Unit 4.4 - Aldehydes and ketones Flashcards
1
Q
What type of compounds are aldehydes and ketones?
A
Carbonyl compounds
2
Q
What is a carbonyl compound?
A
Contains the carbonyl group C=O
3
Q
Carbonyl group
A
C=O
4
Q
C=O group
A
Carbonyl group
5
Q
Where is the carbonyl group in aldehydes?
A
Found at the end of the aliphatic chain
6
Q
Where is the carbonyl group in ketones?
A
Is found at positions in the aliphatic chain other than the end
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)
8
Q
What is formed in the carbonyl group due to the different electronegativities of the carbon and oxygen atoms?
A
A dipole
9
Q
What type of bond is the C=O bond (carbonyl group) and why?
A
Polar covalent bond
A dipole is present
10
Q
What do aldehydes and ketones react with?
A
Nucleophiles
11
Q
How are aldehydes named?
A
By adding the suffix -al to the alkane stem
12
Q
How are ketones named?
A
By adding the suffix -one to the alkane stem
13
Q
Methanal shortened structural formula
A
HCHO
14
Q
Ethanal shortened structural formula
A
CH3CH2O
15
Q
Propanal shortened structural formula
A
CH3CH2CHO
16
Q
Simplest ketone
A
Propanone
17
Q
Propanone shortened structural formula
A
CH3COCH3
18
Q
Butanone shortened structural formula
A
CH3COCH2CH3
19
Q
Pentan-2-one shortened structural formula
A
CH3COCH2CH2CH3
20
Q
What do we need to do after butanone?
A
Specify the position of the C=O bond
21
Q
What type of forces are found between molecules of aldehydes and ketones?
A
Dipole-dipole forces
22
Q
What are dipole-dipole forces?
A
Strong Van der Waal forces
23
Q
Why are dipole-dipole forces found between aldehyde and ketone molecules?
A
Because of the polar nature of the carbonyl group
24
Q
What do aldehydes and ketones have higher boiling points than?
A
Alkanes of similar molar mass
25
What do aldehydes and ketones have compared to alkanes of similar molar mass? Why?
Higher boiling points
Form dipole-dipole forces
26
Why do aldehydes and ketones have higher boiling points than alkanes with a similar molar mass?
Aldehydes and ketones = dipole-dipole forces
Alkanes = only induced dipole-induced dipole forces in alkanes
27
Compare the boiling point of ethanal to that of propane (both with Mr 44) and explain
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
28
What are the names of the forces between alkanes?
Induced dipole-induced dipole
29
What are lower homologous of aldehydes and ketones soluble in?
Not only in polar solvents but also in water
30
Which aldehydes and ketones are soluble in polar solvents and water?
The lower homologues
31
Name an extremely useful solvent
Propanone
32
Why is propanone an extremely useful solvent?
It’s soluble not only in polar solvents but also in water
33
How are aldehydes and ketones prepared?
Aldehydes are formed by the oxidation of primary alcohols
Ketones are formed by the oxidation of secondary alcohols
34
What are formed from the oxidation of primary alcohols?
Aldehydes
35
What are aldehydes formed from the oxidation of?
Primary alcohols
36
What are ketones formed from the oxidation of?
Secondary alcohols
37
What does the oxidation of secondary alcohols form?
Ketones
38
What needs to be included in oxidation equations of forming aldehydes and ketones so that the equations balance?
[O] and H2O
39
Equation for the oxidation of ethanol to form an aldehyde
CH3CH2OH + [O] —> CH3CHO + H2O
40
Equation for the oxidation of propan-2-oil to form a ketone
CH3CH(OH)CH3 + [O] —> CH3COCH3 + H2O
41
Oxidising agent used when oxidising alcohols to aldehydes and ketones
Potassium dichromate (VI) acidified with sulphuric acid
42
What colour change happens to the acidified potassium (VI) dichromate in the reaction from alcohols to ketones and aldehydes? Why?
The orange dichromate (VI) ion is reduced to the dark green chromium (III) ion
43
List all of the chemical reactions of aldehydes and ketones to learn
Reduction
Reaction with 2,4 - dinitrophenylhydrazine (2,4 - DNPH)
Oxidation
Reaction with hydrogen cyanide (HCN)
Iodoform reaction/iodoform test
44
How can aldehydes and ketones be reduced to alcohols?
Using a suitable reducing agent
45
Reducing agent normally used to reduce aldehydes and ketones to alcohols
Sodium tetrahydridoborate (III)/ sodium borohydride (NaBH4)
46
NaBH4
Sodium tetraborohydride (III)/ sodium borohydride
47
What is NaBH4 used for?
As a reducing agent to reduce aldehydes and ketones to alcohols
48
What are the conditions needed to reduce aldehydes and ketones?
Reflux in aqueous solution
49
What are aldehydes reduced to?
Primary alcohols
50
What are ketones reduced to?
Secondary alcohols
51
Which other reducing agent could also be used in the reduction of aldehydes and ketones to alcohols?
LiAlH4
52
What is needed to reduce carboxylic acids to primary alcohols?
LiAlH4
53
What are carboxylic acids reduced to?
Primary alcohols
54
Which reducing agent can’t reduce carboxylic acids to primary alcohols and which reducing agent can?
Can’t —> NaBH4
Can —> LiAlH4
55
What type of reaction is the reaction between aldehydes/ketones and 2,4 - dinitrophenylhydrazine (2,4 - DNPH)?
Addition-elimination (condensation)
56
2,4 - DNPH
2,4 - dinitrophenylhydrazine
57
Under which conditions does 2,4 - DNPH need to be kept and why?
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.
58
Issues with 2,4 -DNPH
Explosive when dry
Toxic
59
Hydrazine functional group
N2H4
60
What is used as a test for the presence of aldehydes and ketones?
Addition of 2,4 - DNPH
61
What is the addition of 2,4-DNPH used as a test for?
The presence of aldehydes and ketones
62
What do aldehydes and ketones react with 2-4-dinitrophenyhydrazine to form?
An orange/yellow solid
63
What is good and bad about the reaction between aldehydes/ketones and 2,4 - DNPH?
Can use it to identify the presence of aldehydes and ketones
Cannot distinguish between aldehydes and ketones
64
How would be distinguish between aldehydes and ketones following a reaction with 2,4 - DNPH?
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
What does the melting points of a solid derivative depend on?
The original aldehyde or ketone which was present
66
How can we characterise carbonyl compounds after a reaction with 2,4 - DNPH?
Using melting points
67
How do we get the melting point of a solid formed after reacting an aldehyde/ketone with 2,4 - DNPH?
A dry sample of the solid product is formed
68
How can we identify the original aldehyde or ketone used in a reaction with 2,4 - DNPH using melting points?
Compare the melting point against known melting point values for the derivatives
69
Why is the reaction between aldehydes/ketones and 2,4 - DNPH classified as a condensation reaction?
Water is lost
70
What is it that forms the orange/yellow precipitate in the reaction between aldehydes/ketones and 2,4 -DNPH?
A solid derivative
71
What form a double bond when aldehydes/ketones react with 2,4 - DNPH and why?
C and N form a double bond
H2O is lost
72
What type of tests are the tests with both Tallen’s reagent and Fehling’s reagent?
Indentification tests
73
What are readily oxidised by both Tollen’s reagent and Fehling’s reagent?
Aldehydes
74
What is formed when aldehydes are readily oxidised by Tollen’s reagent and Fehling’s reagent?
Carboxylic acids
75
Which reagents are aldehydes readily oxidised by?
Tollen’s reagent
Fehling’s reagent
76
why are aldehydes readily oxidised by Tollen’s reagent and Fehling’s reagent?
Due to the presence of an oxidisable hydrogen on the carbon atom bonded to the oxygen (at the end of the aldehyde)
77
What is ehanal converted into when reacted with Tollen’s reagent or Fehling’s reagent?
Ethanoic acid
78
What are *not* oxidised by Tollen’s reagent or Fehling’s reagent?
Ketones
79
Why are ketones *not* oxidised by Tollen’s reagent or Fehling’s reagent?
For ketones to be oxidised, the C-C bond needs to be broken
80
What is the test with Tollen’s reagent?
The silver mirror test
81
What is the positive result with Tollen’s reagent and what is the negative result?
A silver mirror is formed on the side of the test tube with aldehydes
No such mirror is formed with ketones
82
Explain why the silver mirror effect occurs when reacting Tollen’s reagent with an aldehyde?
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
Oxidising agent in the reaction between aldehydes and Tollen’s reagent?
Silver (I) ion
84
Why is the silver (I) ion the oxidising agent in the reaction between aldehydes and Tollen’s reagent?
Will oxidise aldehydes to carboxylic acids
85
What happens when the silver (I) ion oxidises aldehydes to carboxylic acids?
Is reduced to silver metal
86
What forms the mirror effect in the reaction between aldehydes and Tollen’s reagent?
When the silver metal attaches to the walls of the container to produce a mirror
87
Equation to show silver (I) ions being reduced
Ag+ + e- —> Ag
88
Positive test and negative test when reacting aldehydes and ketones with Fehling’s reagent (Benedict’s)
Positive —> bright red precipitate with aldehydes
Negative —> no precipitate with ketones
89
What colour precipitate is formed when aldehydes react with Fehling’s reagent?
Bright red
90
Which ions does Fehling’s reagent contain?
Copper (II) complex ions
91
What do we always need to remember to write out when describing testing for different things?
Always remember to write out the *negative* test too
E.g - no reaction with ketones
92
Oxidising agents in the reaction with Fehling’s reagent and aldehydes
Cu2+ ions
93
Explain how the bright red precipitate forms when aldehydes react with Fehling’s reagent
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
What is the Cu2+ ion in Fehling’s reagent reduced to when oxidising aldehydes to carboxylic acids?
Copper (I) oxide
Cu2O
95
Reactions used to distinguish between aldehydes and ketones in the laboratory
Tollen’s reagent
Fehling’s reagent
96
Hydrogen cyanide
HCN
97
HCN
Hydrogen cyanide
98
What type of reaction is the reaction between aldehydes/ketones and hydrogen cyanide?
Nucleophilic addition
99
What type of reactions do double bonds tend to undergo?
Addition reactions
100
What happens during Nucleophilic addition?
A molecule of the form HX adds across the C=O double bond
101
What does hydrogen cyanide do in aqueous solution?
Is a weak acid and dissociates, partially forming cyanide ions (CN-)
102
Cyanide ions
CN-
103
CN- ions
Cyanide ions
104
What can be donated to form a covalent bond when hydrogen cyanide dissociates in aqueous solution?
The lone pair of electrons on the carbon atom
105
What can the lone pair of electrons on the cyanide ion from the dissociation of hydrogen cyanide be used for?
Can be donates to form a covalent bond
106
Where is the lone pair of electrons on the cyanide ion?
On the carbon atom
107
Equation to show the dissociation of Hydrogen cyanide
HCN —><— H+ + :CN-
108
Equation for the reaction between ethanal and hydrogen cyanide
CH3CHO + HCN —> CH3CH(OH)CN
109
Name CH3CH(OH)CN
2-hydroxypropanenitrile
110
What is CH3CH(OH)CN an example of?
A hydroxynitrile
111
How do we name a hydroxynitrile?
Prioritise nitrile in the nomenclature, refer to the OH group as hydroxy
112
What does the reaction between an aldehyde/ketone and hydrogen cyanide do to the length of the carbon chain?
Increases its length
113
Mechanism for the reaction between hydrogen cyanide and aldehydes/ketones
Nucleophilic substitution
114
Nucleophile in the reaction between aldehydes/ketones and hydrogen cyanide
CN-
115
Where are the CN- and H+ ions from in the reaction between hydrogen cyanide and aldehydes/ketones?
The dissociation of HCN, since this reaction is happening in aqueous solution
116
Explain the mechanism for the reaction between an aldehyde/ketone and hydrogen cyanide
Nucleophile attack on the positive carbon center
Inter mediate reacts with proton
Final produce (a hydroxynitrile)
117
How can a hydroxynitrile be hydrolysed?
By refluxing with dilute sulfuric acid to give a hydroxy acid
118
What forms when a hydroxynitrile is hydrolysed by refluxing with dilute sulfuric acid?
A hydroxy acid
119
What is formed when iodine and sodium hydroxide solution are heated with certain alcohols or carbonyl compounds?
A yellow crystalline solid
120
How does iodoform or triiodomethane appear?
As a yellow crystalline solid
121
What is the yellow crystalline solid that forms when iodine and sodium hydroxide solution are heated with certain alcohols or carbonyl compounds?
Iodoform or triiodomethane
122
Smell of iodoform
Faintly antiseptic smell
123
Formula of iodiform
CHI3
124
CHI3
Iodoform or triiodomethane
125
Which groups does the reaction with iodoform identify?
Compounds which contain…
2-ones
2-oils
Ethanal
Ethanol
126
What does iodine act as in its reaction to form iodoform?
An oxidising agent
127
What is the oxidising agent in the reactions with iodine to form iodoform?
Iodine
128
What does iodine oxidise in the reaction when heated alongside NaOH with certain alcohols or carbonyl compounds?
Will oxidise the CH3CHOH group to the CH3CO group
129
What is a positive test obtained with when reacting iodine and sodium hydroxide o form iodoform?
Ethanol
Ethanol
2-ols
2-ones
130
2-ols
Alcohols with the hydroxy group on the 2nd carbon atom (e.g - propan-2-ol)
131
2-ones
Ketones with the carbonyl group on the 2nd carbon atom (e.g - propanone)
132
Ethanol
CH3CH2OH
133
Ethanal
CH3CHO
134
CH3CHO
Ethanal
135
CH3CH2OH
Ethanol
136
What will negative tests occur with when reacting with iodine and NaOH solution?
With 1-ols and 1-ones
137
Reagents for the iodoform reactions
Iodine and NaOH
138
Product of the iodoform reactions
Iodoform
139
Reagents and products of the iodoform reactions
Reagents = iodine and NaOH
Product = iodoform
140
Colour of Cu2+ ions vs Cu1+ ions
Cu2+ = blue
Cu1+ = red
141
How do we suggest starting materials for different reactions?
Just go through the mechanism in reverse
142
Empirical formula
Simplest who,e number ratio of atoms of each element
143
Molecular formula
Actual number of atoms of each element present
144
What do we need to do first before taking the melting point of a solid?
Need to filter and purify the solid first
145
Why is 2,4 - DNPH an appropriate reagent to identify aldehydes and ketones?
The derivatives formed using 2,4 - DNPH have suitable melting temperatures for identification (precise melting temperatures)
146
How do we go from a halogenoalkane to a cyanide?
Add KCN or NaCN
147
What do we do after a 2,4 - DNPH test?
Take the melting temperature
