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A level Chemistry > Unit 4.2 - Aromaticity > Flashcards

Unit 4.2 - Aromaticity Flashcards

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1
Q

When do radicals form?

A

Hen a covalent bond breaks

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2
Q

Give an example of when a radical forms

A

When you crack a large alkane

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3
Q

Describe a radical

A

Has an unpaired electron and is very reactive

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4
Q

What type of reactions do radicals form?

A

Very rapid ones

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5
Q

How are radicals represented?

A

With a dot

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6
Q

Nucleophiles

A

Ions or molecules which can donate a lone pair of electrons, forming a covalent bond

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7
Q

What are Nucleophiles attracted to?

A

Positive charges or positive centres of charge

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8
Q

Examples of positive centres of charge

A

Nuclei, carbon-halogen bonds

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9
Q

How are Nucleophiles charged?

A

Negatively or neutrally

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10
Q

What do we need to remember to show on Nucleophiles?

A

Lone pairs of electrons

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11
Q

Three types of reagent in organic chemistry

A

Radicals
Nucleophiles
Electrophiles

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12
Q

Electrophiles

A

Ions or molecules which can accept a lone pairs of electrons, forming a covalent bond

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13
Q

What are electrophiles attracted to?

A

Negative charges or negative centres of charge

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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

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15
Q

Negative centre of charge of an alkene

A

Pi bond

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16
Q

How are electrophiles charged?

A

Positively or neutrally

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17
Q

With what do we need to show lone pairs - Nucleophiles or electrophiles?

A

Nucleophiles

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18
Q

What are the 5 types of reactions we need to learn?

A

Additional
Substitution
Elimination
Oxidation/reduction (redox)
Hydrolysis

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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

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20
Q

Example of an addition reaction

A

Electrophilic addition

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21
Q

What type of reactions do alkenes always undergo?

A

Electrophilic addition

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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

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23
Q

Examples of substitution reactions

A

Nucleophilic substituion
Free radical substitution

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24
Q

Example of a free radical substation reaction

A

Photo chlorination of methane

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25
Q

Elimination reaction description

A

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)

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26
Q

During which type of reaction does a double bond often form?

A

Elimination

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27
Q

What would an elimination reaction be known as if water was the eliminated molecule?

A

Dehydration

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28
Q

Redox reaction description

A

Consists of both oxidation and reduction - can’t have one without the other

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29
Q

All of the possible ways of defining oxidation

A

Loss of electrons
Gain of oxygen
Loss of hydrogen

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30
Q

All of the possible ways of defining reduction

A

Gain of electrons
Loss of oxygen
Gain of hydrogen

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31
Q

3 examples of redox reactions

A

Combustion
Total oxidation of primary alcohols
Dichromate (VI) reduced to Cr3+

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32
Q

Colour change when dichromate (VI) is reduced to Cr3+

A

Orange to dark green

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33
Q

Hydrolysis reaction description

A

Chemical reaction between the organic compound and water
Very slow = needs a catalyst

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34
Q

Examples of catalysts used in hydrolysis reactions + why they’re used

A

Sodium hydroxide
HCl
Very slow reactions

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35
Q

Example of a hydrolysis reaction

A

Nucleophilic substitution

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36
Q

What type of compound is benzene?

A

Aromatic

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37
Q

Simplest aromatic compound

A

Benzene

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38
Q

Describe benzene at room temperature

A

A colourless liquid

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39
Q

Give 2 features of benzene

A

Distinctive smell
Highly toxic

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40
Q

What is benzene soluble in and what isn’t it soluble in and why?

A

Insoluble in water (immiscible)
Soluble in non-polar solvents
It’s a non-polar molecule

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41
Q

Describe benzene in water

A

Insoluble (immiscible)

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42
Q

Immiscible meaning

A

Doesn’t mix

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43
Q

What type of intermolecular forces are formed when benzene in dissolved in non-polar solvents?

A

Van der Waals

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44
Q

What type of molecule is benzene?

A

Non-polar

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45
Q

What type of molecules do polar solvents dissolve?

A

Polar molecules (and vice versa for non-polar solvents and molecules)

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46
Q

Empirical formula of benzene

A

CH

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47
Q

Molecular formula of benzene

A

C6H6

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48
Q

What structure for Benzene did August Kekule suggest?

A

A six-membered carbon ring of alternating double and single bonds

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49
Q

What were the issues with the Kekule model for Benzene?

A

Could not explain some of its reactions

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50
Q

Which of benzene’s reactions could the Kekule model not explain?

A

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)

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51
Q

What did Kekule propose to explain why his benzene model did not react as an alkene as it should?

A

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

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52
Q

Resonance arrow

A

<——>

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53
Q

Give an example of when a resonance arrow is needed

A

To show the two resonance Kekule forms of benzene

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54
Q

How does the actual structure of benzene compare to the structure first predicted?

A

It’s a lot more stable

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55
Q

Cyclohexene formula

A

C6H10

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56
Q

What type of molecule is cyclohexene?

A

An alkene

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57
Q

What happens to cyclohexene on hydrogenation?

A

It forms the cyclic alkane cyclohexane

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58
Q

Cyclohexane formula

A

C6H12

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59
Q

Why would it be expected that the enthalpy of hydrogenation of benzene would be 3x that of cyclohexene?

A

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

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60
Q

How does the actual value for the enthalpy change of hydrogenation of benzene compare to the theoretical value? Why?

A

It’s considerably less than calculated
Benzene is more stable than predicted

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61
Q

Why is benzene more stable than predicted?

A

Because of the stabilisation provided by pi electron delocalisation

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62
Q

How do we work out the stabilisation energy of benzene?

A

It’s the difference between the theoretical value and the calculated value of its enthalpy of hydrogenation

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63
Q

How would we work out the enthalpy of formation of benzene using bond energies?

A

Remember that benzene contains 3 C-C and 3 C=C bonds and remember BERP

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64
Q

Chemical formula of benzene

A

C6H6

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65
Q

Structure of benzene

A

Planar cyclic structure

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66
Q

How are the carbon atoms arranged in benzene?

A

In a regular hexagon

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67
Q

What do the electrons of the 6 different carbon atoms in benzene do?

A

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

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68
Q

What type of framework is given rise to from the arrangement of the outer electrons of each carbon atom in benzene?

A

A Trigonal planar framework

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69
Q

What does the Trigonal planar framework of benzene give rise to and what is this similar to?

A

The hexagonal structure
Is a bit similar to graphite

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70
Q

Why does each carbon have an electron left that are found in the p orbitals of benzene?

A

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

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71
Q

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?

A

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

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72
Q

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?

A

The 1 electron from each carbon atom (6 in total) that are not involved in forming single covalent bonds

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73
Q

What is responsible for stabilising benzene?

A

The pi electron delocalisation

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74
Q

What does the pi electron delocalisation do to benzene?

A

Stabilises it

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75
Q

How can the pi electron delocalisation be imagined in benzene?

A

As a charge cloud above and below the plane of the ring

76
Q

Why is benzene not a conductor of electricity even though there are delocalised electrons?

A

The electrons are delocalised around the ring

77
Q

How is the delocalised bond in benzene represented in images?

A

By a circle

78
Q

How could the benzene ring be described in terms of electrons?

A

An area of high electron density

79
Q

Describe the ring in the middle of benzene

A

Electron rich

80
Q

What does a benzene ring attract due to it being an area of high electron density?

A

Electrophiles

81
Q

What does delocalisation do to benzene compared to a structure containing alternate single and double bonds?

A

Stabilises it

82
Q

What stabilises benzene?

A

Delocalisation

83
Q

Explain the three forms of evidence for the delocalisation in benzene

A
  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
Q

Why can’t benzene undergo addition reactions and what type of reactions does it undergo instead?

A

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
Q

What type of reactions does benzene undergo rather than what? Why?

A

Substitution rather than addition
To retain the stabilisation afforded to the molecule by pi electron delocalisation

86
Q

Pi electron delocalisation

A

Pi electrons at 90 degrees to the plane of the molecule

87
Q

What type of reactions are the two main reactions of benzene?

A

Electrophilic substitution

88
Q

What happens during the nitration of benzene?

A

A nitro group (-NO2) replaces a hydrogen atom in the benzene ring

89
Q

Nitro group

A

-NO2

90
Q

-NO2

A

Nitro group

91
Q

Reagents for the nitration of benzene

A

Concentrated sulphuric acid and concentrated nitric acid

92
Q

Conditions for the nitration of benzene

A

Reflux (heat), <50 degrees Celsius

93
Q

Product of the nitration of benzene

A

Nitrobenzene, C6H5NO2

94
Q

Equation for the nitration of benzene

A

C6H6 + HNO3 ——> C6H5NO2 + H2O

95
Q

How is nitrobenzene formed?

A

As a yellow oily liquid

96
Q

What type of compound is nitrobenzene?

A

Toxic

97
Q

Can benzene be nitrated further after its nitration?

A

Yes under more extreme conditions

98
Q

Mononitration

A

Nitration with only 1 nitro group

99
Q

What type of reaction is the mononitration of benzene an example of?

A

Electrophilic substitution

100
Q

Electrophile during the mono nitration of benzene

A

The nitryl cation, NO2+

101
Q

How is the nitryl cation NO2+ formed in the reaction mixture during the nitration of benzene?

A

Formed when the concentrated sulphuric acid reacts with the concentrated nitric acid

102
Q

Which area attracts the electrophile during the nitration of benzene? Why?

A

The delocalised pi electrons
Provide an electron rich area

103
Q

What does the electrophile form a bond with during the nitration of benzene?

A

With the delocalised pi electrons (an electron rich area)

104
Q

When is delocalisation partially lost during the nitration of benzene?

A

When an unstable intermediate is formed

105
Q

How is stability restored after the unstable intermediate during the nitration of benzene?

A

A hydrogen atom is lost to give the product in order to restore the delocalisation and hence the systems stability

106
Q

Where exactly do we draw the arrow during the intermediate stage of the nitration of benzene?

A

Start it from the centre of the C-H bond

107
Q

What does the dotted line represent during the reaction mechanism of the nitration of benzene?

A

The broken delocalised ring

108
Q

Curly arrows

A

Represent the movement of a pair of electrons

109
Q

In which direction are curly arrows always from and to?

A

Always from negative to positive

110
Q

What happens during the halogenation of Benzene?

A

A halogen atom (Cl, Br) replaces a hydrogen atom on the benzene ring

111
Q

What type of reaction is the halogenation of benzene?

A

Substitution

112
Q

Will benzene decolorize bromine on its own?

A

No

113
Q

What is the catalyst required for in the halogenation of benzene known as?

A

A halogen carrier

114
Q

Reagents for the halogenation of benzene

A

Chlorine or bromine

115
Q

Catalyst for the halogenation of benzene

A

Aluminium chloride or iron (III) chloride (halogen carrier)

116
Q

Conditions for the halogenation of benzene

A

Room temperature, dry

117
Q

Possibly products of the halogenation of Benzene

A

If Cl2 was used —> chlorobenzene C6H5Cl
If Br2 was used —> bromobenzene C6H5Br

118
Q

Equation for the halogenation of benzene (using Cl2)

A

C6H6 + Cl2 ——> (C6H5Cl) + HCl

119
Q

Describe the mechanism for the chlorination of benzene

A

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
Q

Where acts as the electrophile during the halogenation of benzene?

A

The partial positive charge of the chlorine atom

121
Q

What is the halogenation of benzene a good example to show in terms of catalysts? Explain

A

Shows how catalysts remain unchanged during a reaction
AlCl3 shows this

122
Q

What happens during the Friedel-Crafts Alkylation reaction?

A

An alkyl group replaces a hydrogen atom in the benzene ring

123
Q

Typical alkyl groups

A

CH3 methyl
CH3CH2 ethyl
CH3CH2CH2 propyl

124
Q

All of the benzene reactions to learn

A

Nitration
Halogenation
Friedel-Crafts Alkylation
Alkaline hydrolysis of 1-chlorobutane and chlorobenzene

125
Q

Equation for the Friedel-Crafts Alkylation reaction

A

C6H6 + CH3Cl ——> C6H5CH3 + HCl

126
Q

What is used as a catalyst/halogen carrier during the Friedel-Crafts Alkylation reaction? Why?

A

Aluminium chloride
Since we need to remove the chlorine from the initial halogen carrier

127
Q

Catalyst for the Friedel-Crafts Alkylation reaction

A

Aluminium chloride

128
Q

Conditions for the Friedel-Crafts Alkylation reaction

A

Room temperature, dry

129
Q

Product of the Friedel-Crafts Alkylation reaction

A

Methylbenzene C6H5CH3 with chloromethane

130
Q

What would the product of the Friedel-Crafts Alkylation reaction be if the reagent is chloroethane?

A

Ethylbenzene

131
Q

What is the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction for?

A

A comparison reaction to compare their reactivity with alkaline NaOH

132
Q

1-chlorobutane equation

A

CH3CH2CH2CH2Cl

133
Q

Chlorobenzene equation

A

C6H5Cl

134
Q

Method for the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction

A

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
Q

What are we comparing in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction?

A

How easily chlorine substitutes for an OH in both of the compounds

136
Q

What does OH- act as in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction?

A

A nucleophile

137
Q

What should react the strongest in the alkaline hydrolysis of 1-chlorobutane and chlorobenzene reaction? Give the expected observations

A

1-chlorobutane should react more readily (white ppt) than the chlorobenzene (no reaction)

138
Q

Explain in detail why 1-chlorobutane reacts more readily than chlorobenzene in the experiment

A

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
Q

How is the C-Cl bond strengthened in chlorobenzene?

A

The lone pair of electrons on chlorine are partially delocalised into the ring system

140
Q

What does a stronger C-Cl bond lead to for chlorobenzene?

A

It means that the chlorine is less likely to be substituted with the OH-

141
Q

Which reaction does chlorobenzene not undergo like 1-chlorobutane and why?

A

Hydrolysis
As the C-Cl bond is stronger in chlorobenzene and not as easily broken

142
Q

what was the issue with the kekule structure of benzene?

A

if it had the kekule structure, it would have different bond lengths and so no regular hexagon

143
Q

which halogens will react when heating an aromatic halogeno with aqueous sodium hydroxide?

A

the chlorines attached to the aliphatic carbon will react, but those attached to benzene remain

144
Q

how do we work out what reaction occurred to produce a product?

A

we think of the normal reaction that would occur and work in reverse

145
Q

which halogens react and which don’t when heating an aliphatic halogeno with aqueous sodium hydroxide?

A

the halogens attached to the aliphatic carbon will react, but those attached to the benzene remain

146
Q

% yield equation

A

Actual/theoretical x 100

147
Q

Why will benzene not react with OH- ions to give phenol as one of the products?

A

The delocalised ring of electrons in benzene will repel the negative OH- ion

148
Q

Write an equation for the formation of the species that attacks the benzene ring when chlorobenzene is formed

A

AlCl3 + Cl2 ——> AlCl4 + Cl+

149
Q

Why is it important in the nitration of benzene that the temperature does not exceed 50 degrees Celsius?

A

As further substitution/nitration into the ring can take place

150
Q

Why would it be expected that the enthalpy of hydration of benzene is 3x that of cyclohexene?

A

Since benzene has 3 double bonds

151
Q

Name two neutrally charges nucleophiles

A

H2O and NH3

152
Q

melting point of phenol and solid or liquid?

A

low
solid

153
Q

melting point of phenol and solid or liquid?

A

low
solid

154
Q

solubility of phenol

A

insoluble in cold, soluble in hot water

155
Q

why is phenol only soluble in hot water?

A

it can form hydrogen bonds with water but the overall molecule is polar

156
Q

toxicity of phenol

A

highly toxic

157
Q

is phenol corrosive?

A

yes

158
Q

equation for the reaction between phenol and NaOH

A

C6H5OH + NaOH —><— C6H5ONa + H2O

159
Q

what colour does litmus not turn with phenol and why?

A

red
not acidic enough

160
Q

why is phenol more acidic than alcohols?

A

it’s easier for phenol to donate its proton

161
Q

conditions for the reaction of phenol with ethanol chloride

A

room temperature, dry

162
Q

observations of the reaction of phenol with ethanoyl chloride + explanation

A

white musty fumes
HCl

163
Q

conditions for the reaction between phenol and bromine

A

room temperature, aqueous solution

164
Q

what forms when aqueous bromine reacts with phenol

A

a white precipitate of 2,4,6 -tribromophenol

165
Q

how do you know that 2,4,6-tribromophenol has been formed during the reaction between phenol and bromine?

A

there’s a smell of antiseptic

166
Q

what can the reaction with neutral aqueous iron (III) chloride be used for?

A

to differentiate between a phenol and an alcohol

167
Q

ethanoyl oxy structure

A

C2O2H3

168
Q

systematic name for aspirin

A

2-ethanoyloxybenzenecarboxylicacid

169
Q

uses of aspirin

A

anaesthetic, pain relief, lower temperate, anti-inflammatory, prevention of strokes and heart attacks

170
Q

why does aspirin prevent strokes and heart attacks?

A

it’s a blood thinner so it stops clotting

171
Q

disadvantages of aspirin

A

bleeding risks, aspirin poisoning from overdosing, ulcers, allergic reaction, overdose can be fatal

172
Q

(CH3CO)2O name

A

ethanoic anhydride

173
Q

ethanoic anhydride

A

(CH3CO)2O

174
Q

what does ethanoic anhydride do during the production of aspirin?

A

esterifies the phenol group

175
Q

Delocalisation energy of benzene

A

The extra stability that benzene has as a result of having delocalised electrons (experimental - predicted)

176
Q

Give a large scale use of a named organic halogen containing compound if your choice

A

DDT
Pesticide

177
Q

Why is chlorine by itself not an organic halogen?

A

It’s only an organo halogen compound if the carbon and chlorine are bonded together

178
Q

When reacting things with benzene rings, e.g acidified potassium dichromate (VI) or NaOH, which elements react and which don’t?

A

Only those that are parts of the aliphatic chains react, not those directly bonded to the benzene ring

179
Q

Example of a neutrally charged electrophile

A

Br2

180
Q

How does the Br-Br (for example) bond become polarised in the halogen action of benzene?

A

By electron attraction from the pi electron system and from polarisation from the electron deficient atom in the catalyst

181
Q

Catalyst for the bromination of benzene

A

FeBr3

182
Q

Why is there little reaction between benzene and bromine under normal conditions in the absence of a catalyst?

A

In the absence of a catalyst, there is little induced polarisation of the bromine molecular by the pi electron system

183
Q

Catalyst for the hydrogenation of alkenes and benzene?

A

Nickel

184
Q

Reagents for the nitration of benzene

A

Concentrated sulphuric and concentrated nitric acid

185
Q

Radical

A

Species with an unpaired electron

A level Chemistry (39 decks)

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