Aromatic Compunds Flashcards
Main Character
The magical ring of benzene
Describe Benzene
- Ring of Carbon atoms
- Molecular formula of C6H6
- Planar cyclic structure of six carbons joined together in a flat ring
- Each carbonic n atom forms single covalent bonds to the carbons on either side of it and to one hydrogen atom
- The final unpaired electron is located in the p-orbital that sticks out above and below the plane if the ring
- Forms a ring of delocalised electrons
- Length of the bonds is 140 pm
• What is the length of a single C-C bond?
• What is the length of a double C=C bond?
• C-C bond: 154 pm
• C=C bond: 135
Note
Hydrogens sticking out all in a flat plane
Draw the three structures of benzene
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What makes Benzene stable?
The delocalised ring of electrons
How can we investigate the stability of benzene
• Comparing the enthalpy change of hydrogenation
Describe the process of investigating the stability of benzene and Cyclohexene
- Cyclohexene has one double bond. When it’s hydrogenated, the enthalpy change is -120 kjmol-1. If benzene had three double bonds, you’d expect it to have an enthalpy of hydrogenation of -360 kjmol
- But the experimental enthalpy of hydrogenation of benzene is
- 208kj mol-1, far less exothermic than expected
- Energy is put in to break bonds and released when bonds are made. So more energy must have been put in to break the bonds in benzene than would be needed to break the bonds in a theoretical Cyclohexene
- The difference indicates that benzene is more stable than Cyclohexene would be. This thought to be due to the delocalised electrons
What compounds are derived from benzene
Aromatic Compounds
What are compounds containing benzene rings called?
Arenes or aromatic compounds
Draw the following structures:
• Chlorobenzene
• nitrobenzene
• 1,3-dimethylbenzene
• phenol
• phenylamine
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What type of reactions do arenes undergo?
Arenes undergo electrophilic substitution
Explain why arenes undergo electrophilic substitution reactions
- Benzene ring is a region of high electron density, attracts electrophiles
- The ring is stable, it doesn’t undergo electrophilic addition reactions which would destroy the delocalised ring of electrons, it undergoes electrophilic substitution
- One of the hydrogens is substituted for an electrophile
What are the two type of electrophilic substitution mechanisms you must know
- Friedel-crafts acylation
- Nitration reaction
What substance is produced during Friedel-crafts acylation
Phenylketones
Describe how phenylketones are produced
- Add an acyl group (RCO-) to the benzene ring
- Once added, side chains can be modified using further reactions to make useful products
- Electrophile has to have a strong positive charge to attack stable benzene ring
- Not strong enough positive charges are made into stronger electrophiles using a catalyst and a halogen carrier
- Friedel-Crafts acylation uses an acyl chloride as an electrophile and a halogen carrier, AlCl3
Note 2
- Useful chemicals such as dyes and pharmaceuticals contain benzene rings
- Benzene is so stable that its fairly unreactive
Describe how AlCl3 makes an acyl chloride electrophile stronger and draw diagrams
- AlCl3 accepts a lone pair of electrons from the acyl chloride
- The polarisation in the acyl chloride increases, forms a carbocation
- Makes it a much stronger electrophile, strong enough charge to react with the benzene ring
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Describe how an electrophile is substituted into a benzene ring and draw the diagram (friedel-crafts acylation)
- Mechanism has 2 steps
- Electrons in the benzene ring are attracted to the positively charged carbocation
- Two electrons from the benzene bond with the carbocation
- This partially breaks the delocalised ring and gives it a positive charge
- The negatively charged AlCl4- ion is attracted to the positively charge ring
- One chloride ion breaks away from the aluminium chloride ion and bonds with the hydrogen ion.
- This removes the hydrogen from the ring to form HCL
It also allows the catalyst to reform - The reactants need to be heated under reflux in a non-aqueous solvent
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How are explosives and dyes manufactured
Nitration
Describe how nitrobenzene is made and give the equation
-This is when you warm benzene with concentrated nitric and sulfuric acids.
-Sulfuric acid acts as the catalyst, it helps to make nitronium ion NO2+ an electrophile
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Draw the electrophilic substitution mechanism of nitration
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What temperature is required for mononitration (only one NO2) GROUP ADDED
Below 55C
Why are nitrations useful?
- Nitro compounds can be reduced to form aromatic amines
- These are used to manufacture dyes and pharmaceuticals
- Some nitro compounds can be used as explosives such as 2,4,6 trinitromethylbenzene, trinitrotoluene TNT (check physical flashcards)
In practice, ethanoic anhydride is used in the industrial synthesis rather than ethanoyl chloride
- Give one reason why ethanoyl chloride is not used in the industrial synthesis
HCL fumes/ corrosive/ strong acid
- Explain the bonding in and the shape of a benzene molecule
- Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule.
Stage 1 Bonding
1a) Each C has three (covalent) bonds
1b) Spare electrons (in a p orbital) overlap (to form a π cloud)
1c) delocalisation
Stage 2 Shape
2a) Planar
2b) Hexagon/6 carbon ring/120° bond angle
2c) C–C bonds equal in length / C–C bond lengths between single and double
bond
Stage 3 Stability
3a) Expected ∆Ho hydrogenation of cyclohexatriene = –360 kJ mol–1
3b) ∆Ho hydrogenation benzene (is less exothermic) by 152 kJ mol–1
3c) Benzene lower in energy than cyclohexatriene / Benzene is more stable
(b) Value within range –239 to –121
When methylbenzene reacts with ethanoyl chloride and aluminium chloride, a similar substitution reaction occurs but the reaction is faster than the reaction of
benzene.
Suggest why the reaction of methylbenzene is faster.
- methyl group has positive (induced) effect or increases electron density on the benzene ring
- Electrophile attracted more/ benzene ring a better nucleophile
The hydrocarbons benzene and cyclohexene are both unsaturated compounds. Benzene normally undergoes substitution reactions, but cyclohexene normally
undergoes addition reactions.
(a) The molecule cyclohexatriene does not exist and is described as hypothetical.
Use the following data to state and explain the stability of benzene compared with
the hypothetical cyclohexatriene.
- Benzene is more stable than cyclohexatriene
- Expected ΔHο hydrogenation of C6H6 is 3(–120)
= –360 kJ mol-1 - actual ΔHο hydrogenation of benzene is
152 kJ mol-1 (less exothermic) - Because of delocalisation or electrons spread out or resonance
Draw the structure of methyl benzene
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