Unit 4.2 - Aromaticity Flashcards
1
Q
When do radicals form?
A
Hen a covalent bond breaks
2
Q
Give an example of when a radical forms
A
When you crack a large alkane
3
Q
Describe a radical
A
Has an unpaired electron and is very reactive
4
Q
What type of reactions do radicals form?
A
Very rapid ones
5
Q
How are radicals represented?
A
With a dot
6
Q
Nucleophiles
A
Ions or molecules which can donate a lone pair of electrons, forming a covalent bond
7
Q
What are Nucleophiles attracted to?
A
Positive charges or positive centres of charge
8
Q
Examples of positive centres of charge
A
Nuclei, carbon-halogen bonds
9
Q
How are Nucleophiles charged?
A
Negatively or neutrally
10
Q
What do we need to remember to show on Nucleophiles?
A
Lone pairs of electrons
11
Q
Three types of reagent in organic chemistry
A
Radicals
Nucleophiles
Electrophiles
12
Q
Electrophiles
A
Ions or molecules which can accept a lone pairs of electrons, forming a covalent bond
13
Q
What are electrophiles attracted to?
A
Negative charges or negative centres of charge
14
Q
Which part of an alkene will be attacked by an electrophile and why?
A
The pi bond since it’s a negative centre of charge
15
Q
Negative centre of charge of an alkene
A
Pi bond
16
Q
How are electrophiles charged?
A
Positively or neutrally
17
Q
With what do we need to show lone pairs - Nucleophiles or electrophiles?
A
Nucleophiles
18
Q
What are the 5 types of reactions we need to learn?
A
Additional
Substitution
Elimination
Oxidation/reduction (redox)
Hydrolysis
19
Q
Additional reaction description
A
2 molecules are brought together to produce 1 molecule
A + B ——> C
Must normally be a double bond available
20
Q
Example of an addition reaction
A
Electrophilic addition
21
Q
What type of reactions do alkenes always undergo?
A
Electrophilic addition
22
Q
Substitution reaction description
A
An atom or group of atoms replaced another atom or group of atoms in a molecule
Normally the of the group which is introduced must be similar to the group which is replaced
23
Q
Examples of substitution reactions
A
Nucleophilic substituion
Free radical substitution
24
Q
Example of a free radical substation reaction
A
Photo chlorination of methane
25
Elimination reaction description
A simple molecule such as water is eliminated from the molecule
AB ——> A + B
A double bond often forms as a result of the reaction (opposite of addition)
26
During which type of reaction does a double bond often form?
Elimination
27
What would an elimination reaction be known as if water was the eliminated molecule?
Dehydration
28
Redox reaction description
Consists of both oxidation and reduction - can’t have one without the other
29
All of the possible ways of defining oxidation
Loss of electrons
Gain of oxygen
Loss of hydrogen
30
All of the possible ways of defining reduction
Gain of electrons
Loss of oxygen
Gain of hydrogen
31
3 examples of redox reactions
Combustion
Total oxidation of primary alcohols
Dichromate (VI) reduced to Cr3+
32
Colour change when dichromate (VI) is reduced to Cr3+
Orange to dark green
33
Hydrolysis reaction description
Chemical reaction between the organic compound and water
Very slow = needs a catalyst
34
Examples of catalysts used in hydrolysis reactions + why they’re used
Sodium hydroxide
HCl
Very slow reactions
35
Example of a hydrolysis reaction
Nucleophilic substitution
36
What type of compound is benzene?
Aromatic
37
Simplest aromatic compound
Benzene
38
Describe benzene at room temperature
A colourless liquid
39
Give 2 features of benzene
Distinctive smell
Highly toxic
40
What is benzene soluble in and what isn’t it soluble in and why?
Insoluble in water (immiscible)
Soluble in non-polar solvents
It’s a non-polar molecule
41
Describe benzene in water
Insoluble (immiscible)
42
Immiscible meaning
Doesn’t mix
43
What type of intermolecular forces are formed when benzene in dissolved in non-polar solvents?
Van der Waals
44
What type of molecule is benzene?
Non-polar
45
What type of molecules do polar solvents dissolve?
Polar molecules (and vice versa for non-polar solvents and molecules)
46
Empirical formula of benzene
CH
47
Molecular formula of benzene
C6H6
48
What structure for Benzene did August Kekule suggest?
A six-membered carbon ring of alternating double and single bonds
49
What were the issues with the Kekule model for Benzene?
Could not explain some of its reactions
50
Which of benzene’s reactions could the Kekule model not explain?
It should react as an alkene and easily undergo addition reactions with Br2 and decolorize bromine water (no decolorisation = not classified as a true alkene)
51
What did Kekule propose to explain why his benzene model did not react as an alkene as it should?
That benzene had 2 forms and suggested that one form changed to the other so quickly that an approaching molecule would have no time to react with it by addition
52
Resonance arrow
<——>
53
Give an example of when a resonance arrow is needed
To show the two resonance Kekule forms of benzene
54
How does the actual structure of benzene compare to the structure first predicted?
It’s a lot more stable
55
Cyclohexene formula
C6H10
56
What type of molecule is cyclohexene?
An alkene
57
What happens to cyclohexene on hydrogenation?
It forms the cyclic alkane cyclohexane
58
Cyclohexane formula
C6H12
59
Why would it be expected that the enthalpy of hydrogenation of benzene would be 3x that of cyclohexene?
Because cyclohexene only has 1 C=C bond whereas benzene is a cyclic alkene that’s a triene, therefore it has 3 C=C bonds
60
How does the actual value for the enthalpy change of hydrogenation of benzene compare to the theoretical value? Why?
It’s considerably less than calculated
Benzene is more stable than predicted
61
Why is benzene more stable than predicted?
Because of the stabilisation provided by pi electron delocalisation
62
How do we work out the stabilisation energy of benzene?
It’s the difference between the theoretical value and the calculated value of its enthalpy of hydrogenation
63
How would we work out the enthalpy of formation of benzene using bond energies?
Remember that benzene contains 3 C-C and 3 C=C bonds and remember BERP
64
Chemical formula of benzene
C6H6
65
Structure of benzene
Planar cyclic structure
66
How are the carbon atoms arranged in benzene?
In a regular hexagon
67
What do the electrons of the 6 different carbon atoms in benzene do?
3 of the 4 outer electrons in each carbon atom are involved in forming single covalent bonds - two between the carbon atoms and one between the carbon and hydrogen atoms
68
What type of framework is given rise to from the arrangement of the outer electrons of each carbon atom in benzene?
A Trigonal planar framework
69
What does the Trigonal planar framework of benzene give rise to and what is this similar to?
The hexagonal structure
Is a bit similar to graphite
70
Why does each carbon have an electron left that are found in the p orbitals of benzene?
Because 3 of the 4 electrons in each carbon atom are involved in forming single covalent bonds, and so each carbon atom has one left
71
What do the total of 6 electrons left for each carbon atom (those that are not involved in forming single covalent bonds) in benzene do?
These electrons are found in p orbitals at 90 degrees to the plane of the molecule and they interact to give a pi electron delocalised bond around the molecule
72
Which electrons are found in p orbitals at 90 degrees to the plane of the molecule and they interact to give a pi electron delocalised bond around the molecule of benzene?
The 1 electron from each carbon atom (6 in total) that are not involved in forming single covalent bonds
73
What is responsible for stabilising benzene?
The pi electron delocalisation
74
What does the pi electron delocalisation do to benzene?
Stabilises it
75
How can the pi electron delocalisation be imagined in benzene?
As a charge cloud above and below the plane of the ring
76
Why is benzene not a conductor of electricity even though there are delocalised electrons?
The electrons are delocalised around the ring
77
How is the delocalised bond in benzene represented in images?
By a circle
78
How could the benzene ring be described in terms of electrons?
An area of high electron density
79
Describe the ring in the middle of benzene
Electron rich
80
What does a benzene ring attract due to it being an area of high electron density?
Electrophiles
81
What does delocalisation do to benzene compared to a structure containing alternate single and double bonds?
Stabilises it
82
What stabilises benzene?
Delocalisation
83
Explain the three forms of evidence for the delocalisation in benzene
1. The C-C bonds in benzene are of the same length and intermediate in length between C-C and C=C bonds (hence “regular hexagon”)
2. Benzene unlike the alkenes does not undergo addition reactions but undergoes substitution reactions. If benzene underwent addition reactions, the stability afforded by the pi electron delocalisation would be lost (the ring would break). The stability is retained when substitution reactions occur. Thus benzene does not undergo addition reactions with bromine or hydrogen bromide
3. Benzene is not oxidised by potassium manganate (VII) solution, unlike the alkenes.
84
Why can’t benzene undergo addition reactions and what type of reactions does it undergo instead?
If it underwent addition reactions, the stability afforded by the pi electron delocalisation would be lost (the ring would break)
Substitution reactions happen instead, where the stability is retained
85
What type of reactions does benzene undergo rather than what? Why?
Substitution rather than addition
To retain the stabilisation afforded to the molecule by pi electron delocalisation
86
Pi electron delocalisation
Pi electrons at 90 degrees to the plane of the molecule
87
What type of reactions are the two main reactions of benzene?
Electrophilic substitution
88
What happens during the nitration of benzene?
A nitro group (-NO2) replaces a hydrogen atom in the benzene ring
89
Nitro group
-NO2
90
-NO2
Nitro group
91
Reagents for the nitration of benzene
Concentrated sulphuric acid and concentrated nitric acid
92
Conditions for the nitration of benzene
Reflux (heat), <50 degrees Celsius
93
Product of the nitration of benzene
Nitrobenzene, C6H5NO2
94
Equation for the nitration of benzene
C6H6 + HNO3 ——> C6H5NO2 + H2O
95
How is nitrobenzene formed?
As a yellow oily liquid
96
What type of compound is nitrobenzene?
Toxic
97
Can benzene be nitrated further after its nitration?
Yes under more extreme conditions
98
Mononitration
Nitration with only 1 nitro group
99
What type of reaction is the mononitration of benzene an example of?
Electrophilic substitution
100
Electrophile during the mono nitration of benzene
The nitryl cation, NO2+
101
How is the nitryl cation NO2+ formed in the reaction mixture during the nitration of benzene?
Formed when the concentrated sulphuric acid reacts with the concentrated nitric acid
102
Which area attracts the electrophile during the nitration of benzene? Why?
The delocalised pi electrons
Provide an electron rich area
103
What does the electrophile form a bond with during the nitration of benzene?
With the delocalised pi electrons (an electron rich area)
104
When is delocalisation partially lost during the nitration of benzene?
When an unstable intermediate is formed
105
How is stability restored after the unstable intermediate during the nitration of benzene?
A hydrogen atom is lost to give the product in order to restore the delocalisation and hence the systems stability
106
Where exactly do we draw the arrow during the intermediate stage of the nitration of benzene?
Start it from the centre of the C-H bond
107
What does the dotted line represent during the reaction mechanism of the nitration of benzene?
The broken delocalised ring
108
Curly arrows
Represent the movement of a pair of electrons
109
In which direction are curly arrows always from and to?
Always from negative to positive
110
What happens during the halogenation of Benzene?
A halogen atom (Cl, Br) replaces a hydrogen atom on the benzene ring
111
What type of reaction is the halogenation of benzene?
Substitution
112
Will benzene decolorize bromine on its own?
No
113
What is the catalyst required for in the halogenation of benzene known as?
A halogen carrier
114
Reagents for the halogenation of benzene
Chlorine or bromine
115
Catalyst for the halogenation of benzene
Aluminium chloride or iron (III) chloride (halogen carrier)
116
Conditions for the halogenation of benzene
Room temperature, dry
117
Possibly products of the halogenation of Benzene
If Cl2 was used —> chlorobenzene C6H5Cl
If Br2 was used —> bromobenzene C6H5Br
118
Equation for the halogenation of benzene (using Cl2)
C6H6 + Cl2 ——> (C6H5Cl) + HCl
119
Describe the mechanism for the chlorination of benzene
The chlorine molecule is polarised as it gets close to the benzene molecule. The partial positive charge of the chlorine acts as the electrophile.
Aluminium chloride accepts a chloride ion to form AlCl4- (aluminium is electron deficient). Also an electron is given back from C-H in the middle to regain stability
The aluminium chloride is re-formed
120
Where acts as the electrophile during the halogenation of benzene?
The partial positive charge of the chlorine atom
121
What is the halogenation of benzene a good example to show in terms of catalysts? Explain
Shows how catalysts remain unchanged during a reaction
AlCl3 shows this
122
What happens during the Friedel-Crafts Alkylation reaction?
An alkyl group replaces a hydrogen atom in the benzene ring
123
Typical alkyl groups
CH3 methyl
CH3CH2 ethyl
CH3CH2CH2 propyl
124
All of the benzene reactions to learn
Nitration
Halogenation
Friedel-Crafts Alkylation
Alkaline hydrolysis of 1-chlorobutane and chlorobenzene
125
Equation for the Friedel-Crafts Alkylation reaction
C6H6 + CH3Cl ——> C6H5CH3 + HCl
126
What is used as a catalyst/halogen carrier during the Friedel-Crafts Alkylation reaction? Why?
Aluminium chloride
Since we need to remove the chlorine from the initial halogen carrier
127
Catalyst for the Friedel-Crafts Alkylation reaction
Aluminium chloride
128
Conditions for the Friedel-Crafts Alkylation reaction
Room temperature, dry
129
Product of the Friedel-Crafts Alkylation reaction
Methylbenzene C6H5CH3 with chloromethane
130
What would the product of the Friedel-Crafts Alkylation reaction be if the reagent is chloroethane?
Ethylbenzene
131
What is the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction for?
A comparison reaction to compare their reactivity with alkaline NaOH
132
1-chlorobutane equation
CH3CH2CH2CH2Cl
133
Chlorobenzene equation
C6H5Cl
134
Method for the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction
1.) 1cm3 of each compound is places in a test tube with 2cm3 of ethanol and 2cm3 of aqueous sodium hydroxide and placed in a water bath at 60 degrees Celsius for 10 minutes
2.) to this mixture is added a mixture of 2cm3 of dilute nitric acid and 2cm3 of aqueous silver nitrate solution similarly warmed for 10 minutes
3.) the mixture is allowed to stand to observe any changes
135
What are we comparing in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction?
How easily chlorine substitutes for an OH in both of the compounds
136
What does OH- act as in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction?
A nucleophile
137
What should react the strongest in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction? Give the expected observations
1-chlorobutane should react more readily (white ppt) than the chlorobenzene (no reaction)
138
Explain in detail why 1-chlorobutane reacts more readily than chlorobenzene in the experiment
Chlorobenzene does not react readily with Nucleophiles such as the hydroxide ion because the lone pair of electrons on the chlorine atom interacts with the pi delocalised electron system of the benzene ring. The lone pair of electrons on the chlorine atom are partially delocalised into the ring system. The C-Cl bond is strengthened.
139
How is the C-Cl bond strengthened in chlorobenzene?
The lone pair of electrons on chlorine are partially delocalised into the ring system
140
What does a stronger C-Cl bond lead to for chlorobenzene?
It means that the chlorine is less likely to be substituted with the OH-
141
Which reaction does chlorobenzene not undergo like 1-chlorobutane and why?
Hydrolysis
As the C-Cl bond is stronger in chlorobenzene and not as easily broken
142
what was the issue with the kekule structure of benzene?
if it had the kekule structure, it would have different bond lengths and so no regular hexagon
143
which halogens will react when heating an aromatic halogeno with aqueous sodium hydroxide?
the chlorines attached to the aliphatic carbon will react, but those attached to benzene remain
144
how do we work out what reaction occurred to produce a product?
we think of the normal reaction that would occur and work in reverse
145
which halogens react and which don’t when heating an aliphatic halogeno with aqueous sodium hydroxide?
the halogens attached to the aliphatic carbon will react, but those attached to the benzene remain
146
% yield equation
Actual/theoretical x 100
147
Why will benzene not react with OH- ions to give phenol as one of the products?
The delocalised ring of electrons in benzene will repel the negative OH- ion
148
Write an equation for the formation of the species that attacks the benzene ring when chlorobenzene is formed
AlCl3 + Cl2 ——> AlCl4 + Cl+
149
Why is it important in the nitration of benzene that the temperature does not exceed 50 degrees Celsius?
As further substitution/nitration into the ring can take place
150
Why would it be expected that the enthalpy of hydration of benzene is 3x that of cyclohexene?
Since benzene has 3 double bonds
151
Name two neutrally charges nucleophiles
H2O and NH3
152
melting point of phenol and solid or liquid?
low
solid
153
melting point of phenol and solid or liquid?
low
solid
154
solubility of phenol
insoluble in cold, soluble in hot water
155
why is phenol only soluble in hot water?
it can form hydrogen bonds with water but the overall molecule is polar
156
toxicity of phenol
highly toxic
157
is phenol corrosive?
yes
158
equation for the reaction between phenol and NaOH
C6H5OH + NaOH —><— C6H5ONa + H2O
159
what colour does litmus not turn with phenol and why?
red
not acidic enough
160
why is phenol more acidic than alcohols?
it’s easier for phenol to donate its proton
161
conditions for the reaction of phenol with ethanol chloride
room temperature, dry
162
observations of the reaction of phenol with ethanoyl chloride + explanation
white musty fumes
HCl
163
conditions for the reaction between phenol and bromine
room temperature, aqueous solution
164
what forms when aqueous bromine reacts with phenol
a white precipitate of 2,4,6 -tribromophenol
165
how do you know that 2,4,6-tribromophenol has been formed during the reaction between phenol and bromine?
there’s a smell of antiseptic
166
what can the reaction with neutral aqueous iron (III) chloride be used for?
to differentiate between a phenol and an alcohol
167
ethanoyl oxy structure
C2O2H3
168
systematic name for aspirin
2-ethanoyloxybenzenecarboxylicacid
169
uses of aspirin
anaesthetic, pain relief, lower temperate, anti-inflammatory, prevention of strokes and heart attacks
170
why does aspirin prevent strokes and heart attacks?
it’s a blood thinner so it stops clotting
171
disadvantages of aspirin
bleeding risks, aspirin poisoning from overdosing, ulcers, allergic reaction, overdose can be fatal
172
(CH3CO)2O name
ethanoic anhydride
173
ethanoic anhydride
(CH3CO)2O
174
what does ethanoic anhydride do during the production of aspirin?
esterifies the phenol group
175
Delocalisation energy of benzene
The extra stability that benzene has as a result of having delocalised electrons (experimental - predicted)
176
Give a large scale use of a named organic halogen containing compound if your choice
DDT
Pesticide
177
Why is chlorine by itself not an organic halogen?
It’s only an organo halogen compound if the carbon and chlorine are bonded together
178
When reacting things with benzene rings, e.g acidified potassium dichromate (VI) or NaOH, which elements react and which don’t?
Only those that are parts of the aliphatic chains react, not those directly bonded to the benzene ring
179
Example of a neutrally charged electrophile
Br2
180
How does the Br-Br (for example) bond become polarised in the halogen action of benzene?
By electron attraction from the pi electron system and from polarisation from the electron deficient atom in the catalyst
181
Catalyst for the bromination of benzene
FeBr3
182
Why is there little reaction between benzene and bromine under normal conditions in the absence of a catalyst?
In the absence of a catalyst, there is little induced polarisation of the bromine molecular by the pi electron system
183
Catalyst for the hydrogenation of alkenes and benzene?
Nickel
184
Reagents for the nitration of benzene
Concentrated sulphuric and concentrated nitric acid
185
Radical
Species with an unpaired electron
