A-level Chemistry: 3.3.10 Aromatic Chemistry
This flashcard set covers the molecular formula, structural characteristics, bonding, and stability of benzene. It highlights benzene’s planar cyclic structure, delocalised electron system, uniform bond lengths, and the reasons behind its exceptional chemical stability.
What is the molecular formula of benzene?
C6H6
Key Terms
What is the molecular formula of benzene?
C6H6
State the structure of benzene
planar cyclic
(6 carbons are joined together in a flat ring)
Draw the skeletal formula for benzene
Describe the structure of benzene
Each C atom forms single covalent bonds to C on either side and to 1 H
Final unpaired electron on each C atom is located in ...
Describe the carbon-carbon bonds in benzene
All carbon-carbon bonds in ring are same = so same length (140 pm)
Lies between the length of a single C-C bond (154...
Why is benzene very stable?
∵ delocalised ring of electrons
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| Term | Definition |
|---|---|
What is the molecular formula of benzene? | C6H6 |
State the structure of benzene | planar cyclic (6 carbons are joined together in a flat ring) |
Draw the skeletal formula for benzene | |
Describe the structure of benzene |
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Describe the carbon-carbon bonds in benzene | All carbon-carbon bonds in ring are same = so same length (140 pm) Lies between the length of a single C-C bond (154 pm) and a double C=C bond (135 pm) |
Why is benzene very stable? | ∵ delocalised ring of electrons |
State how you can prove that benzene is far more stable than theoretical compound cyclohexa-1,3,5-triene (where ring would be made up of alternating single and double bonds) | By comparing enthalpy change of hydrogenation for benzenes with enthalpy change of hydrogenation for cyclohexene |
Cyclohexene has 1 double bond and when it’s hydrogenated, the enthalpy change is -120 kJ mol-1. State what the theoretical enthalpy of hydrogenation would be if benzene had 3 double bonds. | |
Describe how the experimental enthalpy of hydrogenation of benzene is different to the theoretical value of -360 kJ mol-1 |
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Explain the difference between the theoretical enthalpy of hydrogenation of benzene and the experimental value. |
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What are aromatic compounds or arenes? | Compounds containing a benzene ring |
In other molecules the benzene ring can be regarded as a substituent side group on another molecule, like alkyl groups are. The C6H5 - group is known as the phenyl group | |
Explain why arenes attract electrophiles | Benzene ring is a region of high electron density so it attracts electrophiles |
Explain why arenes undergo electrophilic substitution opposed to electrophilic addition |
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Give 2 examples of useful chemicals that contain benzene rings | Dyes and pharmaceuticals |
Why is it tricky to make chemicals that contain benzene | ∵ benzene is so stable = fairly unreactive |
What does Friedel-Crafts acylation do to benzene rings? |
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Why can’t most electrophiles attack a benzene ring? | Most aren’t polarised enough |
How can electrophiles be made into stronger electrophiles? | By using a catalyst called a halogen carrier |
State what electrophile Friedel-Crafts acylation uses | Acyl chloride |
Give an example of halogen carrier that Friedel-Crafts acylation could use | AlCl3 |
Describe how AlCl3 makes the acyl chloride electrophile stronger |
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Draw a diagram showing how AlCl3 makes an acyl chloride electrophile stronger | |
Draw the mechanism for Friedel-Crafts acylation (electrophilic substitution) | |
State the conditions for Friedel-Crafts acylation to occur | Reactants need to heated under reflux in non-aqueous solvent (like dry ether) for reaction to occur |
Describe nitration briefly | When you warm benzene with concentrated nitric and sulfuric acids = nitrobenzene |
State the catalyst used in nitration | Sulfuric acid |
Nitration Why is sulfuric acid used as a catalyst? | Helps to make nitronium ion NO2+ = electrophile |
Nitration Write equations showing how the nitronium ion, electrophile, is made | |
Draw the electrophilic substitution mechanism for nitration of benzene | |
Nitration State what you can do, if you want only 1 NO2 group (mononitration) substituted? | Need to keep temperature below 55°C Above this temperature = get lots of substitutions |
Name 2 uses of nitration reactions |
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What happens when methylbenzene is nitrated with a mixture of nitric and sulfuric acid? | Produces mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene (More vigorous conditions + more nitro-groups substituted) |
How can a mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene be separated? | By thin layer chromatography |
Sulfonation is an important step in… | synthesis including the manufacture of surfactant and sulfonamides |
State the electrophile in sulfonation | SO3 |
Why is SO3 polarised? | ∵ oxygen is more electronegative than sulfur |
Where is SO3 present? i.e. in what acid |
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Draw the electrophile SO3 | |
Draw the mechanism (electrophilic substitution) for sulfonation of benzene | |
State the product when benzene is sulfonated | benzenesulfonic acid |
Write an equation showing when benzene is sulfonated | |
Describe how alkylation (variant of Friedel Crafts acylation reaction) generates an electrophile | Uses chloroalkane as reagent with catalyst of aluminium chloride to generate electrophile |
Alkylation Write an equation showing how the electrophile is generated from aluminium chloride and a chloroalkane (e.g. CH3Cl) | CH3Cl + AlCl3 → CH3+ + AlCl4- |
What products do alkylation produce? | alkyl benzene e.g. benzene reacts to form methylbenzene |
State the mechanism used in alkylation | electrophilic substitution |
Write an equation showing how the alkylation benzene yields methylbenzene e.g. with electrophile = CH3+ | |
Describe how methylbenzene reacts compared to benzene | Reacts similarly to benzene but is more reactive than benzene towards electrophilic substitution |
Explain why methylbenzene is more reactive than benzene towards electrophilic substitution | ∵ methyl group releases electrons onto benzene ring = electrophile attracted more |
What does a sulfonation of methylbenzene produce? | Mixture of 2- and 4- substituted products |
What does free-radical attack by chlorine result in for benzene? | Ring addition to benzene |
What does free-radical attack by chlorine result in for methylbenzene? | Side chain substitution in methylbenzene |
Reaction of benzene with chlorine when exposed to UV is an _ reaction | addition |
Free Radical Reactions What is the final product when benzene reacts with chlorine that has been exposed to UV? | 1,2,3,4,5,6-hexachlorocylohexane | (Exists as number of isomers) |
Free Radical Reactions Draw an equation showing how benzene reacts with chlorine that has been exposed to UV | |
Free Radical Reactions Describe what products are produced when methylbenzene is boiled in presence of chlorine exposed to UV | Methylbenzene produces substitution products of methyl group Behaves like an alkane rather than arene |
Free Radical Reactions Draw a diagram showing how methylbenzene reacts when boiled in presence of chlorine exposed to UV |
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Reactions of Halogen-Substituted Arenes Alkylbenzenes (e.g. methylbenzene) have halogen substituents on ring or alkyl group. The reactivities of these compounds differ according to… | where halogen atom is |
Describe how Cl is substituted in phenylchloromethane |
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Describe how Cl is substituted in 1-chloromethylbenzene (isomer of phenylchloromethane) |
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Explain why it's harder to substitute the chlorine atom in 1-chloromethylbenzene than in phenylchloromethane |
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How do substituents affect the reactivity of benzene ring towards electrophiles? | By releasing electrons onto ring or by withdrawing them |
Outline the electrophilic substitution mechanisms of alkylation using CH3+ as an electrophile | |
Friedel-Crafts Acylation Write an equation to show the role of aluminium chloride as a catalyst in this reaction (with ethanoyl chloride) | CH3COCl + AlCl3 → CH3CO+ + AlCl4- |
Friedel-Crafts Acylation State the product formed | Phenylketone |
Explain why methylbenzene does not react easily with nucleophiles such as ammonia (2) |
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The formula of the ester is (CH3)C6H4COOCH2CH2CH3. State why this is not a structural formula. (1) | the CH3 group can be in different positions on the (aromatic) ring |