?

A

Aldehydes are formed by the oxidation of primary alcohols
Ketones are formed by the oxidation of secondary alcohols

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34
Q

What are formed from the oxidation of primary alcohols?

A

Aldehydes

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35
Q

What are aldehydes formed from the oxidation of?

A

Primary alcohols

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36
Q

What are ketones formed from the oxidation of?

A

Secondary alcohols

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37
Q

What does the oxidation of secondary alcohols form?

A

Ketones

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38
Q

What needs to be included in oxidation equations of forming aldehydes and ketones so that the equations balance?

A

[O] and H2O

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39
Q

Equation for the oxidation of ethanol to form an aldehyde

A

CH3CH2OH + [O] —> CH3CHO + H2O

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40
Q

Equation for the oxidation of propan-2-oil to form a ketone

A

CH3CH(OH)CH3 + [O] —> CH3COCH3 + H2O

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41
Q

Oxidising agent used when oxidising alcohols to aldehydes and ketones

A

Potassium dichromate (VI) acidified with sulphuric acid

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42
Q

What colour change happens to the acidified potassium (VI) dichromate in the reaction from alcohols to ketones and aldehydes? Why?

A

The orange dichromate (VI) ion is reduced to the dark green chromium (III) ion

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43
Q

List all of the chemical reactions of aldehydes and ketones to learn

A

Reduction
Reaction with 2,4 - dinitrophenylhydrazine (2,4 - DNPH)
Oxidation
Reaction with hydrogen cyanide (HCN)
Iodoform reaction/iodoform test

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44
Q

How can aldehydes and ketones be reduced to alcohols?

A

Using a suitable reducing agent

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45
Q

Reducing agent normally used to reduce aldehydes and ketones to alcohols

A

Sodium tetrahydridoborate (III)/ sodium borohydride (NaBH4)

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46
Q

NaBH4

A

Sodium tetraborohydride (III)/ sodium borohydride

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47
Q

What is NaBH4 used for?

A

As a reducing agent to reduce aldehydes and ketones to alcohols

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48
Q

What are the conditions needed to reduce aldehydes and ketones?

A

Reflux in aqueous solution

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49
Q

What are aldehydes reduced to?

A

Primary alcohols

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50
Q

What are ketones reduced to?

A

Secondary alcohols

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51
Q

Which other reducing agent could also be used in the reduction of aldehydes and ketones to alcohols?

A

LiAlH4

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52
Q

What is needed to reduce carboxylic acids to primary alcohols?

A

LiAlH4

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53
Q

What are carboxylic acids reduced to?

A

Primary alcohols

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54
Q

Which reducing agent can’t reduce carboxylic acids to primary alcohols and which reducing agent can?

A

Can’t —> NaBH4
Can —> LiAlH4

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55
Q

What type of reaction is the reaction between aldehydes/ketones and 2,4 - dinitrophenylhydrazine (2,4 - DNPH)?

A

Addition-elimination (condensation)

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56
Q

2,4 - DNPH

A

2,4 - dinitrophenylhydrazine

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57
Q

Under which conditions does 2,4 - DNPH need to be kept and why?

A

If 2,4-DNPH is kept too dry, there’s an explosion risk. It’s therefore stored in water and this contained stored in water and is used in aqueous solution.

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58
Q

Issues with 2,4 -DNPH

A

Explosive when dry
Toxic

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59
Q

Hydrazine functional group

60
Q

What is used as a test for the presence of aldehydes and ketones?

A

Addition of 2,4 - DNPH

61
Q

What is the addition of 2,4-DNPH used as a test for?

A

The presence of aldehydes and ketones

62
Q

What do aldehydes and ketones react with 2-4-dinitrophenyhydrazine to form?

A

An orange/yellow solid

63
Q

What is good and bad about the reaction between aldehydes/ketones and 2,4 - DNPH?

A

Can use it to identify the presence of aldehydes and ketones
Cannot distinguish between aldehydes and ketones

64
Q

How would be distinguish between aldehydes and ketones following a reaction with 2,4 - DNPH?

A

Purify the sample and take its melting point to work out if its an aldehyde or ketone
We would compare the value with known literature values of melting points

65
Q

What does the melting points of a solid derivative depend on?

A

The original aldehyde or ketone which was present

66
Q

How can we characterise carbonyl compounds after a reaction with 2,4 - DNPH?

A

Using melting points

67
Q

How do we get the melting point of a solid formed after reacting an aldehyde/ketone with 2,4 - DNPH?

A

A dry sample of the solid product is formed

68
Q

How can we identify the original aldehyde or ketone used in a reaction with 2,4 - DNPH using melting points?

A

Compare the melting point against known melting point values for the derivatives

69
Q

Why is the reaction between aldehydes/ketones and 2,4 - DNPH classified as a condensation reaction?

A

Water is lost

70
Q

What is it that forms the orange/yellow precipitate in the reaction between aldehydes/ketones and 2,4 -DNPH?

A

A solid derivative

71
Q

What form a double bond when aldehydes/ketones react with 2,4 - DNPH and why?

A

C and N form a double bond
H2O is lost

72
Q

What type of tests are the tests with both Tallen’s reagent and Fehling’s reagent?

A

Indentification tests

73
Q

What are readily oxidised by both Tollen’s reagent and Fehling’s reagent?

74
Q

What is formed when aldehydes are readily oxidised by Tollen’s reagent and Fehling’s reagent?

A

Carboxylic acids

75
Q

Which reagents are aldehydes readily oxidised by?

A

Tollen’s reagent
Fehling’s reagent

76
Q

why are aldehydes readily oxidised by Tollen’s reagent and Fehling’s reagent?

A

Due to the presence of an oxidisable hydrogen on the carbon atom bonded to the oxygen (at the end of the aldehyde)

77
Q

What is ehanal converted into when reacted with Tollen’s reagent or Fehling’s reagent?

A

Ethanoic acid

78
Q

What are not oxidised by Tollen’s reagent or Fehling’s reagent?

79
Q

Why are ketones not oxidised by Tollen’s reagent or Fehling’s reagent?

A

For ketones to be oxidised, the C-C bond needs to be broken

80
Q

What is the test with Tollen’s reagent?

A

The silver mirror test

81
Q

What is the positive result with Tollen’s reagent and what is the negative result?

A

A silver mirror is formed on the side of the test tube with aldehydes
No such mirror is formed with ketones

82
Q

Explain why the silver mirror effect occurs when reacting Tollen’s reagent with an aldehyde?

A

In the course of the reaction, the silver (I) ion is the oxidising agent. This will oxidise aldehydes to carboxylic acids, and in the process is reduced to silver metal. The silver metal then attaches to the walls of the container to produce a mirror.

83
Q

Oxidising agent in the reaction between aldehydes and Tollen’s reagent?

A

Silver (I) ion

84
Q

Why is the silver (I) ion the oxidising agent in the reaction between aldehydes and Tollen’s reagent?

A

Will oxidise aldehydes to carboxylic acids

85
Q

What happens when the silver (I) ion oxidises aldehydes to carboxylic acids?

A

Is reduced to silver metal

86
Q

What forms the mirror effect in the reaction between aldehydes and Tollen’s reagent?

A

When the silver metal attaches to the walls of the container to produce a mirror

87
Q

Equation to show silver (I) ions being reduced

A

Ag+ + e- —> Ag

88
Q

Positive test and negative test when reacting aldehydes and ketones with Fehling’s reagent (Benedict’s)

A

Positive —> bright red precipitate with aldehydes
Negative —> no precipitate with ketones

89
Q

What colour precipitate is formed when aldehydes react with Fehling’s reagent?

A

Bright red

90
Q

Which ions does Fehling’s reagent contain?

A

Copper (II) complex ions

91
Q

What do we always need to remember to write out when describing testing for different things?

A

Always remember to write out the negative test too
E.g - no reaction with ketones

92
Q

Oxidising agents in the reaction with Fehling’s reagent and aldehydes

93
Q

Explain how the bright red precipitate forms when aldehydes react with Fehling’s reagent

A

The Fehling’s reagent contains Cu2+ ions, which are mild oxidising agents. The copper (II) ion is reduced to copper (I) oxide (Cu2O) when oxidising aldehydes to carboxylic acids.

94
Q

What is the Cu2+ ion in Fehling’s reagent reduced to when oxidising aldehydes to carboxylic acids?

A

Copper (I) oxide
Cu2O

95
Q

Reactions used to distinguish between aldehydes and ketones in the laboratory

A

Tollen’s reagent
Fehling’s reagent

96
Q

Hydrogen cyanide

97
Q

HCN

A

Hydrogen cyanide

98
Q

What type of reaction is the reaction between aldehydes/ketones and hydrogen cyanide?

A

Nucleophilic addition

99
Q

What type of reactions do double bonds tend to undergo?

A

Addition reactions

100
Q

What happens during Nucleophilic addition?

A

A molecule of the form HX adds across the C=O double bond

101
Q

What does hydrogen cyanide do in aqueous solution?

A

Is a weak acid and dissociates, partially forming cyanide ions (CN-)

102
Q

Cyanide ions

103
Q

CN- ions

A

Cyanide ions

104
Q

What can be donated to form a covalent bond when hydrogen cyanide dissociates in aqueous solution?

A

The lone pair of electrons on the carbon atom

105
Q

What can the lone pair of electrons on the cyanide ion from the dissociation of hydrogen cyanide be used for?

A

Can be donates to form a covalent bond

106
Q

Where is the lone pair of electrons on the cyanide ion?

A

On the carbon atom

107
Q

Equation to show the dissociation of Hydrogen cyanide

A

HCN —><— H+ + :CN-

108
Q

Equation for the reaction between ethanal and hydrogen cyanide

A

CH3CHO + HCN —> CH3CH(OH)CN

109
Q

Name CH3CH(OH)CN

A

2-hydroxypropanenitrile

110
Q

What is CH3CH(OH)CN an example of?

A

A hydroxynitrile

111
Q

How do we name a hydroxynitrile?

A

Prioritise nitrile in the nomenclature, refer to the OH group as hydroxy

112
Q

What does the reaction between an aldehyde/ketone and hydrogen cyanide do to the length of the carbon chain?

A

Increases its length

113
Q

Mechanism for the reaction between hydrogen cyanide and aldehydes/ketones

A

Nucleophilic substitution

114
Q

Nucleophile in the reaction between aldehydes/ketones and hydrogen cyanide

115
Q

Where are the CN- and H+ ions from in the reaction between hydrogen cyanide and aldehydes/ketones?

A

The dissociation of HCN, since this reaction is happening in aqueous solution

116
Q

Explain the mechanism for the reaction between an aldehyde/ketone and hydrogen cyanide

A

Nucleophile attack on the positive carbon center
Inter mediate reacts with proton
Final produce (a hydroxynitrile)

117
Q

How can a hydroxynitrile be hydrolysed?

A

By refluxing with dilute sulfuric acid to give a hydroxy acid

118
Q

What forms when a hydroxynitrile is hydrolysed by refluxing with dilute sulfuric acid?

A

A hydroxy acid

119
Q

What is formed when iodine and sodium hydroxide solution are heated with certain alcohols or carbonyl compounds?

A

A yellow crystalline solid

120
Q

How does iodoform or triiodomethane appear?

A

As a yellow crystalline solid

121
Q

What is the yellow crystalline solid that forms when iodine and sodium hydroxide solution are heated with certain alcohols or carbonyl compounds?

A

Iodoform or triiodomethane

122
Q

Smell of iodoform

A

Faintly antiseptic smell

123
Q

Formula of iodiform

124
Q

CHI3

A

Iodoform or triiodomethane

125
Q

Which groups does the reaction with iodoform identify?

A

Compounds which contain…
2-ones
2-oils
Ethanal
Ethanol

126
Q

What does iodine act as in its reaction to form iodoform?

A

An oxidising agent

127
Q

What is the oxidising agent in the reactions with iodine to form iodoform?

128
Q

What does iodine oxidise in the reaction when heated alongside NaOH with certain alcohols or carbonyl compounds?

A

Will oxidise the CH3CHOH group to the CH3CO group

129
Q

What is a positive test obtained with when reacting iodine and sodium hydroxide o form iodoform?

A

Ethanol
Ethanol
2-ols
2-ones

130
Q

2-ols

A

Alcohols with the hydroxy group on the 2nd carbon atom (e.g - propan-2-ol)

131
Q

2-ones

A

Ketones with the carbonyl group on the 2nd carbon atom (e.g - propanone)

132
Q

Ethanol

133
Q

Ethanal

134
Q

CH3CHO

135
Q

CH3CH2OH

136
Q

What will negative tests occur with when reacting with iodine and NaOH solution?

A

With 1-ols and 1-ones

137
Q

Reagents for the iodoform reactions

A

Iodine and NaOH

138
Q

Product of the iodoform reactions

139
Q

Reagents and products of the iodoform reactions

A

Reagents = iodine and NaOH
Product = iodoform

140
Q

Colour of Cu2+ ions vs Cu1+ ions

A

Cu2+ = blue
Cu1+ = red

141
Q

How do we suggest starting materials for different reactions?

A

Just go through the mechanism in reverse

142
Q

Empirical formula

A

Simplest who,e number ratio of atoms of each element

143
Q

Molecular formula

A

Actual number of atoms of each element present

144
Q

What do we need to do first before taking the melting point of a solid?

A

Need to filter and purify the solid first

145
Q

Why is 2,4 - DNPH an appropriate reagent to identify aldehydes and ketones?

A

The derivatives formed using 2,4 - DNPH have suitable melting temperatures for identification (precise melting temperatures)

146
Q

How do we go from a halogenoalkane to a cyanide?

A

Add KCN or NaCN

147
Q

What do we do after a 2,4 - DNPH test?

A

Take the melting temperature

A level Chemistry (39 decks)

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